Faulting mechanisms for two oceanic earthquake swarms

Abstract

A study of the relative excitation of long-period surface waves from earthquake swarms near the Delfin basin in the northern Gulf of California and near the Queen Charlotte Islands fault suggests that a prominent mechanism of faulting for these swarm-type events was strike slip. Both swarms were apparently located on fracture zones near active spreading centers. Earlier studies have suggested that normal faulting may dominate oceanic earthquake swarms and that this type of faulting may be related to the occurrence of swarms. However, the present observations imply that the mechanisms for generating swarm-type earthquakes are probably due to factors other than the nature of the faulting involved. The suggested faulting mechanisms were determined by an examination of the relative amplitudes of Love and Rayleigh waves with periods between 10 and 50 s for some 60 events, assumed to be shallow (h < 15 km), that occurred in the northern Gulf of California. These surface waves were recorded at a new very long period seismograph station at Albuquerque, New Mexico. Bathymetry and seismicity of the northern Gulf of California suggest that faulting is occurring at short spreading centers (dip-slip faulting) and along transform faults (strike-slip faulting). Theoretical radiation patterns and relative amplitudes of Love and Rayleigh waves determined for both of these fault mechanisms were compared with the observed surface wave amplitudes recorded at Albuquerque. This technique provides a means of discriminating between the two possible types of focal mechanisms. The observations suggest that strike slip, possibly with a small component of dip slip, was the dominant faulting mechanism for many of these swarm-type events. Further support for this suggestion is provided by the observation of S-P times at a local (Δ < 100 km) short-period seismograph station, and thus the epicentral locations of these events are restricted to a portion of a mapped transform fault. This surface wave technique was also applied to an earthquake swarm that occurred near the Queen Charlotte Islands fault, and a similar conclusion was drawn.