San Andreas Fault Wouldn't Stop Statewide California Quake
January 12, 2013

Researchers Discover Statewide California Earthquake Might Be Possible

redOrbit Staff & Wire Reports — Your Universe Online

Assumptions that the San Andreas Fault would automatically prevent an earthquake from affecting both northern and southern California at the same time may not necessarily be true, according to a new study published Wednesday in the online edition of the journal Nature.

According to Eryn Brown of the Los Angeles Times, geologists have assumed for several years that the fault line would act as a barrier, preventing seismic activity from being felt statewide at the same time.

Now, however, researchers from the California Institute of Technology (Caltech) and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) have discovered a statewide earthquake may not necessarily be impossible after all.

"What we have found, based on laboratory data about rock behavior, is that such supposedly stable segments can behave differently when an earthquake rupture penetrates into them,” study co-author and Caltech mechanical engineering and geophysics professor Nadia Lapusta said in a statement. “Instead of arresting the rupture as expected, they can actually join in and hence make earthquakes much larger than anticipated.”

Lapusta and co-author, JAMSTEC scientist Hiroyuki Noda, believe that may have been what happened during the highly destructive, magnitude 9.0 Tohoku-Oki earthquake that struck Japan in 2011.

Regardless of speed, fault slip results from the interaction between stresses acting on the fault, as well as its resistance to slip, the researchers explained. Both phenomena depend upon several different factors, including the behavior of fluids within the rocks found in the Earth´s crust.

To take all of these factors into account, Lapusta, Noda, and their colleagues crafted fault models combining laboratory-based friction law knowledge and fluid behavior, using them to formulate computational procedures that allow them to simulate the impact that stress will have on those faux-faults.

"The uniqueness of our approach is that we aim to reproduce the entire range of observed fault behaviors — earthquake nucleation, dynamic rupture, postseismic slip, interseismic deformation, patterns of large earthquakes — within the same physical model; other approaches typically focus only on some of these phenomena," Lapusta said.

In addition to recreating the 2011 Tohoku-Oki earthquake, the researchers also used previous laboratory experiments, realistic fault properties, and their simulations to take a closer look at the 1999 magnitude 7.6 Chi-Chi earthquake that took place in Taiwan.

"We find that the model qualitatively reproduces the behavior [of the Japanese earthquake]", according to Lapusta, “with the largest slip occurring in a place that may have been creeping before the event. All of this suggests that the underlying physical model, although based on lab measurements from a different fault, may be qualitatively valid for the area of the great Tohoku-Oki earthquake, giving us a glimpse into the mechanics and physics of that extraordinary event."

“If creeping segments can participate in large earthquakes, it would mean that much larger events than seismologists currently anticipate in many areas of the world are possible,” the researchers explained. “That means“¦ that the seismic hazard in those areas may need to be reevaluated. For example, a creeping segment separates the southern and northern parts of California's San Andreas Fault.

“Seismic hazard assessments assume that this segment would stop an earthquake from propagating from one region to the other, limiting the scope of a San Andreas quake,” they added. “However, the team's findings imply that a much larger event may be possible than is now anticipated — one that might involve both the Los Angeles and San Francisco metropolitan areas.”

Something like this actually occurring along the San Andreas Fault anytime soon is extremely unlikely, according to Caltech seismologist Kate Hutton. Hutton told a Los Angeles television station such an occurrence would happen once every 1,000 years or so, Aaron Sankin of the Huffington Post said. Were it to happen, though, it could be destructive enough to alter the shape of the California coastline.