HR: 0800h
AN: T21A-0449    [Abstracts]
TI: Crack Damage in Core Samples From the San Andreas and Nojima Faults
AU: * Lockner, D A
EM: dlockner@usgs.gov
AF: US Geological Survey, ms/977 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Morrow, C
EM: cmorrow@usgs.gov
AF: US Geological Survey, ms/977 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Moore, D
EM: dmoore@usgs.gov
AF: US Geological Survey, ms/977 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: Crack densities, grouped by crack length, have been measured from thin sections of granodiorites from the SAFOD pilot hole (2200 m depth) adjacent to the San Andreas fault near Parkfield, California and the NIED scientific drillhole crossing the Nojima fault (Kobe 1998 earthquake) on Awaji Island, Japan (1279 m depth). Crack densities were determined for crack lengths ranging from 0.046 to 10 mm in an attempt to quantify the mechanical integrity of these rocks taken from active fault zones. As much as possible, only broken, unhealed cracks were counted. Results were compared to crack densities measured in undeformed Eureka quartzite and in both undeformed and fractured Westerly granite. A simple but apparently robust measure of the critical crack density needed for significant crack interaction is the ratio S/L, where S is the distance between crack centers and L is crack length. Because crack-induced stresses decrease as (distance)^-2, S/L=1 is a good indicator of strong crack interactions. For all samples, the smallest cracks (0.046 to 0.100 mm) had the highest crack densities. Yet these cracks were not interacting with each other since their densities were only 20 to 40 percent of the densities needed for significant stress field interactions. By contrast, for both the SAFOD and Nojima borehole fault zone samples, cracks longer than 0.2 mm had crack densities sufficiently high to result in strong crack interactions leading to mechanically weak grain structures. In other words, both samples were in a state of structural failure for scales larger than the grain size. Both the SAFOD and Nojima samples can be considered representative of off-fault damage zone material from their respective faults. The samples look like `typical' granodiorites to the naked eye, with no evidence of permanent shearing. Yet both samples have been shattered and have essentially lost cohesion. Both samples show repeated episodes of fracture and hydrothermal healing. We interpret these properties as resulting from periodic passage of high-amplitude dynamic stress waves associated with earthquake rupture.
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: Fall Meeting 2005