Great earthquakes on the San Andreas fault produce stress shadows that last for years. (Joint with USGS)

The great 1857 Ft. Tejon earthquake (magnitude 7.8) which ruptured the San Andreas fault from central California to Cajon Pass was the largest earthquake to have hit southern California during the historic period. This great earthquake was followed by approximately 50 years of seismic quiet in the region west of Cajon Pass, including the Los Angeles basin.

Calculations of stress changes produced by this earthquake indicate that many nearby faults were relaxed by the slip in the earthquake. This relaxation resulted in a "shadow" effect by creating regions in which future earthquakes were delayed. Earthquakes continued in other regions, especially on faults southeast of Cajon Pass including the San Jacinto fault, where stresses were increased or changed only slightly.

Scientists hope to use these correlations of earthquakes with stress changes to better estimate changes in seismic hazard after a large earthquake.

Contacts:

Ruth Harris, USGS (650) 329-4842; Jishu Deng, Caltech, (626) 395-6948.