SCEC Award Number 19209 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Exploiting Vertical Motions to Test for Slip Heterogeneity at Depth
Name Organization
Michael Oskin University of California, Davis
Other Participants Dr. Chris Milliner, Nasa JPL
SCEC Priorities 1c, 1d, 2a SCEC Groups Geodesy, FARM, SDOT
Report Due Date 04/30/2020 Date Report Submitted 11/20/2020
Project Abstract
Fault-rupture studies using high-resolution differential topography and image correlation reveal distributed coseismic deformation at the surface, with varying degrees of heterogeneity. Whether this heterogeneity reflects incomplete rupture through unconsolidated surface materials, versus slip variation at greater depth, remains poorly understood. We proposed to develop and test a method utilizing near-field vertical motions at different distances from a fault surface rupture to detect the degree of slip heterogeneity at shallow depth (upper 1 km), below the surface layer. As this project got underway, the 2019 Ridgecrest earthquake sequence offered us the opportunity to assess the contribution of variations of coseismic slip along the rupture to the generation of distributed fractures and aftershocks, and thus to the distribution of inelasticity. We used on-fault measurements of surface slip and distributed deformation derived from optical image correlation and compared these to fracture density mapped from airborne lidar, and aftershock distribution from the Ridgecrest QTM catalog. Surficially expressed fracture and aftershock (<5 km depth) density show moderate to strong correlation with each other throughout the deformed volume. We analyzed the density of fractures and aftershocks in sections of the surface rupture located in bedrock and unconsolidated sediment and found a systematic difference in character and density, where the rupture tends to localize along bedrock and distribute into en-echelon segments in sediment. We found no correlation between slip gradients and lithology, though this was complicated to test because most of the rupture occurred through unconsolidated sediment.
Intellectual Merit Understanding coseismic slip heterogeneity contributes to several important problems in earthquake science. It is perhaps as direct a measure as possible of stress heterogeneity, present either before or in the aftermath of an earthquake, though which may be the case can be uncertain. The surface effects of slip heterogeneity include uncertainty in slip magnitude, and promoting distributed deformation through surrounding volume of rock. Both of these effects impact longer-term geologic records of fault slip and contribute to fault displacement hazard. This project seeks a way to better constrain slip heterogeneity through vertical motions in the region surrounding fault ruptures, which may be measured with high precision using differential airborne lidar. During our work on this project, the occurrence of the 2019 Ridgecrest earthquake sequence afforded us the opportunity to directly apply our efforts in the context of the earthquake response. Though differential lidar was not available for this event, directly measured slip gradients permitted us to test whether distributed deformation and vertical motions are correlated. This effort is ongoing in conjunction with other aspects of our research of the surface ruptures, fracture distribution, and near- and far-field geodesy of these events
Broader Impacts This project contributes directly to understanding fault displacement hazard, which is important for the resilience of fault-crossing lifelines. This project provided research fellowship support for University of California, Davis graduate student Alba M. Rodriguez, including travel to research conferences and analyses that will contribute to one research publication.
Exemplary Figure Figure 1: Left: slip gradients (dots) calculated over a 50m moving window and fracture density (boxes) calculated over a 5km window. Right: slip gradients (dots) and aftershock density (boxes).