Coseismic Rupture Process of 2019 Ridgecrest Earthquake Sequence Computed Using Regional Back-projection with a 3D Crustal Velocity Model

Abstract:

Dense spatial coverage of local seismic networks in Southern California provide us high-quality datasets for studying the detailed coseismic rupture processes for the 2019 Ridgecrest earthquake sequence. By incorporating the geometric mean imaging condition, we develop a novel multi-azimuth back-projection approach, and apply it to image time-lapse coseismic rupture propagation for the 2019 Ridgecrest earthquake sequence. A 3D crustal velocity model for Southern California is used to compute P-wave traveltimes, which automatically takes lateral velocity heterogeneities within the crust into account, and avoids additional empirical traveltime calibrations as required in classical back-projection schemes. The images of accumulated energy computed using this novel approach for the Mw6.4 foreshock and Mw7.1 main shock agree with the fault geometry delineated by the distribution of aftershocks from the Southern California Earthquake Data Center catalog. Back-projection images at different propagation times illustrate the detailed rupture processes of these two events. For instance, the Mw6.4 foreshock initialized close to the hypocenter, then the rupture propagated a short-distance along a northwest-trending fault with an average velocity of 1.2 km/s, and finally jumped to a large southwest-trending fault and propagated about 20 km with an average velocity of 2.0 km/s. In contrast, the Mw7.1 mainshock initialized near the hypocenter, then first propagated to the northwest upon arriving at the Coso volcanic area, and later turned to the southeast and propagated along the main fault zone with an average velocity of 1.0 km/s.