Relocation of the M2.6 Portland, Oregon, earthquake of November 5, 2006
Robert Butler, Jenda Johnson, John C. Lahr, and Ruth S Ludwin
(Authors listed in alphabetical order.)
(Revised 11/30/2006)
During June, 2006, the authors gave a seismology workshop for twelve teachers from 11 public schools in Portland. This year the
teachers are sharing 6 AS-1 seismic systems that were provided by IRIS. Half-way through the year the instruments will change
classrooms.
Four of the six instruments were running when the November 5th event occurred, including the instrument located at Sellword Middle
School (SMOR) just 5 km from the epicenter. Although absolute timing is not maintained on these instruments, the SMOR record
displayed clear P and S arrivals with a time separation of 1.8 seconds. Since WPO, the nearest station of the Pacific Northwest
Seismic Network (PNSN), is located 13 km away, the S minus P interval from SMOR may provide better depth constraint because it
is located much closer.
The computer program HYPOELLIPSE can use of S minus P intervals, along with P and S arrival times to locate earthquakes, so that
program was used to relocate the Portland earthquake.
The results are summarized in this table and details from the HYPOELLIPSE program are given in the appendix. Three different
velocity models are in use by the PNSN, so three sets of locations were computed.
Location source
Origin Time
Latitude
Velocity Model C3 (North of Portland)
Longitude
Depth (km)
Horizontal
PNSN Catalog with
free depth
PNSN with fixed
depth
Hypoellipse without
S-P
Hypoellipse with SP
05:34:35.69
45N30.94 45.5157
05:34:35.80
05:34:35.62
Location source
Origin Time
05:34:35.79
Standard Error (km)
Vertical
15.67
45N30.86
122W38.87122.6478
122W38.89
45n30.78
45.5130
45n30.55
45.5091
122w39.08
-122.6513
122w39.08
-122.6513
18.05
0.9
2.4
13.29
0.8
0.9
Latitude
13.
Velocity Model O3 (South of Portland)
Longitude
Depth (km)
Horizontal
Standard Error (km)
Vertical
PNSN with free
depth
PNSN Catalog with
fixed depth
Hypoellipse without
S-P
Hypoellipse with SP
05:34:35.11
45N30.99
122W38.94
16.62
05:34:35.26
45N30.96
122W38.81
13.0
05:34:35.62
45W30.90
122W38.84
18.05
0.9
2.6
05:34:35.79
45N30.39
122W39.064
11.61
0.8
0.9
Location source
Origin Time
Latitude
Velocity Model P3 (Puget Lowland)
Longitude
Depth (km)
Horizontal
PNSN with free
depth
PNSN with fixed
depth
Hypoellipse without
S-P
Hypoellipse with SP
Standard Error (km)
Vertical
05:34:35.
45N30.89
122W38.85
14.43
05:34:35.
45N30.88
122W38.79
13.0
05:34:35.
45W30.75
122W39.04
16.88
0.9
2.6
05:34:35.86
45N30.57
122W39.10
13.45
0.8
0.9
The computed depth when the S minus P interval was included varied from 11.6 to 13.5 km.
The HYPOELLIPSE relocations did not use the station corrections developed by the PNSN. Perhaps the best solution consistent with
past PNSN locations are those above obtained by fixing the depth at 13 km and then relocating this event with the standard PNSN
location program.
As a check on the constraint provided by a nearby S minus P interval, I’ve computed the S minus P interval for an event located
directly below a station. These intervals were computed for depths ranging from 1 to 20 km and for all three velocity models. If an
uncertainty of plus or minus 0.1 s is assumed for the S minus P time, and if the true velocity model is bracketed by these three models,
then the observed S minus P of 1.8 seconds implies a maximum depth between 11.2 and 15 km. If the earthquake epicenter is located
not directly under station SMOR but rather 5 km away, then the depth range becomes:
Sqrt(11.2**2 – 5**2) to Sqrt(15**2 – 5**2) or 10.2 to 14.1 km
These depths are also a function of the ratio of the P velocity to the S velocity. A value of 1.78 has been assumed.
layer
1
2
3
4
5
6
velocity model C3 (North of Portland)
velocity
depth
thickness
km/sec
km
km
5.100
6.000
6.600
6.800
7.100
7.800
.000
1.000
10.000
18.000
34.000
43.000
1.000
9.000
8.000
16.000
9.000
1000.000
vpvs
1.780
1.780
1.780
1.780
1.780
1.780
layer
1
2
3
4
5
6
layer
1
2
3
4
5
6
7
velocity model O3 (South of Portland)
velocity
depth
thickness
km/sec
km
km
2.900
4.700
6.000
6.400
6.800
7.700
.000
1.300
3.400
8.000
30.000
42.000
1.300
2.100
4.600
22.000
12.000
1000.000
velocity model P3 (Puget Lowland)
velocity
depth
thickness
km/sec
km
km
5.400
6.380
6.590
6.730
6.860
6.950
7.800
.000
4.000
9.000
16.000
20.000
25.000
41.000
4.000
5.000
7.000
4.000
5.000
16.000
1000.000
vpvs
1.780
1.780
1.780
1.780
1.780
1.780
vpvs
1.780
1.780
1.780
1.780
1.780
1.780
1.780
The white rectangle indicates the range of possible depth, given an uncertainty of 0.1 s in the observed 1.8 s S minus P interval.
