Bulletin of the Seismological Society of America, Vol. 103, No. 3, pp. 1640–1661, June 2013, doi: 10.1785/0120120194
Advanced Seismic Analyses of the Source Characteristics
of the 2006 and 2009 North Korean Nuclear Tests
by J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
Abstract
In 2006 and 2009, North Korea conducted underground nuclear tests at a
remote mountainous site in the northeastern part of the country. Both events were
small, leading to uncertainties for location, yield estimation, and discrimination based
on seismic observations. The objectives of the studies described in this paper have
been to more fully exploit data from the International Monitoring System (IMS) network and other unclassified sources (e.g., Incorporated Research Institutions for Seismology [IRIS], Japanese National Research Institute for Earth Science and Disaster
Prevention [NIED]) to analyze these nuclear tests. We have used differential waveform
interferometry (DWIF) to refine relative locations for the tests and extended the methodology using observations from regional stations to estimate relative emplacement
depths, which are consistent with explosion source model predictions for broadband
regional P-wave spectral ratios. Topographic data from the test site were analyzed to
constrain relative locations and depths needed for containment of the two tests based
on normal testing practice. Network-averaged teleseismic P-wave spectra were inverted using a model-based approach and provide explosion yield estimates in the
range 0.6–1.0 kt for the 2006 test and 2.0–4.8 kt for the 2009 test, depending on
assumed emplacement depth. Detailed spectral analysis of broadband regional P observations are most consistent with theoretical explosion models with yields of 0.9 and
4.6 kt for the 2006 and 2009 tests, respectively, with associated source depths of 200
and 550 m. Consistent with previous studies, the surface-wave M S magnitudes for the
2006 and 2009 tests provide anomalously large M S yield estimates and problematic
M S =mb identification characteristics. Finite-difference simulations of long-period
surface-wave signals incorporating tensile prestress, consistent with the regional tectonics, lead to predicted M S values that agree much better with the observed values.
Introduction
In October 2006 and May 2009 the Democratic Peoples
Republic of Korea (DPRK)—North Korea (NK)—conducted
two nuclear explosion tests at the remote Punggye test site
located at Mount Manthap in northeastern North Korea. Both
events produced rather small seismic magnitudes, which
resulted in limited detections (particularly for the first event)
leading to location uncertainties (Murphy et al., 2010, 2011;
Selby, 2010; Wen and Long, 2010) and raising issues about
the yield estimates as well as the robustness of discrimination
methods based on MS versus mb (Kim and Richards, 2007;
Bonner et al., 2008, 2011; Koper et al., 2008; Patton and
Taylor, 2008; Zhao et al., 2008, 2012; Chun et al., 2011).
The objective of the study described here has been to more
fully exploit the data from the International Monitoring System (IMS) network and other data sources to conduct comprehensive, advanced analyses of the characteristics of these
two North Korean nuclear tests. In particular, the focus here
is on refining the event locations, estimating source depths,
determining seismic yields, and better understanding the surface-wave excitation for these two explosions through careful analysis and comparisons of the seismic data combined
with additional constraints from geologic/tectonic information and nuclear test containment practice.
Prior studies have used observations from teleseismic
(Selby, 2010) and regional (Wen and Long, 2010) seismic
stations to refine the original locations reported by the
U.S. Geological Survey (USGS, 2006, 2009) and International Data Centre (IDC, 2006, 2009) for the North Korean
nuclear tests. The locations determined by the various seismic methods generally provide reasonable estimates of the
epicenter locations for the two tests, which are consistent
with locations of the tunnel complex (ground truth) used for
emplacing the explosions based on satellite imagery analyses
(Global Security website, see Data and Resources; Bennett
et al., 2006). These prior analyses, however, have focused
mainly on traditional methods for resolving the epicenter
1640
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
and relative locations with little attention to other factors that
can be used to constrain the source location. In the following,
we apply a location technique based on waveform interferometry (Snieder, 2002; Snieder and Vrijlandt, 2005) to determine relative locations for the 2006 and 2009 explosions,
and we further analyze the residual differential times of
common event-station-phase pairs after correcting for origin
time and fixed epicenter to determine relative depths. We then
analyze the resulting relative location and depth estimates with
respect to topography in the site region to determine areas in
the vicinity of the Mount Manthap tunnel complex where nuclear explosions could be emplaced satisfying normal nuclear
explosion containment practices for depth of burial.
As noted above, previous studies have found that the
relatively small magnitudes of both North Korean nuclear
tests corresponded to low seismic yield estimates but raised
issues about procedures used in assigning yield based on
different types of magnitude measurements (Bonner et al.,
2008, 2011; Koper et al., 2008; Patton and Taylor, 2008). In
particular, long-period surface-wave observations (MS ) from
both of the North Korean nuclear tests produced yield estimates significantly larger than the corresponding yield estimates based on teleseismic body-wave measurements (mb ),
raising questions about potential bias in one or the other of
the observations. In the following, we have used a modelbased approach based on P-wave spectra (Murphy, 1989; Murphy and Barker, 2001) to confirm the low yield estimates from
seismic body-wave observations. The spectral model incorporates a formal mechanism to account for variations in the
source medium, depth of burial, and upper-mantle attenuation,
which are known potential causes of bias in yield estimates
based on seismic body waves from uncalibrated source areas.
Although the P-wave spectral model enables improved
understanding of potential biases, the focal depth for shallow
nuclear-explosion sources remains unresolved for the limited
frequency band of teleseismic P observations, and traditional
seismic location techniques provide little constraint on
source depth for these small shallow events. To help determine depths for the North Korean nuclear explosions, we
have used spectral measurements from broadband P signals
at regional distances. Spectral ratios at common stations
essentially remove the path/propagation effect, and the resulting regional P spectral ratios are then compared with
theoretical spectral ratios predicted by an explosion source
model (Mueller and Murphy, 1971) to determine consistency
with alternative source depth hypotheses. Constrained analyses of regional phase arrival-time data are then used to confirm the differences in depths for the two explosions inferred
from the broadband spectral ratio analysis.
As noted in the previous section, the relatively large M S
magnitudes associated with the two North Korean nuclear
tests cause apparent inconsistencies in the explosion yield
estimates and also for traditional M S -versus-mb discrimination. To help isolate potential sources of these inconsistencies,
we performed moment-tensor analysis of a secondary source,
corresponding to tectonic release triggered by the explosion,
Russia
Mongolia
Eurasian
Plate
Sino-Korean
Craton
China
1641
North
American
Plate
Mt. Changbai
NK Test
Site
East Sea/
Sea of Japan
Pacific
Plate
West Sea/
Yellow Sea
South
Sea
Philippine
Plate
Figure 1.
Google Earth map illustrating the geotectonic setting
of the Korean peninsula with approximate plate boundaries (after
Lee and Yang, 2006) and location of the North Korean nuclear tests.
to determine effects on long-period (LP) surface-wave observations and the associated M S bias. We further applied a finitedifference simulation to develop a better understanding of how
the explosion yields, depths of burial, and prestress could have
affected nonlinear behavior in the near-source region.
Geologic/Tectonic Environment of the NK Test Site
The Mount Manthap area where the NK nuclear tests
were conducted is part of the Sino-Korean craton (Fig. 1).
The Sino-Korean craton has been generally stable following
several periods of metamorphism and intrusion (USGS, 1967)
during the Proterozoic (2500–540 Ma) with a later phase of
graben formation accompanied by intrusions and volcanism
during the Mesozoic (225–65 Ma) and Cenozoic (65 Ma to
present). The overall stable platform/massif nature of the
region surrounding the NK test site is confirmed in several
older publications (e.g., Paek et al., 1993). The most recent
tectonic activities affecting the area surrounding the NK test
site have been related to the opening of the East Sea/Sea of
Japan and the ongoing volcanic activity to the north in the
vicinity of the North Korea/China border. The former is associated with the most recent phase of plate tectonics, which
started in Miocene time (∼25 Ma) and resulted in the pulling
away of the Japanese island arc from the mainland forming a
back-arc basin.
Mount Changbai is an active volcano to the northwest of
the NK test site (∼110 km away) located on the North Korea/
China border. Although volcanic activity near Mount Changbai is interpreted as having continued throughout Cenozoic
time (Don, 1993), the nature of the volcanism changed
significantly over the Miocene, as the plate boundary pulled
farther away. Some current interpretations for the volcano’s
1642
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
Figure 2. (a) North–south and (b) east–west cross sections illustrating the relative change in Moho depth under the NK test site based on
older analyses of Bouguer gravity and other geophysical field measurements (Song and Thae, 1993; Song et al., 1993).
active status suggest that a continental hotspot is located in
the upper mantle beneath the mountain. The general relative
stability for the remainder of the Korean peninsula is evidenced by a lack of significant earthquake activity (Lee and
Lee, 2003; Lee and Yang, 2006), which is particularly true
for northeastern North Korea. Instrumentally recorded earthquakes on the Korean peninsula have had magnitudes as
great as 5–6 M L , and the larger events occur on major faults
associated with Mesozoic orogenies. Lower levels of seismicity in the northeastern part of the peninsula (including
the NK test site area) are interpreted as indicative of less
crustal disturbance in that area during the Mesozoic. So,
overall indications are that the North Korean test site is located in a massif that is part of a stable platform or cratonic
area. The tectonic setting is analogous to that of other nuclear
explosion test sites in platform areas (e.g., Semipalatinsk,
Lop Nor) and unlike that of more active tectonic test site
areas (e.g., U.S. Nevada Test Site [NTS]).
