Seismic Site Response in Central Oklahoma: Low-frequency Resonances from the Great Unconformity

Abstract:

Low-frequency (f<2 Hz) ground motions from induced earthquakes in Oklahoma exceed predictions and observations from tectonic earthquakes across the U.S. by more than 50 percent. We invert ground velocity spectra using a generalized inversion method to investigate site response. We constrain the inversion with a source spectrum, computed by using aftershocks as empirical Green’s functions to deconvolve the effects of wave propagation and site amplification. Site spectra exhibit strong, low-frequency resonances. We model the site spectra using vertically propagating SH-waves and find that the resonances require deep (1–2-km) impedance contrasts. Regional seismic velocity and geologic models indicate that the impedance contrast is at, or in near proximity to, the Great Unconformity, which marks the interface between Precambrian basement rocks and overlying Paleozoic sedimentary rocks and whose depth increases to the southwest. Frequencies of the fundamental and first higher-modes of the site spectra are anticorrelated with basement depth. Because these resonances are not included in regional ground motion models, current hazards assessments likely underestimate potential low-frequency ground-shaking. These results highlight the need for modeling regional effects, such as the strong impedance contrasts observed in this study, in order to improve ground motion predictions and seismic hazard assessments.