A simple explanation for the occurrence of the 1911 Morgan Hill Earthquake in the stress shadow of the 1906 San Francisco Earthquake

Abstract

We propose a new model to overcome an apparent stress shadow paradox. The stress shadow of the 1906 San Francisco earthquake was so effective in inhibiting earthquake activity that no M > 6 earthquakes occurred in the San Francisco Bay area for more than 70 years after the 1906 earthquake, with one exception: the 1911 Morgan Hill earthquake. How could this earthquake occur only 5 years after the 1906 earthquake in an area where earthquake activity should have been inhibited? Our model is characterized by localized locked patches on the Morgan Hill segment of the Calaveras fault surrounded by steadily creeping patches. According to this model, the tectonic stressing rate on the Morgan Hill segment is so high that the stress shadow caused by the 1906 earthquake can be dissipated within several years. This simple model is consistent with both the surface deformation data and the apparent earthquake recurrence at Morgan Hill, in contrast to two previous models which are either poorly constrained or have difficulty explaining such observations.