The Big One in L.A. Basin May Have Been an Orange County Event
A recently discovered fault may have been responsible for the largest recorded earthquake in the Los Angeles Basin--a seismic event more than 200 years ago so cataclysmic that it raised the ground by as much as 11 feet.
That is the premise of a UC Irvine professor’s study to be published next month in the Bulletin of the Seismological Society of America, one of the premier journals of earthquake science.
“It was a fairly large earthquake,” said Lisa Grant, an assistant professor of geology, whose study links the Portola earthquake of 1769 to the San Joaquin Hills fault in South Orange County. “What I find the most interesting and important about this is that it shows the fault to be active.”
Some scientists argue that the newly found fault is a slow-moving one unlikely to generate a major earthquake more often than, say, every 2,500 years.
“What’s been demonstrated is that there has been a very recent large uplift event,” said Tom Rockwell, a professor of geology at San Diego State University and leader of the geology group at the Southern California Earthquake Center. “One consequence is that a repeat of this earthquake is not likely to occur very soon--not in the next few decades, probably not for at least several hundred years.”
A 1999 study by Grant first drew attention to the fault, which stretches about 25 miles from Newport Bay to Dana Point along the coast and inland as far as the San Diego Freeway. Previously the only major fault known in Orange County was the Newport-Inglewood fault, which runs parallel to the coast and generated the deadly 1933 Long Beach earthquake.
Some scientists remain skeptical that the San Joaquin Hills fault exists, said Chris Wells, senior engineering geologist for the California Geological Survey. But there is enough evidence that his agency has recommended its inclusion on an upcoming revision of the national seismic hazard map.
“There is consensus that it is a structure we need to consider,” Wells said. “If it were in a desert it wouldn’t be significant, but it is important because of where it is.” The general area includes heavily populated areas of Costa Mesa, Irvine and Laguna Beach.
In the latest study, Grant and several assistants relied, among other things, on diaries kept by members of the historic Gaspar de Portola expedition, a group of Spanish explorers who camped by the Santa Ana River in what is now north Orange County. Their accounts describe a “severe” earthquake on July 28, 1769, followed by aftershocks over the next several days, each measured by the number of Hail Marys the explorers could utter during each tremor.
Grant concludes that the earthquake chronicled by Portola’s men probably happened along the San Joaquin Hills fault and may have had a magnitude of 7.3, considerably larger than the 6.7-magnitude Northridge earthquake of 1994 that killed 61 people and caused $40 billion in damage.
