SEISMICITY SUMMARY FOR WASHINGTON AND OREGON
From: PNSN QUARTERLY NETWORK REPORT 2000-D
October 1 through December 30, 2000


Pacific Northwest Seismograph Network
University of Washington Dept. of Earth and Space Sciences
Box 351310
Seattle, Washington 98195-1310

Information contained in this report is preliminary, and should not be cited for publication.

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 1434-HQ-98-AG-01937 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, 2000

The PNSN operates seismograph stations in Washington and Oregon.
  • Figure 1a. (19.7K). shows short-period and broad-band seismograph stations operated by the PNSN during the fourth quarter of 2000.


    PNW SEISMICITY

  • Figure 2 (16.5K). shows seismicity in Washington and Oregon during the fourth quarter.

    There were 1,455 events digitally recorded and processed at the University of Washington between October 1 and December 31, 2000. Locations in Washington, Oregon, or southernmost British Columbia were determined for 863 of these events; 746 were classified as earthquakes and 117 as known or suspected blasts. The remaining 592 processed events include teleseisms (172 events), regional events outside the PNSN (65), and unlocated events within the PNSN. Unlocated events within the PNSN include very small earthquakes and some known blasts. Frequent mining blasts occur near Centralia, Washington and we routinely locate only some of them.

    This quarter the PNSN imported ShakeMap from the Southern California TriNet project and began adapting it to our region. ShakeMap generates maps showing instrumentally measured shaking effects. During the quarter, two felt earthquakes larger than magnitude 3.0 occurred in the Puget Sound (on October 15 and Nov. 1) both were well recorded on the strong-motion network, and Shake Maps were made.

    Shake Maps:http://spike.ess.washington.edu/shake/index.html

    Last quarter the PNSN shifted the collection and storage of "felt" reports to the USGS "Community Intensity Internet Map (CIIM)". "Felt" reports are reports from people about how intense the shaking was. The CIIM m ap turns the "felt" reports into a numeric Intensity value, and shows average intensity by zip code. This quarter there were three earthquakes with enough felt reports to generate CIIM maps; the October 15 and Nov. 1 Puget Sound events mentioned above; and a third event near Entiat on Dec. 24.

    CIIM Maps: http://pasadena.wr.usgs.gov/shake/pnw/

    TABLE 3: EARTHQUAKES REPORTED FELT, 4th QUARTER, 2000 
      DATE-(UTC)-TIME   LAT(N) LON(W)  DEPTH   MAG  COMMENTS					CIIM	 SHAKEMAP
  yy/mm/dd hh:mm:ss   deg.   deg.    km    

00/10/15 14:30:05    47.84N 123.00W  48.5   3.6   31.9 km WNW of Poulsbo, WA 		x  	x
00/11/01 08:37:17    48.27N 122.53W  21.8   3.3   21.6 km SW of Mount Vernon, WA 	x  	x
00/11/05 13:10:01    49.47N 119.63W   0.0   3.0   4.7 km  SW of Penticton, BC    
00/11/10 09:12:39    48.45N 123.23W  25.2   2.5   9.5 km ENE of Victoria, BC    
00/11/25 10:01:39    48.83N 119.33W   2.0   3.1   55.2 km NNE of Okanogan, WA    
00/12/24 17:04:58    47.73N 120.28W   9.4   3.6   9.7 km NNW of Entiat, WA  		x  
00/12/31 18:07:44    47.50N 121.67W  12.6   2.9   8.6 km E of North Bend, WA

    OREGON SEISMICITY

    During the fourth quarter of 2000 a total of 29 earthquakes were located in Oreg on between 42.0° and 45.5° north latitude, and between 117° and 125° west longitude. The most interesting activity in Oregon thi s quarter was a deep earthquake that occurred on December 30 about 25 km SSW of Salem. The magnitude 2.6 earthquake was located at about 62 km, which is an unu sual depth for this part of Oregon. The earthquake was presumably located withi n the Juan de Fuca plate; the focal mechanism was indicative of a normal fault w hich is not surprising because the Juan de Fuca plate beneath Washington and Ore gon appears to be under tension as it sinks into the mantle. October 21 provided a day of interesting activity. About 20 felt reports were r eceived from Oregon coastal areas including Coos Bay, Bandon, Lincoln City, Wald port, North Bend, and Newport. These were actually due to military overflights and not earthquake activity. An interesting note this quarter: no earthquakes occurred in the Klamath Falls a rea. This is the first quarter that no earthquakes have occurred. Since 1994, most earthquakes in the Klamath Falls area have been considered aftershocks of a pair of damaging earthquakes in September of 1993. The 1993 earthquakes were f ollowed by a vigorous aftershock sequence which decreased over time.

    WESTERN WASHINGTON SEISMICITY

    During the fourth quarter of 2000, 642 earthquakes were located between 45.5° and 49.5° north latitude and between 121° and 125.3° west longitude. Four earthquakes were felt this quarter in western Washington. Details are in Table 3.

    The first felt earthquake was on October 15, located about 32 km WNW of Poulsbo, WA. The magnitude 3.6 earthquake was was well-recorded by our network of strong-motion instruments. The earthquake was felt across Kitsap and Jefferson counties, including the cities Poulsbo, Brinnon, Silverdale, Sequim, Bainbridge Island , and Mountlake Terrace.

