HR: 08:00h
AN: S31D-01 INVITED     [Abstracts]
TI: Gouge Formation by Dynamic Pulverization During Earthquakes
AU: * Reches, Z
EM: reches@gcn.ou.edu
AF: School of Geology and Geophysics, University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73019 United States
AU: Dewers, T
EM: tdewers@ou.edu
AF: School of Geology and Geophysics, University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73019 United States
AB: The formation of fine grain gouge along mature faults is usually attributed to quasi-static grain crushing, but there is no model for gouge formation that accounts for the dynamic effects of a fast propagating earthquake rupture. The present analysis examines the deformation field close to the tip of a mode II fracture by using the asymptotic solution for an in-plane propagating fracture in an isotropic elastic solid (Freund, 1990). In the solution, the propagation velocity may not exceed the Rayleigh wave velocity, Cr. The parameters that weakly depend on the fracture velocity (density, Poisson's ratio, shear modulus and elastic wave velocities), are assumed constant with values of moderately competent granite; the stress intensity factor, KII, that could vary with velocity is considered in its full range. The calculations show that when a fracture propagates at V $>$ 0.85 Cr, intense deformation conditions develop at a finite distance of 1-3 mm from the tip: At V of about 0.99 Cr, the tensile stresses approach 10 GPa within a 6 mm wide zone around the tip, and the volumetric dilation rates alternate between $10^5 s^-^1$ expansion and similar absolute value of contraction. Similar extreme conditions are known in nature only for the shock wave at impact sites. This model can be bounded with our recent results of the gouge texture from an earthquake rupture zone in a South African mine (Reches et al., this meeting, session T20). The surface area of gouge approaches $80 m^2/g$ that corresponds to a surface energy of 0.2-0.36 $MJ/m^2$ for a 1 mm thick gouge zone. Our mapping of the rupture zone reveals tens of subparallel fractures that are ~1 mm thick and filled with gouge; summation for 10-30 fractures yields surface energy of 3-10 $MJ/m^2$. By assuming that this surface energy is a lower limit on the fracture energy G, we estimate from the model that this earthquake propagated at V $>$ 0.9 Cr with maximum tensile stress exceeding 5 GPa, and dilation rate exceeding $5*10^5 s^-^1$. We propose that the extreme deformation conditions at the tip region of a fast propagating earthquake rupture pulverize the fault rocks by processes known from shock analyses. Further, while we analyzed three earthquake rupture zones in South African mines, only two of them display large amounts of pulverized gouge. According to our model, the third rupture zone propagated at V $<$ 0.85 Cr, corresponding to stresses and dilation rates that are too low to pulverize the rocks. We finally propose that the intensity of pulverization can serve as a paleo-velocity criterion of earthquake rupture.
DE: 8123 Dynamics, seismotectonics
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting