Source, path, and site effects and their role on earthquake ground motions

Haiyang Kehoe, USGS

Seismograms contain information of an earthquake source, its path through the earth, and the local geologic conditions near a recording site. Ground shaking felt on Earth’s surface is modified by each of these contributions–the spatiotemporal evolution of rupture, three-dimensional subsurface structure, and site conditions all have a substantial impact on hazards experienced by exposed populations. In this talk, I highlight three studies that have improved our understanding of ground motion variability arising from source, path, and site effects. First, I describe the rupture process of the 2017 Mw 7.7 Komandorsky Islands earthquake, which reached supershear speeds following a rupture jump across a fault stepover, and demonstrate the enhanced hazard associated with supershear ruptures across Earth’s complex transform fault boundaries. Second, I compare high-frequency wavefield simulations of Cascadia earthquakes using various tomography models of the Puget Sound region, Washington State to highlight the role of basin structure on ground motion amplification. Third, I show horizontal-to-vertical spectral ratio maps of the continental United States and emphasize the continued importance of region-specific constraints on site characterization. While each study demonstrates progress towards understanding the individual roles of source, path, and site effects on damaging earthquake ground motions, together they underscore distinct challenges for improving seismic hazard models and their uncertainties.