This will be the first E-W refraction profile to extend completely across the Cascades. A total of 17 shotpoints were used, with a seismometer array of about 1600 seismometers spaced about 200m apart. To cover the full length, two separate deployments of the seismometer array were necessary. The first extended from Beverly to Packwood, and the second from Packwood to Oysterville. The locations given in our catalog were computed from wave arrival-times using our standard velocity models.
Actual locations are available on request.
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. The remaining events were scattered around the cone of Rainier as seen in Fig. 4. The largest event at Mount St. Helens, magnitude 2.3, was at a shallow depth of less than 3 km. This quarter's activity represents a continued change from the first quarter of this year, which was extremely quiet and had only 18 events, and only 6 deeper than 4 km. This increase in seismic activity is very small compared to the activity that preceded each of the explosive and dome-building eruptions between 1980 and 1986.