Earth’s crust may feel rigid beneath our feet, but it responds elastically to temperature gradients, atmospheric pressure, and hydrological loads. Everything from heavy rain and snow to human activities like groundwater pumping can deform the crust on seasonal scales. Researchers are particularly interested in such deformations when they occur near plate boundary zones, like in California, where they can influence seismicity rates.
Previous studies using continuous GPS (cGPS) network data have shown that California’s crust compacts during the wet winter months and rebounds during the dry summers and that these seasonal shifts have influenced seismicity. But California also experiences years-long shifts in precipitation patterns, such as droughts, that could influence deformation from year to year.
Kim et al. quantified the influence of multiyear phenomena on the horizontal strain field in California’s crust. The team tapped cGPS data collected between 2007 and 2019 by the Network of the Americas system and evaluated horizontal strain patterns across the San Andreas Fault zone, the Great Valley, the Sierra Nevada, and the Eastern California Shear Zone.
The long-wavelength, transient strain model they produced shows that crustal deformations varied significantly from year to year and with variations in precipitation. Indeed, periods of heavy rain and drought appeared to influence the normal patterns of strain and deformation. During the drought that plagued the state from 2012 through 2015, for example, the average dilation of the crust was less than half that of normal years.
On its own, the team’s long-wavelength model, which required a heavy damping in the inversion of the cGPS data to match the elastic response to loading, would not have detected local anomalies, such as the one that occurred in the Long Valley Caldera, which was the only region east of the Sierra Nevada that saw year-round dilation of the crust during drought times. However, the anomaly was caught by a shorter-wavelength, higher-amplitude model that, when combined with the long-wavelength model, created a more complete picture of the seasonal horizontal strain field.
The study is one of the first to clearly show how multiyear variations in precipitation influence deformation over time and space. The authors suggest that the findings will help researchers better understand how seasonal deformation may change over time and what impact such changes may have on seismicity in active regions like California. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2020JB019560, 2021)
—Kate Wheeling, Science Writer