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Over 1,000 shallow earthquakes detected in California

By deploying a series of seismic arrays in California’s Long Beach and Seal Beach, a team of researchers from the California Institute of Technology (Caltech) managed to detect over a thousand tiny earthquakes during an eight-month period, most of them located at surprisingly shallow depths of less than two kilometers below the surface.

These findings suggest that the region’s portion of the Newport-Inglewood fault splays widely at these shallow depths, providing evidence for the first time that it may spread out by more than a kilometer.

To detect these shallow earthquakes, the scientists used three dense nodal seismic arrays containing approximately 100 sensors per square kilometer. Since the constant hum of urban areas can make it challenging to identify small-magnitude earthquakes, the data was recorded during the night, allowing the detection of 1262 nighttime earthquakes, many of which were previously undetected by the larger regional network.

According to the experts, the massive 6.4 Long Beach earthquake from 1933 has likely partly ruptured on the Newport-Inglewood fault. Yet, shallow seismicity as the one recently detected indicates that there are many possible paths for a rupture to propagate to the surface.

In this densely populated area – part of the greater Los Angeles metropolitan region – shallow seismicity could have important implications for earthquake hazard planning. The regulatory zone surrounding the surface traces of active faults in California – known as the Alquist-Priolo zone and spanning a minimum of 50 feet, or 0.01 kilometers – should be re-considered by experts and policymakers.

“Our results suggest the zone of high hazard at the surface may therefore be much wider than the Alquist-Priolo zone indicates,” the study authors explained.

To better characterize seismic hazard, other faults in southern California should be examined for such shallow earthquakes. “The Newport-Inglewood fault along its entire length, as well as the whole Los Angeles Basin, could benefit from such studies. This would help to see if there are faults that have not been detected with the permanent seismic network or through geologic mapping,” concluded lead author Yan Yang, a seismologist at Caltech.

The study is published in the journal Seismological Research Letters.

By Andrei Ionescu, Staff Writer

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