SEISMICITY SUMMARY OF WASHINGTON AND OREGON FROM QUARTERLY NETWORK REPORT 95-C
July 1 through September 30, 1995


University of Washington, Pacific Northwest Seismograph Network 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 1434-95-A-1302 and Westinghouse Hanford Company Contract MLA-SVV-208775

CONTENTS

Seismograph Stations operating during the third quarter, 1995

The PNSN operates seismograph stations in Washington and Oregon.

  • Figure 1. shows seismograph stations operated by the PNSN during the third quarter of 1995 (19.7K).
  • Figure 1a. shows seismograph stations operated by the PNSN in the vicinity of Mount Rainier during the third quarter of 1995, some stations are part of a special study (15.8K).

    Figure 2 shows seismicity in Washington and Oregon during the third quarter

  • Figure 2 (16.5K).


    OREGON SEISMICITY

    During the third quarter of 1995, no earthquakes were reported felt in Oregon. A total of 138 earthquakes were located in Oregon between 42.0 degreees and 45.5 degreees north latitude, and between 117 degreees and 125 degreees west longitude. All but 9 of these were located in the Klamath Falls area, where a pair of damaging earthquakes in September of 1993 (Sept. 21, 03:29 and 05:45 UTC; magnitudes 5.9 and 6.0 respectively) were followed by a vigorous aftershock sequence which has decreased over time. This quarter, 19 earthquakes of magnitude 1.6 or larger were located in the Klamath Falls area.

    WESTERN WASHINGTON SEISMICITY

    During the third quarter of 1995, 693 earthquakes were located between 45.5 degreees and 49.5 degreees north latitude and between 121 degreees and 125.3 degreees west longitude (because of activity between 124.9 and 125.2 degreees west longitude, the western limit was extended from 125 to 125.3). There were no earthquakes deeper than 60 km this quarter. Two small earthquakes were reported felt in western Washington during the third quarter of 1995.

    On July 13, at 10:28 UTC, a magnitude 3.7 earthquake at a depth of approximately km occurred approximately 10 km west-southwest of Mount Rainier on the "Western Rainier Seismic Zone" (WRSZ). It was felt by a few people in Eatonville, at Crystal Mountain, and in Orting, and was followed by 15 very small aftershocks during July. Only one aftershock was larger than magnitude 1. Our detection and location capability in this area is excellent.

    On July 15, at 14:36 UTC, a magnitude 3.0 earthquake at a depth of approximately 25 km was located beneath Bellevue, and was felt by a few individuals in Kirkland and Bellevue. This earthquake was preceded by four foreshocks on July 14 and 15 (magnitudes ranging from 0.7 to 2.7 and depths similar to that of the mainshock) and followed by eight aftershocks (magnitudes 0.6 to 1.9 and depths around 25 km) between July 15 and August 5.

    Last quarter, in June, four earthquakes (two of them about M 3.0) were located about 20 km offshore of the northern Olympic Peninsula; about 60 km west of Forks, Washington; and 9 km from the nearest seismograph station, OSP. This quarter, six small earthquakes (none larger than M 2.4), all in July, cluster in the area of the June activity. Several small earthquakes were also located onshore near Forks in July.

    Also apparent in

  • Figure 2(16.5K). is a small cluster of events in southern Washington, east of the Columbia River between Woodland and Vancouver, Washington. This quarter, a minor swarm of 16 very small events (magnitudes 0.2 to 2.0) occurred at depths of approximately 17 km between August 16 and Sept. 11.

    The 1995 Pacific Northwest Seismic Refraction Experiment

    From September 7th through September 16th 1995, the 1995 Pacific NorthWest Seismic Refraction Experiment (PNW '95) took place in western Washington. It was jointly run by the USGS, Oregon State University, the University of Texas at El Paso, and the University of British Columbia, with additional field assistance provided by the University of Washington. The refraction line extended westward from Beverly, in eastern Washington, to Oysterville on Washington's Pacific Coast. objective of this experiment is to construct a 2-D velocity profile across the Cascadia margin. This will be the first E-W refraction profile to extend completely across the Cascades. Specific goals include: tracking the position of the Juan de Fuca slab as it subducts beneath North America (providing important constraints on slab depth and curvature), looking for major crustal velocity discontinuities that might reveal locations of buried faults, mapping out the boundaries of the Southern Washington Cascades Conductor (a major subsurface structure that appears to control seismicity and volcanism in the southern Washington Cascades), and providing a velocity model that can be used to better determine earthquake locations in southern Washington.

