M 7.2 - 153 km W of Big Lagoon, California

Tectonic Summary

The June 15, 2005, M 7.2 earthquake off the coast of northern California occurred as the result of shallow strike-slip faulting within a deformed section of the southernmost oceanic crust of the Juan de Fuca plate, commonly called the Gorda plate, west of the Cascadia subduction zone. Focal mechanism solutions indicate that rupture occurred on either a left-lateral northeast-southwest-striking fault or on a right-lateral southeast-northwest-striking fault. At the location of the earthquake, the Juan de Fuca plate moves to the northeast relative to the North America plate at a velocity of about 30 mm/yr.

Preliminary analysis of the quake indicates that it resulted from slip on a northeast-striking left-lateral strike-slip fault, a type of mechanism that has been documented for other earthquakes located in the interior of the Gorda plate. This earthquake did not produce a tsunami. Earthquakes with strike-slip mechanisms are less likely to produce tsunamis because they cause relatively little vertical ground displacement.

Earthquakes are common in the Gorda plate, which is subjected to north-south compression due to the northwest-moving Pacific plate that collides with the southern boundary of the Gorda plate along the east-west Mendocino fracture zone. This quake occurred approximately 110 km west of the epicenter of the November 8, 1980, M 7.2 earthquake. Three other quakes with a magnitude greater than 6 have occurred within an 80-km radius of the epicenter of this quake (August 16 and 17, 1991, and July 24, 1996).

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)