Source‐Dependent Amplification of Earthquake Ground Motions in Deep Sedimentary Basins

Deep sedimentary basins amplify long-period shaking from seismic waves, increasing the seismic hazard for cities sited on such basins. We perform 3-D simulations of point source earthquakes distributed around the Seattle and Tacoma basins in Washington State to examine the dependence of basin amplification on source azimuth, depth, and earthquake type. For periods between 1 and 10 s, the pattern of amplification is spatially heterogeneous and differs considerably with the source-to-site azimuth. For close-in earthquakes, the greatest basin amplification occurs toward the farside of the basin and ground motions from crustal earthquakes experience greater amplification than those from more vertically incident, deeper intraplate earthquakes. Love and Rayleigh waves form similar spatial patterns for a given source location, although the magnitude of amplification varies. The source dependence of basin amplification is an important factor for seismic hazard assessment, in both the Seattle and Tacoma basins, and by extension for deep sedimentary basins worldwide. Plain Language Summary Sedimentary basins amplify the ground shaking due to an earthquake. This increases the seismic hazard for cities located above such basins. Therefore, determining the amount and distribution of shaking amplification is critical to estimating the hazard associated with future earthquakes. We use computer simulations to determine how an earthquake's location, depth, and focal mechanism affect ground shaking for communities above the Seattle and Tacoma sedimentary basins, Washington State. We find that the amount of shaking amplification strongly depends on where the earthquake is located, as well as the earthquake type. This has important implications for seismic safety (e.g., for buildings and infrastructure) located above deep sedimentary basins worldwide.