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, relative source time functions are obtained for the April 25, 1992, Cape Mendocino thrust earthquake (M_w = 7.2), its two large strike-slip aftershocks, and two large strike-slip events in the Gorda plate that occurred in 1991. Analyzing the directivity effects observed in the source time functions, rupture directions and corresponding fault orientations are resolved for the four largest events and constraints are placed on the smaller aftershock. 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. -from Authors