California faces threat of back-to-back mega-earthquakes

The mega-earthquakes in Turkey this week show how a magnitude 7.8 quake could trigger a magnitude 7.5 aftershock on another fault, with 60 miles apart between epicentres.

A similar earthquake scenario could occur in California.

Mega-earthquakes that could rupture the San Andreas South Fault near the Mexican border through Los Angeles County and beyond could trigger major aftershocks and shake cities as far apart as Sacramento and San Francisco, according to documents and interviews.

In a 2008 US Geological Survey report detailing a hypothetical magnitude 7.8 earthquake in Southern California, scientists said a plausible aftershock scenario included a magnitude 6 earthquake, 95 that would rock Sacramento and Modesto three days after the main shock, endangering the stability of the levees, which are crucial to maintaining flood control and the movement of water from the northern Sierra Nevada to cities in the state. .

Significant and distant aftershocks have already occurred in California.

The Great 1906 earthquake, best known for destroying much of San Francisco, also triggered quakes much further afield on the same day, said seismologist Lucy Jones, a research associate at Caltech. They included magnitude 5.5 in Santa Monica Bay and magnitude 6 in the Imperial Valley near the Mexican border, Jones said.

Supersized earthquakes are more likely to cause oversized aftershocks – and they can occur much further away than smaller earthquakes.

For example, the Great Earthquake of 1906 ruptured a large swath of the North San Andreas Fault from Humboldt County near Eureka, across the San Francisco Bay Area and approaching San Andreas County. Benito, east of Monterey.

The length of the ruptured fault – nearly 300 miles – is significant.

Earthquake scientists say a subsequent quake that is generated “a broken fault length” away from the main shock can be considered an aftershock.

This means that the Santa Monica Bay earthquake, about 250 miles from the southern end of the ruptured San Andreas Fault, would be considered an aftershock of the 1906 San Francisco earthquake.

Additionally, subsequent earthquakes of a distance approximately four times that of the main shock’s ruptured fault length are considered “triggered” earthquakes.

So the length of the fault that ruptured in the Turkey earthquake – about 125 to 185 miles long – would produce a greater chance of follow-up earthquakes up to 620 miles than the ruptured length of the flaw of the main shock, according to Jones.

“So something like 1,000 kilometers [620 miles], we have an increased risk of having earthquakes,” Jones said. “We’re not going to see it everywhere at 1,000 kilometres, but we’re going to see it.”

It is therefore plausible that an 8.2 magnitude earthquake on the South San Andreas Fault, rupturing near the Mexican border, through Los Angeles County and terminating in Monterey County, could result in a subsequent earthquake in San Francisco, Jones said.

“If we get the biggest break, and it stretches north to Parkfield [in Monterey County] — then the whole Bay Area is within one fault length,” Jones said.

Simulations of huge earthquakes in California show how aftershocks can occur away from the main tremor. Large aftershocks can occur for months, years, and even decades across a wide region, with a mega-earthquake potentially ushering in a generation of increased seismic activity.

Another plausible scenario: a magnitude 7.8 earthquake on the South San Andreas Fault, breaking off near the Mexican border to Lake Hughes near Santa Clarita, triggers a magnitude 7.2 aftershock on the San Andreas Fault. Cucamonga, breaking between Cajon Pass and Monrovia.

“This event would cause further substantial damage to the San Gabriel Valley, possibly increasing financial losses and fatalities by 20% to 30%,” the US Geological Survey said in the 2008 report.

A 7.71 magnitude aftershock could affect communities such as Palm Springs, Palm Desert, Rancho Mirage, Coachella, Thermal, Mecca, Imperial Valley, Brawley and El Centro. Significant damage could extend to San Diego and Imperial counties.

And a magnitude 7 earthquake rupturing 52 miles from the Hayward Fault east of San Francisco could produce large, damaging aftershocks farther from the hardest-hit areas beneath Oakland, Berkeley, Hayward and Fremont. One scenario includes a magnitude 6.4 earthquake in Cupertino, in the heart of Silicon Valley, more than five months after the main shock, according to a USGS report.