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 2nd Quarter of 1999 DATE-(UTC)-TIME LAT(N) LON(W) DEPTH MAG COMMENTS yy/mm/dd hh:mm:ss deg. deg. km 99/04/03 17:29:22 48.35N 123.23W 46.3 3.1 11.4 km SE of Victoria, BC 99/04/11 00:05:37 48.18N 122.75W 20.3 2.6 39.5 km SW of Mount Vernon, WA 99/04/17 07:31:09 46.85N 121.95W 9.8 3.6 15.2 km W of Mount Rainier 99/06/29 04:51:43 48.93N 123.05W 19.6 2.7 20.6 km S of Vancouver,BC 99/06/29 13:31:46 48.91N 123.05W 19.6 2.2 23.6 km S of Vancouver,BC
There were 776 events digitally recorded and processed at the University of Washington between April 1 and June 30, 1999. Locations in Washington, Oregon, or southernmost British Columbia were determined for 469 of these events; 407 were classified as earthquakes and 62 as known or suspected blasts. The remaining 307 processed events include teleseisms (120 events), regional events outside the PNSN (78), 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.
A swarm of 31 earthquakes, ranging in magnitude from 1.5 to 3.8 were located about 20 km east south-east of Christmas Valley, Oregon. Estimated depths are ar ound 20 km. Because these earthquakes lie at the edge of the PNSN network, thei r locations and depths are not very well constrained. The largest event in the cluster, magnitude 3.8, occurred on April 28 at 08:06 U TC. The Christmas Valley area is rural, and these earthquakes were not reported to have been felt. In addition to the 3.8 earthquake there were two magnitude 3. 0 earthquakes. Near Christmas Valley, there is an interesting geologic feature called "Crack-in-the-Ground", which consists of a NNW to SSE oriented two-mile-long tension fracture in basalt. The width of the crack varies from 10 to 15 feet, and the depth is as great as 70 feet in some parts. "Crack-in-the-Ground" appears to have been caused by normal faulting in the strata underlying the basalt. (The Ore Bin, V. 26, No. 9, pp. 158-166.). The focal mechanism for the largest of this quarter's shocks is consistent with thi s interpretation - it shows normal faulting on NNW-SSE striking planes.
In the Klamath Falls area, 14 earthquakes were located this quarter. Most earthquakes northwest of Klamath Falls are aftershocks of a pair of damaging earthquakes in September of \fI1993\fR (Sept. 21, 03:29 and 05:45 UTC; magnitudes 5.9 and 6.0 respectively). These earthquakes were followed by a vigorous aftershock sequence which has decreased over time.
During the second quarter of 1998, 277 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 a magnitude 1.0 earthquake at about 77 km depth. It occurred on June 12 at 13:44 UTC, about 9 km south of Darrington, WA. This location is about 50 km north of the area near Hyak where such deep events are most frequently located.
The largest earthquake in Washington this quarter was a magnitude 3.6 earthquake
on April 17, located in the
"Western Rainier Seismic Zone" (WRSZ). It was
felt in Randle and Ashford, WA. Four other earthquakes were felt
in western Washington or southern British Columbia this quarter. Three of them
were actually closer to Canada than to the US. Details are provided in Table 4.1
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 no events flagged "L" or "S" events were located at Rainier this quarter, 22 "L" or "S" events there were too small to locate reliably
A total of 31 events (three 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, 18 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 near Mt. Rainier this quarter was the 3.6 April 17 earthquake discussed above. See Table 4 for details.
This quarter, there were 6 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.
One type "S" or "L" event was located at Mount St. Helens, and 28 "S" events too small to locate were recorded.
During the second quarter of 1999, 45 earthquakes were located in eastern Washington. The largest was a magnitude 2.6 earthquake on June 16 at a depth of about 2 km which located near Ritzville, WA in an area where seismicity is rare.
Times, locations, and depths of felt earthquakes in the PNSN region are given in Table 4.