Figure 1. Google Earth map showing the location of the 6 schools with AS-1 seismic systems, the PNSN earthquake
location and relocated location based on C3 and O3 velocity models. East Bank fault (purple) and Portland Hills fault (blue).
Figure 2. Sellwood Middle School (SMOR) unfiltered record with S minus P interval of 1.8 seconds.
The first motion is impulsive compression.
Conclusions
Data from the classroom seismic station located at Sellwood Middle School by Rick Oleksak has helped to constrain the depth and
location of the Portland earthquake of November 5, 2006. The relocation places the event at a depth of 13 km approximately
equidistant from the mapped surface traces of the Portland Hills and East Bank faults. The interpretation of the S minus P interval is
dependent on the assumed velocity model. In the vicinity of the relocated epicenter, the velocity models in use at the PNSN constrain
the depth to be between 10.2 and 14.1 km. If the Portland Hills fault dips to the west and the East Bank fault dips to the East, then this
event must have occurred on an unmapped fault. If a future sequence of well-recorded earthquakes occurs on this fault, then it should
be possible to determine its location and dip.
Acknowledgements
Many thanks are due to Roy Chambers and the teachers who have been running AS-1 seismic stations in their classrooms. The
teachers include Joanne Barda, Edwin Wack, Gerilyn Nichols, Tom Bright, Ric Oleksak, Rod Kitson, Joanne Barta, and Edwin Wack.
Support for this study as well as the initial teacher workshop and the AS-1 instruments was provided by the Incorporated Research
Institutions for Seismology (IRIS). The Pacific Northwest Seismic Network shared their arrival-time data and location parameters,
which made this study possible.
Appendix
Event location from HYPOELLIPSE using velocity model C3 and the S minus P interval at SMOR:
horizontal and vertical single variable standard deviations (68% - one degree of freedom; max 99 km)
seh =
.59
seh =
.80
sez =
.91
quality = a
az =
-62.
az =
28.
se of orig =
.09; # of iterations =
8; dmax =
event type = " "; processing status = " "
closest station did not use both p and s
date
20 61106
seh
.8
origin
lat
long
534 35.79 45n30.55 122w39.08
sez q sqd
.9 b b a
depth
13.29
150.00; sequence number =
mag no d1 gap d
2.502 35 5 57 1
rms
.1357
avwt
1.00
adj in nr
avr aar nm avxm mdxm sdxm nf avfm mdfm sdfm
.21 0 56 -.010 .092 0
.0 9 2.50 2.50
.1
-- travel times and delays -stn c pha remk p p-sec s-sec resid std-er
dist
smor z
iZU9 c 36.00
5.2
azm ain
179 157
tc c vthk
1
se
.10
vpvs
.000
ttob-ttcal-dlay-edly=resid rmk stn pha sources
.21 2.35
-2.14
smor
smor
smor
wpo
wpo
huba
pgo
ross
perl
g04a
g04a
g04a
eyes
bull
bull
bull
colt
vg2
f04a
g03a
g03a
g03a
lvp
rvw
tdh
sso
vll
fl2
cdf
shw
sep
f03a
f03a
f03a
edm
sos
elk
gul
vfp
h04a
tdl
f05a
f05a
f05a
h03a
asr
bow
lcw
z
z
z
z
z
z
z
z
z
e
e
z
z
n
n
z
z
z
n
n
z
z
z
z
z
z
z
z
z
z
z
n
n
z
z
z
z
z
z
z
z
n
n
z
z
z
z
s
smp
s
s
s
s
s
s
S0 c
iZ 1
S2
iZ 1
iZU1
iZ 1
iZ 1
iZD1
iE 4
S2
iZ 1
iZ 1
iN 4
S1
iZ 0
iZD1
iZ 2
iN 4
S2
iZD0
