Summary. The project supported rapid geologic, geodetic and seismological field studies associated with the 2019 Ridgecrest earthquake sequence in the Eastern California Shear Zone (ECSZ), which included M6.4 and M7.1 events and numerous aftershocks that extend to the north toward the Owens Valley and to the south toward and along the Garlock fault. The rapid studies allowed capturing seismic and geodetic fields that decay fast with time, and fragile geological evidence that become obscure fast. The studies included field data gathering, instrumentation deployment and remote sensing and imaging surveys. Seismic field work included deployments of over 450 sensors in dense across-fault linear arrays and more regional 2D arrays. The analyses of the seismic data led to detection of numerous small seismic events not included in the regional catalog, detailed imaging of subsurface structures, and information on earthquake source properties before the sequence, between the M6.4 and M7.1 events, and after the Ridgecrest mainshock. The data and follow up studies clarify earthquake processes and structures in the highly active and complex Eastern California Shear Zone.

Intellectual Merit. Earthquake sequences with M > 7 events with clear access for detailed field and areal measurements are relatively rare, so it is important to record rapidly the ample and fast decaying data associated with the Ridgecrest earthquake sequence. The available observations include the following puzzling features. (i) generation of near-orthogonal left- and right-lateral ruptures and lines of seismicity in the July 4 M6.4 event (typical conjugation is associated with about 60 degrees between the two ruptures). (ii) Initiation of the M 7.1 earthquake in close spatial proximity to the NW end of the M6.4 event (the short time separation between the events implies a strong arresting structure at the NW end of the M6.4 event). (iii) Transition from highly localized M6.4 rupture to the distributed rupture of the M7.1 event to the NW (again implying a strong arresting structure of the M6.4 event). (iv) Possible interactions of the sequence with the Garlock fault to the south and the Coso geothermal area and Owens Valley to the north. (v) Space-time variations of seismicity (including foreshocks) and transient strain-rates at the surface. 

Broader Impacts. The data collections facilitated by this RAPID project and distributed via OpenTopography serve very broad communities of scientists working on earthquake processes and effects. The obtained multi-disciplinary observations allow addressing numerous outstanding issues associated with earthquake ruptures, spatio-temporal seismicity patterns, fault zone and crustal structures, intensity of ground shaking and postseismic deformation. The recorded data and follow up studies contribute boradly to improved understanding of tectonic deformation in the ECSZ. The field studies will provide valuable experience for students and the recorded data create many opportunities for students and early career scientists. The field work and derived results generate excellent material for the NSF and SCEC E&O activities and media outreach.

Last Modified: 08/27/2021
Modified by: Viswanath Nandigam