SEISMICITY SUMMARY FOR WASHINGTON AND OREGON
From: PNSN QUARTERLY NETWORK REPORT 2000-A
January 1 through March 31, 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 first quarter, 2000

The PNSN operates seismograph stations in Washington and Oregon.
  • Figure 1. (19.7K). shows seismograph stations operated by the PNSN during the first quarter of 2000.


    PNW SEISMICITY

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

    EARTHQUAKES REPORTED FELT, 1st QUARTER, 2000 
    Felt Earthquakes during the 1st Quarter of 2000
  DATE-(UTC)-TIME   LAT(N) LON(W)  DEPTH   MAG  COMMENTS
  yy/mm/dd hh:mm:ss   deg.   deg.    km    
00/01/16 15:07:58   47.80N  122.74W   17.6  3.0   11.1 km  NW of Poulsbo, WA
00/01/30 19:10:23   45.19N  120.12W    0.0  4.1    6.5 km  SE of Condon, OR
00/01/30 20:46:06   45.18N  120.10W    0.0  3.4    8.8 km  SE of Condon, OR
00/01/30 20:52:28   45.18N  120.10W    1.8  2.8    8.5 km  SE of Condon, OR
00/01/31 03:48:29   48.30N  121.62W    4.1  1.2    5.9 km NNW of Darrington, WA
00/02/01 09:25:36   45.18N  120.11W    0.0  2.8    7.7 km  SE of Condon, OR
00/02/29 18:36:37   45.18N  120.11W    0.0  2.5    7.4 km  SE of Condon, OR

    There were 944 events digitally recorded and processed at the University of Washington between January 1 and March 31, 2000. Locations in Washington, Oregon, or southernmost British Columbia were determined for 507 of these events; 447 were classified as earthquakes and 60 as known or suspected blasts. The remaining 437 processed events include teleseisms (136 events), regional events outside the PNSN (125), 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 and retrieve broad-band data for some of them.

    OREGON SEISMICITY

    During the first quarter of 2000 a total of 88 earthquakes were located in Oregon between 42.0°-45.5° north latitude and 117°-125° west longitude.

    The most interesting activity in Oregon this quarter was a cluster of earthquakes near Condon. Between January 5 and Feb. 29, twenty-three ear thquakes magnitude 2.0 or larger were located in the vicinity. The largest event in the sequence w as magnitude 4.1. Five of these earthquakes were reported felt near Condon. Table 3 gives details on the felt events.

    In the Klamath Falls area, 17 earthquakes were located this quarter. Since 1994, most earthquakes northwest of Klamath Falls have been considered aftershocks of a pair of damaging earthquakes in September of \fI1993\fR (Sept. 21, 03:29 and 05:45 UTC; @ M sub c @ 5.9 and 6.0 respectively). The 1993 earthquakes were followed by a vigorous aftershock sequence which decreased over time.

    WESTERN WASHINGTON SEISMICITY

    During the first quarter of 2000, 296 earthquakes were located between 45.5°-49.5° north latitude and 121°-125.3° west longitude.

    Two earthquakes were felt this quarter in western Washington, The largest was a magnitude 3.0 earthquake that took place on January 16 near Poulsbo, WA. Table 3 gives details.

    This quarter, the deepest event recorded by the PNSN was a magnitude 1.6 at a depth of a bout 54 km. It occurred on February 3 at 21:11 UTC and was located about 13.5 km ESE of Tacoma, WA.

    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). "L" and "S" type events are listed in the catalog, but not shown in Figure 4. Although no events flagged "L" or "S" events were located at Mount Rainier this quarter, 24 "L" or "S" events were recorded, but were too small to loc ate reliably.

    A total of 27 tectonic events (7 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, 11 were 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 earthquakes near Mt. Rainier this quarter was magnitude 2.1.

    This quarter, there were 10 higher-frequency tectonic-style earthquake 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 95 earthquakes were located at Mt. St. Helens in the area shown in Fig. 5. Of these 32 were magnitude 0.0 or larger and 18 were deeper than 4 km, including 6 larger than magnitude 0.0. The largest tectonic earthquake at Mount St. Helens this quarter was magnitude 1.3

    Twelve type "S" or "L" event were located at Mount St. Helens, and 63 "L" or "S" events too small to locate were recorded.


    EASTERN WASHINGTON SEISMICITY

    During the first quarter of 2000, 61 earthquakes were located in eastern Washington in the area described in Table 4. The largest earthquake in eastern Washington this quarter occurred about 2 km west of Coulee City, Washington on March 16 at 09:44 UTC. It had a magnitude of 3.2 and a depth of about 2 km and was not reported to have been felt. .PP Times, locations, and depths of felt earthquakes in the PNSN region are given in Table 3.