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

Caje A. Kindred Weigandt, William A. Griffith, & Thomas K. Rockwell

Published September 11, 2022, SCEC Contribution #12405, 2022 SCEC Annual Meeting Poster #099

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 confining 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 pore fluid and of accumulated damage during multiple earthquake cycles. The onset of grain breakage in dry Bautista sediments is marked by a confining pressure of approximately 0.7-3.0 MPa, consistent with lower-level damage found at an exhumation depth of 70m; however, fracture densities produced by a single experimental loading event 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. Once the critical confining pressure is satisfied, faster strain rates and longer durations tend to produce greater fracture densities. Our results suggest sub-grain scale inelastic deformation in the bulk (i.e., not localized in deformation bands) occurs preferentially at high strain rates. Furthermore, 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.

Key Words
Pulverized, damage, directivity

Citation
Kindred Weigandt, C. A., Griffith, W. A., & Rockwell, T. K. (2022, 09). Role of Confinement in Coseismic Pulverization: Testing the Rock Record of Rupture Directivity on the San Jacinto Fault. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology