SCEC Award Number 20027 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Role of confinement in coseismic pulverization of sediments: Testing the rock record of rupture directivity on the San Jacinto Fault
Name Organization
W. Griffith Ohio State University Thomas Rockwell San Diego State University
Other Participants
SCEC Priorities 2c, 1e, 3d SCEC Groups Geology, FARM, SDOT
Report Due Date 03/15/2021 Date Report Submitted 03/11/2021
Project Abstract
At Rockhouse Canyon, the San Jacinto fault juxtaposes Cretaceous tonalite against weakly to unconsolidated sediments of the Pleistocene Bautista Formation. Recent observations at Rockhouse Canyon have suggested that during earthquakes on the San Jacinto fault, Bautista Formation sediments transition from granular flow to grain crushing under a confining pressure between 1.4 and 2.4 MPa. Understanding this process informs us how to interpret the rock record of past earthquakes and provides constraints on the coseismic constitutive response of sediments. We test the hypothesis of a dynamic coseismic source of brittle damage at Rockhouse Canyoun using a modified Split-Hopkinson Pressure Bar (SHPB) apparatus, where sediments are packed into a cylindrical confining cell and plugged on both ends. A circumferentially mounted strain gauge monitors the dynamic confining pressure throughout an experiment, and traditional SHPB methods are used to calculate the axial stress, strain, and strain-rate data. Post-deformation experimental samples are preserved in epoxy resin for thin section preparation. Through this process, we seek to: (1) determine, experimentally, the stress states and strain-rates necessary to pulverize weakly to unconsolidated sediments, (2) compare experimental results to naturally deformed field samples in order to investigate past stress states and strain rates experienced along the San Jacinto fault, and (3) test the rock record along the San Jacinto fault for the potential of a preferred rupture directivity.
Intellectual Merit This project aligns with the SCEC5 thematic area of “Understanding Earthquake Processes”, particularly under the topical element of “Beyond Elasticity”, and also aligns with key research strategies and priorities of both Earthquake Geology and FARM disciplinary committees including (1) The Earthquake Geology priority of quantifying variations in fault slip, roughness, complexity, strain localization, and damage in relation to the rupture propagation processes, including the extent, magnitude, and mechanisms of off-fault deformation and potential indicators of paleoseismic rupture direction, as well as the FARM priorities to (2) Determine how damage zones, crack healing and cementation, fault zone mineralogy, and off-fault yielding govern strain localization, slip stability, interseismic strength recovery, and rupture propagation, and (3) Investigate the relation between material, geometrical, and dynamic (deformation-induced) on- and off-fault heterogeneity, its effect on rupture initiation, propagation, and arrest, and implications for radiated energy, slip and rupture speed distributions, their scaling, and ground motion.
Broader Impacts This project has supported a new collaboration between PIs Griffith (Ohio State) and Rockwell (SDSU) and supported OSU MS student Caje Kindred. Griffith, Kindred, and PhD Student Michael Braunagel travelled to SDSU to do field work with Rockwell in February 2020. Kindred, as well as fellow OSU MS student Zachary Smith presented their experimental work at the 2020 virtual SCEC Annual Meeting. This project has enhanced the research infrastructure in Griffith's lab by supporting the implementation of a new experiental apparatus capable of conducting dynamic triaxial compression experiments on unconsolidated sediments.
Exemplary Figure Figure 3. (A,B) Preliminary mapping of microfractures (blue) in quartz grains (red) of experimental Ottawa Sand sample OS_12. Hertzian fractures dominate and a crushed grain exists in the bottom left of the image. (C,D) Preliminary mapping of microfractures (blue) in quartz grains (red) of experimental Bautista Formation sample BS_05. Note the concentration of phyllosilicates. Damage appears fundamentally different than Hertzian fractures, with limited crack nucleation from grain contacts. In both materials, fracture interpretations are mapped over PPL images but were constructed using both XPL and PPL.