An Analysis of Seismicity at Mt. Hood, Oregon
Michelle A. Currie, CEOAS, Oregon State University
Abstract:
Mt. Hood, in north-central Oregon, has experienced 13 M ≥ 3.0 earthquakes since 1980.
1812 earthquakes that range in magnitude from -1.1 to 4.5 have been recorded in the region
during the same time frame. This paper analyzes four of the larger earthquakes to provide
moment tensor solutions and relocated depths. Moment tensor analysis reveals that the larger
earthquakes are located on dominantly north-south striking, normal faults with a minor strikeslip component. An analysis of the Mt. Hood earthquake catalog reveals a sinusoidal pattern to
seismicity that repeats approximately every 8-10 years. This cyclical pattern persists when the
data set is constrained to view only magnitude 1+ and 2+ events.
Introduction:
The purpose of this paper is to determine the primary mechanism of earthquakes on Mt.
Hood, Oregon and to identify patterns based upon frequency and location of earthquakes. Mt.
Hood is an active volcano in the Pacific Northwest, located approximately 75 kilometers east of
Portland, Oregon. Recent publications regarding the composition of the magma chamber have
been published, but little research has been recently conducted regarding the timing and
significance of earthquakes at this location. The larger number of seismograms in the region
coupled with the increasing activity on and around the volcano makes Mt. Hood an excellent
location to increase the body of knowledge about this High Cascades peak and the relationship
between its tectonic and volcanic activity.
2
For the purposes of this research an earthquake data set was compiled from all
earthquakes reported to the Pacific Northwest Seismic Network (PNSN) in a region bounded by
latitude 45.28° to 45.46° and longitude -121.55° to -121.85° . Earthquake seismograms for
events M ≥ 3.0 were requested from the Incorporated Research Institutions for Seismology data
center (IRIS), the Berkeley Digital Seismic Network (Berkeley), and the Canadian National Data
Center (Canada).
Geology and History of Mt. Hood Region:
Mt. Hood is a stratovolcano located in north-central Oregon at latitude 45. 37°
and longitude -121.696 and is part of the High Cascades, a chain of volcanoes that run parallel to
the Juan de Fuca-North American subduction zone. They were created by subduction-driven
volcanism that extends from British Columbia to Northern California. The Cascade Range is an
active volcanic province that has episodically produced primarily andesitic peaks for the past 35
million years (Williams, et. al., 1982). The volcano stands at 3429 meters above sea level and
occupies an area of approximately 238 km2,with a volume of approximately 188 km2 (Wise,
1969). A young volcano, less than 700,000 years old (Williams, Hull, Ackermann, & Beeson,
1982), the last eruption at Hood was in the early 1800’s and it is still active, as evidenced by a
fumarole field in the crater that provides an active degassing outlet (Koleszar, 2011). The Mt.
Hood region, and most of the Pacific Northwest, is transected by a series of northwest-southeast
trending, right-lateral, normal faults (Williams, et. al., 1982). Mt. Hood is most likely located at
the extreme northern end of the Basin and Range province where the High Cascades transition
into Basin and Range (Jones & Malone, 2005). The Basin and Range province, a region of
3
extension characterized by horst and graben structures that encompasses much of western North
America is characterized by normal and right-lateral faulting (Christiansen and Lipman, 1972).
Moment Tensor Modeling Procedures:
The procedure used to analyze the selected earthquakes was that earthquakes in the region
bounded by latitude 45.28° to 45.46° and longitude -121.55° to -121.85° were identified using
the PNSN website. The search area was chosen after a preliminary search of the region revealed
extremely few results outside of this approximate region. The study area has an extent of 471
km2, approximately twice the size of the areal extent of the mountain itself to accommodate
regional seismicity. A search for all events yielded 1812 earthquakes of magnitudes from -1.1 to
4.5 between Jun 8, 1980 and April 20, 2014 (Fig 1).