Additional information on the overall crustal structure of
the Korean peninsula and in the vicinity of the NK test site
comes from some older studies of geophysical measurements
(e.g., Bouguer gravity, geomagnetic, and seismic data)
analyzed by the North Koreans (Song and Thae, 1993; Song
et al., 1993). Based on interpretation of these data, depth to
the Moho tends to be rather uniform over much of the Korean
peninsula with an average depth of 29–32 km, a fairly typical
value for continental areas, throughout the southern and
western parts. In contrast, the crustal thickness changes
rapidly over short distances in the northeastern part of the
peninsula, where the NK test site is located. Figure 2 shows
an interpretation of crustal thickness profiles for the vicinity
of the NK test site based on the older geophysical measurements. The depth to the Moho at the NK test site has a value
of ∼33 km (again a fairly nominal value for continental
crust) but increases rapidly to the northwest to ∼37–38 km
near the China/North Korea border and decreases rapidly to
the southeast to ∼28 km along the East Sea/Sea of Japan
coastline. Farther to the east, the same geophysical data
indicate a strong gradient in crustal thickness extending into
the East Sea/Sea of Japan, transitioning into oceanic crust
with thicknesses of only ∼8–9 km near the central basin
(∼200 km offshore). Such changes in crustal thickness
would be expected to principally affect the timing and transmission of regional seismic phases propagating within the
crustal waveguide, although there could be some influences
also on excitation of LP signals by seismic sources within the
crust. In particular, the dipping of the Moho to the northwest
would be expected to introduce significant timing anomalies
(delays) for Pn regional phases propagating to stations in that
direction. Such timing anomalies potentially introduce location errors or increase uncertainties. The rapid changes in
crustal thickness would also be expected to introduce significant attenuation and/or blockages for regional phases like Lg
and Pg for transmission along such source-station paths. The
latter potentially affects detection capability as well as yield
estimates from signal measurements at regional seismic
stations.
Little information is available from published sources on
specific details of the rock types and their properties in the
vicinity of the NK test site. Smaller scale geologic maps
(Fig. 3) for the Korean peninsula indicate the presence of
Archean (3950–2500 Ma) and other Precambrian (> 540 Ma)
basement rocks in the general vicinity of the test site along
with Mesozoic (Jurassic–Triassic, 250–145 Ma) intrusive
igneous rocks, as well as Cenozoic and Quaternary
(< 2:5 Ma) extrusive volcanic rocks extending toward the test
site area from Mount Changbai and adjacent areas to the
northwest (Steinshouer et al., 1997). Best estimates indicate
that most rocks in the NK test site area are low-porosity, dense,
intrusive and extrusive igneous rocks, including granites
(considered the most likely source rock), basalts, and rhyolites, which are Mesozoic or younger. These are viewed as
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
1643
42°N
42°N
40°N
32°N
128°E
122°E
130°E
130°E
Figure 3. General geology of the Korean peninsula (adapted from Steinshouer et al., 1997). A indicates rocks of Archean age, pϵ Precambrian age, JTR Jurrasic–Triassic age, PZu Upper Paleozoic age, CZ Cenozoic age, and Q Quaternary age. NK test site, magenta circle.
providing the best environment for containing nuclear explosion tests; rocks older than Mesozoic tend to be more fractured
and less competent. Thus, the surface rock types identified
from the limited available literature sources for the NK test
site area suggest competent hard rocks, consistent with the
inference that the nuclear tests were conducted in “good coupling” media.
Because of the relatively stable tectonic setting, seismicity has been rather limited in the Korean peninsula and corresponding knowledge from earthquake focal mechanisms is
sparse. Figure 4 shows locations of 27 earthquakes with M ≥
4:0 during the time period 1936–2004 analyzed by Jin and
Park (2006) and the corresponding composite mechanism,
which can be used to infer the general state of stress in
the region. The principal tension axis is nearly horizontal
and oriented approximately north-northwest/south-southeast, whereas the principal compression axis is also nearly
horizontal and oriented approximately east-northeast/westsouthwest. This stress regime appears to be consistent with
the recent plate motion associated with extension forming the
back-arc basin and opening of the East Sea/Sea of Japan
(noted above). As described below, this stress regime has
possible implications for surface-wave generation from
source mechanisms combining the NK explosions with tectonic strain release.
Location
Our objective in this phase of the study was to obtain the
best estimate of the locations of both nuclear tests using
seismic methods. The lack of historical calibration data from
Russia
China
NK Test
Site
Extension
200 km
Figure 4. Google Earth map illustrating locations of 27 earthquakes 1936–2004 with M ≥ 4:0, orange circles, with interpretation
of stress regime based on composite focal mechanisms (after Jin and
Park, 2006). Mechanisms are consistent with the tectonic model
indicating recent extension with back-arc basin formation in the
East Sea/Sea of Japan.
the North Korea test site severely limits the accuracy of
single-event methods. Even with good regional structure
models, biases of several kilometers can be expected. Our
approach was to determine the relative locations between
the events. We used three relative location algorithms that
1644
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
2009/05/25 00-54-45.17
complement each other in terms of the data, earth structure
model, and objective function (Table 1).
3.00
2.50
41.32˚
2.00
Differential Waveform Interferometry (DWIF)
1.50
Coda-wave interferometry has been demonstrated to
constrain the separation between pairs of closely spaced
events by exploiting the fact that the cross correlation falls
off with increasing source separation (Snieder, 2002; Snieder
and Vrijlandt, 2005). The DWIF method builds on the basic
concept that the relative location of a new event with respect
to one or more reference events can be obtained from differential times of common event-station-phase pairs, that is, it
requires pairs of events observed at common stations. Rather
than pick the lag of the maximum of the correlation to obtain a
differential time, the DWIF method involves time shifting the
correlation traces for a given event location hypothesis using a
slowness model of the source region and stacking the correlation traces from all event-station pairs to produce a network
stack. A grid search is performed to determine the event origin
time and hypocenter that maximizes the objective function,
here defined as the maximum of the network stack.
The individual correlation traces are weighted by the
statistical significance of the correlation results, thus, eventstation-phase pairs that do not correlate well are implicitly
down weighted and no a priori rejection of data or outlier
rejection is required. The correlation processing is performed
using distance and phase-dependent rules (filter band, window length) allowing the use of all common body-wave
portions of the waveform (regional, primary, secondary).
Seventy-two stations with waveform recordings and good
signal-to-noise-ratio (SNR) signals for both events were used
in the processing. This included stations at regional and teleseismic distances. The grid search found that the objective
function was maximized for the 25 May 2009 event located
2.3 km to the west and 0.5 km to the north of the 2006 event.
To assess the uncertainty in the solution we performed a
bootstrap experiment. For each case, we randomly selected
half of the stations and performed the relative location using
the same approach as for the full station network. The
locations for each of those cases are plotted as small white
circles in Figure 5. The error ellipse plotted in Figure 5 was
1.00
41.30˚
0.50
0.00
-0.50
-1.00
41.28˚
-1.50
-2.00
-2.50
5
0
41.26˚
129.02˚
129.04˚
129.06˚
129.08˚
Figure 5.
Color-contoured slice through the 3D objective function grid at constant relative depth (0) at a fixed origin time. Location of the 9 October 2006 event that was used as a fixed reference
point in the algorithm, red star. Results of a boot-strap experiment
involving subsets of stations to quantify the uncertainty, white
circles. A 90% confidence ellipse based on the bootstrap results,
black ellipse.
defined as the ellipse that enclosed 90% of the bootstrap experiment locations. To gain further confidence in the result,
we applied the algorithm to various subsets of the network.
Table 2 summarizes the results for the various station subsets.
As an alternative to the correlation processing method,
arrival times for two events recorded with adequate SNR at a
given station can often be measured precisely by manual
waveform alignment (see e.g., Fisk, 2002). We measured
such relative arrival times manually for all stations with recordings of both explosions. Data were prefiltered with a
passband typically between 1 and 2 Hz at most stations beyond regional distances to improve SNR. Alignment focused
on the initial part of the signals to reduce possible bias due to
the 0.4 magnitude difference between the two explosions.
Relative arrival-time measurements were made for 56 stations at distances between about 3.5° and 85° from the North
Korean test area.
