SEISMICITY SUMMARY FOR WASHINGTON AND OREGON
Pacific Northwest Seismograph Network
Information contained in this report is preliminary,
and should not be cited for publication. The on-line version of the quarterly isabbreviated. The complete quarterly is available in paper version on request.
Seismograph network operation in Washington and Oregon is supported
by the state of Washington and the following contracts:
U.S. Geological Survey Joint Operating Agreement 01-HQ-AG-0011
and Contract 259116-A-B3 from the Pacific Northwest National Laboratory,
operated by Battelle for the U.S. Dept. of Energy
CONTENTS
Seismograph Stations operating during the fourth quarter, 2002
The PNSN operates seismograph stations in Washington and Oregon.
There were 2,027 events digitally recorded and processed
at the University of Washington
between July 1 and September 30, 2002.
Locations in Washington, Oregon, or southernmost British Columbia
were determined for 837 of these events;
696 were classified as earthquakes and 141 as
known or suspected blasts. The remaining 1190 processed events
include teleseisms (176 events),
regional events outside the PNSN (99),
and unlocated events within the PNSN.
Unlocated events within the PNSN include surficial
events on Mt. St. Helens and
Mt Rainier, very
small earthquakes, and blasts.
Frequent mining blasts occur near Centralia, Washington and
we routinely locate them.
Table 3A is a listing of all earthquakes reported to have been felt during
this quarter, events for which
ShakeMaps or Community Internet Intensity Maps (CIIM) are noted.
ShakeMap shows instrumentally measured shaking using data from accleromete
rs in the network. Peak ground acceleration (PGA) values between accelerometers
are modeled using information about local geology.
CIIM maps are made using "felt" reports relayed via Internet.
These "felt" reports are converted into numeric intensity values, and the CIIM map shows the average intensity by zip code.
TABLE 3A: EARTHQUAKES REPORTED FELT, 4th QUARTER, 2002
During the fourth quarter of 2002, a total of 42 earthquakes were located in Oregon
between 42.0° and 45.5° north latitude, and between
117° and 125° west longitude.
The largest earthquake in Oregon this quarter was magnitude 2.8 on October 14 UTC, located about 50 km north-east of Baker at a depth of about 5 km. Elsewhere
in Oregon a small cluster of 5 earthquakes, magnitudes 2.2-2.7, occurred about 10 km east-southeast of Condon from Oct. 20 to Oct. 25.
In the Klamath Falls area, 3 earthquakes
were located this quarter. None was larger than magnitude 2.0.
Since 1994, most earthquakes in the Klamath Falls area have occurred in the aftershock zone of a pair of damaging earthquakes in September, 1993. The 1993 earthquakes were followed by a vigorous aftershock sequence which decreased over time.
CASCADE VOLCANOS - OREGON
During the fourth quarter of 2002, 427 earthquakes were located between
45.5° and 49.5° north latitude and between
121° and 125.3° west longitude.
Six earthquakes were felt this quarter in western Washington. Details are in Table 3A.
The largest felt earthquake in western Washington was a magnitude 3.8 earthquake
on November 29 located near Point Roberts, WA, about 22 km S of Vancouver B.C.,
at a depth of about 13 km.
The deepest quake in western Washington was a magnitude 1.3 earthquake at about
51 km depth, located about 27 km east-southeast of Friday Harbor, WA.
An abstract on the effects of the Denali, Alaska earthquake in the Seattle area
was presented at the fall AGU meeting:
The Mw7.9 Alaska earthquake of 3 November, 2002, damaged a large number
of houseboats by initiating a seiche in Lake Union in Seattle, Washington.
These houseboats were possibly damaged by motion during the surface waves,
which were the largest arrivals from this earthquake. To better understand
the causes of this seiche and estimate its hazard in future earthquakes, we
examined ground shaking on strong-motion recorders from the Pacific
Northwest Seismic Network (PNSN). Maps of peak ground acceleration on all
three components of motion on the PNSN strong-motion instruments show
substantially increased amplitudes coincident with the Seattle sedimentary
basin. This basin is a 30km-wide by 90 km-long depression in the volcanic
basement rocks, filled with up to 9 km of consolidated-to-unconsolidated
sediments. The south edge of the basin is formed by the Seattle fault.
Taking advantage of the long-period ground motion from this earthquake, we
computed spectral ratios with respect to nearby bedrock sites at periods of
1 sec to 100 sec. The basin sites showed maximum amplification by factors
of 8 to 10 at periods of 5 and 8 sec. Amplifications were greater than a
factor of four throughout the band from 2.5-14.0 sec period. The data thus
show strong amplification of long-period waves by the Seattle basin
compared to bedrock sites or sites on thin sediments immediately outside
the basin. The recorded wave periods may be amplified in part by vertical
resonance in the basin sediments (1-D effects), but we speculate that they
are primarily amplified by horizontal resonance. The amplification and long
duration were adequate to cause the damaging seiche.
Figure 4 shows earthquakes near Mount Rainier.
The number of events in close proximity to the cone of Mt. Rainier
varies over the course of the year, since the source of much of the
shallow activity is presumably ice movement or
avalanching at the surface, which is seasonal in nature.
Events with very low frequency signals (1-3 Hz)
believed to be icequakes are assigned type "L" in the catalog.
