Coulomb stress interactions amongM≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault

Abstract

The Gorda deformation zone, a 50,000 km2area of diffuse shear and rotation offshore northernmost California, has been the site of 20M≥ 5.9 earthquakes on four different fault orientations since 1976, including fourM≥ 7 shocks. This is the highest rate of large earthquakes in the contiguous United States. We calculate that the source faults of six recentM≥ 5.9 earthquakes had experienced ≥0.6 bar Coulomb stress increases imparted by earthquakes that struck less than 9 months beforehand. Control tests indicate that ≥0.6 bar Coulomb stress interactions betweenM≥ 5.9 earthquakes separated by <9 months are unlikely to occur by random chance, suggesting that the multiple short‐term stress interactions observed among the recent Gorda zone earthquakes are not an apparent effect. In all well‐constrained ≥0.2 bar Coulomb stress interactions between earthquakes that occurred within 4 years of each other, the second earthquake is promoted. On longer timescales, calculated stress changes imparted by the 1980Mw= 7.3 Trinidad earthquake are consistent with the locations ofM≥ 5.9 earthquakes in the Gorda zone until at least 1995, as well as earthquakes on the Mendocino Fault Zone in 1994 and 2000. Coulomb stress changes imparted by the 1980 earthquake are also consistent with its distinct elbow‐shaped aftershock pattern. From these observations, we derive generalized static stress interactions among right‐lateral, left‐lateral and thrust faults near triple junctions.