University deployed two arrays of three sonobuoys over the Blanco fracture zone and two arrays of four sonobuoys over the Gorda ridge to detect and locate microearthquakes. Microearthquake activity predicted by plate tectonic theory for the Blanco fracture zone and Gorda ridge was observed by these arrays to originate from these features. Microearthquake activity of one event per hour was observed along the southern flank of the ridge associated with the Blanco fracture zone near 128°10'W. Similar seismicity was observed in Cascadia Gap and associated with the fault scarp and basin of the gap. Cascadia Gap, which is located near the center of the Blanco fracture zone, exhibits ridge-like seismic activity based on a b-value of 1.5 determined from microearthquake observations. The seismic activity on the Gorda ridge, including swarm events, averaged 3.5 events per hour. The microearthquakes originated from the median valley floor, valley walls, and on top of the crestal hills. Other events, which could not be located, appeared to originate from the surrounding hills with a predominance of events from west of the Gorda-Blanco intersection. Focal depths at the intersection area are 6.5 to 9 km below a 3.5 km datum, while those farther to the south at 42°41'N range from 2.5 to 3 km below the datum. A composite fault plane solution for the events on the Gorda ridge indicates high angle faulting on the eastern valley wall with the eastern side down. Since microearthquake and earthquake activity are usually closely associated, the data obtained by this study suggests that the 20-30 km eastward offset of large earthquake epicenters from the topographic features is due to an incorrect use of excessively high lithospheric velocities for seismic stations lying predominantly to the east. / Graduation date: 1976

The seismicity at the northern section of the Gorda Ridge has been studied with Ocean Bottom Seismographs. Nearly one hundred earthquakes were located with RMS travel time residuals less than 0.5 seconds. Most of these events lie within or near the array and they are probably associated with the dynamics of crustal formation which creates the observed bench-like features in the Gorda Ridge. An average of 5 microtremors per hour can be detected in the median valley with a lower activity outside of the ridge. Considerable clustering in time can be seen and it is typical of a main-shock sequence with the largest shock at or very near the beginning followed by a progressive decay in number. Clustering was spatial as well as temporal and the records indicate that they have similar mechanisms. A brittle zone of 18 km thickness at 42°N and 13 km at 42.5°N was found implying any possible magma chamber must be at greater depth. These thicknesses are larger than those observed in the Mid Atlantic ridge and in the East Pacific Rise. They may be explained by the depression of isotherms due to the heat lost by the contact of the ridge with the old and cold plates across the Blanco and the Mendocino Fracture Zones. Comparison of earthquakes locations using only OBS with those obtained from land stations indicate a major P-delay for the Gorda Basin. The seismic activity appears to decrease markedly to the south of 42°N. This drop in activity indicates that the southern part of the ridge is more stable than the northern part. This decrease in activity is not consistent with a southward propagating rift if we accept that higher seismicity levels are expected at the tip of the propagating ridge segments. / Graduation date: 1982