SEISMICITY OF THE MENDOCINO TRIPLE JUNCTION AND THE GORDA PLATE[c6, p169]

The San Andreas fault terminates at its north end in a transform/transform/trench triple junction just seaward of Punta Gorda. Major earthquake activity lies along the Mendocino Fracture Zone, where it is an active transform fault, and to the north within the Gorda plate, which is undergoing intense internal deformation. The Wadati-Benioff zone is well defined to a depth of 30 km and can be traced eastward to a depth of more than 80 km (see Figure 5.5; Walter, 1986). Strong earthquakes within the Gorda plate locate off shore and span the position of the megathrust; these events appear to lie entirely within the oceanic lithosphere. The 1980 Eureka earthquake, for example, ruptured the Gorda plate from the landward to the seaward side of the megathrust. Despite the high level of seismicity, under-thrusting events are rare.