Bylerly in 1937 made the observation of early or precursory energy arriving before the S-wave for offshore earthquakes in Northern California, which he referred to as False S. 70 years later there is still no adequate explanation of this unusual observation. One of the reasons is that the seismic station coverage in the region was rather sparse. The recently installed USArray stations that complemented the existing Northern California Seismic Network (NCSN) and Berkeley Digital Seismic Network (BDSN) have greatly improved the situation and have provided numerous new observations of the False S phase in just the past few years. We are relocating the offshore events using a double-difference algorithm, and then inverting for the seismic moment tensor. With the location and source parameters constrained we then model the broadband waveforms using 2D and 3D velocity structures in order to find the origin of the False S arrivals. Our 2D modeling is showing that the geometry and velocity structure of the subducting Gorda plate plays an important role in the generation of strong phases with travel times between direct P and S. In this work the False S observations are further modeled by developing a network of 2D structures to explain False S on all the stations leading to 3D structure modeling. Possible source mechanism dependence of False S is also studied. The modeling results will provide important constraints on the geometry and velocity structure of the subducting Gorda plate to the north of the Mendocino Triple Junction (MTJ) as well as on the structure of the overlying Franciscan crust. To the south of the MTJ, the results could provide constraints on the depth, shape, and velocity structure of Gorda plate remnants. A more detailed knowledge of the geometry of the Gorda plate and velocity structure of the crust in the greater MTJ region will contribute to the understanding of the tectonic evolution of the western edge of the North America. The results will also provide constraints for future 3D regional velocity models that will provide more reliable ground motion estimates for possible offshore and onshore events in northern California and southern Oregon.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Gregory J. Anderson
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University of Minnesota Duluth
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