Near-Field Spectra of Large Earthquakes

Abstract

We have studied the spectra of large subduction earthquakes in Chile at short epicentral distances. In this work, we concentrate on the Mw 8.2, Iquique earthquake of 1 April 2014. For such a large event, in most stations we cannot separate P and S waves; which arrive very close together and interfere to produce the static displacement field. We find that the displacement spectra observed in all the accelerograms are significantly different from the usual far-field Brune spectrum. Displacement spectra have clear omega-1 decay at low frequencies that we prove to be associated with the dominant role of near and intermediate field waves in the ground motion. The origin of the omega-1 decay is that at short distances, displacement contains a finite static displacement. We confirm this spectral behavior by comparing the spectra computed from accelerograms integrated to velocity with co-located GNSS recordings. Both spectra are flat at low frequencies and the low-frequency asymptote of the velocity spectrum is proportional to the static displacement determined from GNSS. We explore the transition from these near-field records to far-field ones and find that the usual omega-2 spectrum appears only at large distances compared to the size of the dynamic rupture zone. For large earthquakes, the displacement spectra have a term proportional to the moment spectrum and another proportional to moment rate spectrum. The term proportional to moment dominates the low-frequency behavior of accelerograms in the near and intermediate field, so that they do not follow the omega-square decay of the moment rate spectrum.