Source Characteristics of a 5.5 Magnitude Earthquake that Occurred in the Transform Fault System of the Delfin Basin in the Gulf of California

by Cecilio J. Rebollar, Luis Quintanar, Raul R. Castro, Steven M. Day, Juan Madrid, James N. Brune, Luciana Astiz, and Frank Vernon

Abstract

Portable and permanent broadband seismic stations in the neighborhood of the Gulf of California recorded a moment magnitude M_w 5.5 event on 26 November 1997. This is the first time that a moderate event located in the Gulf of California extensional province was well recorded by local broadband seismic stations. The event was located at 29.754^o N and 113.708^o W and at a focal depth of 5.0 km in the southeastern end of the transform fault that connects the lower and upper Delfin basins. The hypocentral location and the results of the wave modeling indicate that this is a complex event that originated in the pull-apart Delfin basin. The focal mechanism estimated from first motions ( = 310^o, δ = 83^o, = 97^o) and body-wave modeling of P waves in the frequency band 0.05-0.5 Hz suggests that the rupture started with dip-slip (reverse faulting) motion and ended releasing the bulk of energy through strike-slip motion. Synthetics of surface waves in the frequency band 0.05-0.1 Hz were also calculated using a triangular source-time function of 3 sec. The best match between the synthetics and observed surface waves recorded at 90 km from the epicenter was obtained using a fault geometry defined by a strike of 330^o +/- 15, dip 85^o +/- 5, and slip of 165^o +/- 15.

The spectral analysis of the Lg phase recorded at stations in the Peninsular Ranges gives a seismic moment of 1.28 x 10¹⁷ N m (1.28 x 10²⁴ dyne cm), a source radius of 6.3 km and a stress drop of 0.22 MPa (2.2 bar). The source parameters inferred with S-wave spectra and the same model (Brune, 1970) give similar values.

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