Seismic activity in California varies with the seasons

Earthquakes are a result of the shifting of the Earth's tectonic plates, which create jolts as they move past one another during the release of accumulated stress. In addition to tectonics, other natural phenomena can significantly affect seismic activity. In particular, hydrological dynamics, such as fluctuations in groundwater and snowpack levels, exert stress on geological faults. A recent study from the California Institute of Technology (Caltech) indicates that variations in groundwater levels correlate with an increase in seismic activity, particularly during wet seasons. This research is documented in the journal Science Advances.

California's shifting groundwater and quakes

Conducted in the laboratory of Jean-Philippe Avouac, Earle C. Anthony Professor of Geology and Mechanical and Civil Engineering, this study examined the relationship between seismic activity and groundwater across various California regions, including Los Angeles, the Bay Area, the Imperial Valley, and the Eastern Sierra, over the time frame from 2006 to 2022. Due to geographical diversities, rainfall levels differ considerably throughout the state; for instance, Northern California receives substantially more precipitation than its southern counterpart, resulting in more pronounced variations in groundwater levels with the seasons.

Employing innovative data analysis techniques, the researchers discovered that areas with more significant shifts in groundwater levels experienced distinct seasonal variations in seismic activity. In Northern California, for example, changes in groundwater were linked to an uptick in seismic events of up to 10%. It is crucial to note that these hydrological fluctuations did not lead to immediate seismic activity—the maximum increase in earthquakes occurred approximately half a month following the peak changes in groundwater. This research sheds light on the underlying factors that contribute to the initiation of earthquakes.

An accompanying animation depicts how stress rates on faults in both Northern and Southern California over several decades are affected by groundwater changes. In the northern regions, which experience broader seasonal variability in groundwater levels, the changes in seismic stress are more pronounced compared to the drier regions of Southern California.

Additionally, this study offers insights into how human activities, such as groundwater extraction for consumption or oil extraction, can influence 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," states Krittanon (Pond) Sirorattanakul, Ph.D., the lead author of the study and a former graduate student at Caltech. "These constraints are particularly useful when developing new oil or gas fields, which often involves the removal of subsurface fluids located above faults."

From forecasting efforts to tidal forces

While the prediction of earthquakes remains unattainable, the Avouac laboratory is focused on comprehending the mechanics of faults and their responses to stress variations. This new study aids in identifying the most significant parameters that contribute to earthquakes, thus enhancing models for forecasting seismic activity. As Avouac explains, forecasting does not equate to prediction; instead, it involves a systematic measurement of seismic trends and averages over time.

Moreover, the study also explored another potential factor influencing seismic events: the Earth's tides. The gravitational pull from the moon's orbit around Earth causes slight stretching and deformation of our planet, a phenomenon known as tidal forcing.

Avouac and Sirorattanakul found that while the tidal stress exerted on faults is comparable in magnitude to seasonal hydrological stress, it does not significantly elevate earthquake activity. The researchers suggest that the timescale of the forces is a crucial element; tidal forces fluctuate on a cycle of approximately 12 hours, whereas the accumulation and dissipation of groundwater generally transpire over the course 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 remarks. "Thus, tidal stress variations essentially average out from the perspective of the fault."

Ongoing research remains warranted to better detect how faults respond to stress variations imparted by both tidal and hydrological changes. Some laboratory studies have indicated that materials nearing failure are more reactive to harmonic oscillations—regular, cyclical forces such as tides or seasonal hydrological loads. Certain scholars argue that the response of seismicity to minor harmonic stresses becomes increasingly significant for faults that are 'critically stressed' and, thus, more predisposed to triggering major earthquakes.

The research team intends to persist in their investigations to deepen the understanding of the fundamental properties governing fault mechanics and to refine models of the seismic cycle.

Publication details

Krittanon Sirorattanakul et al, Seismic rhythms: Earthquake response to tectonic, hydrological, and tidal forcing in California, Science Advances (2026). DOI: 10.1126/sciadv.adz5711
Journal information: Science Advances

Key concepts

• earth tides
• tectonophysics
• tides (oceanic)
• time series analysis
• faulting (geologic)
• induced seismicity

Provided by California Institute of Technology