Field Research:
Conducting fieldwork near Bear Creek in the central High Sierra Nevada
Field investigations of geologic structures, whether direct via exhumed faults or indirect (e.g. remote sensing, boreholes, seismic reflection), are necessary to construct, constrain, and validate models (including boundary conditions and underlying assumptions about the relevant governing equations) which describe the mechanical behavior of such structures. Our focus in this field has been to study structures that may be used to distinguish between seismic and aseismic slip in the rock record. We focus a great deal of energy studying exhumed faults containing pseudotachylyte (solidified melt produced during seismic slip), because they provide evidence that ancient earthquakes were hosted by the faults. For example, Griffith et al. (2008) described the occurrence of pseudotachylyte on faults in the High Sierra Nevada, and used this to learn more about the short term dynamics of fault slip as it relates to the longer term fault growth (Griffith et al., J. Struct. Geol., 2008). These observations have been combined with detailed mapping along faults to calculate the co-seismic stress drop for small earthquake ruptures (Griffith et al., J. Geophys. Res., 2009). Such detailed mapping also allows for quantitative studies of the effects of non-planar fault geometries on slip and damage production. Griffith et al., J. Geophys. Res. (2010) showed that melting can occur in contractional bends while opening occurs in extensional bends along a fault. This slip behavior causes the entire fault to be weak as if melting was occurring along the entire fault. Such a slip process also results in concentrated off-fault fracturing in the extensional bends due to larger tensile normal stresses there. When in the form of melt, pseudotachylyte intrudes fractures in the wall rock in a process akin to hydraulic fracture. The resulting pseudotachylyte injection veins record information of the constitutive behavior of the rocks in situ (Griffith et al., EPSL, 2012), the directivity and velocity of earthquake rupture (Ngo et al., 2012), and the slip weakening distance characterizing the frictional behavior of the fault (Griffith and Prakash, in prep).