    On November 1 an earthquake was located about 21 km SW of Mount Vernon, WA with a magnitude of 3.3. This event was felt in areas around Whidbey Island, Camano Island, San Juan Islands, Stanwood, Marysville, and also Victoria, BC. It was well-recorded by our network of strong-motion instruments. The third felt earthquake this quarter occurred on November 10 and was actually located 10 km ENE of Victoria, BC. The PGC reported a magnitude of 2.5 and received felt reports across greater Victoria, BC. The last felt earthquake in western Washington for this quarter occurred on December 31 about 9 km E of North Bend, WA. The magnitude 2.9 earthquake was reported felt by several people in North Bend.

    Mount Rainier Area

    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). The third quarter of 2000 showed a large increase in the number of "L" and "S" events because of improved recording at Mount Rainier, where summit station RSU allowed the recording of many small events. Station RSU is no longer operating; however, having the improved recording for the time we did helped us to refine our classification system. We are still paying close attention to the "L" and "S" events and are careful in our classification of these events. Consequently, the number of "L" and "S" events from this quarter and following quarters will seem greater than in previous quarters. 90 events flagged "L" or "S" events were located at Mount Rainier this quarter and an additional 143 "L" or "S" events were recorded, but were too small to locate reliably. "L" and "S" type events are listed in the catalog, but not shown in Fig. 4.

    A total of 85 tectonic events (25 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. Of these, 31 were tectonic events located in the "Western Rainier Seismic Zone" (WRSZ), a north-south trending lineation 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). The largest tectonic earthquake located near Mt. Rainier this quarter was a magnitude 2.5 and was located about 20 km SE of Mt. Rainier at a depth of 3.7. This earthquake was one of 29 events in this area 20 km SE of Mt. Rainier that occurred between November 11 and November 22, 2000. The cluster of earthquakes began on November 11 with a magnitude 2.4 earthquake. The magnitudes ranged from -0.3 to 2.5, with 16 events having a magnitude >= 1.0. This cluster can be seen in Fig. 4.

    This quarter, there were 20 higher-frequency tectonic-style earthquakes within 5 km of the summit. The remaining events were scattered around the cone of Rainier as seen in Fig. 4.


    Mount St. Helens Area

    This quarter 227 earthquakes were located at Mt. St. Helens in the area shown in Fig. 5. Of these 85 were magnitude 0.0 or larger and 14 were deeper than 4 km. One of these deep events occurred on December 5 and was a rare deep, low-frequency event. The event was located about 4 km ESE of Mount St. Helens at a depth of 30 km and had a magnitude of 1.1. The largest tectonic earthquake at Mount St. Helens this quarter was a magnitude 1.6 event located 0.4 km NE of Mount St. Helens.

    Three type "S" or "L" event were located at Mount St. Helens, and 65 "L" or "S" events too small to locate were recorded.


    EASTERN WASHINGTON SEISMICITY

    During the fourth quarter of 2000, 73 earthquakes were located in eastern Washington. The fourth quarter of 2000 presented some interesting seismicity in eastern Washington: A small cluster of earthquakes, visible in Fig. 2, began on December 21, 2000 about 18 miles west of the Tri-Cities (Richland, Pasco, and Kennewick, WA). The epicenters are near the north end of the Horse Heaven Hills, 8 miles east of Prosser and 25 km WSW of Richland. The cluster occurred between December 12, 2000 through December 18, 2000, and included 28 located earthquakes with magnitudes between -0.5 and 2.1. Earthquakes in this part of the Columbia Plateau are common and often occur in swarms. More than 90 percent of swarm earthquakes near the Tri-Cities region are very shallow, at less than 8 km depth. However, the depths of this quarter's swarm are 8-10 km below the surface. Focal mechanisms of Columbia Plateau earthquakes usually indicate a north-south compressive thrust regime perpendicular to, and consistent with, the east-west trend of the anticlines of the Yakima fold belt. This quarter's earthquakes occur within a NE trending zone of earthquakes in the Horse Heaven Hills. However, the mechanisms of the earthquakes in the current cluster determined by P-wave first motions, and the spatial pattern of the hypocenter locations suggest that the earthquakes are occurring on a fault perpendicular to this trend, possibly on a northwest striking reverse fault.

    The other interesting seismicity in eastern Washington were two felt earthquakes. The first was located 55 km north-northeast of Okanogan, WA. The magnitude 3.1 earthquake was reported felt by residents living 50 miles north of Okanogan. The second felt earthquake occurred near Entiat, WA. The magnitude 3.6 earthquake occurred on December 24, 2000 at 1705 UTC time. It was located about 10 km north-northwest of Entiat at a depth of about 9 km. The felt reports included areas around Entiat, Wenatchee, and Chelan. This area is a very persistent temporal and spatial cluster of earthquake activity. Instrumental coverage was not very good in the Chelan cluster area until around 1976. Since 1976, the rate of seismicity has been quite steady, with between 27 and 69 earthquakes of magnitude 1.0 or larger each year. An earthquake occurred on November 5 in Penticton, B.C. The PGC reported a magnitude 2.6 and received reports that it was felt in the Penticton area.

    Times, locations, and depths of felt earthquakes in the PNSN region are given in Table 3.