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-HQ-98-AG-01937 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 1998 DATE-(UTC)-TIME LAT(N) LON(W) DEPTH MAG COMMENTS yy/mm/dd hh:mm:ss deg. deg. km 1998/10/10 07:55:12 48.90N 122.18W 0.1 1.7 9.4 km N of Deming, WA 1998/10/21 05:25:24 45.93N 122.06W 9.4 3.1 30.0 km SSE of Mount St. Helens 1998/11/03 22:40:48 47.51N 122.76W 23.4 3.1 11.4 km WSW of Bremerton, WA 1998/11/20 17:39:20 48.83N 122.18W 1.6 2.0 3.3 km NE of Deming, WA
There were 1,373 events digitally recorded and processed at the University of Washington between October 1 and December 31, 1998. Locations in Washington, Oregon, or southernmost British Columbia were determined for 772 of these events; 729 were classified as earthquakes and 43 as known or suspected blasts. The remaining 601 processed events include teleseisms (128 events), regional events outside the PNSN (70), 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, Washington and we routinely locate and retrieve broad-band data for some of them.
In the Klamath Falls area, 22 events were located this quarter. Most earthquakes in the Klamath Falls 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). These earthquakes were followed by a vigorous aftershock sequence which has decreased over time.
During the fourth quarter of 1998, 620 earthquakes were located between 45.5°-49.5° north latitude and 121°-125.3° west longitude.
This quarter, the deepest event recorded by the PNSN was not in Washington, but in British Columbia just north of the border and about 30 km north of Deming, Wa shington. The magnitude 1.1 earthquake occurred at a depth of about 84 km on Dec . 9th at 23:09 UTC.
The largest earthquake in Washington this quarter
was not reported felt to the PNSN. It was a magnitude 4.0 earthquake at a very
shallow depth (about 3 km) that occurred at 16:43 UTC on October 9
in a fairly remote area 46 km south-southwest of Yakima, on the
Yakama Indian Reservation.
Four smaller earthquakes were reported felt in western Washington this quarter.
Details are provided in Table 4.
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 three events flagged "L" or "S" events were located at Rainier this quarter, 78 additional "L" or "S" events there were too small to locate.
A total of 69 events (30 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, 24 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 north latitude and 121.83-122 west longitude). The largest tectonic earthquake this quarter was magnitude 2.8.
This quarter, there were 32 higher-frequency, tectonic-style earthquakes within 5 km of the summit. The remaining events were scattered around the cone of Rainier as seen in Fig. 4.
This quarter, during a few days in mid-November, station RCS on Mount Rainier recorded some interesting, very tiny, repeating "clone" events. These events are called "clones" because the wave forms of a set of events look nearly identical. On-line "webicorder" records from RCS provide a very interesting record of these events. Because the webicorder wraps to a new line every 15 minutes, events with about 15 minute periodicity line up very prominently. On November 16, starting at around 14:00 UTC, "clone" events occur every 16+ minutes for several hours. At around 20:00, the interval between quakes gradually shortens to 15 minutes and then the periodicity is lost. On the following day, November 17, another clone sequence, beginning at about 02:12 UTC, begins with events a little more than 15 minutes apart, but the event spacing soon becomes a bit less than 15 minutes. This is very distinctive on the 11/17 RCS webicorder record as chevron-shaped pattern of tiny events. Additional clone sequences continued for several days. The cause of these events is unknown, but a glacially related source is considered likely.
See web pages:
/SEIS/PNSN/WEBICORDER/INTERESTING/RCS_EHZ_UW.19981116.gif
/SEIS/PNSN/WEBICORDER/INTERESTING/RCS_EHZ_UW.19981117.gif
/SEIS/PNSN/WEBICORDER/INTERESTING/RCS_EHZ_UW.19981118.gif
Although only six type "S" or "L" events were located at Mount St. Helens, 115 "S" events too small to locate were recorded.
During the fourth quarter of 1998, 60 earthquakes were located in eastern Washington between 45.5-49.5 degrees north latitude and 117-121 degrees west longitude. The largest earthquake recorded this quarter in eastern Washington was a magnitude 4.0 earthquake at a very shallow depth (about 3 km) that occurred that occurred at 16:43 UTC on October 9 in a fairly remote area 46 km south-southwest of Yakima, on the Yakama Indian Reservation.
Times, locations, and depths of felt earthquakes are usually given in Table 4, but no felt events were reported this quarter.
