Bay Area Overdue for Major Seismic Event? Yep.

It's always in the back of the mind of anyone living here. Or at least it should be.

“The extent of fault creep, and therefore locking, controls the size and timing of large earthquakes on the Northern San Andreas fault system,” said James Lienkaemper, a co- author of the study and research geophysicist at U.S. Geological Survey (USGS). “The extent of creep on some fault sections is not yet well determined, making our first priority to study the urban sections of the San Andreas Fault system, which is directly beneath millions of Bay Area residents.”  

Understanding the amount and extent of fault creep directly impacts seismic hazard assessments for the region. The San Andreas Fault system in Northern California consists of five major branches that combine for a total length of approximately 1250 miles. Sixty percent of the fault system releases energy through fault creep, ranging from 0.1 to 25.1 mm (.004 to 1 inch) per year, and about 28 percent remains locked at depth, according to the authors.  

The monitoring of creep on Bay Area faults has expanded in recent years. Alignment array measurements made by the San Francisco State University Creep Project and recently expanded GPS station networks provide the primary data on surface creep, which the authors used to estimate the average depth of creep for each fault segment. Where available, details of past ruptures of individual faults, unearthed in previous paleoseismic studies, allowed the authors to calculate recurrence rates and the probable timing and size of future earthquakes. 

According to the study, four faults have accumulated sufficient strain to produce a major earthquake. Three creeping faults have large locked areas (less than 1 mm or .04 inches of creep per year) that have not ruptured in a major earthquake of at least magnitude 6.7 since the reporting of earthquakes by local inhabitants: Rodgers Creek, northern Calaveras and southern Green Valley. The southern Hayward fault, which produced a magnitude 6.8 earthquake in 1868, is now approaching its mean recurrence time based on paleoseismic studies.

Seismological Society of America: Press Release

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