Seismotectonics of the April 25, 1992, Petrolia earthquake and the Mendocino triple junction region

The April 25, 1992, Petrolia earthquake (M s 7.1) occurred at the southern tip of the Cascadia subduction zone. This is the largest thrust earthquake ever recorded instrumentally in the Cascadia subduction zone. The earthquake was followed by two large strike‐slip aftershocks (both Ms 6.6). Moment release of each of the earthquakes is as follows: 4.0 × 1019 Nm in the first 10 s for the mainshock, 0.7 × 1019 Nm in the first 8 s for the first aftershock, and 0.9 × 1019 Nm in the first 2 s for the second aftershock. These indicate that the mainshock and each of the aftershocks may have different tectonic backgrounds. The best depth estimates of the mainshock and the two aftershocks are 14 km, 18 km, and 24 km, respectively. The slip direction of the mainshock is between N75°E and N80°E. This slip direction is not consistent with either the relative motion of the North American and Juan de Fuca plates (N60°E) or between the North American plate and the Gorda deformation zone (N40°E). It has been suggested that the North American‐Pacific plate motion is accommodated by right‐lateral slip on both the San Andreas and Maacama‐Rodgers Creek‐Hayward fault systems; the intervening block is the Humboldt plate. If we modify the relative motion of the southernmost Gorda deformation zone to conform with the seismicity trends and allow the Humboldt‐Pacific plate motion to be about half the total North American‐Pacific motion, then the Gorda deformation zone‐Humboldt relative motion matches the direction of the Petrolia slip vector. Also, the mixture of focal mechanisms in the two distinct aftershock clusters can be explained by motion between the Gorda deformation zone and Pacific plate and the Humboldt and North American plates. The Gorda deformation zone is subducting beneath the Humboldt plate in the Cape Mendocino area, and the Petrolia earthquake ruptured the entire subduction segment between the Gorda deformation zone and Humboldt plate.