Cecilio J. Rebollar, Luis Quintanar, Raul R. Castro, Steven M. Day, Juan Madrid, James N. Brune, Luciana Astiz, and Frank L. Vernon
Source characteristics of a 5.5 magnitude earthquake that occurred in the transform fault system of the Delfin Basin in the Gulf of California
Bulletin of the Seismological Society of America (August 2001), 91(4):781-791

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 degrees N and 113.708 degrees 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 (phi = 310 degrees , delta = 83 degrees , lambda = 97 degrees ) 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 degrees + or -15, dip 85 degrees + or -5, and slip of 165 degrees + or -15. The spectral analysis of the Lg phase recorded at stations in the Peninsular Ranges gives a seismic moment of 1.28X10 ¹⁷ N m (1.28X10 ²⁴ 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.