DO MICROSTRUCTURES IN NEAR-SURFACE SEDIMENT FROM ACTIVE FAULTS PRESERVE A SIGNATURE OF COSEISMIC SLIP?

Evaluation of microstructures in unlithified near-surface sediments may help differentiate between earthquake rupture-related (coseismic) and creep-related (aseismic) structures in active fault zones. Studies of microstructures at paleoseismic research sites on creeping and rupture traces of the San Andreas Fault (SAF), the New Madrid North fault (NMNF) in southeastern Missouri, and the McKinleyville fault (McKF) in northern California, show contrasting fault zone structures and microstructures in Late Pleistocene to Late Holocene near-surface sand. Deformation bands occur in granular deposits on the 1906 rupture of the SAF, the NMNF, and the McKF, but are absent at the creeping SAF site. Deformation bands for the right-lateral SAF and NMNF have broken and disaggregated grains, preferred grain orientations ~30° counterclockwise from the fault (viewed in the slip-parallel direction), smaller average grain size and, locally, both lower and higher porosity than control samples. In contrast, faulted late Holocene sediment at the creeping SAF site lacks deformation bands, lacks broken grains, has preferred grain orientations ~ 10° clockwise from the fault, and has comparable grain size but lower porosity than control samples. These microstructures record different deformation mechanisms in near-surface sediment: brittle, localized slip and cataclasis at the cosiesmic slip sites, and distributed deformation at the creeping fault site. Microstructures in near-surface fault zone sediments provide clues to rupture characteristics, modes of failure, degree of slip localization, and slip histories of active faults. These microstructures also provide observational evidence for processes that operate in granular media in real fault zones; in effect, they bridge the gap between field-scale fault studies and smaller-scale theoretical and laboratory investigations of fault friction and fault zone processes.