Field, laboratory, and theoretical investigations of fault rupture dynamics

Examination of faults exhumed from seismic depths to the surface of the earth provides high resolution, continuous access to meso- and micro-scale structure that is difficult or impossible to resolve for faults at depth. Quantitative integration of field observations of fault structure with analytical, theoretical and laboratory models allow structural geologists to provide constraints on the mechanics of earthquake rupture in a dynamic sense.;Field maps and thin section observations document the occurrence of pseudotachylyte (solidified melt produced during seismic slip) on small, sub-vertical strike-slip faults in granitoid rocks of the central Sierra Nevada. Measurements from these faults are used to provide constraints on ambient conditions during seismic faulting. The pseudotachylytes are less than 0.3 mm thick and are found in faults typically up to 1 cm in thickness, and total measured left-lateral offset along sampled faults is approximately 20 cm. Field and microstructural evidence indicate that the faults exploited pre-existing mineralized joints and show the following overprinting structures: mylonites more or less coeval with quartz veins, cataclasites and pseudotachylytes more or less coeval with epidote veins, and zeolite veins. Based on observations of the microstructural textures of faults combined with theoretical heat transfer and energy budget calculations, it is suggested that only a fraction (<30%) of the total offset was associated with seismic slip. The elusive nature of these pseudotachylytes demonstrates that observations in outcrop and optical microscope are not sufficient to rule out frictional melting as a consequence of seismic slip in similar fault rocks.;The static stress drop is estimated on small exhumed strike-slip faults in the vicinity of the faults described in the first chapter. The faults are exposed in outcrop along their entire tip-to-tip lengths of 8-12 m. The contribution of seismic slip to the total slip along the studied cataclasite-bearing small faults may be further constrained than the previous chapter estimate (<7 cm) by measuring the length of epidote-filled, rhombohedral dilatational jogs (rhombochasms) distributed semi-periodically along the length of the faults. This affords measurement of both the rupture length and slip, yielding stress drop calculations ranging from 90 to 250 MPa, i.e., one to two orders of magnitude larger than typical seismological estimates for earthquakes. These inferred seismic ruptures occurred along small, deep-seated faults, and, given the calculated stress drops and observations that brittle faults exploited joints sealed by quartz-bearing mylonite, we conclude that these were "strong" faults.;The Bear Creek fault zones localize outcrop-scale damage into tabular zones between two sub parallel boundary faults, producing a fracture-induced material contrast across the boundary faults with softer rocks between the boundary faults and intact granodiorite outside. Using detailed mapping and microstructural analysis of small fault zones to build and constrain numerical effective medium experiments, the effect of mesoscopic (outcrop-scale) damage zone fractures on the effective isotropic elastic moduli of the fault rocks is evaluated, showing that the bulk response of the fractured damage zone is strain-weakening, and can be as much as 75% more compliant than the unfractured granodiorite.;Observations of the geometry and distribution of pseudotachylyte veins along faults in multiple field areas motivated the investigation of the growth of tensile microcracks in Homalite-100 around sub-rayleigh experimental shear ruptures (Laboratory Earthquakes) propagating along an interface with frictional and cohesive strength. Opening microcracks were produced only along one side of the interface where they were associated with transient tensile stress perturbation due to the propagating shear rupture. The orientation of microcracks is related to the rupture velocity and the ambient static stress field. The results of this study provide a rationale for interpreting observations of tensile fractures along exhumed faults and create diagnostic criteria for interpreting the velocity and directivity of past ruptures.