The next magnitude 8.0 or greater earthquake to hit California could come a little sooner than expected. 
 
 The estimated chance of whether such a mega-earthquake would hit California in the next three decades was raised to 7% from about 4.7%, the U.S. Geological Survey said Tuesday. 
 
 An 8.0 earthquake hitting a populated area would be devastating — producing 89 times more energy than the magnitude 6.7 Northridge earthquake in 1994. Such powerful quakes are relatively rare — scientists believe the last mega-quake to hit Southern California was a magnitude 7.9 earthquake in 1857. 
 
 Here's a look at how scientists figured out the new forecast. 
 
 Why is the chance of a magnitude 8.0 quake greater than previously forecast? 
 
 Scientists in recent decades have begun to realize that earthquake faults are interconnected. In fact, earthquakes can begin on one fault and jump to another. Multiple faults can rupture in a simultaneous mega-quake. 
 
 "A simplification we made in the past were these faults were separate and isolated. In other words, they couldn't link up together into truly large earthquakes," said U.S. Geological Survey seismologist Ned Field, the lead author of the report. 
 
 But as scientists identified more faults in California, "it has become increasingly apparent that we are not dealing with a few well-separate faults, but with a vast interconnected fault system," Field said. "In fact, it has become difficult to identify where some faults end and others begin, implying many more opportunities for multi-fault ruptures." 
 
 USGS forecast: Chances of magnitude-8 earthquake up to 7 percent USGS forecast: Chances of magnitude-8 earthquake up to 7 percent SEE MORE VIDEOS 
 
 Have scientists observed earthquakes jumping faults before? 
 
 Yes. All three of the largest, most recent earthquakes in California have pushed beyond fault boundaries, "jumping from one fault to another," Field said. Two hit the sparsely-populated Mojave Desert: the magnitude 7.3 Landers earthquake in 1992 and the magnitude 7.2 Hector Mine earthquake in 1999. 
 
 More recently, the magnitude 7.2 earthquake in 2010 that was centered near the California-Mexico border on Easter Sunday jumped faults. Scientists said the Baja California quake directed tectonic stress northward, putting Southern California at a higher risk for a future quake. The original quake and aftershocks triggered movement on at least six faults, including the Elsinore and San Jacinto faults, which run close to heavily populated areas in eastern Los Angeles County and the Inland Empire. 
 
 The magnitude 9.0 earthquake that hit off the Japanese coast in 2011 also jumped fault boundaries, "resulting in a much larger fault-rupture area and magnitude than expected, and contributing to the deadly tsunami and Fukushima nuclear disaster," the USGS report said. 
 
 How often do quakes rupture along multiple faults in a simultaneous mega-quake? 
 
 A magnitude 8.0 or greater quake in California is now expected once about every 500 years. An older forecast calculated the rate of magnitude 8.0 or greater earthquakes at about once every 600 years. 
 
 Where are these faults located? 
 
 Many are in the Los Angeles area, in highly urban areas. During the Whittier Narrows earthquake, a magnitude 5.9 struck on the Puente Hills thrust fault system Oct. 1, 1987. Three days later, a magnitude 5.6 aftershock hit on a different fault. That aftershock killed one person, twisted several chimneys and broke windows. Damage was reported in Whittier, Pico Rivera, Los Angeles and Alhambra. 
 
 Which fault is ripe for the Big One? 
 
 The southern San Andreas is "most likely to host a large earthquake," the forecast said. This section of the fault has a 19% chance of having a magnitude 6.7 or larger earthquake in the next 30 years centered in California's Mojave Desert. 
 
 Why do scientists use 30 years to forecast the chance of the next big earthquake? 
 
 It's the typical term a homeowner is paying off the mortgage on a house. 
 
 Does the forecast help scientists predict the exact date and time the next Big One will hit? 
 
 No. Experts say they can't predict when exactly the next big earthquake will come, but they're getting better at modeling the possibilities. Tuesday's forecast considered more than 250,000 fault-based earthquakes; the previous forecast considered about only 10,000. The latest calculations use about 300 earthquake faults; the 2007 forecast relied on 200 faults, and the original 1988 report was based on only 16. 
 
 Why does this matter to me? 
 
 "The message to the average citizen hasn't changed. You live in earthquake country, and you should live every day like it's the day a Big One could hit," Field said. "But what it really does help us do is refine our estimates for those designing critical facilities: hospitals, schools, bridges." 
 
