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
Felt Earthquakes during the 4th Quarter of 1997 DATE-(UTC)-TIME LAT(N) LON(W) DEPTH MAG COMMENTS yy/mm/dd hh:mm:ss deg. deg. km _ 1. 97/10/14 18:20:49 47.58N 122.60W 6.5 2.3 3.8 km NE of Bremerton, WA 2. 97/10/19 23:06:19 47.76N 121.85W 5.4 3.1 9.5 km ENE of Duvall, WA 3. 97/11/18 01:53:06 46.13N 120.46W 15.6 3.9 50.4 km S of Yakima, Wa 4. 97/11/26 00:05:34 47.78N 123.06W 46.6 3.7 32.7 km WNW of Poulsbo, WA 5. 97/12/23 20:22:45 47.21N 123.83W 00.0 2.1 27.3 km N of Aberdeen, WA
There were 1,058 events digitally recorded and processed at the University of Washington between October 1 and Dec. 31, 1997. Locations in Washington, Oregon, or southernmost British Columbia were determined for 681 of these events; 614 were classified as earthquakes and 67 as known or suspected blasts. The remaining 377 processed events include teleseisms (160 events), regional events outside the PNSN (71), 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.
In the the Klamath Falls area, 35 earthquakes (6 of magnitude 1.6 or larger) were located this quarter. This represents a modest increase of seismicity (only 8 were located there last quarter). Most earthquakes in the Klamath Falls are are aftershocks of a pair of damaging earthquakes 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 fourth quarter of 1997, 506 earthquakes were located between 45.5° and 49.5° north latitude and between 121° and 125.3° west longitude. This quarter, one event in western Washington was deeper than 50 km. It occurred at a depth of about 93 km, had a magnitude of 1.4 , and was located near Hyak. About 15 other earthquakes deeper than 80 km, all smaller than magnitude 2.0, have located in the same general vicinity since 1990.
Four earthquakes were reported felt in western Washington this quarter. Time, location, and depth details are given in Table 4.
A large rock avalanche occurred at 12:31 AM PDT Monday, October 20, 1997 on the east side of Mount Adams, Washington. The avalanche originated at about 11,200 ft. elevation on the south face of The Castle, a prominent topographic knob at the head of Battlement Ridge. The source area forms an obvious, near-vertical scar roughly triangular in shape with sides about 300 meters in length. The avalanche traveled beyond the end of the Klickitat Glacier and continued roughly 2 kilometers down the valley of Big Muddy Creek, a tributary of the Klickitat River. The length of the avalanche track totals about 5 kilometers, with an average width of .5 km. The volume of debris is estimated to be between 1 and 5 million cubic meters.
The avalanche deposit temporarily blocked the flow of Big Muddy Creek, avalanche debris. By noon on October 21 the avalanche dam had breached, and flow in Big Muddy Creek did not appear unusual.
This avalanche appears unrelated, except in the broadest fashion, to a similar-s ized avalanche that occurred on the western flank of Mount Adams about seven wee ks earlier (August 31, 1997). Both avalanches originated in areas composed of ro cks evidently weakened by intense hydrothermal alteration. Neither avalanche was triggered by regional earthquake or volcanic activity. The information provided above is from the USGS Cascade Volcano Observatory web-page:
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 two events flagged "L" or "S" events were located at Rainier this quarter, 53 additional "L" or "S" events were too small to locate.
A total of 46 events (13 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, 31 (11 smaller than magnitude 0.0; events smaller than magnitude 0.0 a re 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 8 tectonic-style earthquakes. The remaining events were scattered around the cone of Rainier as seen in Fig. 4.
During the fourth quarter of 1997, 56 earthquakes were located in eastern Washington. One earthquake was reported felt. Four earthquakes were larger than magnitude 3.0. Of these, three were located near Horse Heaven Hills as part of a sequence of 13 quakes that began with a magnitude 3.1 on Oct. 13, and included the largest earthquake this quarter, magnitude 3.9, which was felt at Satus Pass on Nov. 18. Times, locations, and depths of felt earthquakes are given in Table 4.
