Vertical Crustal Stability in 23 Years Since the 1992 Cape Mendocino M 7.1 Earthquake: Benchmark Survey Results, Interpretations and Tectonic Implications

Abstract:

In 1992, a M 7.1 earthquake occurred near Petrolia, CA within the area of the Mendocino triple junction and the southern terminus of the Cascadia subduction zone (CSZ). This earthquake resulted in ~1.4 m of coastal uplift, accompanied by inland subsidence. The focal mechanism of the earthquake indicated motion along a low-angle, east-dipping thrust plane, consistent with the subduction zone interface or a subsidiary fault. If the earthquake occurred on the megathrust itself, we might expect that high strain rates resulting from Gorda/North American plate convergence would cause measurable vertical interseismic deformation in the 23 years since the event. Along other subduction zones with historic ruptures, large amounts of deformation had occurred within 10-50 years after the mainshock, resulting in crustal recovery in a direction opposite that of the coseismic deformation. However, a subsidiary fault would not accumulate strain as quickly, thus little to no vertical deformation should be observed after just 23 years. Using high resolution static GPS relocation of benchmarks and comparing these positions to the 1992 leveling elevations, we have found that there has been little to no vertical deformation since the leveling in 1992. These results indicate that the 1992 event was likely not a subduction zone rupture but rather occurred along a low strain rate subsidiary fault.