California earthquake faults are highly sensitive to solid tides, scientists say
Almost everyone is familiar with the ebb and flow of ocean tides driven by Earth’s daily interactions with the moon and sun. What many people don’t realize is that with the same gravitational drive, land also experiences tides. Land is harder than water and therefore less undulating, but it does move, by as much as 20cm in some places. This is called a solid tide, and it creates stress — including within seismic faults. Is that enough to affect those glitches? In some cases, the answer is yes, scientists say.
The fault wobbled slightly in the Ridgecrest region of southwestern California as tidal stress reached its periodic maximum a year and a half ago, researchers reported at the Seismological Society of America annual meeting in April Series of large earthquakes in July 2019.
After 2018 “tidal modulation signals became unusually strong,” says Eric BossPostdoctoral Fellow, Columbia Climate Institute Lamont-Doherty Earth Observatorywho helped lead the study.
The July 2019 Ridgecrest earthquake included three mainshocks of magnitude 6.4, 5.4 and 7.1, each followed by a series of smaller aftershocks. (Courtesy of the US Geological Survey)
The July 4-5, 2019 sequence included three mainshocks of magnitude 6.4, 5.4, and 7.1, and many perceived aftershocks in subsequent days. In the region where the magnitude 7.1 shock occurred, an increased rate of seismicity was detected during peak tidal stress a year and a half ago. It was the state’s strongest earthquake in 20 years. No one was killed, but that and other major quakes damaged homes and roads and cut off water and gas supplies to thousands.
“This connection does not mean that very small tidal stresses compared to other tectonic stresses trigger earthquakes,” Beauce cautioned. “But it could be a sign that something is starting to happen on the fault zone, which is an indicator of an upcoming earthquake.”
Although researchers have known about these tiny stress changes for more than a century, they have had difficulty extracting the signals from seismic records and determining whether they somehow modulate seismic activity. In the past decade, however, better earthquake detection and analysis techniques have made it possible to search for tidal stress signals by searching catalogs of earthquakes from past years.
Beauce and his colleagues used machine learning algorithms and other techniques to build a rich, high-resolution earthquake catalog covering a decade of microseismic activity in the Ridgecrest region. (Microseismic activity usually refers to earthquakes of magnitude 2.0 or less.)
“There is suggestive evidence that peaks in seismicity occur when tidal stress is greatest,” Bosch said. “But this modulation is weak, and because it’s weak, it’s just a suggestion.”
Other researchers looked at records preceding the deadly 2004 Indian Ocean earthquake and the 2011 Tohoku earthquake and found that increases in seismicity were associated with tidal stress, which occurred decades before the quakes. Some scientists have also been able to produce similar results in laboratory-created seismic experiments.
The tidal discovery has no direct impact on earthquake predictions, Beauce said. “We don’t know if we’re looking at a general phenomenon or just one for the Ridgecrest earthquake,” he said. “I see this as a way to place new observational constraints on the physics of earthquakes, possibly earthquake preparation and nucleation.”
Adapted from a press release from the Seismological Society of America.
