Role of confinement in coseismic pulverization: Testing the rock record of rupture directivity on the San Jacinto fault, Southern California

Abstract

The San Jacinto fault juxtaposes Pleistocene Bautista Formation sediments against Cretaceous tonalite near Anza, California. While the tonalites show typical features of pulverized fault zone rocks on the NE side of the fault, the weakly consolidated Bautista Formation shows evidence of pulverization at a depth of 120 m, yet only background level deformation at 70 m, consistent with the expectation that a minimum confining pressure is required to initiate pulverization in poorly consolidated sediments. We test this hypothesis by simulating pulverization using a modified Split Hopkinson Pressure Bar apparatus, in which dry and wet sediments are subjected to up to three repeated dynamic triaxial compressive load cycles. We suggest unconsolidated sediments require a minimum confining pressure under axial compression to initiate pulverization and multiple seismic cycles to reach the observed fracture densities. Companion experiments conducted on Ottawa sand 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 evidence of tensile fragmentation in cross-fault tonalite, supports a preferred northwest rupture direction along the southern section of the San Jacinto fault.