Repeating Nontectonic Seasonal Stress Changes and a Possible Triggering Mechanism of the 2019 Ridgecrest Earthquake Sequence in California

초록

Here we characterize the 13-year history of nontectonic horizontal strain anomalies across the regions surrounding Ridgecrest, CA, using cGPS data from January 2007. This time-dependent model reveals a seasonality in the nontectonic strain anomalies and the associated Coulomb stress changes of similar to +/- 0.5-2 kPa. In the area surrounding the epicenters of the 2019 Ridgecrest earthquake sequence of July, we find that the seasonal preseismic Coulomb stress changes peaked every early summer (May and June) during the last 13 years including during June 2019, a month prior to the large events. In addition, our statistical tests confirm that more strike-slip earthquakes (M-w >= 2) occur during times when seasonal stress changes are increasing on right-lateral faults in comparison with times when stresses are decreasing. These results suggest that the timing of the 2019 Ridgecrest earthquakes may have been modulated by nontectonic seasonal stress changes. The dynamic source of the seasonal nontectonic strain/stress anomalies, however, remains enigmatic. We discuss a possible combination of driving forces that may be attributable for the seasonal variations in nontectonic strain/stress anomalies, which captured in cGPS measurements. Plain Language Summary Global navigation satellite system (GNSS) has provided precise records of the Earth's surface motions, from which crustal deformation patterns can be estimated. In July 2019, two large earthquakes (magnitudes of 6.4 and 7.1) struck Ridgecrest, California. Using GNSS data, we estimate a 13-year history of horizontal deformation patterns and associated stress loads on faults in the region surrounding Ridgecrest leading up to the 2019 Ridgecrest sequence. We detect seasonal variations in the horizontal deformation patterns in the Ridgecrest region, and we find the associated seasonal stress loads peaked every early summer during the last 13 years including during June 2019, a month prior to the 2019 Ridgecrest earthquakes. We further investigate a possible correlation between the seasonal stress changes and timing of small earthquakes by carrying out statistical analyses. This additional test reveals that more earthquakes occur when the seasonal stress is loading the faults than when it is unloading them. These results suggest that the timing of earthquakes, including the 2019 Ridgecrest events, may have been modulated by seasonal stress changes. There is a strong need to continue gathering GNSS data, together with other geodetic observations, for the purpose of the timedependent seismic hazard modeling.

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