    A total of 17 shotpoints were used, with a seismometer array of about 1600 seismometers spaced about 200m apart. To cover the full length, two separate deployments of the seismometer array were necessary. The first extended from Beverly to Packwood, and the second from Packwood to Oysterville. The series of blasts for the first deployment were set off on September 11 from 07:00-07:15 UTC and 10:00-10:15 UTC with 16 shotpoints occupied, The series of blasts for the second deployment were set off during the same UTC hours on September 15, with 15 shotpoints occupied. The PNSN recorded 28 of these blasts, 26 of which were located. The located blasts are flagged "X" in our catalog 3, and are shown as diamond shapes in

  • Figure 3 (13.6K). The locations given in our catalog were computed from wave arrival-times using our standard velocity models. Actual locations are available on request.

    Mount Rainier Area

  • Figure 4 (11K) 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 not shown in Figure 4.

    A total of 84 events (46 of them smaller than magnitude 0.) were located within the region shown in Fig. 4. Of these, 57 (including the magnitude 3.7 earthquake felt on July 13 and discussed above) 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 degreees N latitude, 121.83-122 degreees W longitude). Closer to the summit (within 5 km), there were 17 earthquakes and 2 type "L" or "S" events this quarter (types L and S are not shown in Fig.4). The remaining events were scattered around the cone of Rainier as seen in Fig. 4.

    Mount St. Helens Area

  • Figure 5a (11K) shows tectonic earthquakes near Mount St. Helens. Low frequency (L) and avalanche or rockfall events (S) are not shown. In the third quarter, 318 events (including 70 magnitude 0. or larger), were located at Mt. St. Helens in the area shown in Fig. 5. This quarter, two surficial earthquakes (type "S" or "L") were located. Of the earthquakes this quarter, 188 (30 of them larger than magnitude 0.) were deeper than 4 km. The largest event at Mount St. Helens, magnitude 2.3, was at a shallow depth of less than 3 km.
  • Figure 2a (11K) shows a cross-section of Mount St Helens showing the depth of earthquakes under the volcano during the third quarter of 1995.

    This quarter's activity represents a continued change from the first quarter of this year, which was extremely quiet and had only 18 events, and only 6 deeper than 4 km. In second quarter there were 104 events, 45 of which were deeper than 4 km.

    This increase in seismic activity is very small compared to the activity that preceded each of the explosive and dome-building eruptions between 1980 and 1986. During these eruptions, earthquake activity was clearly associated with the rise of magma (molten rock) into the volcano and its eruption at the surface. The dome-building eruptions were preceded and accompanied by intense shallow earthquake activity, located less than 2 miles (3.2 km) beneath the crater. In contrast, the recent earthquakes were smaller and originated at depths between about 1.5 - 10 km deep. There is no evidence to suggest that they indicate an upward rise of magma.

    The USGS Cascade Volcano Observatory offers a

  • special report on activity at Mount Saint Helens the first 3 quarters of this year, and both
  • background and update information on Mt. St. Helens.

    EASTERN WASHINGTON SEISMICITY

    During the third quarter of 1995, 90 earthquakes were located in eastern Washington, none reported felt. The largest was a magnitude 3.1 earthquake on August 29 at 13:02 UTC located at approximately 15 km depth on the Hanford Reservation about 10 km west of Prosser, WA. It was preceded by 3 foreshocks on August 28, with magnitudes between 1.4 and 1.9, all at about 13 km depth. It was followed by one aftershock, magnitude 1.6, depth 13 km, on the 29th at 18:49 UTC.