 "We are fortunate that seismic activity in California has been relatively low over the past century. But we know that tectonic forces are continually tightening the springs of the San Andreas fault system, making big quakes inevitable," Tom Jordan, director of the Southern California Earthquake Center and a coauthor of the study, said in a statement. 
 
 ron.lin@latimes.com 
 
 rosanna.xia@latimes.com ||||| LOS ANGELES (AP) — Scientists are virtually certain that California will be rocked by a strong earthquake in the next 30 years. Now they say the risk of a mega-quake is more likely than previously thought. 
 
 FILE - This Jan. 17, 1994 file photo shows several trailer homes that were burned to the ground at the Oak Ridge Trailer Park in Sylmar, Calif., after a magnitude-6.7 earthquake struck southern California.... (Associated Press) 
 
 FILE - In this Jan. 17, 1994 file photo, the covered body of Los Angeles Police Officer Clarence Wayne Dean, 46, lies near his motorcycle which plunged off the State Highway 14 overpass that collapsed... (Associated Press) 
 
 FILE - In this Jan. 17, 1994 file photo, a Los Angeles police officer stands in front of the Northridge Meadows Apartment building, after the upper floors of the structure collapsed onto the open garages... (Associated Press) 
 
 The chance of a magnitude-8 quake striking the state in the next three decades jumped to 7 percent from 4.7 percent, mainly because scientists took into account the possibility that several faults can shake at once, releasing seismic energy that results in greater destruction. 
 
 While the risk of a mega-quake is higher than past estimates, it's more likely — greater than 99 percent chance — that California will be rattled by a magnitude-6.7 jolt similar in size to the 1994 Northridge disaster. The chance of a Northridge-size quake was slightly higher in Northern California than Southern California — 95 percent versus 93 percent, according to a report released Tuesday by the U.S. Geological Survey. 
 
 "California is earthquake country, and residents should live every day like it could be the day of a big one," USGS geophysicist and lead author Ned Field said. 
 
 The latest seismic calculations largely mirror previous findings issued by the USGS in 2008. Back then, scientists also determined that California faced an almost certain risk of experiencing a Northridge-size quake. 
 
 The new report included newly discovered fault zones and the possibility that a quake can jump from fault to fault. Because of this knowledge, the odds of a catastrophic quake — magnitude 8 or larger — in the next 30 years increased. 
 
 There is a 93 percent chance of a magnitude 7 or larger occurring over the same period and a 48 percent chance of a magnitude 7.5 — similar to previous estimates. 
 
 Thousands of quakes every year hit California, sandwiched between two of Earth's major tectonic plates, the Pacific and North American plates. Most are too small to be felt. 
 
 Of the more than 300 faults that crisscross the state, the southern segment of the San Andreas Fault — which runs from central California to the Salton Sea near the U.S.-Mexico border — remains the greatest threat because it hasn't ruptured in more than three centuries. 
 
 The report found there is a 19 percent chance in the next 30 years that a Northridge-size quake will unzip the southern section compared to a 6.4 percent chance for the northern section, partly because it last broke in 1906. 
 
 The southern San Andreas is "ready to have an earthquake because it's really locked and loaded," Field said. 
 
 The report is a forecast, but it is not a prediction. Experts still cannot predict exactly where or when a quake will hit anywhere in the world. 
 
 In recent years, the USGS and several universities have been testing an early warning system designed to detect the first waves of a jolt and send out an alert before the slower-moving damaging waves. Proponents have said a few seconds of notice can allow trains to slow down, utilities to shut off gas lines and people to duck for cover. The public alert system — still in pilot phrase — needs more funding before it can be rolled out statewide. 
 
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– The latest US Geological Survey forecast for California doesn't contain much that will reassure people worried about earthquakes—unless they find fatalism reassuring. The report says there is not only a 99% chance of a 6.7 magnitude or greater quake like the 1994 Northridge one hitting the state in the next 30 years, the chances of a mega-quake of 8 magnitude or greater are significantly higher than earlier thought, the AP reports. "California is earthquake country, and residents should live every day like it could be the day of a big one," says lead author Ned Field, a USGS geophysicist. He says that out of the state's hundreds of faults, the "locked and loaded" San Andreas Fault is still the biggest threat. Field says the odds of a mega-quake in the next three decades have risen to 7% from 4.7% since the 2008 forecast because scientists now have a better understanding of the fault system and how quakes can jump from fault to fault, the Los Angeles Times reports. In California, "it has become increasingly apparent that we are not dealing with a few [well-separated] faults, but with a vast interconnected fault system," he says. The USGS still can't predict exactly when an earthquake will hit, but it's testing an early warning system that will give authorities vital seconds to do things like shut off gas lines and slow down trains, the AP reports. (Another federal study warns that small, fracking-linked quakes in Oklahoma and Kansas are making a big one more likely.)