Ground Motions along the Cascadia Margin Revealed by Virtual Earthquakes

Abstract:

The Cascadia subduction zone is the potential locality of devastating earthquakes that could cause severe damages to the Pacific Northwest coastal area. Contemporary low seismicity rate along the Cascadia margin precludes the use of conventional methods to study the ground motion hazard in this region. We thus use ambient-noise seismology to characterize the propagation of surface waves along the Cascadia margin, particularly the accretionary wedge. In particular, we combine 227 Ocean Bottom Seismometers and 228 onshore stations available between 2010 and 2015. Offshore seismic data is polluted by ocean noise, so that we remove the tilt and compliance noise and corrected the orientations for horizontal components. We compute the 9-component correlation functions for 1-hour window, design an optimal stacking technique to improve wave coherence, and focus on the frequency band between 0.02 Hz and 5 Hz. We find that surface waves travel through the accretionary wedge at quite low velocities, less than 1 km/s, indicating that shear wave speed is lower than expected from common values of Vp/Vs ratios. Additionally, we explore the peak amplitudes of the surface waves, which demonstrate strong variations along the Cascadia margin at different frequencies. From virtual earthquakes on land, we observe relatively large peak amplitudes at the Olympia peninsula and the Portland basin. The surface waves from offshore virtual earthquakes are amplified at the accretionary wedge. The preliminary results from this study demonstrate the localization of amplified earthquake ground motions in the Cascadia region and its dependency on the direction of wave propagation.