Emergent, very long duration signals, probably due to rockfalls or
avalanches, are assigned type "S" (see Key to Earthquake Catalog).
There were four located events flagged "L" or "S" at Mount Rainier this quarter
although 143 "L" or "S" events were recorded, but were too small to locate reliably.
Type L and S events are not shown in Fig. 4.
A total of 107 tectonic events (45 of these were smaller than magnitude 0.0, and
thus are
not shown in Fig. 4) were located within the region shown in Fig. 4.
The largest tectonic earthquake located near Mt. Rainier this quarter was a magnitude 2.8 earthquake on Nov. 22 at 04:14 UTC, located about 17 km northwest of
the summit at about 3 km depth. This quarter, 62 tectonic earthquakes were located in the "Western Rainier Seismic Zone" (WRSZ), a north-south trending lineatio
n of seismicity approximately 15 km west of the summit of Mt. Rainier (for counting purposes, the western zone is defined as 46.6-47 degrees north latitude and
121.83-122 west longitude).
Within 5 km of the summit, there were 33 (16 of them smaller than magnitude 0.0
and thus not shown in Fig. 4) higher-frequency tectonic-style earthquakes, and
the remaining events were scattered around the cone of Rainier as seen in
Fig. 4.
This quarter, 91 earthquakes were located at Mount St. Helens in the area shown in Fig. 5.
Of these earthquakes, 24 were magnitude 0.0 or larger and 6
were deeper than 4 km.
The largest tectonic earthquake at Mount St.
Helens this quarter was a magnitude 2.1 event at about 2 km depth on Nov. 20 UTC. It was located beneath the dome in the crater.
No type "S" or "L" events were
located at Mount St. Helens, but
37 "L" or "S" events too small to locate
were recorded.
During the fourth quarter of 2002, 105 earthquakes were located in eastern
Washington in the area between 45.5-49.5 degrees north latitude and 117-121 degrees west longitude.
In the Spokane area, only one tiny quake (magnitude 0.0), was recorded. It was also reported felt! The largest earthquake in eastern Washington this quarter was magnitude 2.9, on Oct. 22. It located at a depth of about 3 km approximately 19 km south-east of Vantage. A small cluster of 6 earthquakes (magnitudes 1.4-2.6, and depths around 10 km) was located in an
area of generally low seismicity about 70 km due north of Walla Walla. Elsewhere in eastern Washington, about 20 earthquakes were located in the Entiat area, a
locale with peristent seismicity.
The largest Entiat-area quake this quarter was magnitude 2.7, at a very shallow
depth (less than 1 km), on Oct. 25 UTC.
Near Yakima, 9 earthquakes, magnitudes 0.9-2.1 were located at about 10 km depth
between Nov. 29 and Dec. 27 UTC.
From: PNSN QUARTERLY NETWORK REPORT 2002-D
October 1 through December 31, 2002
Dept. of Earth and Space Sciences
Box 351310
Seattle, Washington 98195-1310
FIGURES:
DATE-(UTC)-TIME LAT(N) LON(W) DEPTH MAG COMMENTS CIIM SHAKEMAP
yy/mm/dd hh:mm:ss deg. deg. km
02/10/04 17:31:32 47.65 120.25 3.0 2.5 2.3 km WSW of Entiat, WA
02/10/08 10:40:23 47.58 121.84 17.5 2.9 4.2 km ENE of Fall City, WA x
02/10/18 10:13:57 47.00 121.99 15.7 2.0 21.7 km S of Enumclaw, WA
02/10/31 10:50:46 48.49 122.88 0.1 2.4 10.1 km ESE of Friday Harbor, WA
02/11/04 05:02:03 48.16 122.56 26.7 3.1 33.3 km SSW of Mount Vernon, WA x x
02/11/23 09:41:17 47.69 117.40 0.0 0.6 2.3 km N of Spokane, WA
02/11/29 11:52:35 48.93 123.04 13.3 3.8 22.0 km S of Vancouver,BC x x
02/12/07 04:26:40 48.10 123.34 8.8 2.2 7.8 km E of Port Angeles, WA
02/12/27 06:38:06 48.08 123.35 8.9 2.4 7.5 km ESE of Port Angeles, WA
OREGON SEISMICITY
Mount Hood, Oregon:
WESTERN WASHINGTON SEISMICITY
This quarter 20 earthquakes were located in the area
around Mount Hood from 45.25 to 45.45 N latitude and 121.6 to 121.8 W
longitude. The largest earthquake near Mt. Hood this quarter was magnitude 2.7,
and occurred on Dec. 12 at a depth of about 4 km.
Local Amplification of Seismic Waves from the Mw7.9 Alaska Earthquake
and a Damaging Seiche in Lake Union, Seattle, Washington,
CASCADE VOLCANOS - WASHINGTON
Barberopoulou, A, Creager, K., Qamar, A., Thomas, G., Steele, W., and Pratt, T,
2002, EOS Trans. AGU, V. 834 No. 47, Fall Meet. Suppl., Abstract S72F-1355.
Mount Rainier Area
EASTERN WASHINGTON SEISMICITY
Low frequency (L) and avalanche or rockfall events (S) are not shown.
Low frequency (L) and avalanche or rockfall events (S) are not shown.
