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
The PNSN operates seismograph stations in Washington and Oregon.
Figure 2 shows seismicity in Washington and Oregon during the third quarter
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.
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
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
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.
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
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.