Table 1
Comparison of Relative Location Algorithms
Differential Waveform
Interferometry (DWIF)
Stations
Phases
Travel-time model
Measurements
Objective function
Algorithm
0°–85°
Regional, teleseismic P
Source slowness model derived
from IASPEI91
Waveform cross correlation
“Best” stack of the correlation
traces after slowness correction
Snieder and Vrijlandt (2005)
-3.00
129.10˚
Joint Hypocenter
Determination (JHD)
0°–85°
First arriving P only
IASPEI91 travel-time tables
Manual waveform alignment
Weighted rms residual of measured
versus theoretical after removal of
static station corrections
Dewey (1972)
Double
Difference (DD)
< 10°
First arriving P only
Layered 1D Korean
Peninsula model
Manual waveform alignment
Weighted rms residual of
double-difference times
Waldhauser and Ellsworth (2000)
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
1645
Table 2
Relative Locations of the 25 May 2009 Event with Respect to the 9 October 2006 Event
Network*
IMS,
IMS,
IMS,
IMS,
IMS,
IMS,
IRIS reg teleseismic
IRIS, NIED, regional, and teleseismic
IRIS first P only
IRIS regional only
IRIS, regional only, using high-frequency filters
IRIS, NIED, regional only
Time of 2009/05/25
ΔEast(km)
ΔNorth (km)
Latitude (°)
Longitude (°)
N-Sta
00:54:45.17
00:54:45.17
00:54:45.18
00:54:45.17
00:54:45.18
00:54:45.17
−2.3
−2.3
−1.8
−2.4
−2.2
−2.3
0.5
0.5
0.8
0.5
0.4
0.5
41.2955
41.2955
41.2982
41.2955
41.2946
41.2955
129.0575
129.0575
129.0635
129.0563
129.0587
129.0575
47
87
32
16
16
56
*Location differences based on DWIF processing for the various station network subsets are shown with respect to the location of the 9 October 2006
(41.291° N, 129.085° E) reported by Bennett et al. (2006).
Table 3
Comparison of the Relative Location Results
Method*
Fixed
DD
JHD
DWIF
Date Time (yyyy/mm/dd hh:mm:ss.ss)
2006/10/09
2009/05/25
2009/05/25
2009/05/25
01:35:29.90
00:54:44.90
00:54:45.10
00:54:45.17
Latitude (°)
Longitude (°)
Smaj
Smin
Az
41.291
41.2986
41.2968
41.2955
129.085
129.0616
129.0605
129.0575
0.13
0.13
0.25
0.23
0.12
0.12
0.20
0.26
90
90
62
2
*The double-difference solution (DD) used regional stations, including NIED station. JHD used
regional and teleseismic IMS and IRIS stations.
Joint Hypocenter Determination (JHD)
We calculated relative epicenters of the two explosions
using the algorithm developed by Dewey (1972). Only first
arrival P phases were used. The epicenter of the 2006 explosion was fixed at 41.291° N and 129.085° E and at zero depth
throughout the calculations. To detect and remove possible
gross errors in the data, we applied Grubbs’ (1950) outlier test.
This test, which assumes normality of the underlying distribution, tests the null hypothesis that there are no outliers in the
data set. The test was applied iteratively to the arrival-time
residuals of successive JHD runs. Grubbs’ test detects one outlier at a time. Arrivals for the stations with an outlying residual
(p value > 0:05) were removed in each iteration before the
next JHD run with the arrivals of the remaining stations. This
iteration continued until no outlying residuals were detected.
Double Difference (DD) Locations
We also applied the double difference method (Waldhauser and Ellsworth, 2000) to data from stations within
∼10°. Convergence of the calculations was found to be dependent on the depths of the starting solutions, but epicenters
of converging solutions generally agreed with JHD solutions.
The epicenter of the 2009 explosion for starting depths at
zero for both explosions was 41.2986° N 129.0616° E, after
the epicenter solutions of the two explosions were shifted so
that the 2006 explosion coincided with the location fixed
in the JHD calculations. In all, 41 stations were used in this
solution after applying an outlier cutoff at 1.96 standard
deviations (95%).
Relative Epicenters
Table 3 summarizes the relative epicenters of the 2009
event assuming a fixed epicenter of the 2006 event (Fig. 6).
The differences between the solutions are quite small
(< 0:5 km) and are likely due to differences in the station
network for each method. We take the DWIF result as our
preferred solution because it included the largest number of
contributing stations and was based on cross correlations,
thereby reducing measurement error. This places the epicenter of the 25 May 2009 event ∼2:5 km to the west-northwest
of the 9 October 2006 origin and is consistent with the findings of most other researchers (Table 4).
Yield Estimation
The yields of underground nuclear explosions are typically estimated from the related measure of seismic magnitude
or similar measures of seismic signal strength. This is accomplished using empirical relationships between the known
yields of fully tamped nuclear explosions that have been conducted at a variety of globally dispersed test sites and various
measures of the strength of the recorded seismic signals (e.g.,
mb , M S ). Note that in the following discussions we assume
that both explosions were fully tamped and not evasively
tested, which seems appropriate given the fact that the North
Koreans announced these tests as signs of their power and,
consequently, are unlikely to have deliberately done anything
to reduce their apparent size. The simplest and most widely
used method for determining explosion yield is based on the
correlation with teleseismic body-wave magnitude, mb . Extensive data sets from all known nuclear test sites have been
1646
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
Figure 6. Locations of the 25 May 2009 event assuming a fixed
location for the 9 October 2006 event based on Bennett et al.
(2006). It places the 2009 event beyond the area of maximum overburden relative to the adit, indicating a probable bias in the fixed
location for the 2006 event.
analyzed to calibrate this relationship, and it has been found
that the applicable mb =yield relation depends in part upon the
upper-mantle attenuation characteristics beneath the test site
region. More specifically, results of studies conducted using
data from a number of test sites worldwide suggest that the
mb =yield relations for explosions conducted at nominal containment depth (i.e., ∼120 m=kt1=3 ) in “good coupling”
source media (i.e., hard-rock or water-saturated media) can
be bounded by the results for such tests at the NTS and the
former Soviet Semipalatinsk test site (Semi), with the relations given by Murphy (1981, 1996)
mb 3:94 0:81 log WNTS
mb 4:45 0:75 log WSemi;
(1)
where W is the yield in kt.
It should be noted that these empirical mb =yield relations were originally calibrated using NEIS magnitude estimates and, consequently, the NEIS mb values are used herein
for yield estimation purposes. In our later discussion of the
MS =mb identification criterion, we use the corresponding
IDC mb values, as the IDC event screening line was explicitly calibrated for use with IDC magnitudes (Fisk et al.,
2002). Using the NEIS mb estimates for the two North
Korean tests (i.e., mb 2006 4:3; mb 2009 4:7), to
which the relations of equation (1) apply, gives yield estimates ranging from about 0.6 to 2.8 kt for the 2006 explosion
and approximately 2.2–8.7 kt for the 2009 explosion. As will
be shown later, evidence from spectral data suggest that the
lower estimates in these ranges obtained using the Semipalatinsk mb =yield relation are more appropriate for the North
Korean test site. Comparable yield estimates are obtained
using other short-period magnitude regional measures, such
as mb Lg.
Although yield estimates based on MS have historically
been found to be less precise than those based on shortperiod mb and mb Lg values, they have been used to estimate explosion yields since the beginning of underground
nuclear testing, and these yields have generally been found
to be reasonably consistent, on average, with mb -based estimates. In the case of the North Korean tests, however,
the observed M S magnitude values provide dramatically different yield estimates from those obtained from the corresponding mb values. The MS =yield relation determined on
the basis of calibration with globally distributed historical
explosion data has the form (Stevens and Murphy, 2001)
M S 2:1 1:0 log W:
By using the M S estimates discussed later for these two
tests (i.e., M S 2006 2:93; M S 2009 3:66), this relation gives yield estimates of 6.8 kt for the 2006 explosion
and 36 kt for the 2009 explosion. That is, the yields inferred
from the observed M S values are more than a full order of
magnitude larger than those obtained from the corresponding
mb values using the Semipalatinsk mb =yield relation. As will
be shown in the following model-based spectral analysis, it
Table 4
Comparison of Relative Location Results with Other Published Sources
Date Time (yyyy/mm/dd hh:mm:ss.s)
Latitude (°)
Longitude (°)
Author
2006/10/09 01:35:29.9
41.2867
41.26386
41.2874
41.2925
41.275
41.299567
41.2939
129.0902
129.08591
129.1083
129.0657
129.089
129.054578
129.0817
This study
Rodgers*
Wen†
This study
Selby‡
Rodgers*
Wen†
2009/05/25 00:54:45.3
*Rodgers et al. (2010)
and Long (2010)
‡
Selby (2010)
†Wen
(2)
2009 Event Relative to 2006
2.3
1.8
2.3
2.2
km
km
km
km
west,
west,
west,
west,
0.5
0.3
4.0
0.7
km
km
km
km
north
north
north
north
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
Figure 7. Comparison of the network-averaged, teleseismic
P-wave spectra determined from data recorded at a common set
of stations from the North Korean nuclear tests of 25 May 2009
and 9 October 2006. It can be seen that the spectral amplitude levels
for the 2009 test are about a factor of four larger, on average, than
those for the 2006 test over the frequency band extending from 0.5
to 2.5 Hz.
has been concluded that the short-period yield estimates are
more appropriate and, consequently, that the long-period surface waves generated by these two North Korean explosions
were anomalously large. Possible reasons for this anomaly
are addressed in the following surface-wave analysis section.
