Recent large earthquakes near Cape Mendocino and in the Gorda plate: Broadband source time functions, fault orientations, and rupture complexities

The northward migration of the Mendocino Triple Junction is associated with complex faulting within the Gorda plate and in the convergent zone between the Gorda and North American plates. This region has experienced substantial recent large earthquake activity, and quantification of these faulting processes is essential for understanding the evolution of the triple junction. Using an empirical Green function deconvolution method for teleseismic and regional surface waves and body waves, we obtain relative source time functions for the April 25, 1992, Cape Mendocino thrust earthquake (M_w = 7.2), its two large strike-slip aftershocks on April 26, 1992 (M_w = 6.5, 6.6), and two large strike-slip events in the Gorda plate that occurred July 13, 1991 (M_w = 6.8), and August 17, 1991 (M_w = 7.1). The removal of propagation effects using empirical Green functions provides unusually detailed source rupture information and indicates that all the ruptures are less than 16 s in duration. Analyzing the directivity effects observed in the source time functions, we resolve rupture directions and corresponding fault orientations for the four largest events and place some constraints on the smaller aftershock. Significant differences in the rupture duration and stress drop are observed between the Cape Mendocino mainshock and its two largest aftershocks. The mainshock, which ruptured westward on a shallow dipping plane either on or paralleling the interplate contact between the Gorda and North American plates, had a smooth 9-s-long rupture with a 0.9-4.6 MPa static stress drop. The two largest aftershocks occurred within the uppermost mantle of the Gorda plate and have more complex ruptures with total durations of 14-15 s and static stress drops of less than 0.3 MPa, perhaps reflecting a difference between mantle and crustal earthquakes in this region. The aftershocks appear to have ruptured conjugate strike-slip faults, accounting for differences in their damage patterns. The August 17, 1991, Gorda plate event ruptured a southwest striking fault, while the July 13, 1991, event ruptured a southeast striking fault, with both events relieving north-south compression within the deforming Gorda plate. The existence of multiple active faults in the region constitutes a significant earthquake hazard associated with the complex stress environment of the migrating triple junction.