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Crustal structure and seismicity of the Gorda RidgeSolano-Borrego, Ariel E. 28 September 1984 (has links)
We have determined the seismic crustal structure of the
northern part of the Gorda Ridge using signals generated by
explosive charges and recorded on Ocean Bottom Seismometers. The
shot pattern forms two parallel lines, one on the east flank and the
other along the median valley. Inversion of the travel time data
and synthetic modelling of the signals resulted in two compressional
velocity structures: the model for the flank indicates a 1.6 km
thick upper crust characterized by high velocity gradients and 3.6
km thick lower crust characterized by a low gradient. A sharp
mantle transition exists at 5.2 km depth with an upper mantle
velocity of 7.6 km/sec. The median valley velocity model has a
thicker high gradient upper crust of 3.0 km and a lower crust of at
least 3.5 km thickness. No upper mantle velocities were detected
beneath the median valley.
We have also monitored the seismicity of the ridge during 15
days with two arrays of OBS and detected ~4 events/hour. Epicentral
coordinates were determined for 140 earthquakes. Most of them lie
within the median valley and show spatial clustering. Intraplate
seismicity was also detected in the Gorda Basin with three of the
earthquakes big enough to be reported by land stations. They
suggest that the Gorda Plate is presently undergoing deformation.
Good control over the focal depth was possible for ~80 earthquakes
occurring on the ridge, and there the seismic activity appears to be
pervasive throughout the upper 20 km suggesting that the the brittle
lithosphere is at least this thick.
From the earthquake shear-wave data we have obtained a value of
1.73 for the Vp/Vs ratio. Moments of the well constrained events
derived from the spectra of the waveforms are of the order 10²⁰
dyne-cm and suggest an average fault width of 300 m.
The refraction data is consistent with the earthquake results,
and all the evidence suggests that a large magma chamber underlying
the axis of spreading does not presently exist at shallow depths. / Graduation date:1985
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Microseismicity on the Gorda RidgeSolano-Borrego, Ariel E. 23 February 1982 (has links)
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
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