Spectra of some Oaxaca earthquake aftershocks from RESMAC

Abstract

S-wave spectra of the main event and 21 aftershocks of the Oaxaca (November 29, 1978) earthquake, registered digitally on a RESMAC fixed seismic telemetry station 500 km from the epicentral area, have been calculated. Due to the bandwidth of the RESMAC system, the corner frequency of the shear wave spectrum of the main event is not observed. We do see two slopes at high frequencies (-2.0 and -2.8) intersecting at a frequency of 1.4 HZ. Comparison of the spectra of aftershocks with those obtained from the nearby portable digital stations shows a shift of comer frequency to the lower frequencies, typically of the order of 0.3 HZ and in general, a smaller value, typically 4.3 smaller, for the seismic moment. The corner frequencies of the P-wave spectra are outside the observed frequency band.