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This report is prepared as a final summary of the work performed for the U.S. Geological Survey in the Mount St. Helens, Olympic Mountains and northern Oregon areas. Information contained in this report is only of a summary nature and should not be cited directly for publication. The original reports and listed publications should be refered to for details of the work.
The University of Washington operates the Pacific Northwest regional seismograph network with financial support of several organizations. During the period 1988-1989 the network was supported by the U.S.G.S external program for ~26 stations in west central Washington, by Westinghouse Hanford Company for the Department of Energy for ~26 stations in eastern Washington, and by the U.S.G.S. (this contract) for ~44 stations in the Mount St. Helens, Olympic mountains and northern Oregon areas. The composite network is cooperatively operated with all data being recorded and analysed together using the same computer systems and analysis personnel. Field technical maintenance is handled separately in each area though assistance between groups is common.
Summaries of seismic activity recorded by the whole network are published quarterly and distributed to a large mailing list within one month of the end of each quarter. Details of network operations, unusual seismic activity, and important earthquakes as well as a preliminary catalog are included in these reports. Comprehensive earthquake bulletins are published through the State Department of Natural Resources on a yearly basis. Below is a brief summary of the important seismic activity during the period of this contract. Copies of the quarterly reports are included in the appendix to this final technical report.
Individual research projects have been carried out with support of this contract by students and the principal investigators using data from the whole network in some cases or small subsets in other cases. This research has primarily been on the seismic and volcanic activity at Mount St. Helens. Research has involved details of the volcanic structure and plumbing system at St. Helens as well as volcanic earthquake sources and the attenuation structure of the area. Lists of reports and publications detailing this research are included at the end of this report.
During this contract period a major change was made in the data recording and analysis part of our operations. The recording computer system was changed from a DEC PDP-11/34 (running since March 1, 1980) to a Masscomp-5600 system. The new on-line recording system, called HAWK, is much more reliable, versatile, and efficient. It includes alarm capability for both large earthquakes as well as computer failure and produces automatic soulutions for most medium to large earthquakes. It has reduced our operating costs because of the increased efficiency of the analysis personnel, its lower maintenance costs, and its increased computing capability. A detailed users manual for the HAWK system is included as part of the appendix to this report.
A total of 1,565 earthquakes were located in Washington and Oregon during the period January 1, 1988 through February 28, 1989. 76% of these events occurred in western Washington, and 23% occurred in eastern Washington. Seismicity in eastern Washington followed past patterns for the most part with clusters around Entiat, Corfu and Wooded Island. There were a couple of felt events near Entiat with magnitudes of 3.0 and 3.3 in the first half of 1988. Western Washington's 76% value can be broken down into 16% beneath the cone of Mount St. Helens with the remaining 60% elsewhere in western Washington.
Of the 1,188 earthquakes located in western Washington and Oregon, 21% were located beneath Mount St. Helens during the time period covered in this report. Mount St. Helens events will be covered separately later in this report because although it lies within western Washington, the trend in seismicity differs from the rest of this area. The average number of events (excluding Mount St. Helens) during the first four quarters was 198. 143 earthquakes occurred in the first two months of 1989 which falls well within the previous range.
As in previous years, the majority of these events were located in the Puget Lowlands and along the western front of the Cascades, with most depths less than 30km. A distinct concentration of earthquakes along a zone extending several kilometers NNW from Mount St. Helens is a pattern that was observed once again during this period. An increased number of earthquakes with magnitudes large enough to be felt occurred in the Puget Lowlands, along the West Cascade Front and on the Olympic Peninsula during the first quarter of 1989. The average number of western Washington felt earthquakes per quarter in 1988 was three; 19 felt earthquakes had occurred by the end of the first quarter of 1989 in western Washington.
Felt earthquakes were recorded along the West Cascade Front throughout this time period. In the first quarter of 1988 Enumclaw residents were shaken by a magnitude 2.7 shallow event. A magnitude 2.9 magnitude earthquake at nearly 20km rattled North Bend. In the first quarter of 1989, residents in the Packwood and Randall area, approximately 35km south of Mount Rainier in the Cascades, were shaken by a shallow magnitude 3.0 magnitude event. A shallow magnitude 2.5 earthquake was felt near Darrington which has had a history of felt and cluster type earthquakes.
In the second quarter of 1988 a magnitude 3.1 earthquake was felt at Carnation and Duvall; and a magnitude 2.9 event occurred at a depth of 7.5km and shook residents a few kilometers east of Sedro Wooley in the Skagit Valley. Two events again were felt in the third quarter. A near-surface magnitude 3.2 earthquake was located in the Robe Valley area, between Granite Falls and Darrington. Also a magnitude 3.1 event occurred at shallow depth and it was felt near Arlington.
