An ML Scale for the La Paz-Los Cabos Region, Baja California Sur, Mexico

Abstract

We have developed a local magnitude scale to quantify the size of earthquakes in the La Paz–Los Cabos region, Baja California Sur, a seismically active region with little seismic history reported. For this, we used digital data from 501 local and regional earthquakes recorded by temporary networks that were deployed in the region from October 1998 to December 2003. A multiple linear regression analysis applied to 3328 average zero-to-peak synthetic Wood–Anderson amplitudes produced the following results: a −log A attenuation function for the region, the local magnitude for each earthquake, and a magnitude correction for each station. Regarding the attenuation function, our best fit to the regressed data is −log A ;eq (1.2052 ± 0.0177) log ( r /100) + 0.0016 ± 0.0003) ( r − 100) + 3, where r is the hypocentral distance. The ground-motion attenuation represented by this relationship is similar to the attenuations reported for the Peninsular Ranges of Baja California and for the southern California region. In addition, the station magnitude corrections obtained from our regression analysis range from −0.46 to 0.23. As it should be expected, positive station corrections correlate well with hard-rock sites, whereas negative corrections correlate with soft or sedimentary rock sites. The combined use of Richter’s definition of magnitude with the attenuation function and station corrections determined here give reliable and homogeneous estimations of the local magnitude.