Study: Earthquakes in California Can Occur Deeper, Raising Possibility of More Powerful Quakes

The San Andreas fault in California. Researchers have found that tidal forces play a role in the timing of small, deep earthquakes along the fault. (Credit: U.S. Geological Survey)

The San Andreas fault in California. Researchers have found that tidal forces play a role in the timing of small, deep earthquakes along the fault. (Credit: U.S. Geological Survey)

Scientists in California have found that earthquakes can occur much deeper below the Earth’s surface than originally believed, a discovery that alters their understanding of seismic behavior and potential risks.

Seismologists have long believed that earthquakes occur less than 12 to 15 miles underground in the planet’s brittle, rocky crust. But new research has found evidence of quakes deeper than 15 miles under the surface, in the upper mantle, an area where the rock is so hot that it is no longer brittle but creeps, moving around like an extremely hard honey.

Three scientists at Caltech in Pasadena studied data collected over six months from 5,000 state-of-the-art sensors installed in Long Beach atop the Newport-Inglewood fault, one of the most dangerous in the Los Angeles Basin and which caused the magnitude 6.4 Long Beach earthquake of 1933.

Caltech seismology professor Jean Paul Ampuero, one of three authors of the study that was published Thursday in the journal Science, said the research raised the possibility that the Newport-Inglewood fault and others, such as the San Andreas, could see even more powerful earthquakes than expected. The earthquakes he and his colleagues studied were so deep that they were not felt at the surface by conventional seismic sensors.

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