SEISMICITY SUMMARY FOR WASHINGTON AND OREGON
From: PNSN QUARTERLY NETWORK REPORT 98-B
April 1 through June 30, 1998


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 second quarter, 1998

The PNSN operates seismograph stations in Washington and Oregon.
  • Figure 1. (19.7K). shows seismograph stations operated by the PNSN during the second quarter of 1998.


    PNW SEISMICITY

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

    EARTHQUAKES REPORTED FELT, 2nd QUARTER, 1998 
    Felt Earthquakes during the 2nd Quarter of 1998
  DATE-(UTC)-TIME    LAT(N) LON(W) DEPTH MAG COMMENTS
  yy/mm/dd hh:mm:ss   deg.   deg.    km    
1. 98/04/27 07:00:16   48.73N  123.16W   57.8  3.0  26.1 km NNW of Friday Harbor, WA
2. 98/06/24 15:53:17   47.75N  121.88W    4.0  2.5   7.9 km ENE of Duvall, WA
3. 98/06/28 07:35:33   46.50N  116.90W   31.4  3.8  32.3 km  SE of Pullman, WA
4. 98/06/29 01:36:42   47.56N  120.81W    8.4  2.4  41.9 km WNW of Wenatchee, WA

    There were 1,995 events digitally recorded and processed at the University of Washington between April 1 and June 30, 1998. Locations in Washington, Oregon, or southernmost British Columbia were determined for 1,248 of these events; 1,183 were classified as earthquakes and 65 as known or suspected blasts. The remaining 812 processed events include teleseisms (164 events), regional events outside the PNSN (79), and unlocated events within the PNSN.

    OREGON SEISMICITY

    During the second quarter of 1998 a total of 69 earthquakes were located in Oreg on between 42.0° and 45.5° north latitude, and between 117° and 125° west longitude. No earthquakes were reported felt in Oregon this quarter.

    In the the Klamath Falls area, 40 earthquakes (4 of magnitude 1.6 or larger) were located this quarter. Most earthquakes in the Klamath Falls are aftershocks of a pair of damaging earthquakes in September of \fI1993\fR (Sept. 21, 03:29 and 05:45 UTC; M 5.9 and 6.0 respectively) earthquakes were followed by a vigorous aftershock sequence which has decreased over time.

    WESTERN WASHINGTON SEISMICITY

    During the second quarter of 1998, 1,049 earthquakes were located between 45.5° and 49.5° north latitude and between 121° and 125.3° west longitude. Six events in western Washington (or western British Columbia) were deeper than 50 km, including a felt earthquake, magnitude 3.0, located near Friday Harbor (see Table 4). Three of the deep events, none larger than magnitude 1.5, were deeper than 80 km. One was 8 km west-southwest of Skykomish, in an area where over 20 similar small deep events have been noted in the past. Another was 23 km south of Hyak; southeast of previously located deep activity. A third deep event was 40 km north of Deming, WA, in British Columbia; and was further north than where such events have been noted in the past.

    In addition to the deep earthquake near Friday Harbor, which was reported felt i n British Columbia, one other earthquake was reported felt in western Washington this quarter, near Duvall. Time, location, and depth details are given in Table 4.

    Mount Rainier Area

    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 surface-type activity is presumably ice movement or avalanching, 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 only three events flagged "L" or "S" events were located at Rainier this quarter, 103 additional "L" or "S" events there were too small to l ocate.

    A total of 47 events (11 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, 17 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 earthquake this quarter was magnitude 1.8. A very shallow low-frequency event on April 21, probably a rockfall, was assigned a magnitude of 2.8 on the basis of a long coda duration.

    Closer to the summit (within 5 km), there were 18 higher-frequency, tectonic-style earthquakes. The remaining events were scattered around the cone of Rainier as seen in Fig. 4.


    Mount St. Helens Area

    This quarter 776 events (compared to only 293 events last quarter) were located at Mt. St. Helens in the area shown in Fig. 5. Of these, 205 were magnitude 0.0 or larger (compared to 73 in the first quarter). Although no type "S" or "L" events were located at Mount St. Helens, 120 "S" or "L" surficial events too small to locate were recorded. Of this quarter's earthquakes, 503 (215 last quarter) were deeper than 4 km including 141 (57 last quarter) larger than magnitude 0.0. The largest tectonic event at Mount St. Helens this quarter was magnitude 2.2. Although the total number of events at Mt. St. Helens increased considerably beginning in May, most of the events this quarter were very small, with only two events larger than magnitude 2. Two Information Statements, were issued by the PNSN and the Cascade Volcano Observatory (CVO) provide more details, and are included in this report.

  • Figure 5A shows how seismic activity at Mt. St. Helens has varied since the beginn ing of 1997. In the upper plot, average number of events occurring per day (aver aged over 30 days) is shown. The lower plot shows average seismic strain energy release (square root of seismic energy) per day (averaged over 30 days). Seismic strain energy is computed from earthquake magnitude and is more representative of the changes in the deep magma system than simply number of locatable events a lone. For comparison, daily strain energy reached values close to 3*10**6 in 1989-1992, when activity culminated in a series of gas-and-ash emissions. Dome building eruptions in the mid 1980s were in the range of 1 to 27*10**7; and the catastrophic 1980 eruption reached strain energy values of around 7*10**9 The current energy levels are around 1.7*10**6.

    EASTERN WASHINGTON SEISMICITY

    During the second quarter of 1998, 79 earthquakes were located in eastern Washington. The largest earthquake recorded near the PNSN network this quarter was a magnitude 3.8 event on June 28 (UTC), 32 km SE of Pullman, WA (too far east to be shown on Fig. 2) This earthquake was felt in Lewiston and Gennesee Idaho, and in Colton, Washingt on. (This earthquake is listed in Table 5, but is outside the area shown in Fig. 2).

    Times, locations, and depths of felt earthquakes are given in Table 4.