SEISMICITY SUMMARY FOR WASHINGTON AND OREGON
From: PNSN QUARTERLY NETWORK REPORT 96-D
October 1 through December 31, 1996


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 1434-95-A-1302 and Westinghouse Hanford Company Contract MLA-SVV-208775

CONTENTS

Seismograph Stations operating during the fourth quarter, 1996

The PNSN operates seismograph stations in Washington and Oregon.
  • Figure 1. (19.7K). shows seismograph stations operated by the PNSN during the fourth quarter of 1996.


    PNW SEISMICITY

  • Figure 2 (16.5K). shows seismicity in Washington and Oregon during the fourth quarter There were 974 events digitally recorded and processed at the University of Washington between October 1 and December 31, 1996. Locations in Washington, Oregon, or southernmost British Columbia were determined for 476 of these events; 387 were classified as earthquakes and 89 as known or suspected blasts. The remaining 498 processed events include teleseisms (135 events), regional events outside the PNSN (91), 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, and we routinely locate and retrieve broad-band data for some of them.

    The largest earthquake within our network this quarter (on December 15 at 08:34 UTC) was only magnitude 2.9. However, an earthquake of magnitude 6.2 occurred on the Nootka fault, off of the west coast of Vancouver Island, British Columbia . This earthquake occurred at 20:13 UTC on October 6, and was located (by the NEI C) at a shallow depth at 48.8 degrees north latitude and 128.4 degrees west longitu de. It was not reported felt. It is not shown in our figures because it is outside our reporting area. More information is available: 10/6/96 Earthquake - Offshore Vancouver Island, B.C.

    OREGON SEISMICITY

    During the fourth quarter of 1996 a total of 38 earthquakes were located in Oreg on between 42-45.5 degrees north latitude, and 117-125 degrees west longitude. 29 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; @ M sub c @ 5.9 and 6.0 respectively) were followed by a vigorous aftershock sequence which has decreased over time. This quarter, only six earthquakes of magnitude 1.6 or larger were located in the Klamath Falls area. No earthquakes were reported felt in Oregon this quarter.


    WESTERN WASHINGTON SEISMICITY

    During the fourth quarter of 1996, 312 earthquakes were located between 45.5-49.5 degrees north latitude and between 121-125.3 west longitude. The deepest earthquake this quarter was a magnitude 2.7 event on December 17 at 05:53 UTC at a depth of about 57 km. Its epicenter falls between San Juan and Wa ldron Islands.

    Two small earthquakes were reported felt in western Washington this quarter. Both were located at shallow depths (around 5 km) near Duvall, where a magnitude 5.4 earthquake occurred on May 3 at 04:04 UTC. This quarter's larger event, magnitude 2.6, occurred on Oct. 27 at 07:53 UTC and the smaller event was magnitude 2.3 on November 13 at 06:12 UTC.

    In the Duvall area, activity this quarter included 25 earthquakes. For comparison, 382 were located in the same area during the second quarter, and 57 earthquakes during the third quarter. The largest earthquake in the Duvall vicinity this quarter was the magnitude 2.6 felt event discussed above.


    Mount Rainier Area

    A total of 59 events (21 of them smaller than magnitude 0., and thus not shown in Fig. 4) were located within the region shown in Fig. 4. Of these, 16 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 N latitude, 121.83-122 degrees W longitude). Closer to the summit (within 5 km), there were 25 tectonic-style earthquakes, and 5 "L" or "S" events were located 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

    In the fourth quarter, 65 events (only 14 magnitude 0. or larger), were located at Mt. St. Helens in the area shown in Fig. 5. This quarter, no type "S" or "L" events were located. Of this quarter's earthquakes, 35 (7 of them larger than magnitude 0.) were deeper than 4 km. The largest event at Mount St. Helens, magnitude 1.8, on November 16 at 19:09 UT C, was at a depth of about 10 km.

    EASTERN WASHINGTON SEISMICITY

    During the fourth quarter of 1996, 45 earthquakes were located in eastern Washington. None were reported felt. The largest quake in the area that we refer to as "eastern Washington" for reporting purposes, actually occurred in Oregon this quarter. It was on December 14 at 21:50 UTC, had magnitude 2.3 and was located at a depth of about 2 km between Pendleton and Uma tilla.