A sharp increase in number of earthquakes detected is seen beginning in 1998. Jones &
Malone (2005) indicate that the coverage by seismometers has been constant for earthquakes on
the mountain since 1986, but the sharp increase in detection of magnitude <1.0 earthquakes
since 1998 and the continuing disparity between recorded events larger or smaller than
magnitude = 0.0 which should obey the Gutenberg-Richter law suggests that coverage for
smaller events may still be incomplete.
A magnitude minimum of 3.0 for moment tensor analysis was selected because signal
strength attenuates more rapidly for smaller earthquakes, M = 3.0 was believed to be the smallest
magnitude that would likely yield sufficient stations with clear enough signals to allow
processing. The 13 largest earthquakes (Fig. 2 & Table 1), of M ≥ 3.0 were selected to attempt
moment tensor analysis processing. Their physical location in relation to Mt. Hood peak are
indicated on all figures and tables with alphabetic designations A-M. For the 13 earthquakes,
4
seismograms were ordered from IRIS, Berkeley, and Canada. Because signal strength
attenuates with distance from epicenter the stations from which data was requested were limited
to those within a 1000 km radius of the 4.5 magnitude event and a 500 km radius for the M =
3.0-3.8 events. The computer records of the 3-component [Transverse(T-axis), Radial (R-axis),
and Vertical (Z-axis)] seismograms were then processed to make the data compatible with the
moment tensor program and filtered to view the low frequency data. For seismograms where
only high frequency data were available, the signal was down-sampled to provide a consistent
record for evaluation. Low frequency rather than high frequency data sets were utilized for these
models because high frequency data require detailed knowledge of the crustal structure
heterogeneities which we do not know.
Filtering parameters varied from earthquake to earthquake and are used to minimize
noise-to-signal ratios. A variety of options were attempted for each earthquake, the options
which produced the best quality seismograms were selected for final inversion. The filtering
options selected were all between 10-20 s for low-pass frequency filter, 15-50 s for high-pass
frequency filter, and all used a 5-20s taper. The specific selections for each event are listed on
Table 2.
Synthetic seismograms were created using a 1-D crustal velocity model (Table 3) based
upon typical Oregon crustal thicknesses to describe the wave propagation. After low quality
seismograms and components were removed and the filtering criteria adjusted to best fit the
signal strength, as described previously, an iterative inversion process using the methods
described by Nábĕlek and Xia (1995) modeled synthetic seismograms for the selected, highsignal quality observed stations and components. The synthetic seismograms were inverted and
iteratively modeled using the least squares inversion to determine best-fit moment tensor.
5
As in Nábĕlek and Xia (1995), the solution was constrained to be purely deviatoric (i.e., no
volume change in the subsurface) and the source time function parameterized as described by
Nábĕlek (1984). The source time function parameterization controls the rate of signal
attenuation. The timing of the synthetic and observed seismograms were realigned as needed to
correct for phase misalignment. This adjustment corrects for much of the deviation between
synthetic and observed signal start times due to potential errors in hypocentral location and
earthquake initiation time as well as incorrect assumptions about wave propagation speed in the
crustal model. As in Nábĕlek and Xia (1995), each station was realigned as a whole, which did
not allow for alignment adjustments due to Love/Rayleigh wave speed variation induced by
crustal anisotropy.
After inversion and iteration at depths provided from USGS data this best-fit solution for
each earthquake was then modeled for various depths and the best solution selected by the
computer. From the 13 M ≥ 3.0 earthquakes, eight were unsuitable due to low quality/quantity of
seismograms which precluded an acceptable solution being obtained or (for the 1982 earthquake)
no electronic versions being available. The four earthquakes that were successfully analyzed
were: M=4.5 on 6/29/2002, M=3.8 on 6/29/2002, M=3.3 on 7/7/2003, and M=3.0 on 5/13/2010.
These events are indicated on the list of all earthquakes selected for attempted processing in
Table 1 by an asterisk next to the magnitude. Selected data from the analyses including the
recalculated magnitude, relocated depth, and the strike/dip/rake for the four modeled earthquakes
is listed on Table 2.
