SEISMOTECTONICS OF THE APRIL 25, 1992, PETROLIA EARTHQUAKE AND THE MENDOCINO TRIPLE JUNCTION REGION

The April 25, 1992, Petrolia earthquake (Ms 7.1) occurred at the south ern tip of the Cascadia subduction zone. This is the largest thrust ea rthquake ever recorded instrumentally in the Cascadia subduction zone. The earthquake was followed by two large strike-slip aftershocks (bot h Ms 6.6). Moment release of each of the earthquakes is as follows: 4. 0 x 10(19) Nm in the first 10 s for the mainshock, 0.7 x 10(19) Nm in the first 8 s for the first aftershock, and 0.9 x 10(19) Nm in the fir st 2 s for the second aftershock. These indicate that the mainshock an d 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 mainshoc k is between N75 degrees E and N80 degrees E. This slip direction is n ot consistent with either the relative motion of the North American an d Juan de Fuca plates (N60 degrees E) or between the North American pl ate and the Gorda deformation zone (N40 degrees E). It has been sugges ted that the North American-Pacific plate motion is accommodated by ri ght-lateral slip on both the San Andreas and Maacama-Rodgers Creek-Hay ward 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 mech anisms in the two distinct aftershock clusters can be explained by mot ion between the Gorda deformation zone and Pacific plate and the Humbo ldt and North American plates. The Gorda deformation zone is subductin g beneath the Humboldt plate in the Cape Mendocino area, and the Petro lia earthquake ruptured the entire subduction segment between the Gord a deformation zone and Humboldt plate.