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
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29239 |
| Date | 28 September 1984 |
| 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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