Influence of Outcrop Scale Fractures on the Effective Stiffness of Fault Damage Zone Rocks

Abstract

We combine detailed mapping and microstructural analyses of small fault zones in granodiorite with numerical mechanical models to estimate the effect of mesoscopic (outcrop-scale) damage zone fractures on the effective stiffness of the fault zone rocks. The Bear Creek fault zones were active at depths between 4 and 15 km and localize mesoscopic off-fault damage into tabular zones between two subparallel boundary faults, producing a fracture-induced material contrast across the boundary faults with softer rocks between the boundary faults and intact granodiorite outside the boundary faults. Using digitized fault zone fracture maps as the modeled fault geometries, we conduct nonlinear uniaxial compression tests using a novel finite-element method code as the experimental ''laboratory'' apparatus. Map measurements show that the fault zones have high nondimensional facture densities (>1), and damage zone fractures anastamose and intersect, making existing analytical effective medium models inadequate for estimation of the effective elastic properties. Numerical experiments show that the damage zone is strongly anisotropic and the bulk response of the fault zone is strain-weakening. Normal strains as small as 2% can induce a reduction of the overall stiffness of up to 75%. Fracture-induced effective stiffness changes are large enough to locally be greater than intact modulus changes across the fault due to juxtaposition of rocks of different lithologies; therefore mesoscopic fracturing is as important as rock type when considering material or bimaterial effects on earthquake mechanics. These results have important implications for earthquake rupture mechanics models, because mesoscopic damage zone fractures can cause a material contrast across the faults as large as any lithology-based material contrast at seismogenic depths, and the effective moduli can be highly variable during a single rupture event.

Comparison of thermal modeling, microstructural analysis, and Ti‐in‐quartz thermobarometry to constrain the thermal history of a cooling pluton during deformation in the Mount Abbot Quadrangle, CA

“…Christiansen [] suggests that mechanical deformation of biotite grains in fault steps reset the 40 Ar/ 39 Ar systematics. Other evidence for deformation continuing to lower temperatures include the occurrence of lower greenschist facies minerals, cataclasites, and pseudotachylytes within faults [ Martel et al ., ; Griffith et al ., ; Griffith et al ., ; Kirkpatrick and Shipton , ; Kirkpatrick et al ., ].…”

“… Outcrops investigated in the Mount Abbot Quadrangle, mapped with the outline of the Lake Edison pluton from Figure 1, the Bear Creek Kink Band (BCKB, delineated by the dashed dark blue lines), and the Rosy Finch Shear Zone (RFSZ, pink region). Squares indicate outcrops used for TitaniQ and microstructural analyses in this study; circles indicate outcrops presented in previous studies: BJF, Big Juniper Fault [ Griffith et al ., , ]; BCC, Bear Creek Camp [ Martel et al ., ]; WF, Waterfall [ Martel , ]; TJM, Trail Junction Meadows [ Martel et al ., ]; and DBF, Dancing Burn Fault [ Kirkpatrick et al ., ]. Outcrops near “CS” contain prominent brittle and ductile features; thus, it is labeled as both “brittle” and “ductile.” The base map shows the traces of the major lineaments in the southern half of the Mount Abbot Quadrangle [ Davies and Pollard , ; modified from Lockwood and Lydon , ].…”

Experimental demonstration of a semi-brittle origin for crustal strain transients

“…Chemical and metamorphic reactions associated with these fluids are expected to take advantage of the new surface area generated by fractures to form veins and increase fracture stiffness. This process leads to slower fracture propagation rates 24 and, in some cases at high temperature, the formation of ductile fractures 25 . In our experiments we modulate the yield stress by varying the microgel concentration affecting fracture stiffness in the Carbopol, and the type and rate of fracture formation and propagation.…”

Using a vibratory source at Mt. Etna (Italy) to investigate the wavefield polarization at Pernicana Fault

“…Cochran, Vidale and Li () and Griffith, Sanz and Pollard () connected the directional amplifications in fault zones to the stiffness anisotropy of the fractured rock mass, where the predominant seismic motion is expected to be normal to the fractures orientation. Pischiutta et al .…”