All of the magnitude-based yield estimates described
above are based on simple statistical correlations of observed
1647
magnitudes with known yields of past explosions and do not
explicitly account for variations in physical variables that can
affect seismic yield estimates. In an attempt to overcome this
limitation of the empirical approach, a model-based yield estimation procedure based on inversion of observed networkaveraged, teleseismic P-wave spectra was developed and
successfully applied to explosions at a variety of nuclear test
sites (Murphy, 1989; Murphy and Barker, 2001). This formulation provides a formal mechanism for explicitly accounting
for effects due to variations in explosion source medium and
depth of burial, as well as upper mantle attenuation conditions beneath the test site, although it still does require independent estimation of several different model parameters.
Figure 7 shows a comparison of the observed networkaveraged P-wave spectra for the 2006 and 2009 explosions
determined from data recorded at seven common IMS teleseismic stations (MKAR, FINES, AKASG, ASAR, HFS,
NOA, NVAR) having good SNR ratios over the passband
from 0.5 to 2.5 Hz. The estimated 95% uncertainty bounds
on these observed network-averaged spectral amplitudes
average to about 40% over the displayed frequency range.
It can be seen that the average spectral amplitude levels for
the 2009 test are about a factor of four larger than those for
the 2006 test, in general agreement with the observed
differences in magnitudes referenced above. The observed
network-averaged P-wave spectrum for the 2009 test is
shown compared with the best-fitting theoretical predictions
obtained using the Mueller and Murphy (1971; Mueller/
Murphy) granite source model in the left panel of Figure 8,
assuming a nominal source depth of 200 m and frequencydependent distance attenuation models appropriate for nuclear
Figure 8. Comparison of the observed network-averaged teleseismic P-wave spectrum for the 25 May 2009 North Korean nuclear test
with various best-fitting theoretical Mueller/Murphy source models. (a) Comparisons obtained assuming a nominal source depth of 200 m
and attenuation models consistent with tests at the Semipalatinsk and NTS test sites, confirming that the Semipalatinsk model is much more
consistent with the observed spectral data. (b) Comparisons for assumed source depths of 200 and 800 m together with the Semipalatinsk
attenuation model. It can be seen that the observed teleseismic spectral data do not have the resolving power to distinguish between the
alternate hypotheses of a 2.7 kt explosion at a depth of 200 m and a 4.8 kt explosion at a depth of 800 m.
1648
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
tests at the Semipalatinsk and Nevada test sites. It can be seen
that the inferred yield estimates vary by about a factor of two
(i.e., 2.7 kt versus 5.3 kt) depending on the selected attenuation model and that, in this case, the Semipalatinsk model
with an associated yield estimate of 2.7 kt provides a much
better overall fit to the observed spectrum over the 0.5–
2.5 Hz band than does the NTS model. Similar results were
found from the analysis of the observed network-averaged Pwave spectrum for the 2006 explosion (with an associated
yield estimate of ∼0:6 kt for an assumed nominal explosion
depth of 200 m), and, consequently, the Semipalatinsk attenuation model was adopted for all further yield estimation
modeling of the North Korean explosions. Somewhat different yield estimates would, of course, result from the use of a
different seismic source model. Given what is known about
the geology of the North Korean test site, however, and the
fact that this Mueller/Murphy source model has been demonstrated to be applicable to all hard-rock test sites studied to
date (i.e., NTS granite, French Sahara, Degelen, Balapan,
etc.; Murphy and Barker, 2001), a significantly different seismic source model seems unlikely. Moreover, to remain consistent with the observed network-averaged P-wave spectra,
this would require a highly fortuitous coincidence that the t
value differs from Semipalatinsk in just such a way as to offset any frequency-dependent changes in the source function,
which seem implausible.
Unfortunately, the narrowband, teleseismic P-wave
spectral data available for these two shallow, low-yield
explosions do not provide the resolving power needed to
confidently determine source depths, and this leads to significant uncertainties in the seismic yield estimates. This fact is
illustrated in Figure 8b where the network-averaged P-wave
spectrum for the 2009 test is compared with the best-fitting
predictions for hypothetical Mueller/Murphy sources at
depths of 200 and 800 m. It can be seen that these two predictions are essentially identical over the available 0.5–
2.5 Hz band. That is, the observed teleseismic spectral data
cannot be used to distinguish between the alternate hypotheses of a 2.7 kt explosion at a depth of 200 m and a 4.8 kt
explosion at a depth of 800 m. Similar comments apply to the
2006 test, with best-fitting yield estimates ranging from
∼0:6 kt, for an assumed source depth of 200 m, to ∼1:0 kt,
for an assumed source depth of 800 m. That is, uncertainties
in the source depth estimates over the plausible range from
100 to 800 m, implied by the local topographic and satellite
imagery data, translate into significant uncertainties in the
corresponding yield estimates. In the following section we
will attempt to refine the source depth estimates for these
two explosions and then use the resulting depth constraints
to obtain more confident seismic yield estimates.
Estimation of Explosion Source Depths
As was noted in the preceding section, teleseismic Pwave spectral data over the 0.5–2.5 Hz band do not have
the resolving power to distinguish between source depths
45˚
MDJ
40˚
KSRS
INCN
MAJO
TJN
km
35˚
0
125˚
130˚
500
135˚
140˚
Figure 9.
Locations of the regional stations used in the Pn spectral ratio analysis, relative to the location of the North Korean test
site, star.
in the plausible range from 100 to 800 m for the two small
North Korean nuclear tests, and this leads to significant uncertainties in the associated seismic yield estimates. Consequently, an investigation was initiated in an attempt to define
a robust alternate procedure for constraining the source
depths of these two explosions, thereby reducing this yield
estimation uncertainty.
In principle, broadband regional P-wave data recorded
at regional distances from these explosions could provide the
information needed to distinguish between different source
depth hypotheses. To use such data to accurately infer source
characteristics, however, it is necessary to first correct for
frequency-dependent propagation path effects, and that cannot currently be done with confidence for the regional distance stations that recorded the two North Korean nuclear
tests. One approach to eliminating the uncertainties associated with correcting for frequency-dependent propagation
effects is to compute P-wave spectral ratios of the two explosions at common regional stations. For these closely
spaced explosions, the propagation path effects are essentially the same, and computing the P-wave spectral ratios
cancels them out to give estimates of the broadband seismic
source spectral ratio between these two explosions. The individual regional station P-wave spectral ratios can then be
averaged to obtain a robust estimate of the source spectral
ratio that can be compared with the theoretical source spectral ratios predicted by the Mueller/Murphy explosion source
model corresponding to different source depth hypotheses
for the two explosions.
This analysis approach has now been applied to the
broadband P-wave data recorded from the two explosions
at regional stations KSRS, MDJ, INCN, TJN, and MAJO,
the locations of which relative to the North Korean test site
are shown in Figure 9. The average observed P-wave spectral
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
Figure 10.
Comparison of the average observed broadband Pwave spectral ratio, North Korea (2009)/North Korea (2006), and
associated 95% confidence bounds derived from recordings of both
explosions at five common regional stations with the theoretical
Mueller/Murphy source spectral ratios computed assuming that
both explosions were detonated at either 200 m depth or 800 m
depth. It can be seen that the hypothesis that both tests were detonated at the same depth is excluded by the observed highfrequency spectral ratio data.
