Highly Heterogeneous Pore Fluid Pressure Enabled Rupture of Orthogonal Faults During the 2019 Ridgecrest Mw7.0 Earthquake

Here, we show that the 2019 Mw7.0 Ridgecrest mainshock as well as its Mw6.5 foreshock ruptured orthogonal conjugate faults. We invert the waveforms recorded by the dense strong motion network at relatively high frequencies (up to 1 Hz for P; 0.25 Hz for S) to derive multiple‐point source models for both events, aided by path calibrations from a Mw5.4 and a Mw5.5 earthquake. We demonstrate that the mainshock started from a shallow (3 km) depth with a Mw5.2 event and ruptured the main fault branches oriented in the NW‐SE direction. At ~11 s, two Mw6.2 subevents took place on the SW‐NE oriented fault branches that conjugate to the main fault to the NE and SW. The SW branch rupture partially overlapped with the foreshock rupture. We suggest the coseismic rupture on nearly orthogonal faults was enabled by high pore fluid pressure, which greatly weakened the immature fault system in a heterogeneous way. Plain Language Summary Earthquakes, which are caused by shear dislocation processes on faults, often rupture single faults or multiple faults oriented at acute angles. However, rupture of orthogonal faults (i.e., faults oriented at 90° to each other) has until now been considered unfavorable based on the basic Mohr circle stress analysis. Here, we show that two large July 2019 earthquakes (Mw6.5 and Mw7.0) ruptured fault segments that are perpendicular to each other, with one NW trending and the other SW trending. We suggest that the complex fault slip is the result of a young fault system, aided by high heterogeneous pore fluid pressure. Key Points The rupture process of the Ridgecrest Mw7.0 earthquake is represented by six subevents whose source parameters are constrained by local strong motion data Two subevents of the mainshock occurred on the SW striking conjugate fault, while the major rupture propagated on the NW striking fault system Subevents and seismicity demonstrate the highly complex fault geometry, and the orthogonal fault coseismic rupture is most likely the result of high pore fluid pressure