Beyond Resonance: Characterizing Complex Basin Effects Using a Dense Seismic Array

Abstract:

Cross-correlation of the ambient seismic field is now a well-established approach to create high-resolution images of the crust and the upper mantle, to explore the spatial and temporal variations in elastic wave speeds, and to develop images of complex wavefields themselves. Recent ambient-field studies have successfully observed higher-mode surface waves and body wave propagation at various scales of the Earth. These new observations paved the way for a more accurate seismic hazard assessment for which a detailed knowledge of seismic wave propagation is critical, especially in complex media such as sedimentary basins. While the effects of basin resonance are widely appreciated and understood, basin-edge effects are usually less well constrained, but have been used to explain zones of concentrated damage in the 1994 Northridge and 1995 Kobe earthquakes. In this study, we use the dense MeSO-net (MEtropolitan Seismic Observation network) seismic network, deployed in the Tokyo metropolitan area, and the sparse, but high quality, Hi-net (High sensitivity seismograph network) to identify the dominant modes of wave propagation within the Kanto Basin. Our goal is to explore how the wavefield behaves in the vicinity of sharp basin edges. When combined with the ambient seismic field interferometry, dense, 3-component, seismic arrays provide a new opportunity to image such propagation effects. Using array processing techniques, we show that mode conversions, reflection, and diffractions, in particular at basin edges dominate the ground motion in the Kanto Basin. Accurate predictions of strong ground motion, and its variability, must account for these effects.