High tidal stress may trigger big quakes

Los Angeles Times (TNS)

Why small earthquakes stay small, while others grow into monsters is one of the most enduring mysteries in earthquake science.

A group of researchers offered a partial, but tantalizing answer this month: The moon and big tides.

The scientists zeroed in on times of high tidal stress, which can occur twice a month, during the full moon and the new moon. During these moments, high tides are at their highest — flooding the tallest reaches of a beach — and about six hours later, low tides are at their lowest for the month, with seawater retreating to the farthest point toward the ocean.

This produces massive movement of ocean water and produces high tidal stress. And that tidal stress can change the stress on the fault, and, the scientists suggest, help push small earthquakes that happen to grow into very large earthquakes.

“When tides are very large, small earthquakes tend to grow,” said Satoshi Ide, lead author of the report and professor of seismology at the University of Tokyo.

“This suggests that the probability of a tiny rock failure expanding to a gigantic rupture increases with increasing tidal stress levels,” Ide and his co-authors wrote in the report, published in the journal Nature Geosciences.

Or, put another way, the tidal forces give a slight nudge to a fault on the cusp of rupturing.

“It could be just the amount of stress that is the ‘straw that breaks the camel’s back,’ so to speak,” said Nicholas van der Elst, a U.S. Geological Survey geophysicist. “So it makes sense that an earthquake would be more likely to happen, and coalesce into a larger earthquake, if there is just a little, additional, push.”

As a result, when tidal forces are at their largest, under this idea, “earthquakes have a slight tendency to grow larger than they would otherwise,” van der Elst said.

Examples of such earthquakes are the magnitude 9.1 Indonesia earthquake in 2004 and magnitude 8.8 earthquake in Chile in 2010. Both produced damaging tsunamis, occurred around the time of a full moon, close to the peak time of tidal stress, the study said.

The research adds to a growing body of research investigating how tidal forces can affect the earth’s movement. Tidal forces — which besides the oceans also affect solid rock — are also believed to be related to small tremors deep underground along the central San Andreas fault in California Monterey County, according to a study earlier this summer co-authored by van der Elst.

Tidal forces are not the central reasons why an earthquake ruptures. The primary cause of earthquakes is the Earth’s moving tectonic plates, which are constantly grinding against each other. Between the tectonic plates, strain builds up on faults until the pressure is released suddenly by an earthquake.

Ide noted that many earthquakes will still happen when tidal stress is low.

“Earthquakes are nearly a random process,” Ide said. “Tidal forces are just a factor in a complex process. There are a lot of other factors.”