Damage Characterization in Sandstones Along the Mojave Section of the San Andreas Fault With a new Method: Initial Results and Implications for the Depth and Mechanism of Dynamic Rock Fragmentation

Following theoretical expectations that pervasive rock damage during seismic faulting is limited to the top few km of the crust (Ben-Zion and Shi, 2005; Rice et al., 2005), and uncertainties regarding the depth of rock pulverization within fault zones (Dor et al. EPSL 2006), we evaluate the fragmentation intensity in sedimentary rocks that have never been buried deeply and were displaced by the San Andreas Fault (SAF). For the analysis of damage in the sandstones we use a new method that compares the original perimeter length of a grain to the total perimeter length of its fragments, applied to a statistically representative population of grains from each sample. We employ this method on samples from the Juniper Hills formation, a tectono-stratigraphic unit that has been deposited adjacent to active strands of the SAF system. This unit, like many of the other examined sandstones in the vicinity of the Mojave section of the SAF, displays minimal or complete absence of significant SAF-parallel shear, although in places it is not as cohesive as older sandstone units along the fault. Results of a transect on the southwest side of the SAF delineate a damage zone of about a 100 m wide that is likely associated with SAF faulting events. Together with other considerations, the damage content within those sandstones suggest that dynamic fragmentation on the microscale occurs very close to the Earth surface. When we apply the method on three mutually perpendicular sections of a sample collected 10 m from the fault we observe an anisotropic damage pattern. In addition we observe preferred orientation of microfractures and many microscale damage elements likely associated with grain contact pressure. Those observations are compatible with failure in an overall compressional field. The orientation of microfractures in a sample near the fault is normal to the SAF and leaning to verticality, in agreement with modeling of the possible orientation of maximum compressive stress during cyclic loading associated with slip events on rough frictional fault surface (Chester and Chester, 2000). A change in the preferred orientation of microfractures between this sample and a more distant sample can possibly reflect a variability of the stresses throughout the damage zone that may be associated with strong dynamic reduction of normal stress or fault opening.