Voyager: Is a large earthquake on the San Andreas Fault imminent?

Scientists are concerned that a great earthquake is likely to occur on the southern portion of the San Andreas Fault, which runs through much of California, probably in the next 30 years. Geologists, geodesists (scientists who study the dimensions of the earth), and seismologists base this forecast on years of measurements and research.

We know from historical record that the San Andreas Fault experiences great earthquakes every 200 years on average. The fault is more than 800 miles long and divided into three sections: northern, central, and southern. The central portion last ruptured in 1857 during the magnitude 7.9 Fort Tejon earthquake, while the northern section ruptured in 1906 with the 7.9 San Francisco earthquake.

However, there has not been a major earthquake on the southern part of the fault for at least 300 years. The plates in this region of the fault are moving at 22 to 30 millimeters a year.  This shows that the San Andreas Fault system has accumulated from 5.5 to 9.0 meters (18 to 30 feet) of crustal motion without a major earthquake. The record of this movement is called the “slip deficit” and this part of the San Andreas Fault is approaching the maximum slip deficit ever accumulated in its recorded history.

So yes, it is probable that a great earthquake is “imminent,” either on the southern section of the San Andreas or neighboring San Jacinto Fault, which is approaching a similar slip deficit.

Scientists at Scripps Institution of Oceanography at UC San Diego are working to improve their forecast of the next great earthquake in our region and are also developing early-warning systems that may help reduce injuries and damage when they do occur. For example, an early warning system in Japan will automatically slow down high-speed trains just before an earthquake, potentially saving lives and property.

-- Yehuda Bock, Research Geodesist, Institute for Geophysics and Planetary Physics

To learn more about how scientists are getting ready for earthquakes, read our story “A Step Ahead of the Big One”