LOCAL MAGNITUDE AND SOURCE PARAMETERS FOR EARTHQUAKES IN THE PENINSULAR RANGES OF BAJA-CALIFORNIA, MEXICO

Digital three-component seismograms provided by the Northwestern Mexico Seismic Network were used to estimate local magnitudes and source parameters for 81 earthquakes located in the Peninsular Ranges of Baja California. The magnitude values were estimated from peak amplitudes, as measured on equivalent Wood-Anderson seismograms, in combination with the attenuation law proposed by Richter (1958) for the Southern California region. The source parameters were determined from spectral analysis based on the seismic source model proposed by Brune (1970, 1971). A comparison of our local magnitudes (M(LR)) with magnitudes reported by the California Institute of Technology and the U.S. Geological Survey shows residuals with a standard deviation of +/- 0.21. Concerning the source parameters, stress drop determinations yielded values between 1 and 100 bars, while the calculated seismic movements range from 2.4 x 10(19) to 7.6 x 10(21) dyne-cm. Least-square fits of the moment-magnitude data give log M0 = (1.11 +/- 0.02)M(LR) + (17.32 +/- 0.05), for our data for small earthquakes (2.0 less-than-or-equal-to M(LR) less-than-or-equal-to 4.0) and log M0 = 1.63 M(L) + 14.87 for earthquakes in the same area having magnitudes between (4.0 less-than-or-equal-to M(L) less-than-or-equal-to 6.8). These two linear relationships intersect at a magnitude of about 4.6.