San Andreas fault could pose wider risk than thought

Posted by Emma Woollacott

The 'stable' zones thought to inhibit nearby earthquakes may actually make them worse, a new study shows, implying that California's San Andreas fault may be much more dangerous than believed.

Many segments of geological faults move slowly, through a process called creep, and have been believed to act as barriers to fast-slipping, shake-producing earthquake ruptures.

One such exists on the San Andreas fault, and has been seen as partial protection for the cities of San Francisco and Los Angeles.

"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," says Nadia Lapusta, professor of mechanical engineering and geophysics at Caltech.

"Instead of arresting the rupture as expected, they can actually join in and hence make earthquakes much larger than anticipated."

This, she believes, is what happened in the 2011 magnitude 9.0 Tohoku-Oki earthquake, which was unexpectedly large.

Her team developed fault models that incorporate lab-based knowledge of complex friction laws and fluid behavior, along with computational procedures that allow the scientists to numerically simulate how those model faults will behave under stress.

"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," she says.

Using this technique, the team found that the ostensibly stable area would indeed occasionally creep, and often stop seismic events - but not always. From time to time, dynamic rupture would penetrate that area in just the right way to activate dynamic weakening, resulting in massive slip.

They believe that this is what happened in the 1999 magnitude 7.6 Chi-Chi earthquake in Taiwan - indeed, the quake's largest slip occurred in what was believed to be the 'stable' zone.

If creeping segments really can make large earthquakes worse, there are serious implications for many areas of the worldd.

For example, a creeping segment separates the southern and northern parts of California's San Andreas Fault, and previous seismic hazard assessments have assumed that this would stop an earthquake from propagating from one region to the other.

However, the team's findings imply that a much larger quake may be possible - one that might involve both the Los Angeles and San Francisco metropolitan areas.

"We plan to further develop the modeling to incorporate realistic fault geometries of specific well-instrumented regions, like Southern California and Japan, to better understand their seismic hazard," says Lapusta.