SAN ANDREAS FAULT CREEP IN RESPONSE TO THE 2019 RIDGECREST EARTHQUAKE SEQUENCE

On 4 July 2019, the Ridgecrest earthquake sequence began with a M_w 6.4 foreshock. Thirty-four hours later the M_w 7.1 mainshock occurred. The epicenters for both the foreshock and mainshock occurred in the area between the Garlock Fault and the Coso Geothermal Field. The sequence caused stress to be regionally redistributed. Several studies noted that part of the Garlock Fault creeped in the days after the mainshock.

Along the central San Andreas Fault, where aseismic creep is naturally occurring, we calculated the Coulomb stress change associated with the Ridgecrest earthquake sequence. Continuous GPS displacement time series data from 11 plate boundary observatory (PBO) stations adjacent to the creeping section of the fault were used to determine whether the creep rate had changed. The GPS time series data were used to estimate the mean creep rate of two different time spans (month and year). We then calculated the change of the creep rate one month and one year before the Ridgecrest earthquake sequence to after. Our results show no detectable change in the creep rate within instrument uncertainty, suggesting that the creep rate likely did not change.

We found a maximum Coulomb stress decrease of 2 kPa due to the Ridgecrest event along the Parkfield section of the central San Andreas Fault. The lowest Coulomb stress change calculated showed a negligible decrease (less than 0.5 kPa) in the San Juan Bautista section, which is the north end of the creeping section. These results suggest that: (1) there could be a critical value of Coulomb stress change that must be achieved to initiate a detectable difference in the creep rate. This value was not achieved by the Ridgecrest event. Or, (2) the change in the creep rate is below the GPS instrument detection level, indicating any change was not resolvable by our methods. This work highlights the sensitivity of the San Andreas Fault’s creeping section from regional stress induced by a significant earthquake.