Poster #058, Earthquake Geology

Role of Confinement in Coseismic Pulverization of Sediments: Testing the Rock Record of Rupture Directivity on the San Jacinto Fault

Caje A. Kindred Weigandt, W. Ashley Griffith, & Thomas K. Rockwell
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Poster Presentation

2021 SCEC Annual Meeting, Poster #058, SCEC Contribution #11153 VIEW PDF
Recent observations within Anza Borrego State Park, where the San Jacinto Fault juxtaposes Pleistocene Bautista Formation sediments against Cretaceous tonalite, display a stark contrast across the fault and at varying depths. In particular, while the tonalites show typical features of Pulverized Fault Zone Rocks (PFZR), the weakly consolidated Bautista Formation shows evidence of incipient pulverization at a depth of 120m, yet only background levels of deformation at 70m. Pulverization has previously been observed as a shallow crustal phenomenon in well-indurated low porosity rocks, but these observations in the Bautista Formation prompted Whearty et al. (2017) to propose that a minimum conf...ining pressure is required to initiate pulverization in poorly consolidated sediments. We test this hypothesis by simulating the pulverization process in Bautista sediments using a modified Split Hopkinson Pressure Bar (SHPB) apparatus, in which sediments are subjected to a dynamic triaxial stress state. We further investigate the role of accumulated damage during multiple earthquake cycles as well as the role of pore fluid. The onset of grain breakage in dry Bautista sediments is marked by a confining pressure of approximately 0.6-1.3 MPa, consistent with lower-level damage found at an exhumation depth of 70m; however, fracture densities produced by a single experimental loading are much lower than that of the natural specimens at greater depth. Successive experimental loading events to simulate multiple seismic cycles are necessary in order to achieve higher fracture densities. We suggest that unconsolidated sediments require a minimum confining pressure under axial compression to initiate incipient pulverization and multiple seismic cycles to reach the observed fracture densities. The compressive origin of sediment pulverization, along with the evidence of tensile fragmentation in cross-fault tonalite, supports a preferred southeastern to northwestern rupture direction along the San Jacinto Fault.