Complex long-range stress transfers between the Mw7.1 Ridgecrest earthquake, Long Valley Caldera magmatic activity and the Mw6.5 Monte Cristo Range earthquake

Abstract:

The July 6 2019 Mw 7.1 Ridgecrest earthquake triggered a wide-spread seismic and aseismic activity across the Eastern California Shear Zone. Remote aftershocks included three Mw>5 earthquakes, among which the May 15 2020 Mw6.5 Monte Cristo Range earthquake, which occurred 250 km to the northeast, is the strongest and most distant. Here we investigate the co - and post-seismic response of the Long Valley (LV) caldera to the Mw7.1 Ridgecrest earthquake, and assess the triggering effect of the two on the Mw6.5 Monte Cristo Range earthquake. To this end, we analyze daily GNSS solutions from a dense geodetic network composed of 26 stations located within the LV area, located 230 km from Ridgecrest and 100 km from the Mw6.5 epicenter. Our results clearly indicate a complex transient post-seismic deformation within the caldera and along its rims. The transient initiated on July 6, continued for approximately 240 days, and resulted in surface offsets that greatly exceed the co-seismic offsets in LV. Observed subsidence and horizontal offsets indicate a deflation source within the caldera, whose deformation was accompanied by a significant increase in microseismic activity. Given the smallness of the Ridgecrest coseismic stress perturbation in the LV area (< 3 KPa), it is likely that the post-Ridgecrest seismicity increase there is not directly caused by that stress change, but instead is the result of the magmatic activity within the caldera. Interestingly, the seismicity response is delayed by about 75 days relative to the initiation of the aseismic transient, possibly indicating slow aseismic propagation of dikes towards seismically active fault portions in LV. Finally, the timing and proximity of the 2020 Mw6.5 to the 2019 LV transient suggest a causal link between the two. We invert the geodetic data to recover the sources of aseismic deformation, estimate the cumulative geodetic moment associated with the 2019 LV transient, and investigate the possible role of the 2019 LV transient in transferring stresses to 2020 Mw6.5 area, as well as to other faults hosting M>5 remote Ridgecrest aftershocks.