Preliminary seismological analyses of 2019 Mw 6.4 Searles Valley and Mw 7.1 Ridgecrest California earthquakes

Chen Ji, Ralph J. Archuleta, Kenichi Tsuda, & Scott J. Condon

Published August 15, 2019, SCEC Contribution #9718, 2019 SCEC Annual Meeting Poster #259

We have studied the co-seismic rupture processes of the 2019 Mw 6.4 Searles Valley and Mw 7.1 Ridgecrest earthquakes by joint inverting seismic waveforms recorded by local and teleseismic stations, and co-seismic geodetic displacements estimated with high rate GPS sensors. We find that the observation of the Searles Valley earthquake can be reasonably well explained using the rupture on a single subvertical fault orienting N223E. The inverted rupture scenario features long rise time, slow rupture velocity, long rupture duration, low average static stress drop. The Ridgecrest earthquake ruptured a 50 km long curved fault plane in an average rupture velocity of 2 km/s. Most slip occurred on two asperities centering northwest and southeast of its hypocenter. Besides rupture velocity, the weighted average fault slip and rise time are also similar to the estimates of 1999 Mw 7.1 Hector Mine earthquake, which also ruptured bilaterally on multiple fault segments. While the failure of the northwest asperity can be viewed as continuous rupture propagating from the network hypocenter, the initiation of the southeast asperity occurred at a site distant from the inverted rupture front at the detected triggering time. This interesting result motivates us to calculate the dynamic stress perturbation excited by the rupture that occurred before this second initiation. We found that at this site the positive right-lateral shear stress perturbation emerges after P wave arrives, and its amplitude increases with time to a peak of 0.26 MPa right before the SH wave arrives at 2.5 s. The oscillating stress perturbation carried by S wave has larger amplitude but is frequency dependent. For frequency smaller than 1 Hz, the predicted dynamic Coulomb stress perturbation is 0.24 MPa (apparent friction coefficient of 0.4) at the detected rupture initiation time of 4 s. A similar calculation also finds that the Searles Valley earthquake produced larger negative dynamic Coulomb stress perturbation at same site, which prevents immediate fault failure. However, the final static Coulomb stress perturbation is -1.0 MPa. Reconciling these two results suggests that significant fault weakening occurred within the 34 hours between these two earthquakes.

Key Words
Ridgecrest earthquakes, dynamic interaction

Citation
Ji, C., Archuleta, R. J., Tsuda, K., & Condon, S. J. (2019, 08). Preliminary seismological analyses of 2019 Mw 6.4 Searles Valley and Mw 7.1 Ridgecrest California earthquakes . Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes