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
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29324 |
| Date | 23 February 1982 |
| Creators | Solano-Borrego, Ariel E. |
| Contributors | Bibee, L. Dale |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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