Slow-moving section of the San Andreas Fault could produce major activity

The San Andreas Fault is an 800-mile-long sliding tectonic boundary between the Pacific Plate and the North American Plate. It cleaves California from north to south, as the two tectonic plates slowly grind against each other, threatening to give rise to massive earthquakes. 

San Andreas comprises three major sections which can move independently. In the southern and the northern sections, the plates are locked most of the time, causing stresses to build over years, decades, or even centuries, and occasionally leading to large earthquakes, such as the catastrophic one from San Francisco in 1906 that had a magnitude of 7.9 and killed over 3,000 people. 

In the central section though, which separates the other two, the plates slip past each other slowly, at a steady 26 millimeters rate each year. Until recently, scientists thought that this so-called “aseismic creep” prevents stresses from building and does not lead to large earthquakes.

However, a new study published in the journal Geology suggests that this central creeping section of the San Andreas Fault has in fact hosted many major earthquakes throughout history, some of them possibly happening fairly recently. By using novel chemical-analysis methods to calculate the degrees of heating that the sedimentary rocks withstood in the distant past – a sign that earthquakes took place there – the researchers argue that the creep may have witnessed over 100 earthquakes during its history. Some of these quakes may have been even larger than the disastrous 1906 San Francisco one.

“The creeping section is a difficult place to do Paleoseismology, because evidence for earthquakes can be easily erased by the creep,” said Morgan Page, a seismologist at the U.S. Geological Survey. “If this holds up, this is the first evidence of a big seismic rupture in this part of the fault.”   

“Ultimately, our work points to the potential for higher magnitude earthquakes in central California and highlights the importance of including the central (San Andreas Fault) and other creeping faults in seismic hazard analysis,” the study authors added.

However, according to the researchers, although massive earthquakes did occur along the creep and thus they could happen again, since tectonic strain is not currently accumulating at significant rates in the area, no such disastrous events are foreseen in the near future.

“People should not be alarmed,” concluded study co-author Stephen Cox, a geologist at Columbia University.  “Building codes in California are now quite good. Seismic events are inevitable. Work like this helps us figure out what is the biggest possible event, and helps everyone prepare.”

—

By Andrei Ionescu, Earth.com Staff Writer