Source spectra and site response from P and S waves of local earthquakes in the Oaxaca, Mexico, subduction zone

Abstract

We compare P - and S -wave source functions to determine the level of P/S radiation at high frequencies. For this purpose, spectral records from local earthquakes with magnitudes ranging between 2.8 and 4.8 are inverted to separate source and path effects. We used digital records from 16 local events recorded at short hypocentral distances ( r < 91 km) in the region of Oaxaca, Mexico. Since most of the seismograms used were recorded on scale and the three components of velocity are available, this data set offers the opportunity of comparing source spectra and site response of P and S waves. The digital seismograms analyzed were recorded by a temporary network of six seismic stations, five of them located on hard-rock sites and one on sediments. The spectral amplitudes of the records used were corrected for attenuation with a 1/ r geometrical spreading function ( r being the hypocentral distance) and using estimates of Q of P and S waves reported by Castro and Munguia (1993) for the region. The corrected amplitudes were then inverted to separate source and site responses for 15 different frequencies selected between 1 and 25 Hz. The site responses for P and S waves have similar trends. However, the maximum P -wave amplification shifts toward higher frequencies relative to the maximum S -wave amplification, in consistency with the expected VP/VS ratio. While the stations located on metamorphic rocks show small amplifications in narrow frequency bands, the station sited on sediments shows amplification factors ranging from 2 to 10 in a broader frequency band. In general, the P -wave source spectra show corner frequencies greater than those of the S wave. Although the S -wave source acceleration spectra exceed the spectral amplitudes of P at low frequencies ( f < 10 Hz), the P/S source function ratios tend to increase at frequencies greater than 5 Hz. Beyond 10 Hz, P -wave source spectra are frequently above the S -wave source amplitudes. These results indicate that the high P -wave radiation observed in the spectral records is a source effect and also support the hypothesis proposed by Brune et al. (1989) and Brune and Anooshehpoor (1992) that normal vibrations are an important characteristic of the rupture process.