Scientists from the US Geological Survey have determined that the gravitational pull of the moon and the sun can cause low-frequency quakes deep along fault lines, including the San Andreas Fault, offering an opportunity to scrutinize processes deep underground and possibly predict major quakes.

Low-frequency earthquakes are those with magnitudes of less than 1.0 on the Richter scale, occurring at depths between 15-30 kilometers underground.

Those tremors appear to be triggered by gravitation compressing and stretching the Earth’s crust, researchers outlined in a study published in the Proceedings of the National Academy of Sciences.

Having analyzed some 81,000 low-frequency earthquakes on the San Andreas fault, between 2008-2015, the scientists determined that quakes of this nature are most likely to happen during the two-week waxing lunar cycle.

As noted by Nicholas van der Elst, one of the authors of the study, the quakes are more intense during this period, “the moon, when it`s pulling in the same direction that the fault is slipping, causes the fault to slip more – and faster.”

Researchers pointed out that the findings allow the study of deeper levels of the fault that have been previously unreachable.

Given that tremors from a deep fault are normally transferred to shallow regions, information about the intensity of low-frequency quakes is useful for predicting possible major quakes, David Shelly, a USGS seismologist and co-author of the study, said.

“So if all of a sudden, we saw that the deep part of the fault was slipping a huge amount, it might be an indication that there was an increased chance of having an earthquake come at the shallower part of the fault,” he said.

Shelly noted that a dependable earthquake notifying system is yet to be created.
“We don`t quite know yet what it`s going to mean in the long term, whether it`ll result in some sort of warning that an earthquake is coming. We`re going to have to monitor it for a lot longer.”

The San Andreas Fault, stretching roughly 800 miles through California, lies at a boundary between two slowly-moving tectonic plates. It accumulates stress over time and releases it, according to current research, at unpredictable times. The last time stress was released in 1857, causing a destructive earthquake.

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