Estimates of shallow attenuation and apparent stresses from aftershocks of the Oaxaca earthquake of 1978

Acceleration spectra at high frequencies were analyzed to make an estimation of the spectral decay parameter, κ , in the aftershock area of the 1978 Oaxaca earthquake. It was found that κ follows a linear behavior given by κ ( r ) = κ + α r , increasing with source-to-station distance, r , in the frequency band from 8 to 25 Hz. κ values range from 15.7 to 33.9 msec and do not show a strong variation from station to station on different types of crystalline terrain. Estimates of dκ ( r / dr do not indicate a strong lateral variation of the frequency-independent quality factor, Q, in the crust of the Oaxaca region. Apparent stresses were also calculated for a subset of aftershocks, and no clear scaling with seismic moment or κ is observed. Apparent stresses lie between 3.6 to 256 bars.