Seismotectonics and Fault Geometries of the 2019 Ridgecrest Sequence: Insight From Aftershock Moment Tensor Catalog Using 3‐D Green's Functions

Abstract

The 2019 Mw 7.1 Ridgecrest earthquake occurred on 6 July, preceded by the Mw 6.4 foreshock on 4 July 2019. These two earthquakes occurred close in space and time with partially overlapping surface ruptures and aftershock patterns, raising the question of the relationship between the two events. Geological surveys and satellite observations provide important constraints on the surface traces of faulting. However, the subsurface fault geometries, which are important for understanding the regional stress field, earthquake initiation, propagation, and termination, are not well resolved. In this study, moment tensor solutions for 256 earthquakes in the 2019 Ridgecrest sequence were determined by waveform inversion using 3‐D velocity model. The obtained moment tensor solutions show rotations of the stresses after mainshock, indicating the ratio of mainshock stress drop to the background stress to be 0.5–0.9. The obtained moment tensor catalog also facilitates a better understanding of the subsurface fault geometries, including (1) splay faults and antithetic faults in the northwest aftershock zone; (2) shallow flower structures near the Mw 7.1 epicenter; and (3) subparallel faults in the southeast aftershock zone. The aftershocks' studies suggest the very complex surface ruptures near the 2019 Ridgecrest Mw 7.1 epicenter are near‐surface features that linked to a simple large throughgoing fault at >5 km depth. We also found that the southeastern‐most aftershocks, which are located less than a kilometer from the Garlock fault trace, have significant different strike directions from that of the Garlock fault, indicating the central Garlock fault remains seismically quiet.

Real-time determination of earthquake focal mechanism via deep learning

“…Comparing to other studies regarding this earthquake sequence 20,23,43 , the predicted focal mechanisms by our FMNet are essentially consistent with previous results. All these results demonstrate that the proposed FMNet enables us to determine the source focal mechanisms effectively.…”

“…After testing, we nd the conventional methods (such as gCAP) also fail to invert the source focal mechanisms with a 1D velocity model for these smaller earthquakes. With a ner 3D velocity structure and an e cient waveform modeling tool 23 , our method shall be extended to smaller earthquakes as long as the synthetic training data can be well modeled to match the real data. Nonetheless, moderate-to-large earthquakes (Mw>5) especially those that might cause destructive damages are our main target of this study.…”

“…For example, we can use source focal mechanisms to characterize faulting geometry and faulting mechanism 19,20 . A group of focal mechanisms can be used to invert the spatial stress eld distribution [21][22][23] . We can also use the focal mechanism of the mainshock to calculate the static Coulomb stress changes [24][25][26] for examining the earthquake triggering theory of the aftershocks [27][28][29] .…”

Multifault Models of the 2019 Ridgecrest Sequence Highlight Complementary Slip and Fault Junction Instability

“…The waveform fittings and uncertainty analysis of the subevent model are shown in Figures S9–S11. Strike angles of all major subevents (E2–E5) are well constrained (Figure S11) to be ~320°, consistent with the throughgoing aftershock band at depth (X. Wang & Zhan, 2020a), but contradicting the curved surface rupture traces (Figure 1a). To reconcile these observations, we set three fault segments for slip inversion, including two major fault segments F1 and F2 and a shallower subparallel fault branch F3 (Figure 3).…”

“…Examinations of different CVM models suggest that the CVM‐S4.26 model well predicts seismograms up to 0.2 Hz (Lee, Chen, & Jordan, 2014). Application of CVM‐S4.26 on automated moment tensor inversion in the Los Angeles and Ridgecrest regions also demonstrated the model's effectiveness (X. Wang & Zhan, 2020a, 2020b).…”