Some of the limitations involved in moment tensor analysis for smaller earthquakes include
the small number of stations which record the event and the high noise/signal ratio for such small
events. Analysis of the 3.0 earthquake on 4/7/1996 was attempted using 80 stations due to a
6
temporary EarthScope array in the region, but was ultimately unsuccessful due to the low
number of quality signals. Only five stations yielded useable seismograms and an unacceptably
low double couple percentage of 11% was obtained. This number of recording stations for such a
small earthquake is unusual and all others had a much smaller number of stations available for
analysis, consequently even less reliable results were obtained for the other events.
Results of Moment Tensor Analysis:
The moment tensor analysis of four earthquakes along with geological constraints suggest
that motion on Mt. Hood is located on primarily normal, right-lateral, steeply dipping faults.
The stations utilized and results of the analysis for the 6/29/2002 at 14:36 UTC are presented in
Fig. 3. The same information is presented in Fig. 4 for the earthquake on 6/29/20092 at 18:49
UTC, in Fig. 5 for the 7/7/2003 12:40 UTC event, and in Fig. 6 for the 5/14/2010 19:03 UTC
event. The solid black line are the observed seismograms, the red dashed lines are synthetic
models. The double-couple percentage (DC%) listed at the top of this figure and presented in
Table 2 serve as a proxy for the reliability of the results presented and indicates the degree to
which the modeled and observed seismograms are in agreement. The best-fit depth for each of
these earthquakes are presented in Fig. 7-10. The figures each show a graphic analysis of the
fault plane solutions, represented as beach balls, for each analyzed depth against the variance
from best-fit strike, dip, and rake for each earthquake with the best-fit depth presented in red.
Earthquake Catalog Analysis:
In addition to moment tensor analysis of the largest earthquakes, the entire catalog was
analyzed for patterns in seismicity. Looking at the number of earthquakes per year, both for all
7
recorded earthquakes (Fig. 11) and for M ≥ 1.0 (Fig. 12) reveal an sinusoidal variation in annual
number of earthquakes that repeats on a scale of ~8-10 years. The smaller, M ≥ 1.0 data set is
used to reduce the effects of an incomplete catalog for smaller magnitude earthquakes in earlier
years. The patterns persists when M ≥ 2.0 earthquakes are viewed (Fig. 13). No conclusions as
to the source of this variation are made here, but future research into potential relationships
between earthquake frequency cyclicity, fumaroles emissions, and GPS-monitored movement of
the peak are planned.
The Mt. Hood earthquakes can also be examined by assigning them to four clusters based
upon regions of increased activity, with the fifth cluster being all other earthquakes in the data
set (Fig. 14). When viewed by clusters, each region appears to show characteristic patterns to
earthquake depths (Fig. 15).The chart of cluster by depth shows that 68% of earthquakes in the
Peak cluster are shallow , < 5km deep. This seems consistent with chemical analyses of gas
emissions which suggest a relatively shallow (~5 km) magma chamber (Koleszar, 2011). The
West flank cluster shows the largest percentage of deeper earthquakes, the depth of 37% of the
earthquakes in this region is ≥ 7.0 km. Other trends are evident when the regions are viewed by
number of M ≥ 1.0 earthquakes in each cycle (Fig. 16). The number of earthquakes in the west
cluster has increased for the 2007-2014 cycle which is underway, while the SE and SW clusters
show the most activity for the 1995-2006 cycles. The apparent change in preferred earthquake
location from cycle to cycle will be analyzed further, but no conclusions may be made at this
time as to the significance of this movement.
