Three-dimensional simulations of ground motions in the seattle region for earthquakes in the Seattle fault zone

We used the 3D finite-difference method to model observed seismograms of tw o earthquakes (M-L 4.9 and 3.5) in the Seattle region and to simulate groun d motions for hypothetical M 6.5 and M 5.0 earthquakes on the Seattle fault , for periods greater than 2 sec. A 3D velocity model of the Seattle Basin was constructed from studies that analyzed seismic-reflection surveys, bore hole logs, and gravity and aeromagnetic data. The observations and the simu lations highlight the importance of the Seattle Basin on long-period ground motions. For earthquakes occurring just south of the basin, the edge of th e basin and the variation of the thickness of the Quaternary deposits in th e basin produce much larger surface waves than expected from flat-layered m odels. The data consist of seismograms recorded by instruments deployed in Seattle by the USGS and the University of Washington (UW). The 3D simulatio n reproduces the peak amplitude and duration of most of the seismograms of the June 1997 Bremerton event (M-L 4.9) recorded in Seattle. We found the f ocal mechanism for this event that best fits the observed seismograms in Se attle by combining Green's functions determined from the 3D simulations for the six fundamental moment couples. The February 1997 event (ML 3.5) to th e south of the Seattle Basin exhibits a large surface-wave arrival at UW wh ose amplitude is matched by the synthetics in our 3D velocity model, for a source depth of 9 km. The M 6.5 simulations incorporated a fractal slip dis tribution on the fault plane. These simulations produced the largest ground motions in an area that includes downtown Seattle. This is mainly caused b y rupture directed up dip toward downtown, radiation pattern of the source, and the turning of S waves by the velocity gradient in the Seattle basin. Another area of high ground motion is located about 13 km north of the faul t and is caused by an increase in the amplitude of higher-mode Rayleigh wav es caused by the thinning of the Quaternary deposits.