ratio, North Korea (2009)/North Korea (2006), estimated
from the initial 5 s of the recordings at these stations, is
shown in Figure 10 over the frequency range from 1 to
15 Hz, together with approximately 95% confidence bounds
on this logarithmic average. Note that the spectral ratio at
frequencies above 10 Hz in Figure 10 is based on KSRS data
alone, as the data at the other stations are not usable above
10 Hz. The KSRS spectral ratio, however, tracks the average
spectral ratio very closely in the 1–10 Hz band, falling within
the estimated 95% uncertainty bounds over this entire frequency range, suggesting that the KSRS high-frequency
spectral ratio is representative. The plotted 95% uncertainty
1649
bounds are approximate in that it was found that, whereas the
computed standard deviations in the individual observations
show some modest variation as a function of frequency, there
are no consistent trends, and a constant multiplicative factor
of 1.3 provides a conservative upper bound over the 1–10 Hz
band. This uncertainty estimate was simply extrapolated
into the 10–15 Hz band in the absence of a more definitive
estimate. Also shown on Figure 10 are the theoretical
Mueller/Murphy source spectral ratios computed assuming
that both explosions were detonated at either 200 or 800 m
depth, using yield estimates for the two explosions consistent
with the teleseismic, network-averaged, P-wave spectral
inversions illustrated in Figure 8. It can be seen that the
observed spectral ratio is statistically inconsistent with the
hypothesis that both explosions were detonated at the same
depth for any depth in this plausible depth range, in that the
corresponding theoretical solutions fall well outside the estimated 95% confidence bounds at high frequencies. In fact,
it has been found that these spectral ratio data are much more
consistent with the hypothesis that the 2006 test was conducted at a depth of ∼200 m, whereas the 2009 test was conducted at a depth of ∼550 m. This fact is illustrated in
Figure 11a where the observed spectral ratio data are compared with the best-fitting theoretical Mueller/Murphy
source spectral ratio obtained by modeling the 2009 test
as a 4.6 kt explosion at a depth of 550 m and the 2006 test
as a 0.9 kt explosion at a depth of 200 m. Note that these
depth estimates were obtained from the inferred pP delay
times assuming an average overburden elastic P-wave velocity of 5 km=s and, consequently, represent upper-bound
estimates. This is a reasonable first approximation for such
small, overburied explosions at a hard-rock test site, however. In these calculations the effects of the surface reflected
pP phases were included with the pP=P amplitude ratios
held at 0.3, consistent with previous experience for shallow
explosions at a variety of other test sites (Murphy and Barker,
2001). It can be seen from this comparison that these source
models predict a source spectral ratio that agrees remarkably
Figure 11. Comparison of the average observed broadband P-wave spectral ratio, North Korea (2009)/North Korea (2006), and associated 95% confidence bounds with various theoretical Mueller/Murphy source spectral ratios. (a) Best-fitting theoretical source spectral ratio
corresponding to a depth of 550 m (W 4:6 kt) for the 2009 test and a depth of 200 m (W 0:9 kt) for the 2006 test. (b) Alternate
theoretical source spectral ratios computed assuming that the 2006 test was conducted at depths of 100 and 300 m.
1650
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
45˚
45˚
40˚
40˚
35˚
35˚
rms residual (s)
0.15
0.10
0.05
0.00
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Relative depth (km)
30˚
30˚
0
115˚
120˚
125˚
130˚
135˚
500
140˚
Figure 12.
Map of seismic stations within the regional distance
range that recorded both North Korean nuclear tests, red star. The
stations marked with a gray triangle were used in the relative
epicenter calculations but were screened out of the data set for
the relative depth calculation due to data quality, low signal coherence or possible timing errors (identified using a Grubbs outlier
test). The stations marked with a blue triangle had the highest signal
coherence between the events and were used in the relative depth
calculation.
well with the average observed spectral ratio and its associated uncertainty over the entire band extending from 1
to 15 Hz.
The sensitivity of the theoretical solution shown in
Figure 11a to the estimated source depths is illustrated in
Figure 11b, which shows corresponding comparisons
obtained by assuming that the 2006 explosion was detonated
at alternate depths of 100 and 300 m. It can be seen that even
these modest changes in the specified depth of the 2006
explosion relative to the nominal best-fit value of 200 m produce significantly worse fits to the observed spectral ratio
data, particularly at high frequencies where the theoretical
predictions fall well below the estimated lower 95% confidence bound on the average observed ratio. Corresponding
variational analyses of the yield estimates suggest that the
nominal best-fit yield values of 0.9 kt for the 2006 explosion
and 4.6 kt for the 2009 explosion have precisions on the
order of 30%, reflecting the fact that the spectral ratio technique eliminates uncertainties associated with propagation
path effects. The absolute yield estimation accuracy also
depends, of course, on the applicability of the underlying
Mueller/Murphy source model.
As was noted above, the high-frequency (> 10 Hz)
spectral information that provides the strongest constraints
on these source depth estimates is based on data from station
KSRS alone. Therefore, it is appropriate to look for independent evidence supporting these estimates. To accomplish
this, we applied the DWIF procedure described above to
available regional phase observations, fixing the relative epicentral locations to the values determined above. Figure 12
shows a map of the local and regional seismic stations used
Figure 13. Root mean square (rms) of the residuals of the relative arrival times plotted against relative depth hypotheses. The
minimum rms (red circle) is found for a relative absolute depth
of 0.35 km, that is, the data suggest that the 2009 event occurred
350 m deeper than the 2006 event. The pattern of the rms residuals
(red curve) was modeled using the Korean Peninsula travel-time
model, assuming a relative depth between the events of 0.35 km
and a Gaussian distribution of relative arrival-time measurements
errors with a standard deviation of 0.04 s.
in the inversion. Cross correlations were computed for time
windows bracketing the individual regional phases (Pg or Pn
and Lg) using the 9 October 2006 event as the template and
the 25 May 2009 event as the target. We used distance- and
phase-dependent rules (filter band, window length) to optimize the time bandwidth product (reduce the width of the
correlation peak), 3–6 Hz for P waves and 1–5 Hz for Lg.
As described earlier, the DWIF method takes the stack of
the correlation traces of all stations and all phases (network
stack) as the objective function and finds the maximum. The
time-domain lag of the peak of the network stack is taken as
the relative origin time. To further quantify the fit, we return to
the individual correlation traces and make a measurement of
the peak for each individual station-phase pair. After removing the relative origin time (taken from the network stack), the
root mean square (rms) of the residual lags provides a measure
of the fit to the correlation traces. By first correcting for the
relative origin time and fixed epicenters, we make a pick of the
lag off the cross-correlation trace in the time domain. The
search window is very small (< 0:4 s) and therefore much less
susceptible to the errors such as cycle skipping that normally affect cross-correlation methods for picking relative arrival times.
To determine the best relative depth, we computed the
rms of the residuals for relative depths between −2:0 and
2.0 km. Figure 13 shows a distribution of the rms as a function of the relative depth. Although no formal uncertainty
estimate is currently available and the observed minimum is
fairly shallow, the best estimate corresponds to the 2009
event being 0.35 km deeper than the 2006 event. Note that
this estimated difference in source depths is identical to that
obtained using the broadband spectral ratio approach and,
thus, provides additional independent evidence supporting
that solution.
It must be noted that relative arrival-time measurements
(both based on analyst picks and from picking the lag off
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
(a)
(b)
41.30°
41.30°
41.28°
41.28°
0
129.04°
129.06°
129.08°
1
2
129.10°
0
129.04°
129.06°
129.08°
1651
1
2
129.10°
Figure 14.
Candidate areas for (a) the 2006 event (red hatched) and (b) the 2009 event (blue hatched) constrained by the relief requirements and the relative epicentral locations, and the derived relative depth between the events. The solutions derived based on relative epicenter only (2006 event: red star; 2009 event: yellow circle) are not consistent with the topographic analysis.
correlation traces) were made at a precision (milliseconds)
below the sampling rate of the raw data (100 Hz 0:01 s).
This raises the question of whether the relative depth finding
is reliable or whether we are just fitting noise.
By using a 1D travel-time model for the Korean Peninsula (Kohl et al., 2004), we computed the theoretical differential arrival times between the 9 October 2006 and 25 May
2009 explosions assuming fixed epicenters according to
Table 4. Figure 13 also shows the modeled rms of the theoretical differential times for a relative depth of 0.35 km and a
Gaussian distribution of relative arrival-time measurement
errors with a standard deviation of 0.04 s. It can be seen that
this model is very consistent with the observed rms residual
pattern.
Topographic Analysis for Estimation
of Absolute Location
The location for the 2006 event that was used in location
analysis described earlier was initially estimated as being
∼1 km into the mountain from a known tunnel adit in the
direction of maximum relief. This placed the 2006 event directly north of the adit. This assumed location of the 2006
explosion (41.291° N, 129.085° E) results in a location for
the 2009 event on the other side of the ridge from the adit
(Fig. 6). This seems unlikely and suggests that the presumed
location of the 2006 event may be biased. To determine a best
estimate of the absolute locations of the NK tests, we conducted a topographic analysis of the area using ASTER
Global Digital Elevation Model. These terrain data are
sampled with a posting interval of 30 m, and an accuracy
of 7–14 m, making them comparable to NIMA DTED
level 2. We looked for the areas that are consistent with
the following assumptions:
1. The 2006 event was placed with 100–300 m of overburden based on:
• yield of 0.9 kt, typically requires at least 130 m for containment (where scaled depth ∼130 Yield in kt1=3 );
• radionuclides were reportedly detected, suggesting possible shallower emplacement.
2. The 2009 event was placed with 350–750 m of overburden based on:
• yield of 4.6 kt, typically requires at least 220 m for containment (where scaled depth ∼130 Yield in kt1=3 );
• no radionuclide detection, suggests possible deeper
containment;
• relative depth estimate placed the 2009 events 350 m
deeper than the 2006 event.
3. The 2009 event was ∼2:5 km to the west-northwest of
the 2006 event.
