The black dots in the figure above represent epicenters of all recorded earthquakes in California over the last 40 years (Link). The background shows our peak-to-peak seasonal dilatational strain anomalies (8/2017-2/2017).

In addition to steady-state tectonic deformation, there are seasonal strain anomalies in California. Analyses suggest that such transient strain anomalies may have slightly modulated the timing of earthquakes in California (Johnson et al., 2017; Kraner et al., 2018; Kim et al., 2021b).

What causes these seasonal variations in crustal deformation?

California has a Mediterranean climate: wet winters and dry summers! Additionally, the state underwent severe drought between 2012 and 2015, followed by heavy precipitation in 2017 and 2019. Depending on precipitation patterns, the amount of water on the surface (e.g., soil moisture and accumulated snow) varies over time. For instance, during the three years of severe drought, the western U.S. lost ~ 230 gigatons of surface water (Borsa et al., 2014)! The Earth's crust responds to these variations in surface loads (e.g., Argus et al, 2014, 2017; Kim et al., 2021a). 

Model horizontal displacements relative to the North American frame obtained from smoothed fit to seasonal components of GNSS data, with dilatational strain anomalies plotted in the background for the winter and summer of 2017 (Kim et al., 2021a).

Implications on seismic activities

A time series of the seasonal stress changes on faults (pertaining to the seasonal crustal deformation) in the Ridgecrest area surrounding the epicenters of the 2019 Ridgecrest earthquakes.

The seasonal Coulomb stress changes on strike-slip faults in Ridgecrest peaked every May and June from 2007, including June 2019, during the month prior to the 2019 Ridgecrest earthquakes. Furthemore, our statistical analyses support the hypothesis that the background earthquakes may have been modulated by seasonal stress changes there (Kim et al., 2021b)!

We are currently setting up an automated program that computes time-dependent crustal deformation and associated Coulomb stress changes on faults using CRON.

We plan to upload our monthly deformation solutions on a website. Stay tuned!