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 1999 DATE-(UTC)-TIME LAT(N) LON(W) DEPTH MAG COMMENTS yy/mm/dd hh:mm:ss deg. deg. km 99/10/03 10:50:29 46.54N 121.81W 3.7 2.5 28.2 km W of Goat Rocks 99/11/16 20:51:16 47.86N 122.00W 19.7 2.5 2.7 km WNW of Monroe, WA 99/11/21 11:33:45 45.46N 122.07W 13.6 2.6 31.3 km WNW of Mount Hood, OR 99/11/25 14:46:15 45.11N 122.78W 28.3 3.4 6.5 km ESE of Woodburn, OR 99/11/29 04:04:15 42.31N 122.01W 7.0 3.4 22.2 km WNW of Klamath Falls, OR 99/12/11 12:53:40 48.53N 123.24W 49.2 3.7 14.4 km NNE of Victoria, BC 99/12/25 06:49:58 48.70N 125.91W 10.0 4.0 49.0 km S of Tofino, BC 99/12/25 07:01:49 48.62N 125.93W 10.0 3.2 58.3 km S of Tofino, BC
There were 1313 events digitally recorded and processed at the University of Washington between October 1 and Dec. 31, 1999. Locations in Washington, Oregon, or southernmost British Columbia were determined for 660 of these events; 610 were classified as earthquakes and 50 as known or suspected blasts. The remaining 653 processed events include teleseisms (148 events), regional events outside the PNSN (74), 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, 118 earthquakes were located this quarter. This is the largest number of earthquakes located in the Klamath Falls area since the third quarter of 1994.
Since 1994, most earthquakes northwest of Klamath Falls have been considered aftershocks of 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). The 1993 earthquakes were followed by a vigorous aftershock sequence which decreased over time.
The earthquakes this quarter may be independent of the 1993 activity, as at least some of the locations appear to be on a completely different fault segment . This quarter, a felt earthquake of magnitude 3.4 (see Table 4 for details). was followed by 84 aftershocks on November 29 (UTC), all smaller than magnitude 2.0. This quarter's earthquake was the largest near Klamath Falls since 1996.
During the fourth quarter of 1999, 394 earthquakes were located between 45.5°-49.5° north latitude and 121°-125.3° west longitude.
Three earthquakes were felt this quarter in this area, the largest was in British Columbia. On December 11, at 12:53 UTC, a magnitude 3.7 (the Canadia Pacific Geoscience Centre (PGC) reports this event as a magnitude 4.0) earthquake at a depth of nearly 50 km occurred about 14 km NNE of Victoria, British Columbia. This earthquake, in a well-populated area, was felt by quite a few people. PGC reports that most people in Victoria were awakened. Many reported a noise, and some pictures were knocked askew. A few unstable items fell off shelves, but no damage was reported. The earthquake was also felt in greater Vancouver, Abbotsford, Gibsons Bellingham (WA), Duncan, Chemainus, Nanaimo, Sooke, and Mill Bay. Two other smaller events, near Monroe and the Goat Rocks Wilderness, were reported felt in western Washington this quarter. Table 4 gives details.
Also worthy of brief mention are two other felt earthquakes outside the area we usually cover (they are listed in Table 4, but not shown in Figure 2). These were located west of Vancouver Island. The first event, PNSN magnitude 4.0 (the PGC reports this event as magnitude 4.2) on Dec. 25 (UTC), was reported felt in British Columbia (to the PGC). The reporting locations were Ucluelet and Tofino on the west coast of Vancouver Island; and Nanaimo, Duncan, Parksville, and Hornby Island on the east coast of the Island. It was followed by two aftershocks (PGC magnitudes 3.2 and 3.3), one of them felt, within 30 minutes of the mainshock.
This quarter, the deepest event recorded by the PNSN was a magnitude 1.5 earthquake at approximately 64 km depth. It occurred on October 24 at 13:33 UTC, about 14.8 km SW of Maple Valley, WA.
A total of 48 tectonic events (21 of these 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, 30 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 earthquakes near Mt. Rainier this quarter was magnitude 2.0.
This quarter, there were 14 higher-frequency tectonic-style earthquake within 5 km of the summit. The remaining events were scattered around the cone of Rainier as seen in Fig. 4.
Two type "S" or "L" event were located at Mount St. Helens, and 236 "L" or "S" events too small to locate were recorded.
During the fourth quarter of 1999, 62 earthquakes were located in eastern Washington in the area described in Table 4A. The largest earthquake in eastern Washington this quarter occurred about 4 km south of Entiat, Washington on December 25 at 22:49 UTC. It had a magnitude of 3.0 and a depth of about 7 km and was not reported to have been felt. .PP Times, locations, and depths of felt earthquakes in the PNSN region are given in Table 4.