Conclusions and Directions for Further Research:
8
Because of the short travel times between earthquake hypocenter and seismometer
stations, this data set might benefit from re-analysis that attempts to account for subtle variations
in wave velocity with depth and regional anisotropy as described by Kohler, Healy, & Wegener
(1982) and Williams et. al (1982). Another method of moment tensor analysis for small
earthquakes and close seismometers utilizes higher-frequency waves. Analysis using the higher
frequency waves requires a more complete knowledge of the regional structure, but avoids lowfrequency noise from ocean movement. The source of the sinusoidal periodicity of the catalog is
unknown and might be revealed by further analysis of the region. The relationship between
seasonal variations in hydrostatic pore pressure and seismicity has been previously explored by
Saar & Manga (2003), and between volcanic versus tectonic origins by Jones & Malone (2005),
but the catalog of regional earthquakes has grown by approximately 50% since the beginning of
2006. The additional ~10 years of data since these reports provides a great deal of additional
information to expand upon these researches. Further research into the cyclic nature of
earthquake frequency at Mt. Hood is planned.
Acknowledgements:
Many, many thanks to John Nábĕlek for guidance and technical assistance without which
this paper would never have become more than a vague desire to study seismicity.
9
Tables and Figures:
45.46
1980-2014 Earthquakes by Magnitude
45.44
45.42
45.4
45.38
Mt Hood
45.36
45.34
Mag=-1.1--0.1
Mag=0.0-0.9
Mag=1.0-1.9
Mag=2.0-2.9
Mag=3.0-3.9
Mag=>3.9
Mt Hood
45.32
45.3
45.28
-121.85
-121.8
-121.75
-121.7
Figure 1. 1812 earthquakes in data set, by magnitude.
-121.65
-121.6
-121.55
10
Figure 2: Locations of 13 largest earthquakes at Mt. Hood. Alpha designation from Table
1.
11
map
Magnitude
Time UTC
Latitude
Longitude
Depth (km)
45.3348
-121.6863
6.2
45.3423
-121.6798
6.1
45.341
-121.6858
6
45.3727
-121.7068
5.2
45.3717
-121.6967
5.6
45.3273
-121.6857
6.7
3.2
3.2
6/29/2002 14:41
1/14/1999 11:56
Time Local
2002/06/29 07:36:04
PDT
2002/06/29 11:49:58
PDT
1990/10/19 07:13:58
PDT
1989/09/15 03:28:00
PDT
1982/08/18 04:50:37
PDT
2003/07/07 05:40:12
PDT
2002/06/29 07:41:21
PDT
1999/01/14 03:56:47 PST
A
4.5*
6/29/2002 14:36
B
3.8*
C
3.5
6/29/2002 18:49
10/19/1990
14:13
D
3.5
9/15/1989 10:28
E
3.4
8/18/1982 11:50
F
3.3*
7/7/2003 12:40
G
H
45.3275
45.3303
-121.6815
-121.6698
2.7
7.6
I
3.2*
45.3195
-121.6545
6.9
J
K
L
3*
3
3
45.3595
45.3242
45.3232
-121.7522
-121.6638
-121.6543
5.3
5.9
7
M
3
1/11/1999 22:04 1999/01/11 14:04:14 PST
2010/05/14 12:03:04
5/14/2010 19:03 PDT
1/14/1999 16:13 1999/01/14 08:13:42 PST
1/11/1999 16:54 1999/01/11 08:54:11 PST
1996/04/07 06:17:14
4/7/1996 13:17 PDT
45.3588
-121.7153
4.4
Table 1. Earthquake details for 13 largest earthquakes at Mt. Hood, as reported by
USGS.
map
A
B
F
J
recalculat
Magni
ed
tude magnitude Time UTC
4.5*
4.3
6/29/2002 14:36
3.8*
3.7
6/29/2002 18:49
3.3*
3.3
7/7/2003 12:40
3*
3.5
5/14/2010 19:03
Depth
(km)
6.2
6.1
6.7
5.3
Relocated
depth Freqency filter
(km)
& taper (s)
4
20-50s, 20s
4
10-30s, 10s
4
10-30s, 10s
12
10-20s, 10s
Strike/dip/rake
Movement style
155.6/55.7/-129.8 normal right-lateral oblique
135.3/88.5/-160.2
right-lateral strike slip
353.8/66.6/-114.1 normal right-lateral oblique
156.2/58.7/126.1 reverse right-lateral oblique
Table 2. Highlighted details from moment tensor analysis. Map designation refers to location in Fig.2.
