abstract

Three earthquake swarms originating in Gulf of California spreading centers have been studied with sonobuoys.

Two of the sequences were microearthquake swarms detected during a seismicity survey of the Gulf. Both were located beneath spreading center grabens. All events are quite shallow, less than 3 km into the crust. One of the swarms, which consisted of about 1000 events in 8 hr, showed an episodic seismicity which we associate with episodic magma movement and/ or slip along the graben boundary fault.

The third swarm occurred near the Delfin Basin trough. Sonobuoy epicenters show a seismicity trend parallel to local spreading center trends but about 17 km from the trough itself. Surface-wave radiation patterns of the larger events indicate primarily normal faulting. Normal faulting is not predominant in this region, however. Analysis of surface waves recorded at Pasadena indicates that most swarms have strike-slip mechanism.

The data from the Gulf suggest that sediments may play a major role in the character of earthquake swarms there. Some swarm events studied may, in fact, occur in consolidated sediments.

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