Microseismicity on the Gorda Ridge Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m039k727z

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  • 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.
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