Improved Source Parameter Estimation of Earthquakes in the 2019 Ridgecrest Sequence Based on a Global-Optimization Algorithm and Their Implications on Fault Behaviors

Earthquake source parameters including stress drop and corner frequency are useful tools to investigate the earthquake rupture process, and the estimation is usually accompanied by large uncertainties. We implement a novel spectral-decomposition-based approach that is capable of mitigating biases in estimating the source parameters to the 2019 M 6.4 and 7.1 Ridgecrest Sequence using both P-wave and S-wave data, and obtain the source parameters of more than 4000 earthquakes in 14 days before and after the M 7.1 event with a magnitude range of 1-5.5, and attempt to understand the fault behaviors following the M 7.1 mainshock. The P-wave stress drops have a median of around 6.5 MPa with high heterogeneity over space, in which the values at the M 7.1 rupture end to the northwest are systematically lower than that to the southeast. The stress drops are found significantly scaling with depths above 5 km, which apparently correlates with underground material changes and supports a constant strain-drop hypothesis. On the contrary, the stress drops scale weakly with magnitudes, which may indicate possible earthquake self-similarity. The stress drops are also found changing over time differently at different depths with respect to the M 7.1 mainshock, and the more rapid changes generally match the major rupturing depths of the mainshock. Moreover, we find that the S-wave and Pwave corner-frequency ratios at rupture ends are notably larger than those at the fault intersection, which is likely related to strong spatial heterogeneity of the rupture process. Our uncertainty analysis indicates that the measurements are reliable by investigating the data selection strategy, spectral fitting misfits, frequency band selection, and the difference from a previous study. Through this study, we advance our understandings of source physics of earthquakes and complex earthquake rupture processes in southern California and other areas.