DC %
95%
49%
53%
42%
12
P wave S wave
P wave
S wave
thickness velocity velocity density attenuation attenuation
(m)
(m/s)
(m/s)
(kg/m3)
factor
factor
1000
4000
2250
2140
225
100
5000
5500
3090
2560
225
100
16000
6598
3700
2868
225
100
13000
7089
3980
3002
225
100
halfspace
8100
4550
3290
225
100
Table 3. Crustal velocity model
13
14
μ
Figure 3. Results of moment tensor analysis for earthquake on 06/29/2002 at 14:36 UTC.
μ
Figure 4. Results of moment tensor analysis for earthquake on 06/29/2002 at 18:49 UTC.
15
1.29μm
Figure 5. Results of moment tensor analysis for earthquake on 07/07/2003 at 12:40 UTC.
16
0.18 μm
Figure 6. Results of moment tensor analysis for earthquake on 05/14/2010 at 19:03 UTC.
17
Figure 7. Best fit depth for earthquake on 6/29/2002 at 14:36 UTC.
18
Figure 8.Best fit depth for earthquake on 6/29/2002 at 18:49 UTC.
19
Figure 9. Best fit depth for earthquake on 7/07/2003 at 12:40 UTC.
20
Figure 10. Best fit depth for earthquake on 5/14/2010 at 19:03 UTC.
21
350
Annual earthquakes by Magnitude
# of eq per year by magnitude
300
4.0+
3.0-3.9
2.0-2.9
1.0-1.9
0.0-0.9
-1.0--0.1
250
200
150
100
50
0
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014
Figure 11. Earthquakes at Mt. Hood, 1980 through April 20, 2014.
22
160
Annual number of earthquakes, by magnitude
140
120
Annual Earthquakes by Magnitude (M ≥ 1.0)
4.0+
3.0-3.9
2.0-2.9
1.0-1.9
100
80
60
40
20
0
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014
Figure 12. Annual number of M≥1.0 earthquakes, 1980-April 20, 2014 at Mt. Hood.
23
30
Annual Earthquakes by Magnitude (M ≥ 2.0)
4.0+
3.0-3.9
2.0-2.9
# of eq per year by magnitude
25
20
15
10
5
0
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014
Figure 13. M > 2.0 earthquakes at Mt. Hood.
24
peak cluster
45.46
Description of Clusters Used
West flank
cluster
SW cluster
45.44
SE cluster
Other
45.42
Mt. Hood
A
45.4
B
C
45.38
Mt. Hood
D
45.36
J
45.34
M
D
E
E
F
G
C
B
F
A
G H
K
45.32
H
I
L
I
J
K
45.3
L
45.28
-121.85
M
-121.8
-121.75
-121.7
-121.65
-121.6
-121.55
Figure 14. Earthquakes in region grouped by cluster. 13 largest earthquakes are identified
as in Table 1.
25
Earthquake Depth by Region for All
Magnitudes
100%
Depth = 16.0+
80%
Depth = 13.0-15.9 km
60%
Depth = 10.0-12.9 km
Depth = 7.0-9.9 km
40%
Depth = 5.0-6.9 km
20%
Depth = 3.0-4.9 km
Depth = 0-2.9 km
0%
Peak
West
SE
SW
other
Figure 15. Earthquake clusters showing percent of earthquakes in each region by depth.
# of M ≥ 1.0 Earthquake per Cluster, over Time
350
300
2007-2014
1995-2006
1985-1994
250
1980-1984
200
150
100
50
0
Peak
West
SE
SW
other
Figure 16. Number of M ≥ 1.0 earthquakes in each cluster, sorted by time.
26
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27
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