Repeating nontectonic seasonal stress changes and a possible triggering mechanism for the 2019 Ridgecrest Earthquake Sequences in California

Abstract:

We invert cGPS displacements, obtained from position time series of Network of the Americas (NOTA), to quantify horizontal non-steady-state strain anomalies on various length-scales within the plate boundary zone in the western United States. Our long-wavelength strain solution between 2007 and 2020 highlights remarkable seasonal strain anomaly patterns, as well as multi-annual variations of strains during drought and heavy precipitation years (Kim et al., 2020). These long-wavelength strain changes are attributable to the crust’s elastic response to hydrologic loads (e.g. Argus et al., 2014; 2017; Borsa et al., 2014; Kim et al., 2020). However, this solution requires a heavy damping in our inversion of horizontal displacements, which possibly smooths out and masks physically meaningful local features in the horizontal transient strain field. Applying a lower damping, we produce a higher-amplitude, shorter-wavelength horizontal transient strain solution. This lower damping leads to a closer fit to the cGPS, but larger posterior errors in the model strain. However, the shorter-wavelength, higher-amplitude seasonal strain changes repeat reliably over space and time for the duration of our analysis (14 years). For instance, in the region of 2019 M 7.1 Ridgecrest Earthquake, our shorter-wavelength seasonal strain solution predicts that positive Coulomb stress changes recur every late spring, including in June 2019, a month prior to the Ridgecrest event. Although the source of the short-wavelength seasonal stress changes remains enigmatic, this result indicates that the large event may have been triggered by seasonal stress changes (Fig 1a, 1b, 1c). The shorter-wavelength seasonal stress changes may provide important insights into mechanisms that influence seismicity rate changes in California (Christensen et al., 2007; Johnson et al., 2017; Kreemer et al., 2018; Kraner et al., 2018). We also quantify the full horizontal transient fields in the regions near Ridgecrest following the July 2019 sequence.