4. The 2009 event was ∼350 m deeper than the 2006 event.
Figure 14 shows the potential areas consistent with the
above constraints. Under these revised conditions, both
events must have occurred ∼1:5 km farther to the east and
south of the locations determined from the relative location
techniques above (Fig. 15).
Surface-Wave Analysis
The map locations of the six available regional stations
(BJT, ENH, HIA, INCN, KS31, and MDJ) that recorded usable long-period Rayleigh-wave data for both North Korean
nuclear tests are shown in Figure 16. Although Love waves
1652
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
50˚
HIA
45˚
MDJ
41.30˚
40˚
BJT
KS31
INCN
35˚
41.28˚
ENH
30˚
km
0
129.04˚
129.06˚
129.08˚
1
2
0
129.10˚
105˚
Figure 15. Revised best estimates of the North Korea nuclear
explosions, black squares.
110˚
115˚
120˚
125˚
500
130˚
1000
135˚
Figure 16.
Map locations of the six regional seismic stations
that recorded usable long-period data for both North Korean nuclear
tests.
test, a ratio that is quite consistent with the observed ratios of
the corresponding short-period amplitude data. Looking at
the transverse component data displayed in Figure 17, it
can be seen that the background noise level at this station
during the 2006 test was about a factor of five lower than
the observed Love-wave signal level from the 2009 test. That
is, comparable Love waves may have been generated by the
2006 test, but not at detectable levels. In any case, evidence
suggests that the long-period seismic source functions for
these two North Korean nuclear tests contain nonisotropic
(a)
(b) 5000
Relative amplitude
Relative amplitude
were not evident at these stations for the 2006 test, they were
clearly observable for the larger 2009 test. If Love waves,
however, were in fact produced by the 2006 test in the same
proportion to the observed Love waves from the 2009 test as
the corresponding Rayleigh waves, their amplitude levels
would have been just below the noise thresholds at these
observing stations.
This fact is illustrated in Figure 17, which shows comparisons of the vertical and transverse recordings at station
MDJ for the two tests. It can be seen from this figure that the
maximum Rayleigh-wave amplitudes from the 2009 test are
about a factor of five larger than those observed for the 2006
5000
2006
0
−5000
50
0
100
5000
150
2009
0
−5000
50
2006
−5000
50
100
5000
150
2009
0
100
Time (s)
150
−5000
50
100
Time (s)
150
Figure 17. Comparison of (a) long-period vertical and (b) transverse component motions recorded at station MDJ from the 2006 and
2009 North Korean nuclear tests. If Love waves were in fact produced by the 2006 test in the same proportion to the observed Love waves
from the 2009 test as the corresponding Rayleigh waves, they would have been just below the detection threshold associated with the
observed noise level at this station at the time of the 2006 test.
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
1653
Figure 18. (a) Comparison of the MS =mb observations for the North Korean nuclear tests with corresponding MS =mb observations from
other recent nuclear tests and earthquakes. (b) M S =mb values for the two North Korean tests with respect to the event screening line used at
the IDC at the time of the tests.
components that may be affecting the observed Rayleighwave amplitude levels.
Applying the Russell (2006) Butterworth filter surfacewave magnitude algorithm to these observed Rayleigh-wave
data gives M S values of 2.93 and 3.66 for the 2006 and 2009
explosions, respectively. Selby et al. (2012) obtained similar
MS values of 2.83 and 3.58 using the method of Marshall and
Basham (1972). As noted earlier, converting these M S values
to corresponding explosion yield estimates using the nominal
MS =yield relation of equation (2) derived by Stevens and
Murphy (2001) for a statistical analysis of a large, globally
distributed sample of MS =yield observations from underground nuclear explosions gives estimates of about 7 kt for
the 2006 test and 36 kt for the 2009 test, about a factor of 10
larger than the nominal estimates determined from the corresponding short-period mb and mb Lg values.
Not surprisingly, these apparent M S anomalies also have
implications with regard to the performance of the usually
highly reliable M S =mb identification criterion, giving indeterminate values for the two North Korean nuclear tests that
approach previously observed earthquake values. This fact is
illustrated in Figure 18, which shows comparisons of the
observed M S =mb values for the two North Korean tests with
both the nominal IDC event screening line (Fisk et al., 2002)
used at that time to separate earthquake and explosion
populations, as well as M S =mb values observed from some
recent earthquakes and underground nuclear explosions
(Fig. 18a). Figure 18b shows the observed M S =mb values
for just the two North Korean tests with respect to the
IDC event screening line, where it can be seen that the mean
MS =mb value for the 2009 test falls on the “earthquake-like”
side of the decision line, whereas the corresponding value for
the 2006 test falls just below that decision line. In both cases,
the uncertainties in the MS and mb values encompass the decision line, so that both are indeterminate; that is, neither can
be positively identified as nuclear tests based on this M S =mb
criterion. Although alternate short-period discriminants confidently identify these events as explosions (Kim and Richards, 2007; Koper et al., 2008; Chun et al., 2011), the
failure of the normally highly reliable M S =mb identification
criterion for these two North Korean nuclear tests stands as a
troubling unexplained anomaly.
One effect that has been shown to bias M S values observed from explosions at other test sites is the triggering
of the release of preexisting tectonic strain energy by the explosion. That is, tectonic strain energy stored in the medium
prior to the explosion can be released by the effects of the
explosion shock waves, producing a secondary source of
long-period surface waves that can either add to or subtract
from the surface waves produced directly by the explosion,
leading to anomalous M S values. This fact is illustrated in
Figure 19, which shows comparisons of the azimuthal distributions of Rayleigh-wave amplitude for explosion plus
tectonic release sources versus explosion sources alone for
the strike-slip, thrust, and normal faulting modes of tectonic
release. It can be seen from Figure 19 that the strike-slip
mode of tectonic release will generally average out to have
a small effect on the network-averaged MS value, whereas
the thrust mode of tectonic release will decrease M S . Thus,
for example, detailed investigations of the seismic characteristics of Soviet underground nuclear tests at the Semipalatinsk test site revealed very strong thrust-type tectonic
release on some tests that significantly decreased the M S
values (Helle and Rygg, 1984; Ekstrom and Richards,
1654
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
Isotropic + Strike Slip
Isotropic + Thrust
90
90
2.5
60
120
Isotropic + Normal
90
2.5
60
120
2
2.5
60
120
2
1.5
2
1.5
150
1.5
150
30
150
30
30
1
1
1
0.5
0.5
0.5
180
0
330
210
180
0
300
240
330
210
180
0
300
240
330
210
300
240
270
270
270
Strike−slip mechanism
causes positive and negative
amplitude variations
Thrust mechanism
causes amplitude decrease
at all azimuths
Normal mechanism
causes amplitude increase
at all azimuths
(a)
(b)
(c)
Figure 19. Comparisons of azimuthal distributions of Rayleigh-wave amplitudes for explosion plus tectonic release sources versus
explosion sources alone (red) for (a) the strike-slip, (b) thrust, and (c) normal faulting modes of tectonic release.
1994) and, consequently, the explosions discriminated very
well on the M S =mb criterion. It was noted at the time of the
Semipalatinsk analyses, however, that, if the sense of the tectonic release was reversed to normal faulting, then the effect
would also be reversed to increase MS and make M S =mb
discrimination more problematic. Thus, accurate characterization of any secondary sources of surface waves is important in seismic monitoring, and moment tensor inversion
analysis provides a formalism to characterize such secondary
sources.
Most analyses of tectonic release from explosions assume that the secondary source can be modeled with a single
fault mechanism added to the isotropic source. The secondary source, however, need not be limited to a single fault
mechanism. For example, a Compensated Linear Vector
Dipole (CLVD) source with moment tensor components
Mxx and Myy equal causes the Rayleigh waves to increase
or decrease in amplitude uniformly, with no Love waves generated. The CLVD source is axisymmetric and equivalent to
two thrust (or normal) faults rotated 90° from each other.
Because Rayleigh waves are generated primarily by the
horizontal displacements at the source, a secondary CLVD
source that is compressive in the horizontal directions
reduces the net horizontal displacement at the source and reduces Rayleigh-wave amplitudes. Conversely, a secondary
CLVD source tensile in the horizontal directions increases the
net horizontal displacement at the source and increases
Rayleigh-wave amplitudes.
