Double whammy of multi-fault ruptures can unleash stronger earthquakes, Stanford scientists say 
 
 Research based on a new study of California's 1812 earthquake yields insights to improve future seismic hazard predictions. 
 
 Julian Lozos A view of Cajon Pass, where the San Andreas and San Jacinto fault lines run close together. According to a new study, a deadly earthquake that rocked Southern California two centuries ago was likely caused by the slippage of the San Jacinto Fault that then triggered the second fault. 
 
 A deadly earthquake that rocked Southern California two centuries ago was likely caused by the slippage of one fault line that then triggered a second fault, according to a new study. The research not only offers a solution to a long-standing seismic "whodunit," it also demonstrates how the rupturing of multiple faults can trigger more powerful temblors. 
 
 Researchers have previously blamed the sole rumblings of the infamous San Andreas Fault for the 7.5-magnitude quake of December 8, 1812, one of the biggest in Golden State history. The new study implicates an accomplice, however: the lesser-known, nearby San Jacinto Fault, whose potential for provoking serious quakes in tandem with the San Andreas has been underestimated. 
 
 "This study shows that the San Jacinto Fault is an important player that can influence what the San Andreas Fault is doing and lead to major earthquakes," said Julian Lozos, the author of the study, who conducted the work as a postdoctoral fellow at Stanford. "It's important that we learn more about how activity on any single fault can affect other faults." 
 
 For the new study, published in Science Advances, Lozos took a multidisciplinary approach. He ran a series of physics-based computer simulations known as dynamic rupture models. The simulations involved crunching numbers for forces such as stress and friction across fault lines, which are fractures where vast, subterranean rock faces grind against each other. Key outputs from the simulation included how fast the faults move and the ground motion this movement causes, which helps in pegging the destructive power of an earthquake. 
 
 Julian Lozos Seismologists use precariously balanced rocks like these in Southern California to gauge the strength of earthquakes. 
 
 Lozos, who is now an assistant professor of geological sciences at California State University, Northridge, compared the simulations' results to geologic records of slip in prehistoric earthquakes on both faults. Because the 1812 quake occurred before instruments existed to record its severity, Lozos also looked to early 19th-century historical records, jotted down by Catholic missionaries from Spain. The missionaries reported varying degrees of ground shaking and damage at their several outposts, including the collapse of a stone church at Mission San Juan Capistrano that killed 40 people. Lastly, Lozos accounted for present-day examples of boulders precariously balanced atop other rocks in the region, a phenomenon seismologists cleverly use to gauge where an earthquake's local shaking had not been violent enough to topple the delicate rocky arrangements. 
 
 Altogether, the evidence suggested that the San Jacinto Fault slipped first somewhere between the cities of Moreno Valley and San Jacinto. The rupture propagated north and crossed over to the San Andreas Fault near a location called the Cajon Pass, between the San Bernardino and San Gabriel mountains, where the two fault lines run as close as 1.5 kilometers. Together the ruptured faults set Southern California ashudder that early December day. 
 
 "Julian has offered a reasonable and physically plausible explanation for the 1812 earthquake," said geophysicist Greg Beroza, the Wayne Loel Professor at Stanford School of Earth, Energy & Environmental Sciences, who was Lozos' adviser. "Understanding this earthquake in particular, and complex earthquakes in general, is important to quantifying seismic hazard." 
 
 Bill Perry/ Shutterstock Ruins at Mission San Juan Capistrano. The collapse of the stone church during the earthquake of 1812 killed 40 people. 
 
 Lozos' research could help in future preparations of the Uniform California Earthquake Rupture Forecast, or UCERF. Spearheaded by the U.S. Geological Survey, these reports project the likelihoods for major earthquakes occurring in the Golden State over several decades. 
 
 The most recent UCERF, which published a year ago, took into account some of the newer scientific appreciations of multi-fault ruptures. As a result, the estimated chance for a magnitude 8 or larger earthquake to jolt California by 2045 rose from about 4.7 percent in a previous UCERF report to about 7 percent in the latest forecast. 
 
 In addition to improving earthquake modeling, Lozos hopes his work will increase earthquake awareness in the general public – especially for the millions of Californians living in the Inland Empire, undercut by both the San Andreas and San Jacinto fault lines. 
 
 "People shouldn't just be thinking about the San Andreas Fault," Lozos said. "There are lots of other faults, so it's important for everyone in the regions at risk to have their earthquake kits ready." 
 
 Media Contact 
 
 Ker Than, Stanford School of Earth, Energy & Environmental Sciences: (650) 723-9820, kerthan@stanford.edu 
 
 Julian Lozos, California State University, Northridge: julian.lozos@csun.edu 
 
 Greg Beroza, Stanford School of Earth, Energy & Environmental Sciences: (650) 723-4958, beroza@stanford.edu 
 
 Bjorn Carey, Stanford News Service: (650) 725-1944, bccarey@stanford.edu ||||| The San Jacinto fault, a twitchy system that cuts through the East County, could produce larger earthquakes than scientists believed and may rival the San Andreas in power, according to research led by San Diego State University. 
 
