The Mw 6.4 and Mw 7.1 Ridgecrest earthquake sequence occurred on 4 and 5 July 2019 within the eastern California shear zone of southern California. Both events produced extensive surface faulting and ground deformation within Indian Wells Valley and Searles Valley. In the weeks following the earthquakes, more than six dozen scientists from government, academia, and the private sector carefully documented the surface faulting and ground-deformation features. As of December 2019, we have compiled a total of more than 6000 ground observations; approximately 1500 of these simply note the presence or absence of fault rupture or ground failure, but the remainder include detailed descriptions and other documentation, including tens of thousands of photographs. More than 1100 of these observations also include quantitative field measurements of displacement sense and magnitude. These field observations were supplemented by mapping of fault rupture and ground-deformation features directly in the field as well as by interpreting the location and extent of surface faulting and ground deformation from optical imagery and geodetic image products. We identified greater than 68 km of fault rupture produced by both earthquakes as well as numerous sites of ground deformation resulting from liquefaction or slope failure. These observations comprise a dataset that is fundamental to understanding the processes that controlled this earthquake sequence and for improving earthquake hazard estimates in the region. This article documents the types of data collected during postearthquake field investigations, the compilation effort, and the digital data products resulting from these efforts.
Surface rupture from the 2019 Ridgecrest earthquake sequence, initially associated with the Mw 6.4 foreshock, occurred on 4 July on a ∼17 km long, northeast–southwest-oriented, left-lateral zone of faulting. Following the Mw 7.1 mainshock on 5 July (local time), extensive northwest–southeast-oriented, right-lateral faulting was then also mapped along a ∼50 km long zone of faults, including subparallel splays in several areas. The largest slip was observed in the epicentral area and crossing the dry lakebed of China Lake to the southeast. Surface fault rupture mapping by a large team, reported elsewhere, was used to guide the airborne data acquisition reported here. Rapid rupture mapping allowed for accurate and efficient flight line planning for the high-resolution light detection and ranging (lidar) and aerial photography. Flight line planning trade-offs were considered to allocate the medium (25 pulses per square meter [ppsm]) and high-resolution (80 ppsm) lidar data collection polygons. The National Center for Airborne Laser Mapping acquired the airborne imagery with a Titan multispectral lidar system and Digital Modular Aerial Camera (DiMAC) aerial digital camera, and U.S. Geological Survey acquired Global Positioning System ground control data. This effort required extensive coordination with the Navy as much of the airborne data acquisition occurred within their restricted airspace at the China Lake ranges.
The 2019 Ridgecrest, California, earthquake sequence, including an Mw 6.4 event on 4 July and an Mw 7.1 approximately 34 hr later, was recorded by 15 instruments within 55 km nearest-fault distance. To characterize and explore near-field ground motions from the Mw 6.4 foreshock and Mw 7.1 mainshock, we augment these records with available macroseismic information, including conventional intensities and displaced rocks. We conclude that near-field shaking intensities were generally below modified Mercalli intensity 9, with concentrations of locally high values toward the northern and southern termini of the mainshock rupture. We further show that, relative to near-field ground motions at hard-rock sites, instrumental ground motions at alluvial near-field sites for both the Mw 6.4 foreshock and Mw 7.1 mainshock were depleted in energy at frequencies higher than 2–3 Hz, as expected from ground-motion models. Both the macroseismic and instrumental observations suggest that sediments in the Indian Wells Valley experienced a pervasively nonlinear response, which helps explain why shaking intensities and damage in the closest population center, Ridgecrest, were relatively modest given its proximity to the earthquakes.
Further dissemination authorized to the Department of Energy and DOE contractors only; other requests shall be approved by the originating facility or higher DOE programmatic authority. During Phase 2A, 2B, and 2C, the Fallon FORGE site will be instrumented and readied to test new technologies and techniques in Phase 3. In Phase 2A, an Environmental Information Volume will be completed while a schedule to complete the NEPA process and obtain required permits will be completed. Additionally, preliminary telemetered seismic monitoring of the site will be deployed to complement existing seismic monitoring activities at the Fallon FORGE site. During Phase 2B all reviews, permits, and approvals initiated in Phase 2A will be obtained in accordance with NEPA and other local and state regulations. It is anticipated that these permits will be obtained early in Phase 2B and additional site characterization allowed by NEPA and applicable permits to begin. Phase 2B will also include the completion of the Induced Seismicity Mitigation Plan that will incorporate recorded site MEQ data and associated analyses into a Probabilistic Seismic Hazard Analysis, Criteria for Damage and Vibration, and Mitigation Actions for field testing. In 2C the site is brought to readiness for FORGE implementation through, at a minimum, additional surface and subsurface site characterization, deployment of high resolution seismic monitoring, geologic model refinement, and reservoir modeling. Additionally, a Science and Technology Analysis Team (STAT) will be assembled to provide technical guidance to the FORGE team and to ensure DOE objectives are incorporated in FORGE execution. As a result of working with the STAT to assess current technology, establish technical baseline information and performance metrics for FORGE work, and review the FORGE implementation plan, topics for the first round of competitive solicitations will be developed and a draft solicitation produced. Where applicable and appropriate, DOE may elect to have the Fallon FORGE team incorporate testing of methods and tools developed by separately funded DOE researchers into FORGE activities.
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