Recent Stress Orientation and Deformation in the Mendocino Triple Junction Region Determined from the Analysis of Earthquakes

The Mendocino Triple Junction (MTJ), defined by the intersection of the San Andreas fault zone, Mendocino fault, and Cascadia subduction zone as well as the North American plate, Gorda plate, and Pacific plate, has a history of strong earthquakes of magnitude 6.5 or greater, with much of the seismicity within the Gorda plate. We model recent, moderate-sized earthquakes in the MTJ region for focal mechanism and total moment, including the most recent events that occurred in the Gorda plate (Mw=7.2). Including larger events with known focal mechanisms and moment from the Harvard Centroid Moment Tensor (CMT), Northern California Earthquake Data Center (NCEDC) catalogs, and historically studied events to reveal the overall state of stress for the region. We categorize events by magnitude and class of focal mechanism (strike slip, thrust, transtensive, transpressive and normal) and determine the direction of moment as a function of the P-axis (maximum compressional stress). We find that the larger strike-slip events in the Gorda plate align with a P-axis that might be expected for motion along the San Andreas fault; smaller events align with an expected Mendocino fault orientation, and very few events have an orientation related to the Cascadia subduction zone. These results suggest that Gorda plate deformation is related to the San Andreas fault system and the northward migration of the MTJ and not the spreading of the Gorda ridge. This study also leads to a conclusion that deformation of the Gorda plate is not presently relieving stresses associated with the Cascadia subduction zone.