Simultaneous measurement of real contact area and fault normal stiffness during frictional sliding

The tectonic stresses that lead to earthquake slip are concentrated in small regions of solid contact between asperities or gouge particles within the fault. Fault strength is proportional to the contact area within the shearing portion of the fault zone and many fault properties of interest to earthquake hazard research, e.g., occurrence time, recurrence interval, precursory slip, triggered earthquake slip, are controlled by processes acting at the highly stressed contact regions. Unfortunately the contact-scale physical processes controlling earthquake occurrence cannot be easily observed or measured directly. In this pilot study we simultaneously directly measure contact area using transmitted light intensity (LI) [Dieterich and Kilgore, 1994; 1996] and continuously monitor the normal stiffness of the fault using acoustic wave transmission (AT) [Nagata et al., 2008]. The objective of our study is to determine relations amongst contacting area, stiffness, strength, normal stress, shear displacement, and time of contact during sliding. Interface stiffness is monitored using acoustic compressive waves transmitted across the fault. Because the fault is more compliant in compression than the surrounding rock, the fault has an elastic wave transmission coefficient that depends on the fault normal stiffness. Contact area is measured by LI: regions in contact transmit light efficiently while light is scattered elsewhere; therefore transmitted light intensity is presumed proportional to contact area. LI and AT are expected to be correlated; e.g., an elastic contact model suggests that stiffness goes as the square root of contact area. We observe LI and AT for sliding at slip speeds between 0.01 and 10 microns/s and normal stresses between 1 and 2.5 MPa while conducting standard velocity-step, normal stress-step and slide-hold-slide tests. AT and LI correlate during all tests, at all conditions. If the physical relationship, or even an empirical calibration between AT and LI can be established for rough fault surfaces, contact area could be measured with AT for non-transparent materials and at higher normal stresses than in the present experiments.