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The 1911 M6.6 Calaveras Earthquake: Source Parameters and the Role of Static, Viscoelastic, and Dynamic Coulomb Stress Changes Imparted by the 1906 San Francisco Earthquake

Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968

Kim B. Olsen

Department of Geological Sciences, San Diego State University, MC1020, San Diego, California 92182-1020

Fred F. Pollitz and Ross S. Stein

U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025

Shinji Toda^*

Geological Survey of Japan, Advanced Institute of Science and Technology, Higashi 1-1, Tsukuba 305-8567, Japan

^* Order of authorship is alphabetical.

The occurrence of a right-lateral strike-slip earthquake in 1911 is inconsistent with the calculated static stress decrease imparted by the 1906 rupture at that location on the Calaveras fault, and 5 yr of calculated post-1906 viscoelastic rebound does little to reload the fault. We have used all available first-motion, body-wave, and surface-wave data to explore possible focal mechanisms for the 1911 earthquake. We find that the event was most likely a right-lateral strike-slip event on the Calaveras fault, larger than, but otherwise resembling, the 1984 M_w 6.1 Morgan Hill earthquake in roughly the same location. Unfortunately, we could recover no unambiguous surface fault offset or geodetic strain data to corroborate the seismic analysis despite an exhaustive archival search. We calculated the static and dynamic Coulomb stress changes for three 1906 source models to understand stress transfer to the 1911 site. In contrast to the static stress shadow, the peak dynamic Coulomb stress imparted by the 1906 rupture promoted failure at the site of the 1911 earthquake by 1.4–5.8 bar. Perhaps because the sample is small and the aftershocks are poorly located, we find no correlation of 1906 aftershock frequency or magnitude with the peak dynamic stress, although all aftershocks sustained a calculated dynamic stress of 3 bar. Just 20 km to the south of the 1911 epicenter, we find that surface creep of the Calaveras fault at Hollister paused for 17 yr after 1906, about the expected delay for the calculated static stress drop imparted by the 1906 earthquake when San Andreas fault postseismic creep and viscoelastic relaxation are included. Thus, the 1911 earthquake may have been promoted by the transient dynamic stresses, while Calaveras fault creep 20 km to the south appears to have been inhibited by the static stress changes.