12:43 GMT +318 January 2020
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    Using advanced processing techniques, the research team found ten times as many earthquakes in a particular area in California as had been previously observed. Many of them were so weak that they couldn’t be identified in real time, but they can crucially often foreshadow a major quake.

    A new study of seismic activity in southern California over a 10-year span has allowed US geologists to dig deeper into the science of earthquakes and improve quake forecasting.

    According to their findings, published in the peer-reviewed journal Geophysical Research Letters in late July, the geologists studied data from the Southern California Seismic Network – which operates 550 seismic monitoring station in the region – to identify earthquakes that cannot be traced in real time as they are too faint.

    The geologists have identified 1.81 million minor earthquakes that occurred in the area between 2008 and 2017 – which is ten times more than seismologists had previously located in this region and for this period of time.

    This data was crunched by a supercomputer, which found that nearly 72 percent of the biggest earthquakes in southern California (with magnitude 4 or higher) had smaller foreshocks in the days and even weeks leading up to the mainshock. According to earlier studies, these precursor tremors – as small as magnitudes 0 and 1 – were observed only before 10 to 50 percent of mainshocks.

    Most of these quakes can be difficult to spot amid the daily noise recorded by seismic monitoring stations, such as vibration from automobile traffic.

    “It's very difficult to unpack what triggers larger earthquakes because they are infrequent, but with this new information about a huge number of small earthquakes, we can see how stress evolves in fault systems,” co-author Daniel Trugman, from Los Alamos National Laboratory, said in a press release.

    These findings do not necessarily mean that a big quake is on its way every time that a weak shaking is observed, but they do suggest that all moderate and large quakes have smaller foreshocks, and that further study is required in that direction.

    “This new information about triggering mechanisms and hidden foreshocks gives us a much better platform for explaining how big quakes get started,” Trugman concluded.

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