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Sonobuoy and teleseismic study of Gulf of California transform fault earthquake sequences

INSTITUTE OF GEOPHYSICS AND PLANETARY PHYSICS SCRIPPS INSTITUTION OF OCEANOGRAPHY UNIVERSITY OF CALIFORNIA. SAN DIEGO, LA JOLLA. CALIFORNIA 92093
GEOLOGICAL RESEARCH DIVISION SCRIPPS INSTITUTION OF OCEANOGRAPHY UNIVERSITY OF CALIFORNIA. SAN DIEGO, LA JOLLA. CALIFORNIA 92093

Abstract

Sonobuoys have been used in the study of two Gulf of California transform fault aftershock sequences. Uncertainties in epicentral locations are no larger than navigational uncertainties.

The first local study of an oceanic transform fault aftershock sequence followed a M_S = 5.5 event on March 25, 1973. The sonobuoy data suggest that the main event did not lie along the main transform fault scarp. Surface-wave radiation patterns and a P-wave fault-plane solution give a strike of 327, about 20 different from local bathymetric trends. Subsidiary faulting near a locked transform fault, perhaps a result of local tectonic complication, is a possible explanation for this difference.

The second study followed a M_S = 6.3 event on May 31, 1974. Sonobuoy data limit the aftershock zone to be about 25 km long, trending approximately parallel to local bathymetric features.

All earthquakes located with sonobuoys were shallow. Arrival times limit the depths to be less than 7 km, and when good depth control is present, depths were 3 to 5 km. The calculated stress drop for the March 25, 1973 event is 8 to 30 bars.

The historic seismic slip rate along Gulf of California transform faults has been estimated from the published seismicity to be 6.1 to 3.7 cm/yr, assuming fault thicknesses of 3 to 5 km, respectively, in good agreement with estimates of Pacific-American relative plate motion from geologic and geodetic methods. One 240-km section of transform fault appears to have been locked since about 1955, accumulating strain for a large earthquake. The good agreement between the calculated slip rate and the geologic and geodetic estimates supports the sonobuoy-based observation that the earthquake generating zone is very thin, perhaps only 3 to 5 km thick.

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