Seismic moment tensors of western US earthquakes and implications for the tectonic stress field

Using a linear inversion method applied to regional surface wave data, we have determined seismic moment tensors for 50 earthquakes in the western US. The direction of maximum horizontal compressive stress is mainly N-S in northern California and western Oregon. Our results, along with the results of Zoback et al. (1981) for central California, indicate a 60° rotation of the maximum horizontal compressive stress between northern and central segments of the San Andreas fault system. A change in stress state along the northern segment could be related to the thermal and mechanical evolution of the Pacific-North American plate boundary. -from Authors