FRAGMENTATION OF THE SOUTHEAST CORNER OF THE GORDA PLATE [c5, p143-144]

The 1980 Eureka M=7.2 earthquake occurred along a fault break that extended from the continental slope 40 km west of the coastline at lat 41° N. for a distance of 140 km southwestward to the MFZ, virtually cutting off the southeast corner of the Gorda plate. Focal mechanisms of the main shock and largest aftershock (events 1, 3, Figure 5.11A) both indicate left-lateral strike-slip displacement along a vertical fault that coincides with the line of aftershocks. Some early aftershocks, including event 4 and other moderate events farther east along the MFZ, have focal mechanisms that indicate right-lateral slip along the MFZ. Although the main shock occurred beneath the Continental Shelf, it seems clear that the 1980 earthquake primarily involved the Gorda plate because the fault broke well beyond the base of the continental slope and the edge of the North American plate. Moreover, left-lateral slip along the 1980 break stimulated right-lateral slip along the adjacent part of the MFZ. Ongoing right-lateral displacement along the MFZ is also indicated by event 5 (Dec. 1983).

Two moderate earthquakes near Cape Mendocino in 1981 and 1987 (events 6 and 7, respectively) had focal mechanisms similar to that of the 1980 Eureka earthquake, indicating left-lateral strike-slip displacement on steeply dipping, northeast-striking planes. Aftershocks of the 1987 M=5.8 event outlined a narrow, steeply dipping, northeast-trending, 20-km-long zone between about 15- and 25-km depth that extended southwestward from the shoreline just north of Cape Mendocino to the MFZ. This aftershock zone appears to cut off the southeasternmost corner of the Gorda plate just north of the abrupt eastward termination of intense seismicity along the MFZ, at a point that might be taken as the Mendocino triple junction from the viewpoint of seismicity.

Relative horizontal extension at seismogenic depths is suggested by events 2 and 8. Event 2 (Nov. 10, 1980; 7 km deep) was the largest in a detached cluster of shallow aftershocks 20 km east of the 1980 main shock, and event 8 (Apr. 9, 1987; 26 km deep) occurred about 100 km east of the 1980 main shock in the zone of seismicity associated with the subducting Gorda plate.