Earthquakes occur when the tectonic plates of the earth's crust shift, jolting past each other in a release of built-up tension. However, other natural forces can also influence seismic activity: Hydrological dynamics, like changes in groundwater and snowpacks, in particular, put pressure on faults. A new study from Caltech finds that a higher rate of change in groundwater levels leads to a noticeable increase in seismic activity.
The research was conducted in the laboratory of Jean-Philippe Avouac, Earle C. Anthony Professor of Geology and Mechanical and Civil Engineering, and is described in a paper appearing in the journal Science Advances on March 25.
The study examined seismic activity in various regions of California—Los Angeles, the Bay Area, the Imperial Valley, the Eastern Sierra, and so on—from 2006 to 2022. The state's rainfall levels vary by region; for example, Northern California receives significantly more precipitation than Southern California, leading to more pronounced seasonal changes in groundwater levels.
Utilizing new data analysis methods, the researchers saw that regions experiencing more dramatic changes in groundwater levels exhibit a larger seasonal variation of seismic activity. In Northern California in particular, groundwater changes correlated with an increase in seismic activity of up to 10 percent. Notably, these hydrological changes did not instantaneously lead to earthquakes; the peak increase in seismic activity was delayed by about half a month after the peak groundwater changes. The work gives insights into the factors that lead earthquakes to nucleate.
The study also gives insights into how human operations, such as drawing out groundwater for consumption or pumping for oil, can modulate the level of seismic activity.
"This work gives us a new opportunity to better constrain earthquake nucleation models, which is helpful for understanding the physics of earthquakes and factors that might trigger them," says former Caltech graduate student Krittanon (Pond) Sirorattanakul (PhD '24), the study's lead author. "These constraints are particularly useful information when developing new oil or gas fields, which often involves removing subsurface fluids atop faults."
While earthquakes cannot be predicted yet, the Avouac laboratory aims to understand the mechanics of faults and how they respond to variations in stress. The new study helps determine the most important parameters that contribute to earthquakes, which improves forecasting models. Forecasting, Avouac emphasizes, is not prediction; rather, it is a measurement of seismic trends and averages over time.
The study also looked at another possible factor influencing earthquakes—the earth's tides. As the moon orbits the earth, its mass exerts a gravitational pull on our planet, causing it to stretch and deform ever so slightly, a process known as tidal forcing. Avouac and Sirorattanakul found that although tidal stress upon the faults is on the same order of magnitude as seasonal stress due to hydrology, tides do not notably increase earthquake activity. The team suggests that the timescale of force matters. Tidal forcing oscillates on a roughly 12-hour cycle, whereas groundwater accumulation and dissipation occurs on the order of an entire year.
"The simplest explanation is that it takes time for an earthquake to nucleate, much more than the typical twice-daily tidal cycle," Avouac says. "Tidal stress variations simply average out in the eyes of the fault."
There remains considerable interest in detecting how faults respond to stress variations due to tides or hydrology. Certain lab experiments have shown that materials that have been pushed close to the brink of failure are more susceptible to harmonic oscillations—regularly waxing and waning forces like tides or hydrological seasonal loading. Some researchers argue that the seismicity response to small harmonic stresses becomes more consequential for faults that are "critically stressed" and therefore more likely to cause large earthquakes.
The team plans to continue work into understanding the properties underlying fault mechanics and improving models of the seismic cycle.
The paper is titled "Seismic Rhythms: Earthquake Response to Tectonic, Hydrological, and Tidal Forcing in California." Sirorattanakul and Avouac are the study's authors. Funding was provided by the National Science Foundation and the Center for Geomechanics and Mitigation of Geohazards.
