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 . The chance of a magnitude-8 quake striking the state in the next three decades jumped to 7 per cent from 4.7 per cent , 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 per cent chance - that California will be rattled by a magnitude-6.7 jolt similar in size to the 1994 Northridge disaster . Brace : A new U.S. Geological Survey report has increased the likelihood of a magnitude-8 earthquake hitting California over the next 30 years . Above , a photo of an overpass that collapsed in the 1994 magnitude-6.7 Northridge earthquake . The chance of a Northridge-size quake was slightly higher in Northern California than Southern California - 95 per cent versus 93 per cent , 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 per cent chance of a magnitude 7 or larger occurring over the same period and a 48 per cent 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 has n't ruptured in more than three centuries . The report found there is a 19 per cent chance in the next 30 years that a Northridge-size quake will unzip the southern section compared to a 6.4 per cent 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 can not 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 .
Northridge report increases chances of magnitude-8 earthquake in next three decades to 7 per cent , up from 4.7 per cent .