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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 1984-1987 the network was supported by the U.S.G.S external program for ~26 stations in west central Washington, by the U.S. Department of Energy for ~42 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 on volcanic eruption predictions has accompanied the actual prediction of eruptions at Mount St. Helens with good success. Other research has involved the details of the volcanic structure and plumbing system, volcanic earthquake sources and the attenuation structure of the volcano and area. Lists of reports and publications detailing this research are included at the end of this report.
A total of 5,869 earthquakes were located in Washington and Oregon during the period 1984 through 1987. Most of these events occurred in western Washington. Mount St. Helens has been the area of highest seismic activity, though clusters of nonvolcanic types of activity were also prevalent elsewhere. Western Washington and Oregon comprised 42% of the earthquakes during this period excluding Mount St. Helens seismicity. 17% of the earthquakes occurred in eastern Washington. The remaining 41% were located at Mount St. Helens, although this percentage is very conservative. During intense eruptive phases, earthquakes at the volcano are often recorded at a lower sensitivity rate due to saturation of the local subnet by constant seismicity and overlapping events. In addition, among the volcanic events that are recorded, only a representative sampling of them may be located because of work intensive factors that occur during an eruption. Very small magnitude events may simply be saved for processing at a later date and not appear in the catalog as a consequence.
A total of 2,473 earthquakes were located in Western Washington and Oregon, excluding Mount St. Helens, between 1984 and 1987. The majority of these events occurred at depths less than 30km and were located in the Puget lowlands and along the western front of the Cascades. A clear concentration of earthquakes exists along a zone extending tens of kilometers NNW from Mount St. Helens (the St. Helens seismic zone). These events often came in clusters of a dozen or more and usually had a magnitude less than 1.0, and a depth of 7 to 12km.
The area between Battle Ground, Wa., and Portland, Oregon was also active with one or two single felt events of magnitude 2.3 to 3.1 per year. Depths in this area were commonly near 20km. Some of the deepest earthquakes during this time period occurred in southwest Washington near Rainier (65km) and Longview (50km). Sub-crustal earthquakes were also located in the coast ranges near Olympic Hotsprings (40km), near Sequim (55km), south and southwest of Aberdeen (35 and 40km). Such deep events are rare this far south. An extremely unusual seismogram that did not resemble other deeps, (less impulsive and low frequency), occurred at 50km depth near Mist, Oregon in October of 1984. Other Oregon events include several felt events in the Deschutes River Valley, in the vicinity of Mount Hood, and 30km east of Salem.
Earthquake swarms at Mount Rainier and Mount Hood occurred as in previous years. The Mount Rainier swarms typically occurred either under the mountain or along the west side of the park. Event depths were a few kilometers deeper on the west side than those under the mountain. Mount Hood had three swarms in 1987. They were located on the southeast side of the volcano with depths down to 10km and occurred where previous clusters had taken place, including the 1982 activity.
Over 2,500 events were located at Mount St. Helens between 1984 and 1987. Hundreds more were also recorded but not located for one reason or another. Quarterly seismicity ranged from approximately a dozen earthquakes during quiescent periods to nearly a thousand during one eruptive phase. There were five eruptive episodes and several minor degassing events.
In the first quarter of 1984, seismicity became distinctly different from the slow, nearly continuous eruption of 1983 as a classical precursor earthquake swarm developed and culminated in a quiet dome building eruption in February. Seismicity fell to the lowest levels in more than a year until late March when a new precursor swarm began. This new swarm was the most energetic since May 1980 and had two distinct phases preceding and accompanying the birth of the newest lobe of lava. The first phase was a 1981/1982 style swarm of shallow volcanic earthquakes, consisting of about equal numbers of type "m" and "l" events. It peaked early the morning of March 28 with an average of 16-20 countable earthquakes per hour which was twice the rate of the February eruption though average event size was a bit smaller. The second phase began the same morning as a subtle increase in the rate of small, nearly identical shallow events on YEL (the crater station) and this was the dominant event by 1600PST. They merged into a continuous signal appearing to resemble volcanic tremor. Partial collapse of the north side of the dome occurred at 0317PST on March 29 with a nine minute rockfall series which was the first of several over the next few hours. The resultant avalanche scar became the exit path for the new lobe which was confirmed on March 29. Background seismicity returned four days later.
Surface events dominated the the records for the rest of March and April and May. A partial collapse of the west side of the lava dome followed by a vigorous gas emission to 28K feet and a small lateral blast to the west crater wall occurred on May 14. On May 26 a gas and ash plume shot to 22K feet followed by a small mudflow to Spirit Lake. Another plume on May 27 generated no significant water or mudflow. The largest rock and snow avalanche since January 1981 swept down from the SW crater wall on May 23. A new lobe of lava accompanied by lots of avalanching was observed at predawn on June 18. By the end of the quarter seismicity was still at "slightly elevated" and deformation and new lobe growth continued.
Seismicity decreased in July and early August, then picked up again in late August. Increased deformation on the north side of the dome was seen by August 20. The counts of avalanches and "peppercorns" (tiny short duration signals) grew significantly between September 7-10, when the "peppercorns" became larger, relatively high-frequency earthquakes. These larger events disappeared abruptly on September 10 for several hours while the intensity of the volcanic tremor grew steadily until it saturated the near stations during which time vigorous dome growth was assumed to be occurring. By day's end the tremor intensity decreased until many discreet earthquakes were once again visible although their frequency content was lower than that of earthquakes preceding the eruption. Visual confirmation of dome growth was then observed. Levels of seismicity were back to normal by September 13.
Rockfalls and blasts connected with the tunnel project at Spirit Lake plus several earthquakes were all that was recorded until May 1985 when another small lobe was extruded and extensive deformation of the south side of the dome occurred. The earthquake sequence associated with this eruptive activity was the most energetic since the catastrophic May 18, 1980 eruption. Nearly a thousand events were located with a seismicity high on May 23, and earthquakes with magnitudes at or in excess of 2.6 every 5-10 minutes by May29. Seismicity began to decline June 2 and reached background June 17. A notable feature of this eruption was the appearance of two separate families of "multiplets" (nearly identical waveforms). The first occurred between May 24 and 26; the second appeared between May 28 and 29. Plotting these events pointed up the division of events into two magnitude groups between May 29 to June 4 with one group at 2.3-2.8 and the other at 1.0-1.8 with an obvious gap at about magnitude 2.0. It appeared that both magnitude bi-modality and multiplet families were related to a bi-modal depth distribution with one group located in the upper 0.5km and the other at 1.2-1.5km depth. Both multiplets located in the deeper group of events and the deeper events between May 24 and June 4 were, in general, larger than the shallow events.
A fourth eruptive phase took place during April and May 1986. Nearly daily degassing (steam and ash plumes) began April 17, continued into May with the seismic levels rising rapidly from background prior to May 3 to high rates on May 7 and 8, then decreasing to background again by May 19. This eruption resulted in another extrusion of lava at the top of the dome.
The last two weeks of September 1986 heralded another eruptive phase with half of the quarter's 85 Mount St. Helens' earthquakes occurring during that time. Seismicity increased by October 4-5, dropped down to background by October 7, then surged to a high level by October 19. Volcanic tremor began as seismicity decreased on October 21. Tremor amplitude grew then faded until only low frequency earthquakes were recorded after midnight. Early the next morning, October 22, another new lobe extrusion was confirmed at the top of the dome. Background seismicity returned by October 27 and has remained with the result that 1987 was the first year with no eruptive activity since 1980.
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 technaical report.
Other publications are as follows:
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