MICROROUGHNESS OF AN ANCIENT EARTHQUAKE FAULT

Deformation within upper crustal high strain zones occurs principally through seismogenic slip. During an earthquake, the combination of high slip velocity and relatively high ambient pressure and temperature can create extreme physical conditions. Simulation of these conditions in laboratory experiments reveals dynamic weakening associated with a variety of physical and chemical processes that lubricate the fault. Of these mechanisms, frictional melting is the most easily recognizable in the rock record, as it results in the production of pseudotachylytes.

The physics of melt lubrication is controlled by fault surface microtopography. In pseudotachylyte-bearing faults, the solidification of frictional melt soon after earthquake slip ceases preserves a record of the coseismic microroughness; however, it also precludes the development of extensive fault surface exposures that have enabled previous studies of fault roughness. We have overcome this difficulty by imaging the intact 3D geometry of the fault using high-resolution X-ray computed tomography (CT). 2 cm diameter cores from a wavy segment of the Gole Larghe Fault Zone, Italy, were scanned at the University of Texas High Resolution X-ray CT Facility. The geometry of the fault zone and surrounding wall rock is thus captured in a grayscale image volume made up of millions of ~32 μm voxels (3D pixels).

We have extracted fault surfaces (contact between pseudotachylyte bearing fault zone and wall rock) from the CT volumes using integrated manual mapping, automated edge detection, and statistical evaluation. Fault surfaces from both contractional and extensional regions are smoother in the slip direction than in the slip perpendicular direction, likely a result of frictional wear. Only the contractional bend sample, however, shows clear evidence of preferential melting. Fault surface microroughness thus reflects effects of both wear and melting, and fault normal stress may influence the relative balance between these processes.