Why seismologists warned of a big earthquake in California this week

The threat of a magnitude 7.0 or greater earthquake could be as high as 1 percent through Tuesday, say seismologists.

First Look

After a series of small tremors shook the Salton Sea region about 100 miles east of San Diego last week, authorities placed residents of southern California on heightened alert: Be prepared for a much bigger earthquake, they warned.

Seismologists recorded a swarm of more than 200 quakes in quick succession, three of which exceeded magnitude 4.0, which threatened to trigger movement in the San Andreas fault less than four miles away. The chances of a magnitude 7.0 or greater quake striking the fault's southern stretch could be as high as 1 percent through Tuesday, according to the US Geological Survey (USGS).

"Any time there is significant seismic activity in the vicinity of the San Andreas fault, we seismologists get nervous," Thomas H. Jordan, the director of the Southern California Earthquake Center, told The Los Angeles Times.

Even with the low-end USGS estimated probability of 1-in-3,000, the potential that a 7.0 quake could strike, perhaps ripping through half the state, is significantly higher than the standard 1-in-6,000 probability during any regular week.

Dr. Jordan made headlines last May when he said the southern San Andreas fault – part of perhaps the most thoroughly researched and best understood fault lines in the world – is "locked, loaded, and ready to go." 

Eight years ago, researchers simulated what would happen if a magnitude 7.8 earthquake plowed up the San Andreas from the Salton Sea, sending shock waves in every direction. They estimated that it could kill as many as 1,800 people and cause $200 billion in damage, as the Los Angeles Times' Rong-Gong Lin II wrote on Saturday:

By the time the San Andreas fault becomes unhinged in San Bernardino County’s Cajon Pass, Interstate 15 and rail lines could be severed. Historic downtowns in the Inland Empire could be awash in fallen brick, crushing people under the weight of collapsed buildings that had never been retrofitted.

Los Angeles could feel shaking for a minute – a lifetime compared with the seven seconds felt during the 1994 Northridge earthquake. Shaking waves may reach as far as Bakersfield, Oxnard and Santa Barbara. About 1,600 fires could spread across Southern California. And powerful aftershocks larger than magnitude 7 could pulverize the region, sending shaking into San Diego County and into the San Gabriel Valley.

But Lucy Jones, a seismologist who retired in March after three decades with the USGS, said on Twitter that the heightened risk of a major tremor falls off dramatically as time passes, so by this past Saturday – three days before the advisory expires – the risk had returned to normal.

The added precaution taken by seismologists comes after seven prominent earthquake experts in Italy were convicted of manslaughter in 2012, sentenced to six years in prison, and fined $10.2 million – then exonerated in 2014 – for their failure to forewarn residents of an area struck in 2009 by a quake that killed more than 300 people, as The New York Times reported.

The California advisory, which was issued by the Governor's Office of Emergency Services, also came during the same week that Gov. Jerry Brown signed a law to develop an early-warning system statewide, as The Orange County Register reported.

The system aims to begin warning Californians via cellphone, radio, and other devices of impending earthquakes within the next two years.