iZD1
iZD2
iZD2
iZD2
iZD2
iZU1
iZ 2
iZ 2
iZU2
iZ 2
iN 4
S2
iZ 2
iZD1
iZD1
iZD1
iZ 2
iZD0
iZD1
iZ 8
iN 4
S8
iZD1
iZ 2
iZD1
iZ 2
37.80
39.06
41.28
38.97
c 39.52
39.38
39.96
d 42.15
47.60
42.56
42.48
47.59
42.92
d 44.16
43.95
52.02
44.70
46.10
47.10
47.59
47.71
48.50
48.63
48.70
49.09
c 49.61
49.30
d
d
d
d
d
d
c
d
d
d
d
d
60.05
49.56
50.18
50.14
50.95
51.40
51.20
51.37
52.40
65.02
d 52.25
53.07
d 53.63
55.98
-.04
.21
.04
.07
.28
.05
-.11
.06
-42.09
.60
.29
-.06
-42.54
-.22
.10
.14
-.24
-44.70
.37
.00
-.28
-.26
.19
-.07
.42
-.01
-.32
.03
.23
-.09
-49.39
.04
.02
-.13
-.30
.14
.44
-.31
-.24
.43
-51.97
.43
-.26
.09
-.53
-.10
1
.10
.50
----.50
.50
.50
.50
.50
--------.51
.51
--------.10
.52
1.06
--------.11
.57
1.20
1.20
1.22
1.25
.65
1.34
1.34
1.38
1.38
--------1.39
.74
.75
.78
1.57
.16
.85
------------.93
2.02
1.24
3.99
13.0
13.5
16.3
17.0
22.5
36.3
36.3
37.6
39.4
39.4
41.2
49.3
50.4
53.8
53.8
64.8
71.4
71.6
74.1
76.1
79.8
82.3
82.6
84.7
84.8
84.8
85.8
90.9
91.7
94.2
95.2
98.8
99.4
101.8
101.8
105.4
108.5
116.3
129.1
303 131
131
360 130
109 124
358 122
206 113
158 101
158 101
101
238 100
100 99
100 99
99
156 99
143 96
21 96
247 96
96
247 96
17 94
354 94
110 94
168 94
94 93
17 93
35 93
23 93
25 93
304 93
304 93
93
27 93
26 93
15 93
60 93
102 93
158 92
20 92
65 92
65 92
92
209 92
48 92
338 92
358 76
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2.01
1.80
3.27
5.49
3.18
3.73
3.59
4.17
6.36
-35.79
11.81
6.77
6.69
-35.79
11.80
7.13
8.37
8.16
-35.79
16.23
8.91
10.31
11.31
11.80
11.92
12.71
12.84
12.91
13.30
13.82
13.51
-35.79
24.26
13.77
14.39
14.35
15.16
15.61
15.41
15.58
16.61
-35.79
29.23
16.46
17.28
17.84
20.19
4.19
1.84
3.06
5.45
3.12
3.46
3.54
4.29
6.30
6.30
11.22
6.48
6.75
6.75
12.02
7.03
8.24
8.40
8.91
15.87
8.91
10.59
11.58
11.61
12.00
12.29
12.86
13.23
13.28
13.60
13.61
13.61
24.22
13.76
14.53
14.65
15.03
15.18
15.72
15.82
16.18
16.18
28.80
16.73
17.19
18.38
20.29
-2.18
.21
.04
.07
.28
.05
-.11
.06
-42.09
.60
.29
-.06
-42.54
-.22
.10
.14
-.24
-44.70
.37
.00
-.28
-.26
.19
-.07
.42
-.01
-.32
.03
.23
-.09
-49.39
.04
.02
-.13
-.30
.14
.44
-.31
-.24
.43
-51.97
.43
-.26
.09
-.53
-.10
smor
smor
wpo
wpo
huba
pgo
ross
perl
g04a
g04a
g04a
eyes
bull
bull
bull
colt
vg2
f04a
g03a
g03a
g03a
lvp
rvw
tdh
sso
vll
fl2
cdf
shw
sep
f03a
f03a
f03a
edm
sos
elk
gul
vfp
h04a
tdl
f05a
f05a
f05a
h03a
asr
bow
lcw
s
smp
s
s
s
s
s
s
lmw
bro
bkc
pcmd
rer
g06a
huo
rcs
euo
rvc
z
z
z
z
z
z
z
z
z
z
iZU1 c 56.60
iZ 8
56.97
iZ 8
60.64
iZ 1
59.47
iZD2 d 60.48
iZ 8
61.30
iZ 7
62.45
iZ 7
61.88
iZ 7
61.28
iZD7 d 61.72
.14
2.28
-.54
----.89
-----.52 d ----.10 d ----.56
----.81
----.21
-----.48
-----.05
-----
131.7
138.9
154.1
155.7
158.4
160.8
166.9
167.2
167.8
167.8
depth may decrease by
1.00 or increase by
standard error of
.100 and rms limit of
.145
12
173
150
10
23
100
157
25
192
18
76
76
76
76
76
76
76
76
76
76
1
1
1
1
1
1
1
1
1
1
20.81
21.18
24.85
23.68
24.69
25.51
26.66
26.09
25.49
25.93
.93 given a reading
20.67
21.73
23.97
24.20
24.60
24.95
25.85
25.88
25.97
25.98
.14
-.54
.89
-.52
.10
.56
.81
.21
-.48
-.05
lmw
bro
bkc
pcmd
rer
g06a
huo
rcs
euo
rvc