DISTRIBUTED SEISMICITY AND DEFORMATION OF THE PLATE MARGINS [c5, p145]

The two largest earthquakes in California during the interval 1980-86 occurred off the faults of the San Andreas system, and their occurrence emphasizes the importance of deformation within the plate margins along the San Andreas transform boundary. The M=7.2 Eureka event (Nov. 8, 1980), for example, involved deformation internal to the Gorda plate; and the M=6.7 Coalinga event (May 2, 1983) involved crustal shortening with reverse slip perpendicular to the San Andreas fault. These two earthquakes and the many smaller, "off fault" events ( Figure 5.4A) reflect local deviations from the simple rigid-plate approximation of plate tectonics.