To examine possible effects of secondary sources on the
surface-wave amplitudes observed for the two North Korean
nuclear tests, moment tensor inversions were performed for
these two events using the Rayleigh- and Love-wave data
recorded at the six stations shown in Figure 16. This was
accomplished by conducting a search for the best fits to
the data in the 10–20 s period band while varying the isotropic moment, CLVD moment, and shear moment tensor
components, excluding the vertical dip-slip moment tensor
components, which do not contribute to surface-wave amplitudes because the excitation functions for these components
vanish at the free surface. The resulting moment tensor
inversion results are shown in Table 5, together with the associated estimates of the isotropic (M I ) and CLVD (MCLVD )
moments. It can be seen from Table 5 that, although the
CLVD moment estimates are small, they are positive and,
therefore, are predicted to enhance the observed surface
waves. Ford et al. (2009) also performed a moment tensor
inversion for the 2009 explosion and found that the bestfit solution was a pure explosion plus a small CLVD compressive component in the horizontal direction; they also found,
however, that a pure CLVD tensile in the horizontal direction
Table 5
Moment Tensor Inversion Results for North Korean Explosions
(×1014 N·m)
Event (yyyy/mm/dd)
M xx
M yy
Mzz
M xy
MI
M CLVD
Event 1-2006/10/09
Event 2-2009/05/25
4.30
25.12
5.56
27.63
3.68
18.84
−0.50
−4.27
4.51
23.86
1.25
7.54
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
(a) −300
(b) −300
Yield −− at 2.00s
−100
−100
0
0
meter
−200
meter
−200
100
100
200
200
300
300
400
0
200
400
Radial distance (m)
400
600
Tensile crack −− at 2.00s
0
200
400
Radial distance (m)
−500
−500
−400
−400
−300
−300
−200
−200
−100
Figure 20.
−100
0
0
100
100
200
200
0
200
400
600
Radial distance (m)
600
Tensile crack −− at 2.00s
meter
meter
Yield −− at 2.00s
300
1655
800
1000
300
0
200
400
600
Radial distance (m)
800
1000
Regions of (a) yielding and (b) cracking from axisymmetric calculations of the (top) 2006 and (bottom) 2009 North Korean
explosions.
fit the data almost as well. This underscores the fact that,
although our solution found a tensile (horizontally) CLVD
that is consistent with the Ford et al. (2009) results, the
moment tensor is not well constrained by the data.
The predicted effects of the moment tensor solution of
Table 5 on the individual MS values at the six stations of
Figure 16 have been calculated for the 2009 explosion and
found to be small, resulting in an increase in the networkaveraged M S value of only 0.06 magnitude units relative to
the corresponding isotropic explosion prediction. As noted
previously, however, there are significant uncertainties in
these estimates because the CLVD component is not well constrained in the inversion. An explosion with the same isotropic
moment and a larger CLVD would generate larger Rayleigh
waves with the same Love waves, no additional Rayleighwave azimuthal variation, and only a small change in spectral
shape. Therefore, it is appropriate to consider alternate
physics-based models of possible secondary sources for these
two explosions to determine whether they can provide plausible explanations for the observed anomalies.
In an attempt to derive a better physical understanding of
possible secondary surface-wave sources for the two North
Korean nuclear tests, we conducted a series of nonlinear
axisymmetric finite-difference simulations of the explosions
using the yield and depth estimates derived from our broadband, regional P-wave spectral ratio analysis described earlier. These simulations employed a material model for granite
derived from Degelen Mountain explosion data (Stevens
et al., 2003). The calculated regions of nonlinear deformation for the two North Korean explosions in the absence
of tectonic prestress are shown in Figure 20. Because the
explosions are estimated to be overburied for their yields, the
extents of nonlinear deformation away from the explosion
are relatively modest, particularly for the 2009 explosion,
which is nearly spherical except for a small amount of
deformation near the surface.
Tectonic strain release refers to relaxation and motion of
the medium surrounding the explosion caused by local or
regional stresses. In general we expect the vertical stress field
in the earth σ33 to be equal to the overburden stress because
1656
0
Sv
Sh Friction Limit
Sh Calculations
100
200
300
Depth (m)
any differences from this value would cause the earth to be
out of equilibrium with the free surface boundary condition.
The horizontal stress fields σ11 and σ22 may differ from this
value depending on regional tectonic stresses, and the deviatoric horizontal stresses may be either compressive or tensile.
(We assume that the coordinate system is oriented along the
principal horizontal stress directions so that we only need to
consider σ11 and σ22 .) There may be other stresses due to
internal dislocations, but the larger-scale stresses are most
likely to be the major factor affecting long-period surface
waves. There are limitations on the magnitude of tectonic
stresses. In particular, the stresses cannot be so large that the
material fails, creeps, cracks, or slips along preexisting
cracks, as any of these mechanisms will relieve the stress and
bring it back to a lower level. Unless there is some mechanism to relax the prestress, however, it will increase to a critical level just balanced against these forces. Zoback (2010)
shows that borehole measurements of stress indicate that
much of the earth’s crust is exactly balanced against frictional sliding, with coefficients of friction ranging from 0.6
to 1.0. Because the earth is more likely to be in a critically
stressed state than unstressed, at most locations tectonic
strain release can be expected to either increase surface-wave
amplitudes or decrease them depending on the regional
stress state.
Day and Stevens (1986) and Day et al. (1987) showed
through axisymmetric finite-difference calculations of explosions in prestressed media that stresses consistent with
regional stresses near the Nevada Test Site were sufficient
to cause Rayleigh-wave reversals. Rayleigh-wave reversals
were observed in some directions from the explosion
Pile driver, which was specifically modeled in that study.
During an explosion the surrounding material is moved by
both the explosion and the tectonic stresses. For compressive
deviatoric tectonic stresses, the material moves in and up; for
tensile deviatoric stresses the surrounding medium moves
out and down. There is one additional physical effect at
work: tectonic stresses increase (if compressive) or reduce (if
tensile) the overburden pressure as a function of depth. That
is, while the vertical stress is in equilibrium with gravity, the
horizontal stresses add or subtract to the pressure. Because
rock is generally stronger under higher pressure, this increases the amount of rock failure in tension and decreases
it in compression, so the nonlinear deformation and corresponding surface-wave amplitudes will be larger or smaller
due to this effect in addition to tectonic strain release.
We ran two additional calculations for the North Korean
explosions, adding compressive and tensile prestress. We apply an axisymmetric horizontal stress with a maximum value
of 150 bars (1:5 × 107 Pa), and with a maximum absolute
value of three-fourths of the overburden pressure as shown in
Figure 21. Also shown in the figure are the limiting stresses
based on frictional balance as discussed previously. For the
tensile case, the stresses used in the calculation are close to
the minimum stresses possible based on frictional sliding.
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
400
500
600
700
800
0
10
20
30
Stress (MPa)
40
50
Figure 21. Vertical (Sv , dashed) and horizontal (Sh , solid)
stresses used in the calculations. The lower horizontal stress represents tensile tectonic release and the higher value compressive tectonic release. Also shown are the limits on Sh derived from
Coulomb friction (Zoback, 2010), with a coefficient of friction
of 0.8 and assuming a water-table depth of 100 m.
Figure 22 shows computed regions of nonlinear yielding
and cracking for the 2006 North Korean explosion with prestress included, and Figure 23 shows the same for the 2009
explosion. As discussed earlier, compressive prestress has
the effect of reducing the amount of nonlinear deformation,
whereas tensile prestress increases the amount of nonlinear
deformation. This is particularly dramatic for the 2009 explosion calculation with tensile prestress. Note that because of the
limitation defined by overburden pressure, at the 550 m depth
of the 2009 explosion the prestress is considerably higher than
at the shallower depth of the 2006 explosion.
Waveforms from the axisymmetric calculations outside
of the source region can be calculated using the representation theorem (Aki and Richards, 1980):
ui ∯
SM
i
M
fGij ξ; X T M
j ξ − uj ξ Sjk ξ; Xnk gdA:
(3)
Stresses and displacements are saved on a monitoring
surface SM outside the nonlinear region, and integrated together with the Green’s function and Green’s function stress
derivatives over this surface. The seismogram at the receiver
point X outside of the monitoring surface is obtained by
integrating over the monitoring surface in the frequency domain, where Gij ξ; X and Sijk ξ; X are the Green’s function
and the stress tensor on the monitoring surface due to a unit
impulsive force at X in direction i, T M
j is the traction on the
monitoring surface due to the seismic source, u is the displacement on the monitoring surface, and n is the normal to
the monitoring surface. The operator denotes convolution,
and the summation convention is assumed. The monitoring
surface can be any surface, but in practice we use a cylindrical surface for axisymmetric problems.
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
−200
−200
−100
−100
0
0
100
200
300
300
0
200
400
Radial distance (m)
400
600
Yield −− at 2.00s
−300
Tensile crack −− at 2.00s
100
200
400
0
−300
−200
−200
−100
−100
0
0
meter
meter
(b) −300
Yield −− at 2.00s
meter
meter
(a) −300
100
200
300
300
0
200
400
Radial distance (m)
600
200
400
Radial distance (m)
600
Tensile crack −− at 2.00s
100
200
400
1657
400
0
200
400
Radial distance (m)
600
Figure 22.
Regions of (a) yielding and (b) cracking from axisymmetric calculations of the 2006 North Korean explosion with (top) compressive and (bottom) tensile prestress.
Figure 24 shows fundamental mode surface-wave seismograms calculated at a distance of 1150 km from the source
for each of the six calculations. Seismograms were calculated
at the distances of each of the stations that observed the two
explosions and then measured using Russell’s magnitude
procedure as with the data. The results are shown in Table 6.
