COMPREHENSIVE MAPPING OF ACTIVE FAULTS IN THE VICINITY OF THE 2019 RIDGECREST EARTHQUAKE RUPTURES

The area surrounding the July 2019 Ridgecrest earthquake sequence in southeastern California had not been comprehensively examined for active faults prior to those earthquakes. No long continuous fault lines were recognized along the 2019 ruptures beforehand, and only ~35% of the rupture occurred on previously mapped faults. We present a new map of active faults in the region for integration into the USGS Quaternary Fault and Fold Database and California Active Fault Map, based on the observed 2019 ruptures and on active tectonic features recognized in the landscape. We identified these features—tufa lineaments, sheared Quaternary deposits, scarps, deflected drainages, and lineaments and contrasts in topography, vegetation, and ground color—using pre-event high-resolution (<2 m) topography and optical imagery in combination with post-event field observations. These features reveal a network of NE- and NW-trending orthogonal fault strands, a subset of which ruptured in 2019. Neotectonic features are commonly short (<2 km), discontinuous, and display en echelon stepping patterns along both the M6.4 and M7.1 ruptures. Faults are generally better expressed outside the late Pleistocene lake basins and in areas where substantial vertical motion occurred in 2019. Both the NE- and NW-trending active fault systems are subparallel to regional bedrock fabrics that were established as early as ~150 Ma, and may be reactivating these older structures. Overall, we estimate that 50–70% of the 2019 surface ruptures could have been recognized as active faults with detailed inspection of pre-event data. Similar detailed mapping of potential neotectonic features could help improve seismic hazard analyses in other regions of the Walker Lane – Eastern California Shear Zone and elsewhere where faulting is distributed or incompletely mapped. Given the distributed features but observed continuity of rupture, we recommend that in areas where a fault cannot be resolved as a single throughgoing structure, a zone of potential faulting, in addition to or instead of a representative proxy fault, should be used as a hazard model input. This is necessary to adequately represent potential structural coherency and connectivity that may permit larger and more variable earthquake ruptures than would be allowed in a more simplified model.