Hi All. On 11/08/1980 there was a 7.2Ms at 41.12N/124.25W. This is slightly north of the current swarm off the coast of Northern California. There doesn’t appear to have been any foreshocks to that particular quake so it’s hard to say if this current swarm are foreshocks to something larger.

This was the largest earthquake to occur in California during the interval 1980-86. The N. 53° E. trend of its aftershock zone agrees closely with the strike of the left-lateral slip plane inferred from the focal mechanism of the main shock, which was located about a fourth of the way downstrike from the northeastern end of the aftershock zone (Eaton, 1989).

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. Also the left-lateral slip along the 1980 break stimulated right-lateral slip along the adjacent part of the MFZ.

This mornings quakes appear to be occurring along the Mendocino Fault Zone. The Cascadia Subduction Zone is located to the north of this fault zone.

The following is from John Martin and the USGS.

“Two moderate earthquakes near Cape Mendocino in 1981 and 1987 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 southeastern most 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.”

South of this location seismicity drops off very sharply indicating that the northeast corner of the Pacific plate behaves as a rigid block in its interaction with the younger, thinner, and internally deforming Gorda plate (Wilson, 1986; Eaton, 1989). Take Care…Don in creepy town

• Mendocino Quake