Much like ocean waters rise and fall during daily tidal cycles, the Earth’s crust bows outward, then relaxes every day, pulled by the Moon. The ocean also rises and falls on 2-week tidal cycles when the pull of the Sun reinforces the tug of the Moon, a phenomenon also observed in Earth’s crust.
According to new measurements, this bulging of the crust every 2 weeks cyclically increases the numbers of small earthquakes that take place deep in the San Andreas Fault. These temblors occur between 15 and 30 kilometers underground and release too little energy too slowly to be felt by anyone on the surface.
What’s more, the pattern of these slow-moving earthquakes reveals something unexpected—that they “are most common during the week when tides are growing rather when tides are the biggest,” said Nicholas van der Elst, a geophysicist at the Earthquake Science Center at the U.S. Geological Survey (USGS) in Pasadena, Calif. Van der Elst and his team published the findings on 18 July in the Proceedings of the National Academy of Sciences of the United States of America.
The new study shows no connections between these “low-frequency earthquakes,” as researchers refer to them, and the sudden, ground-shaking earthquakes that typically originate much closer to the surface and can cause widespread destruction and death. However, their very presence reveals information about the deep mechanics of the fault.
Recording Fault Slippage
Despite their “low-frequency” label, these earthquakes are detected more frequently than any other class of quakes near Parkfield, Calif.—the section of the San Andreas Fault observed in the study. More than a thousand of these low-level seismic slips register every day at Parkfield, but because of their depth and low magnitudes (<1) people can’t feel them. Scientists refer to them as “low frequency” because they are characterized by slow seismic waves of compression and expansion analogous to low-frequency (i.e., low-pitch) sounds.
Seismologists from the USGS and Northern California Seismic Network recorded these slow quakes near Parkfield with sensitive seismic equipment placed in deep holes. They set out to record small tremors of slightly greater magnitude in areas where the fault is already known to churn with deep activity.
The low-frequency quakes became apparent only when scientists used algorithms to extract weak signals from background noise. By contrast, in the relatively shallow areas where the “big one” might hit, the fault moves far less, allowing stress to build up until a large quake occurs, said van der Elst.
Quake Tally Rises as Biweekly Tide Waxes
Finding that the crustal tide affects deep, little quakes isn’t entirely new. Scientists previously looked into the frequency of earthquakes during daily Earth tides, which sometimes lift rock by a few centimeters over a 12-hour cycle akin to the timing of ocean tides.
Although those past findings proved inconclusive in the shallow regions of the fault where bigger earthquakes strike, results showed that deep, low-frequency earthquakes take place 50% more often when the daily crustal tides hit their maximum heights.
To learn if the total number of small, deep earthquakes rises and falls also with the fortnightly tide, van der Elst and his colleagues returned to data from Parkfield to analyze 81,000 low-frequency quakes and found that their abundance correlated statistically with the 2-week tidal pattern.
It turns out that low-frequency earthquakes are 10% more common during weeks when the biweekly Earth tide is growing. Van der Elst and his team identified two time periods each month when the low-level, deep quakes were most common, and both correlated with the two waxing cycles of the fortnightly tide, when tension builds in the fault, rather than the tidal peaks, when tension is greatest.
The scientists suggest why the number of quakes doesn’t surge again during the waning phase even though the tidal contribution to stress is just as large in that period: According to van der Elst, the boost in earthquake numbers during the waxing period likely relieves fault tension so that little is left to be released during waning. However, during the lull between waxing periods, the movement of the fault’s adjoining tectonic plates builds stress up again. Then, “when the tide begins to rise,” said van der Elst, “the fault is that much closer to failure and produces a bigger crop of low-frequency earthquakes early on.”
Peering into a Fault’s Depths
Although the results “are not directly relevant to forecasting damaging earthquakes, [they] open a new window to deep faulting, which remains mysterious in many ways,” commented John Vidale of the University of Washington in Seattle. Vidale did not participate in the study.
The results may provide valuable insights into the working of the deep fault, according to van der Elst. For example, only a weak fault would respond to the small amount of stress caused by tides, and the sensitivity to tides at deep levels might indicate the presence of pressurized fluids that lubricate the fault far below ground, he suggested.
Also, “variations in fault response over the fortnightly cycle tells you something about how long it takes for the fault to reaccumulate stress after an episode of low-frequency earthquakes,” said van der Elst. Future studies will explore these and other mechanisms.
—Amy Coombs, Editorial Intern