Stress drop validation and triggering process of foreshocks in Southern California

Earthquake stress drop is an important source parameter that directly links to strong ground motion. However, estimating stress drops is often challenging due to many factors, such as data limitations, methodology, and attenuation. Different studies may yield highly inconsistent stress drop values for the same earthquakes, leading to different interpretations of stress drop scaling and spatial patterns. Stress drop is also important for interpretation of earthquake triggering processes. In particular, the roles of foreshocks have continuous debate, and some recent studies show that detailed source parameter analysis is the key. In this study, we combine analyses of three sequences in Southern California using different methods to investigate the resolution of stress drops and the roles of foreshocks. The three sequences include the M7 El-Mayor Cucapah, the 2012 M5 Brawley swarm, and the 2019 M7 Ridgecrest sequence. The Ridgecrest sequence will participate in the Community Stress Drop Validation Study. For each sequence, we apply an improved stacking method and a spectral ratio method. We will use different types of waves: P-wave, S-wave, and Coda-wave. Stress drop results from this study will be compared with available previous studies. We will first discuss the influence of wave type and methodology on stress drop estimations, then we will investigate detailed stress interactions between foreshocks and the mainshock for different types of earthquake sequences (i.e., mainshock-aftershock and swarms that involve aseismic slip or fluid).