Collaborative Research: Low-cost imaging and analysis of the August 24, 2014 M6.1 South Napa California earthquake surface rupture (RAPID)

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Non-Technical ExplanationMeasurements of the change in shape of Earth's surface following a significant earthquake, including ground offsets caused when faults break all the way to the surface, can be used to understand the physics governing fault behavior, including earthquakes, and the properties of the Earth's crust. These features can be quickly eroded by natural effects and people working to repair damaged structures, roads, and other infrastructure. Rapid documentation of fragile earthquake-generated features is essential for both the scientific characterization of such events as well as for informing emergency response and recovery. New laser scanning (LiDAR) and photogrammetric (Structure from Motion) techniques for generating three-dimensional, "point-cloud" feature reconstructions powerfully augment traditional geological field methods. We are working to develop low-cost Structure-from-Motion technology for post-earthquake capture, measurement, and archiving of surface offsets from the 24 August 2014 Mw 6.0 South Napa earthquake. These data are integrated with post-earthquake airborne laser scanning and compared with pre-earthquake scans to accurately capture the surface rupture and adjacent deformation, and to interpret earthquake processes.Technical DescriptionRapid collection of high-resolution imagery after an earthquake is essential in order to map surface ruptures before onset of degradation or repair, as well as to accurately deconvolve coseismic and post-seismic deformation. The distribution of coseismic slip and the presence and nature of after-slip are both fundamental to understanding the frictional and mechanical properties of faults zones. Fortunately, recent advances in low-cost, aerial topographic imaging and differencing have given geologists the tools to collect and interpret sub-meter resolution imagery at unparalleled resolution and in record time. We are using Structure from Motion (SfM) mapping and multi-temporal airborne Light Detection and Ranging (LiDAR) to map surface offsets and the off-fault, 3-dimensional (3-D) displacement field generated by from the 24 August 2014 Mw 6.0 South Napa earthquake. This is the first ground-rupturing earthquake within the US since the advent of SfM mapping in the early 2010s and also the first with pre-event LiDAR coverage. The study addresses three key scientific questions. (1) What is the accuracy and reproducibility of surface offset measurements? (2) To what extent does early post-seismic deformation bias inferences of coseismic slip at the surface, and at depth? (3) How representative are surface offsets of deeper fault slip and what is the nature of the shallow slip deficit? These questions have implications not only for the mechanics of this fault but also for how such observations are interpreted in other events where such data are lacking. The project will showcase best practice in collecting, interpreting and archiving such data. We will also have strong collaborations with active response activities undertaken with US Geological Survey and California Geological Survey colleagues. These rich datasets will enable us to provide virtual educational experiences in immersive environments.