What rock has the biggest impact on the San Andreas? The moon.

Last year the subject of a Hollywood disaster movie, this year the San Andreas Fault is getting attention from geophysicists at NNSA’s Los Alamos National Laboratory.

U.S. Geological Survey and Los Alamos scientists have released a new study detailing how the gravitational tug of the sun and moon produces small, low-frequency earthquakes deep within the fault. These unusual earthquakes, called ‘low-frequency earthquakes’, or ‘tremor’ due to their low rumbling character compared to ordinary earthquakes, were recently discovered along an unexpectedly weak section of California’s 800-mile-long San Andreas Fault. It has been known for several years that these earthquakes surge in response to the daily tides; the new study reports that this surge is strongest during certain phases of the moon, on a cycle that repeats every 14 days.

The geophysicists published their research this month in the Proceedings of the National Academy of Sciences of the United States of America. While these tidally predictable tiny earthquakes can’t be felt by humans on the surface, the observation that the deep roots of seismic faults respond to the tidal cycle opens a window into the workings of plate tectonics.

“These findings provide previously inaccessible information about the San Andreas Fault activity and strength,” said Los Alamos’ Paul Johnson, a geophysicist in Los Alamos’ Earth and Environmental Sciences Division. The continued study of “low-frequency-earthquake and tidal triggering of the San Andreas Fault may give seismologists new warning signals and information about slightly more predictable triggers of quakes to come.”

Large-scale plate tectonic processes are a significant part of the study of geophysics. Besides helping the world predict and prepare for earthquakes, NNSA’s experts in geophysics support national security. From support and training for explosive test ban agreements to ground-based nuclear explosion monitoring, geophysicists by providing technical expertise that directly supports NNSA’s missions.

Learn more about geophysics at Los Alamos, and read the researchers’ paper in full here.