Acceleration spectra for subduction zone earthquakes

Abstract

We estimate the source spectra of shallow earthquakes from digital recordings of teleseismic P wave groups, that is, P + pP + sP, by making frequency dependent corrections for the attenuation and for the interference of the free surface. The correction for the interference of the free surface assumes that the earthquake radiates energy from a range of depths. We apply this spectral analysis to a set of 12 subduction zone earthquakes which range in size from MS = 6.2 to 8.1, obtaining corrected P wave acceleration spectra on the frequency band from 0.01 to 2.0 Hz. Seismic moment estimates from surface waves and normal modes are used to extend these P wave spectra to the frequency band from 0.001 to 0.01 Hz. The acceleration spectra of moderate subduction zone earthquakes, that is, earthquakes whose seismic moments are less than 1027 dyn cm, exhibit ω‐square or Brune‐type spectra, while the acceleration spectra of large subduction zone earthquakes, that is, earthquakes whose seismic moments are greater than 1027 dyn cm, exhibit intermediate slopes where ü(ω) ∝ ω5/4 for frequencies from 0.005 to 0.05 Hz. For this set of earthquakes, spectral shape appears to be a discontinuous function of seismic moment. Using reasonable assumptions for the phase characteristics, we transform the spectral shape observed for large earthquakes into the time domain to fit Ekström's (1987) moment rate functions for the MS = 8.1 Michoacan earthquake of September 19, 1985, and the MS = 7.6 Michoacan aftershock of September 21, 1985.