Prehistoric earthquakes discovered along San Andreas Fault
In the study, USGS geologist Dr. Kate Scharer and her team excavated trenches across the fault – near Frazier Mountain in northeastern Kern County – to investigate the timing of sand, mud, and gravel deposits that were episodically ripped apart by earthquakes along the San Andreas Fault over the last 1200 years. They found evidence of 10 ground-rupturing earthquakes between 800 A.D. and the last rupture in 1857.
“In order to properly design infrastructure, like highways, water and power lines, so that it can survive the next earthquake, we study how often and how big prehistoric earthquakes were,” Scharer said. “This work provides a new long record where we previously had very little data. The record shows that large earthquakes occurred near Tejon Pass on average every 100 years. Most appear to be quite large, between magnitude 7.0 and magnitude 7.5.”
The average time between earthquakes in the record studied is about 100 years. Statistical evaluation of the timing between earthquakes reveals they are not random in time, but show a very weak periodicity. For example, some inter-earthquake periods are as short as 20 years, while at least one period was almost 200 years long. Such patterns are consistent with other locations along the San Andreas Fault where these prehistoric earthquake records are available.
The last earthquake to rupture the southern San Andreas Fault occurred in 1857. That earthquake ruptured a section of the fault that was about 210 miles long, with an estimated magnitude of M7.9. Looking at the prehistoric record along the same stretch as the 1857 earthquake, Scharer and her colleagues find that since 1300 A.D. only one in eight of the past ruptures could have broken the same stretch of the fault. From these data, it appears that most ruptures are slightly smaller than the 1857 event, about M7.5. Official USGS forecasts for California earthquakes, which included data from Frazier Mountain, forecast a 16 percent chance that a M7.5 or larger earthquake will occur on this section of the fault in the next 30 years.
The study, “Ground-rupturing earthquakes on the northern Big Bend of the San Andreas Fault, California, 800 A.D. to present” is available online.