LOS ANGELES — For decades, scientists have assumed the central portion of California’s San Andreas fault acts as a barrier that prevents a big quake in the southern part of the state from spreading to the north, and vice versa. As a result, a mega-quake that could be felt from San Diego to San Francisco was widely considered impossible.
But that key fault segment might not serve as a barrier in all cases, researchers wrote Wednesday in the online edition of the journal Nature.
Using a combination of laboratory measurements and computer simulations, the two scientists showed how so-called creeping segments in a fault — long thought to be benign because they slip slowly and steadily along as tectonic plates shift — might behave like locked segments, which build up stress over time and then rupture.
Such a snap caused the 9.0-magnitude Tohoku-Oki earthquake that hit Japan in 2011, triggering a tsunami, killing nearly 16,000 people and destroying the Fukushima Daiichi nuclear power plant. Forecasters had not believed such a large quake was possible there.
A supposedly stable section of fault also ruptured during the 1999 Chi-Chi quake in Taiwan, a 7.6-magnitude temblor that killed more than 2,400 people.
Afterward, scientists drilled into rocks surrounding the Chelungpu fault there, removing samples and testing them to better understand their geology.
Caltech engineer and geophysicist Nadia Lapusta and a former postdoctoral fellow, Hiroyuki Noda of the Japan Agency for Marine-Earth Science and Technology in Yokohama, used that data in their analysis published in Nature.
They plugged the measured rock properties into a computer model they built that simulated a simple fault with two “patches” of rock — one that was locked and another that was creeping.
As expected, most of the time only the locked patch ruptured. But there were also instances when the simulation resulted in ruptures in the creeping patch. In those cases, the rocks slipped past each other quickly enough to heat up and weaken the fault, allowing it to snap.
The results provide a possible explanation for events that caused the Tohoku-Oki and Chi-Chi quakes, which have puzzled scientists. By extension, they also suggest that the San Andreas might be capable of a more extensive earthquake than was widely assumed.
“The thinking has been that an earthquake could either occur on the southern San Andreas fault or on the northern San Andreas fault — that the creeping segment is separating it into two halves,” Lapusta said. “But this study shows that if an earthquake penetrates that creeping area in a certain way, it could rupture through it.”
The San Andreas wouldn’t necessarily snap as the fault in the model did, she said: “Hopefully the creeping segment is such that it doesn’t have the propensity for weakness. But without examining further, you can’t say.”
Such an investigation might include further computer simulations, laboratory experiments or digging along the creeping portion of the San Andreas to look for evidence of extremely large slips in the ancient past.
By looking at a fault relatively close to its surface — no more than tens of yards deep — paleogeologists can see whether very large earthquakes ever ruptured through to the surface, Lapusta said. Scientists can also drill to greater depths to collect rock samples, as they did to study the Chelungpu fault.
Kenneth Hudnut, a geophysicist at the U.S. Geological Survey in Pasadena who was not involved in the research, said that the study sounded “a warning message.”
“We’re realizing we need to worry more about these things we’ve been calling barriers,” he said, adding that the Tohoku-Oki quake wasn’t the only recent disaster making researchers reconsider fault segments once thought to be “toothless” — temblors in the Indian Ocean, Chile, Haiti and China had also given pause.
“The more big earthquakes we’ve seen around the world, the more we’ve realized that there are some deficiencies in our models,” he said. “Everyone’s taking a second look at what we thought was worst-case.”
Hudnut emphasized that an extreme quake powerful enough to blast through the supposedly stable midsection of the San Andreas was unlikely.
But if one did strike, he said, it might put unexpected strain on California’s emergency response systems. Planners had always assumed that responders from one part of the state would be available to supply aid in the other.
-Los Angeles Times