 "A magnitude 7.5 was generally accepted to be the largest earthquake that would like occur on the fault," said Tom Rockwell, an SDSU seismologist and the study's leader. "We have shown that the central and northern sections of the San Jacinto fault appear to fail together at times, and that would be in the magnitude 7.6 to 7.7 range. 
 
 SDSU's Tom Rockwell is one of the world's foremost authorities on the San Jacinto fault. — Gary Robbins Share Photo  
 
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 Reddit 
 
 ✉ SDSU's Tom Rockwell is one of the world's foremost authorities on the San Jacinto fault. — Gary Robbins 
 
 "If it ruptures onto the San Andreas fault, it could approach a magnitude 8.0, although we don't see evidence that that has happened in the past couple of thousand years. The take home here is that earthquakes on the San Jacinto fault potentially rival those on the San Andreas fault." 
 
 The San Jacinto system is a 130-mile strike-slip fault that stretches from Imperial County through Anza, Ocotillo Wells and Borrego Springs into Riverside County and the San Bernardino Valley. Rockwell oversaw field studies that examined 21 seismic events that have occurred on the fault over the past 4,000 years. The study, recently published in the journal Pure and Applied Geophysics, was centered at Hog Lake, a spot near Anza. 
 
 The San Jacinto is part of the larger San Andreas fault, the dividing line between the Pacific and North American plates. Scientists say that the two faults are responsible for roughly 80 percent of the slippage along the plates. Rockwell has played a major role in revealing the path of the San Jacinto and reconstructing its history. Records show that the fault produced a magnitude 6.5 quake in April 1968. A different strand of the fault, known as Superstition Hills, produced a 6.7 quake in November 1987 that was strongly felt in San Diego. 
 
 Could San Andreas quake destroy the Hoover Dam 
 
 UC San Diego shoots stark aerial image of Salton Sea ||||| The San Andreas is the state's longest fault and perhaps its most famous—well-known enough to be the standalone title of a recent disaster film. And, says the LA Times, citing an earthquake expert speaking at the National Earthquake Conference, it's also the one we need to be keeping our eyes on the most right now. 
 
 The issue is that the fault line has been too quiet for too long. Scientists have found that the "earthquakes should be relieving about 16 feet of accumulated plate movement every 100 years" on the San Andreas, but the fault hasn't let off any steam in way over a century. 
 
 The southern section of the fault hasn't had a big 'quake since 1857, which "ruptured an astonishing 185 miles between Monterey County and the San Gabriel Mountains," near LA, in a 7.9 earthquake. Other nearby sections of the fault are also overdue. A segment by the Cajon Pass (where the 15 Freeway cuts between the San Bernardino and San Gabriel mountains) "have not moved substantially" since an 1812 earthquake, and over by the Salton Sea, it's been seismically silent as far back as 1680. 
 
 That indicates it's long overdue for a rupture. Or, to put it more chillingly, "The springs on the San Andreas system have been wound very, very tight. And the southern San Andreas fault, in particular, looks like it’s locked, loaded and ready to go." 
 
 The expert, Thomas Jordan, director of the Southern California Earthquake Center, went on to say that, with this information, LA should focus on being prepared for a BIG one—a magnitude 8 or so. (For perspective, a 2008 US Geological Survey study warned that a 7.8 earthquake along the southern San Andreas would send shaking far and wide from the Coachella Valley and Inland Empire to East LA and the San Gabriel Valley, and cause about 2,000 deaths. So, stronger than that.) 
 
 Jordan applauded the city's efforts to retrofit buildings vulnerable to collapse in a strong seismic event, saying, "It’s remarkable that this happened...We know politically how difficult it is to make these kinds of changes." LA's also at work making its cell phone network more resilient to serious shaking so that communication post-'quake would still be possible. 
 
 With most Angelenos less than totally prepared for a significant earthquake, there's no time like the present to get started.
– When we think about building-leveling earthquakes in California, it can seem like the San Andreas fault is the only game in town. Heck, The Rock even made a movie about it. But a study published last week in Science Advances finds the San Andreas could have an equally dangerous partner in creating the state's next catastrophic quake. The Los Angeles Times reports geophysics professor Julian Lozos, while trying to solve the mystery of the earthquake of 1812, determined that it's possible the San Andreas fault combined with the San Jacinto fault to cause a magnitude-7.5 earthquake—and could do so again. According to a press release, the 1812 earthquake was one of the biggest in California's history and now appears to have been caused when a rupture in the San Jacinto spread to the San Andreas. "People shouldn't just be thinking about the San Andreas fault," Lozos says in the press release. The lesser-known San Jacinto could actually be a more serious "seismic hazard" because it runs directly under a number of population centers east of Los Angeles. If it combined with the San Andreas to create a "Big One"—as evidence shows it has in the past—the resulting earthquake could kill more than 1,000 people while destroying buildings and starting fires, what the Times calls a "grim seismic scenario." Lozos says it's important to understand how one fault rupturing can trigger ruptures in others. A recent report taking similar multi-fault thinking into account raised California's odds for having a magnitude-8 or larger earthquake by 2045 from 4.7% to 7%.