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Non-technical summary: The paper by Pollitz, Bakun, and Nyst (2004) examines how tectonic stress in the San Francisco Bay area evolves with time. They used Global Positioning System data to calibrate the predicted behavior of the San Andreas Fault (SAF) plate boundary zone based on the physics of stress transfer associated with earthquakes. To predict where stress levels will concentrate in the future they found it necessary to include the effects of many physical processes. These include: (1) The constant tectonic motion of the Pacific plate relative to the North American plate a rate of about 4 cm/yr around the SAF plate boundary zone, (2) the stress changes associated with large earthquakes such as the M 7.8 1906 San Francisco earthquake, (3) the slow re-adjustment of the crustal mechanical system to the sudden stress changes that accompany earthquakes, and (4) creep along faults such as the central SAF and Hayward fault. In order to obtain as accurate a picture as possible, they added up the stress changes contributed by 13 large earthquakes (M greater than about 6.2) that occurred in the Bay area from 1838 up to 1989. They found that using Coulomb failure stress theory, the predicted areas of elevated stress tended to coincide with the locations of M>5.8 earthquakes in the past century and a half. This suggests that physics-based models of stress in the Bay area crust have promise for identifying those areas most likely to experience M>5.8 earthquakes in the future.

Figure 8. Pre-1906 (a) and post-1906 (b) potentially triggered earthquakes. Locations and magnitudes are from Bakun [1998, 1999] and Toppozada and Branum [2002]. Black stars show other earthquakes listed in the Toppozada and Branum catalog which we do not consider. The great majority of these are located on the creeping portions of the Calaveras and San Andreas faults south of 37 N. The triggered earthquakes shown in Figure 8b include all pre-1906 earthquakes of magnitude M 5.8 and post-1906 earthquakes of magnitude M 5.5, with the exception of the M5.2 Yountville earthquake. Note that the 1865 earthquake is placed in accordance with scenario B of Bakun [1998]. Click on image for the whole pdf article or on the link above.