Site response in the Oklahoma region from seismic recordings of the 2011 Mw 5.7 Prague earthquake

Abstract

We invert the shear-wave displacement spectra obtained from 30 three-component, broadband waveforms recorded within 300 km of the 6 November 2011 Mw 5.7 Prague, Oklahoma earthquake to recover the site-response contribution using an inversion method that simultaneously inverts for source, path, and site effects. Site-response functions identify resonant frequencies within a range of 0.1-10 Hz that generally coincide with spectral peaks in H/V curves derived from the recorded waveforms. S-wave velocity profiles available for several sites were also used to calculate theoretical SH transfer functions that predict the site amplification due to the near-surface soil structure down to depths of 30-50 m. These transfer functions are generally flat below about 8 Hz in the frequency range sampled by the spectral inversion process, indicating that the spectral peaks in the site response obtained from the waveform analysis result from deeper velocity variations. A 0.3-Hz spectral peak observed at several stations, for example, coincides with the strong, surface-wave amplitudes observed at 3s periods for induced M ≥ 3 earthquakes in Oklahoma and Kansas, suggesting that this resonant peak may be due to surface waves trapped in the upper sedimentary layer of the crust. Both shallow and deep contributions to the site response are important for the characterization of ground motion from Central and Eastern North America (CENA) earthquakes. We obtain a seismic moment of 4.32 × 1024 dyne-cm and a corner frequency of 0.229, consistent with the magnitude of the event. A frequency-dependent attenuation relation of Q(f)=1107f 0.398 consistent with prior measurements of path properties in CENA is also derived.