Complex Rupture of an Immature Fault Zone: A Simultaneous Kinematic Model of the 2019 Ridgecrest, CA Earthquakes

• Published in: Geophysical research letters Vol. 47; no. 3
• Main Authors: Goldberg, D. E., Melgar, D., Sahakian, V. J., Thomas, A. M., Xu, X., Crowell, B. W., Geng, J.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.02.2020; Wiley
• Subjects: Coalescing; Earthquakes; Fault lines; Fault zones; Geological faults; Interferometric synthetic aperture radar; kinematic slip model; Kinematics; Movement; Plate boundaries; Plate tectonics; Plates (tectonics); Radar data; Ridgecrest sequence; Rupture; Rupturing; SAR (radar); Seismic activity; seismic triggering; Sequencing; Shear zone; Slip; Synthetic aperture radar; Transform faults; United States > US; California
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL086382

The 4 July 2019 Mw6.4 and subsequent 6 July 2019 Mw7.1 Ridgecrest sequence earthquakes (CA, USA) ruptured orthogonal fault planes in a low slip rate (1 mm/year) dextral fault zone in the area linking the Eastern California Shear Zone and Walker Lane. This region accommodates nearly one fourth of plate boundary motion and has been proposed to be an incipient transform fault system that could eventually become the main tectonic boundary, replacing the San Andreas Fault. We investigate the rupture process of these events using a novel simultaneous kinematic slip method with joint inversion of high‐rate GNSS, strong motion, GNSS static offset, and Interferometric Synthetic Aperture Radar data. We model the Coulomb stress change to evaluate how the Mw6.4 earthquake may have affected the subsequent Mw7.1 event. Our findings suggest complex interactions between several fault structures, including dynamic and static triggering, and provide important context for regional seismic source characterization and hazard models. Plain Language Summary The San Andreas is a right‐lateral strike‐slip fault marking the main tectonic boundary between the North American and Pacific Plates. East of the San Andreas, a diffuse region of right‐lateral shear known as the Eastern California Shear Zone accommodates roughly one quarter of the motion between the two tectonic plates. The Eastern California Shear Zone has been proposed to be an immature fault system that in the future will accommodate a greater portion of the region's tectonic forces, coalesce into a simple, throughgoing fault, and eventually replace the San Andreas as the major tectonic boundary. The July 2019 Ridgecrest sequence therefore provides the opportunity to gain valuable insights into the development of tectonic boundaries. We analyze the spatial and temporal history of slip during the two largest events in the sequence, the 4 July Mw6.4 and 6 July Mw7.1 earthquakes, and demonstrate a novel approach to evaluate seismic and geodetic observations for both earthquakes simultaneously. We identify complex interactions between discrete fault segments, including dynamic triggering of one fault by slip on another, and present evidence for static triggering of the 6 July Mw7.1 event by the 4 July Mw6.4 earthquake. Key Points We present a novel kinematic slip inversion method to model the two largest 2019 Ridgecrest events in a single simultaneous inversion step We demonstrate evidence for both static and dynamic triggering within the Ridgecrest sequence Low rupture velocity and complex fault geometry support the view of this region as an incipient transform fault system