Tides in Earth’s crust trigger small, deep quakes

Study of one portion of the San Andreas fault finds that just a little added stress makes a quake more likely

The subtle changes in stress caused by tides in Earth’s crust can trigger small, deep quakes along a seismically active portion of California’s San Andreas fault, a new analysis suggests.

The same forces of attraction that cause ocean tides also cause tides in Earth’s rocks, says Amanda Thomas, a seismologist at the University of California, Berkeley. And although the continual variations in rock stress associated with those tides are minuscule, they apparently are big enough to trigger small tremors along some faults, Thomas and Berkeley colleagues Robert M. Nadeau and Roland Bürgmann report in the Dec. 24/31 Nature.

Evidence for the claim comes from earthquake data collected near Cholame, Calif., a tiny town near the southeastern end of a portion of the San Andreas fault where small, deep tremors are common. The study focused on a 110-kilometer-square area around the town where more than 1,700 minor quakes occurred between July 2001 and May 2008, Thomas says.

When analyzing these quakes, she and her colleagues found that the mini-temblors were much more likely to occur at times when tidal stresses tended to shear the fault in the direction that it normally breaks — that is, when the Pacific tectonic plate is being pulled to the north-northwest relative to the North American tectonic plate, which lies to the east of the fault. In a sense, the added stress on a fault poised to slip acts like the straw that breaks the camel’s back.  

When tidal stresses act in the other direction and therefore tend to relieve stress on the fault, the frequency of small quakes drops substantially. The fault’s extreme sensitivity to small changes in stress suggests that the deep portions of the fault where these small quakes occur is well lubricated, probably by water that is under exceedingly high pressure, the researchers suggest.  

    Previous studies bolster the notion that small changes in stress along faults can set off earthquakes. In December 2004, seismic waves spreading from the tsunami-spawning temblor that occurred off the west coast of Indonesia triggered a flurry of small quakes near Alaska’s Mount Wrangell, a quarter of the way around the globe (SN: 8/27/05, p. 136).

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