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 Contract 259116-A-B3 from the Pacific Northwest National Laboratory, operated by Battelle for the U.S. Dept. of Energy
DATE-(UTC)-TIME LAT(N) LON(W) DEPTH MAG COMMENTS yy/mm/dd hh:mm:ss deg. deg. km 97/06/23 19:13:27 47.58N 122.56W 7.2 4.9 5.7 km NE of Bremerton 97/06/24 14:23:13 48.35N 119.88W 11.2 4.6 23.0 km W of Okanogan 97/06/24 14:40:58 49.25N 123.61W 14.2 4.6 44.2 km WNW of Vancouver,BC
Felt Earthquakes during the 3rd Quarter of 1997 DATE-(UTC)-TIME LAT(N) LON(W) DEPTH MAG COMMENTS yy/mm/dd hh:mm:ss deg. deg. km _ 1. 97/07/03 22:54:43 47.75N 121.80W 0.0 1.7 13.9 km E of Duvall 2. 97/07/04 10:45:38 47.70N 120.03W 8.6 3.7 14.1 km S of Chelan 3. 97/07/11 01:26:01 47.58N 122.53W 1.0 2.2 7.2 km ENE of Bremerton 4. 97/07/11 01:28:55 47.58N 122.53W 6.1 3.5 7.0 km ENE of Bremerton 5. 97/08/01 12:55:03 47.30N 123.73W 0.0 3.4 36.5 km N of Aberdeen 6. 97/09/03 17:17:26 47.68N 120.26W 0.6 3.3 4.9 km NW of Entiat 7. 97/09/24 06:10:22 48.60N 123.08W 10.0 3.4 10.9 km NW of Friday Harbor, San Juan Island
There were 1,167 events digitally recorded and processed at the University of Washington between July 1 and Sept. 30, 1997. Locations in Washington, Oregon, or southernmost British Columbia were determined for 633 of these events; 568 were classified as earthquakes and 65 as known or suspected blasts. The remaining 534 processed events include teleseisms (129 events), regional events outside the PNSN (54), 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.
Last quarter, three geographically separated magnitude 4.5+ earthquakes occurred within a 24 hour period on June 23 and 24. The first, and largest, earthquake occurred within the Puget basin near Bremerton. The second earthquake was east of the Cascade crest near Okanogan, and the third was located in British Columbia's Georgia Strait between Vancouver Island and the mainland.
Aftershocks and update for the Bremerton earthquake, June 23 19:13 UTC, M 4.9
Last quarter, a magnitude 4.9 Bremerton earthquake occurred on June 23, 1997 at 12:13PM PDT (19:13 UTC). The epicenter was approximately 5.7 km east-northeast of Bremerton, Washington. The earthquake, and several aftershocks, were widely felt throughout the greater Seattle area. During the second quarter, a total of 44 events, including the mainshock, were located within a rectangular area 10 km square centered on the mainshock location. In the third quarter 39 earthquakes were located within the same area, including two felt earthquakes about 3 minutes apart on July 11 (UTC) (see Table 4). After the end of July, aftershock activity diminished considerably.
Aftershocks and update for the Okanogan earthquake, June 24 14:12 UTC, M 4.6\fR
A magnitude 4.6 earthquake centered between Okanogan and Twisp, Washington, occurred at 14:40 UTC (6:40 am PST) on June 24, 1997 (Figure 11). It had a depth of about 11 km, and was widely felt in north-eastern Washington. A moderate aftershock (M 3.6), also felt, occurred 12 minutes after the mainshock. A a total of 8 aftershocks were located nearby during the second quarter, and three more aftershocks, none larger than magnitude 2.5, were recorded and located during the third quarter.
Aftershocks and update for the Georgia Strait earthquake, June 24 14:41 UTC, M 4.6
Last quarter, a M 4.6 earthquake occurred beneath the Strait of Georgia. Although out
of
the area normally covered by this report, this event was felt in northwestern Washingt
on.
During the third quarter, one aftershock, magnitude 1.5, was located nearby.
In the the Klamath Falls area, only 8 earthquakes (3 of magnitude 1.6 or larger) were located this quarter. A pair of damaging earthquakes near Klamath Falls in September of 1993 (Sept. 21, 03:29 and 05:45 UTC; Mc 5.9 and 6.0 respectively) were followed by a vigorous aftershock sequence which has decreased over time.
During the third quarter of 1997, 473 earthquakes were located between 45.5° and 49.5° north latitude and between 121° and 125.3° west longitude. No events in western Washington had depths greater than 50 km this quarter.
Five earthquakes were reported felt in western Washington this quarter. Two of these, both on July 11, were aftershocks of the June 23 Bremerton M 4.9 earthqu ake. Elsewhere in western Washington, small earthquakes were reported felt at Duvall, Aberdeen, and Friday Harbor. Time, location, and depth details are given in Table 4.
The one felt earthquakes in the Duvall area this quarter was an aftershock of a magnitude 5.4 earthquake that occurred in May of 1996. The number of earthquakes in the area has diminished greatly, and only 8 earthquakes were located near Duvall this quarter.
In late August, 1997, a section of the Avalanche Glacier 1 km long and 300m wide detached from the upper southeast slope of Mount Adams, generating a large debris avalanche. The avalanche descended the glacier and continued into the heads of Salt and Mud Creeks below timberline, leaving a deposit with an average length of 4 km, exceeding 5 km in places. Field observations made by geologists from the Cascade Volcano Observatory (CVO) indicate that the avalanche deposit consisted mostly of glacier ice and snow, blanketed by a veneer of sand, clay, and rock debris scoured from hydrothermally altered rock underneath the glacier. The bulk volume of the deposit is estimated to be approximately 5 million cubic meters.
The avalanche generated three seismic signals, indicating that glacier ice and rock debris fell from the Avalanche Glacier at least three times- first at 0035 PDT on 8/30, and again a day later at 0631 and 0635 PDT on 8/31. These times agree with reports from hikers, one of whom witnessed the fall of the largest avalanche at 0635. Although they are visible on helicorder records from most stations in southern and central Washington and northern Oregon, these low amplitude signals were not digitally recorded, and were not recognized by seismologists until a search was made for them after the avalanche. The most distant station on which the signal is visible is JCW, 224 km from the avalanche source.. The signal amplitudes and durations increased during the sequence from an average of 60 seconds and 1mm for the first event to 110 seconds and 2mm for the third. The increase in signal size with time indicates that each successive pulse of the avalanche was larger in volume.
The cirque that contains the upper Avalanche Glacier has been the source of two other large avalanches since 1900. An avalanche in 1921 left a deposit of 5 million cubic meters of altered rock debris on the southeast slope of the mountain, and a previous collapse of the upper Avalanche Glacier on July 15, 1983 left a deposit of icy rubble similar to but smaller than the August 1997 deposit.
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, 78 additional events were too small to locate.
A total of 45 events (17 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, 19 (11 smaller than magnitude 0.0; events smaller than magnitude 0.0 are not shown in Fig. 4) 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° N latitude, 121.83-122° W longitude). Closer to the summit (within 5 km), there were 7 tectonic-style earthquakes. The remaining events were scattered around the cone of Rainier as seen in Fig. 4.
During the third quarter of 1997, 67 earthquakes were located in eastern Washington. Three earthquakes were larger than magnitude 3.0, and two of them were reported felt; one near Chelan and another near Entiat. Times, locations, and depths of felt earthquakes are given in Table 4. Although 8 aftershocks were located this quarter near Okanogan, where a magnitude 4.6 earthquake occurred last quarter, the largest was only magnitude 2.5, and none were reported felt.
