Source characteristics of a 5.5 magnitude earthquake that occurred in the transform fault system of the Delfin basin in the Gulf of California

Portable and permanent broadband seismic stations in the neighborhood of th e Gulf of California recorded a moment magnitude M-w 5.5 event on 26 Novemb er 1997. This is the first time that a moderate event located in the Gulf o f California extensional province was well recorded by local broadband seis mic 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 loca tion and the results of the wave modeling indicate that this is a complex e vent that originated in the pull-apart Delfin basin. The focal mechanism es timated 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) mo tion and ended releasing the bulk of energy through strike-slip motion. Syn thetics of surface waves in the frequency band 0.050. 1 Hz were also calcul ated using a triangular source-time function of 3 sec. The best match betwe en the synthetics and observed surface waves recorded at 90 km from the epi center was obtained using a fault geometry defined by a strike of 330 degre es +/- 15, dip 85 +/- 5, and slip of 165 degrees +/- 15. The spectral analysis of the Lg phase recorded at stations in the Peninsula r Ranges gives a seismic moment of 1.28 X 10(17) N m (1.28 X 10(24) dyne cm ), a source radius of 6.3 km and a stress drop of 0.22 MPa (2.2 bar). The s ource parameters inferred with S-wave spectra and the same model (Brune, 19 70) give similar values.