Cluster activity was observed along the West Cascade Front also. In the last quarter of 1988 a cluster near Enumclaw in the 0.5 to 2.7 magnitude range was concentrated in a small area. Most of the events in this cluster (23) occurred between December 11 and 13. During January and February 1989, a cluster near Big Lake which is a couple miles east of Mt. Vernon tallied nine felt events with some damage to a nearby school. The maximum magnitudes events were a 4.0 on February 14 and a 4.2 on March 6 with near-surface depths. The minimum felt event was a magnitude 1.8 earthquake on February 14. Due to the high seismicity and the occurrence of two moderate sized earthquakes in the area, several portable seismographs were installed by the USGS and were allowed to run for several weeks. Also this quarter, two shallow events within thirteen minutes registering magnitudes of 2.7 and 2.9 were felt in the Preston and Fall City area.
Within the Cascade Mountain Range, Mount Rainier experienced a variety of earthquakes well into March 1989. Mount Hood recorded a brief swarm in March 1989. This pattern of activity at both volcanoes was similar to past years. At Mount Rainier, shallow low- and high-frequency events are quite common near the summit. Deeper high frequency earthquakes are common along the west boundary of Mount Rainier National Park. They tend to be located along a north-south trend. The maximum magnitude earthquake occurred in the third quarter of 1988 on the west flank of Mount Rainier and was felt in various towns around the mountain and had a magnitude of 4.1. A magnitude 3.8 aftershock occurred at the same location about an hour later. Two weeks later a magnitude 3.0 earthquake occurred in the same area. The magnitude 4.1 earthquake was the largest earthquake in the Mount Rainier region since April 20, 1974 when a magnitude 4.7 event occurred. A summit swarm of at least a dozen events with the largest magnitude approximately 2.0 occurred on March 20, 1989; and the lineation to the west of Mount Rainier continued active through the first quarter of 1989. A new seismic station is currently planned for near Camp Schurman this spring.
A cluster of earthquakes at Mount Hood in March 1989 consisted of about a dozen events visible on helicorder records in less than an hour with a maximum magnitude of 1.3. They were located near the summit in the area of Timberline Lodge but were not felt. Other notable seismicity in Oregon occurred in the third quarter of 1988. A deep earthquake was located at approximately 55km beneath the Oregon coast with a magnitude of 2.0.
The Olympic Peninsula was also not immune to deep or felt events. In March of 1989 the largest event of the quarter, a magnitude 4.5 event, was located about 40km south of Port Angeles with a depth of 45km which would put it in the subducting Juan de Fuca Oceanic Plate. It was felt over a wide area from Victoria, B. C. in Canada to Olympia. Two days later a magnitude 3.1 earthquake was located farther west near Forks at a depth of 29km placing it still within the North American Crustal Plate. It was felt by several residents. The Forks event was followed a few hours later by an aftershock of 0.7 magnitude. This is in contrast to the larger Port Angeles event which had absolutely no aftershocks. Larger subduction zone earthquakes usually do not have either fore- or after-shocks.
Half of the felt events for the report period occurred in the Puget Lowlands. Most were located at depths less than 15km, and had epicenters in West Seattle, the Mt. Vernon area and Kirkland. Most of this seismicity consisted of clusters in West Seattle and Kirkland. Deeper felt earthquakes occurred at Whidbey Island and Tacoma with the deepest at 65km near Tacoma with a magnitude of 3.9.
A total of 376 events were located in the Mount St. Helens area during 1988 through February 28, 1989. 252 of these were earthquakes located beneath the cone. The average number of events occurring beneath the mountain was 39 per quarter during 1988. The range of these events was 29 to 50. A notable increase in these events began during the last quarter of 1988 and became marked in the first two months of 1989. There were 96 events in this two month period compared with the 1988 average of 39. This was an increase by a factor of 246.
The increase in deeper events, i.e. equal to or greater than 3km, was quite noticable. The 3km depth separates shallow crustal events and volcanic events from deeper in the magmatic system at Mount St. Helens. The average number of deeper earthquakes per quarter for 1988 was 26. The range was 18 to 28. During the first two months of 1989, however, an increase in the number of deeper events at Mount St. Helens became obvious. 80 of the 96 events located under the mountain were at or deeper than 3km. This was a 462% increase during the final quarter during this report period in deep events at the mountain as we moved into 1989. The maximum magnitude of the deeper events was a magnitude 2.2 event on February 19, 1989 at a depth of 4.3km. This is comparable to a magnitude 2.1 earthquake which occurred on November 12, 1987 and was noted as the largest deep earthquake since the summer of 1980. Most are less than 0.5 magnitude. Obviously, the trend of increasing numbers of deep earthquakes at Mount St. Helens which began in mid-1987 continues. There were no eruptions during this report period.
Quarterly Technical reports have been published every quarter and
distributed to a large mailing list. Copies of these reports
are attached as an appendix to this final technical report.
Other publications are as follows:
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