For these calculations, tensile prestress increases M S by
about 0.2 magnitude units in both cases. Compressive prestress, which is not expected in Korea but likely in other
areas, causes a reduction in MS of about 0.15 magnitude
units for both events. Figure 25 shows the calculated magnitudes together with observed M S =yield data from explosions at other test sites. For both events, tensile prestress
increases M S substantially, reducing the difference from
the observed M S from 0.47 to 0.26 magnitude units for the
2006 event and from 0.62 to 0.35 magnitude units for the
2009 event. Although tensile tectonic release provides a
likely explanation for the large MS of the North Korean
events, the calculated surface-wave amplitudes are still only
about half of the observed amplitudes. A possible reason for
this is that the Degelen granite model is quite strong, and the
Korean granite could be weaker, more like NTS granite,
which would increase surface-wave amplitudes. In addition,
there are biases between test sites that cause differences in
surface-wave amplitudes. At Semipalatinsk, surface-wave
amplitudes are reduced by the prevailing prestress, whereas
the lower-velocity media at NTS also reduce surface-wave
amplitudes. Also, the Mueller–Murphy source model predicts a slope of 0.87 instead of 1.0, which increases surfacewave amplitudes for smaller events. Plotted on Figure 25 are
the predicted values of M S for an explosion in granite at the
inferred source depths using the relations in Stevens and Day
(1985). This removes the test site biases mentioned above
and predicts larger M S for the two North Korean explosions.
1658
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
Yield −− at 2.00s
Tensile crack −− at 2.00s
(b)
−500
−500
−400
−400
−300
−300
−200
−200
meter
meter
(a)
−100
−100
0
0
100
100
200
200
300
0
200
400
600
Radial distance (m)
800
300
1000
0
200
−500
−500
−400
−400
−300
−300
−200
−200
−100
0
100
100
200
200
0
200
400
600
Radial distance (m)
1000
800
1000
−100
0
300
800
Tensile crack −− at 2.00s
meter
meter
Yield −− at 2.00s
400
600
Radial distance (m)
800
1000
300
0
200
400
600
Radial distance (m)
Figure 23.
Regions of yielding (a) and cracking (b) from axisymmetric calculations of the 2009 North Korean explosion with (top) compressive and (bottom) tensile prestress.
There is still an anomaly compared to the observations but
closer to the anomaly explainable by tectonic strain release.
Discussion and Conclusions
In October 2006 and May 2009 the DPRK conducted
two small nuclear explosion tests at a remote test site located
at Mount Manthap in northeastern North Korea. The objectives of the studies described in this paper have been to more
fully exploit the data from the IMS network and other unclassified data sources to conduct comprehensive advanced seismic analyses of the characteristics of these two North Korean
nuclear tests. More specifically, the focus has been on refining the event locations, estimating source depths, determining more confident seismic yield estimates, and developing a
better understanding of surface-wave excitation for these two
explosions through careful analysis and comparisons of the
seismic data combined with additional constraints from geologic/tectonic and topographic information and nuclear test
containment practice. The results of these analyses support
the following conclusions:
• A review of the likely geologic environment at source
emplacement depths at the Punggye test site suggests that
the 2006 and 2009 nuclear tests were conducted in “good
coupling” media for purposes of seismic yield estimation.
Moreover, the tectonic environment in the Punggye region
is inferred to be predominantly extensional, indicating that
the release of tectonic prestress by the explosions would be
expected to increase M S values.
• Available seismic arrival-time data from the 2006 and 2009
tests were analyzed using a variety of state-of-the-art
relative location techniques. All of the resulting solutions
yielded very similar locations, indicating that the 2009 test
was conducted ∼2:5 km west-northwest of the 2006 test.
Supplemental topographic data for the site were used to
further constrain the absolute locations.
• Teleseismic P-wave spectral data were inverted using a
model-based procedure to determine the yield of the 2009
test. Because source depth is poorly constrained using conventional seismic techniques, yield estimates were determined at 100 m increments over the plausible depth range
from 100 to 800 m. These yield estimates vary from 2.0
to 4.8 kt.
• Because the uncertainty in source depth leads to considerable uncertainty in the yield estimate, a new technique
based on broadband source spectral ratios was developed
to better constrain the depths of the 2006 and 2009 explosions. The results of this analysis indicate that the two explosions could not have been conducted at any common
depth in the plausible 100 to 800 m range, and, in fact,
the observed spectral ratio data are best modeled by source
Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests
100
1659
NK2006 stress−comp
0
−100
100
NK2006 stress−none
0
−100
100
NK2006 stress−tens
0
−100
600
400
200
0
−200
−400
600
400
200
0
−200
−400
600
400
200
0
−200
−400
NK2009 stress−comp
NK2009 stress−none
NK2009 stress−tens
250
300
350
400
450
500
550
Figure 24.
Surface-wave displacement seismograms calculated at a distance of 1150 km. Labels NK2006 and NK2009 identify the two
events, and stress-comp, stress-none, and stress-tens identify runs with compressive, zero, and tensile prestress, respectively. Seismograms
are passband filtered between 0.04 and 0.1 Hz. Scale differs by a factor of four for the two events.
MS values that agree much better with the observed values
than those obtained previously without tectonic strain
release. The predicted M S values, however, continue to
be low compared to the observations, suggesting the need
M vs. Yield
Ms:Yield Data Fit
5.5
5
Semipalatinsk
NTS
North Korea
NK Calculated
NK Predicted (S&D)
s
4.5
4
3.5
3
2.5
2 −1
10
Table 6
s
6
M
depths of about 200 m for the 2006 test and 550 m for the
2009 test. The corresponding yield estimates for the 2006
and 2009 tests are 0.9 and 4.6 kt, respectively. This estimated difference in the source depths of the two explosions
was shown to be remarkably consistent with that determined from careful DWIF analyses of the differential
regional phase arrival-time data.
• The long-period surface-wave M S magnitudes for both the
2006 and 2009 tests appear to be anomalously large relative to historical experience, producing unreasonably large
M S yield estimates and problematic M S =mb identification
characteristics. A series of theoretical, 2D, axisymmetric
nonlinear finite-difference simulations have been conducted to assess the expected effects of nonlinear free surface interaction and possible tectonic release on the longperiod surface-wave amplitudes and resulting M S
values for the 2006 and 2009 tests. It has been found that
those simulations incorporating tensile prestress consistent
with the regional tectonic environment lead to predicted
0
10
1
2
10
10
3
10
4
10
Yield (kt)
Calculated Surface-Wave Magnitudes
Event
Observed
MS
M S No
Prestress
M S Compressive
Prestress
M S Tensile
Prestress
NK2006
NK2009
2.93
3.66
2.46
3.04
2.32
2.89
2.67
3.31
Figure 25. Calculated MS with and without tectonic release
(from Table 6) plotted together with observed North Korean and
historical observed M S =yield data from other test sites along with
predictions for the North Korean explosions based on Stevens and
Day (1985, S&D). The red squares show the calculated MS for compressive, zero, and tensile prestress, in the order of increasing M S .
1660
J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett
to consider other potential physical mechanisms. Other
factors that contribute to the high MS relative to other test
sites are that much of the existing explosion-generated
surface-wave data come from Semipalatinsk, which are reduced in amplitude by compressive tectonic release, and
NTS, which are lower in amplitude because of lower source
material velocities.
Data and Resources
The seismic data analyzed in this study were obtained
from the CTBTO International Monitoring System (IMS)
global network, the Incorporated Research Institutions for
Seismology (IRIS) data management center, the Ocean
Hemisphere Project Data Management Center (OP HDMC),
and the Japanese National Research Institute for Earth Science and Disaster Prevention (NIED). Geologic and tectonic
information regarding the North Korean test site region came
from published sources listed in the references. The U.S.
Geological Survey/National Earthquake Information Center
Online Bulletin of the Earthquake Hazards Program was accessed for seismic bulletin information associated with the
North Korean tests at http://earthquake.usgs.gov/regional/
neic/ (last accessed January 2013). Information and imagery
analyses from the GlobalSecurity.org website were accessed
at http://www.globalsecurity.org/wmd/world/dprk/nuke-test
.htm (last accessed January 2013).
Acknowledgments
We would like to acknowledge the direction and support for this
project provided by Assistant Secretary Gottemoeller and her staff (particularly those from Rongsong Jih) of the U.S. Department of State. We would
also like to acknowledge the contribution of Hans Israelsson to the waveform
correlation processing used to support the DWIF location analyses. We also
thank the Associate Editor and two anonymous reviewers for their thoughtful comments and suggestions for improving this paper. This research was
sponsored by the U.S. Department of State under Contract Number
SAQMMA09C0250. The views and conclusions in this report are those
of the authors and should not be interpreted as representing the official
policies, either expressed or implied, of the U.S. Department of State or
the whole U.S. government.
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Manuscript received 6 June 2012