CONTRIBUTIONS AND CHALLENGES TO UNDERSTANDING COSEISMIC FAULT-ZONE STRUCTURES

The implications of fault-zone structures have involved numerous historic studies and continue to pose challenges for the future.

Historically, field studies of faults documented their geometric and lithologic properties offering insights of their long-term mechanical responses but this emphasis changed with experimental studies on the frictional properties. Early rock-on-rock experiments showed cyclic stick-slip responses similar to the recurrence intervals of many earthquakes and stimulated investigations into the mechanics of slip. The use of simulated fault gouge led to studies of fabric development and the localization of slip, which include not only mechanical data, but also petrographic information of changes during the slip history. Comparing the latter to field observations are critical in extrapolating the mechanical data to nature, leading to a new emphasis on detailed field studies not only of surface exposures, but also in mines and through drill cores.

The questions of mechanical response during seismic cycles led to other areas of laboratory investigations such as variations in lithology, pore pressure, localized heating and melting, potential chemical influences, and fluid flow during seismic slip and in the aseimic interval. Issues of slip weakening and strengthening emerged with these studies. Recent extension of the experimental displacement rates to those approaching seismic slip velocities is offering new perspectives.

The insights gained have concurrently raised issues for future investigations, which increasingly require interaction between subdisciplines. These include but are clearly not limited to: 1. The distribution of seismic energy into heat, melting, deformation of fault asperities, and the damage zone encasing the slip zone. 2. The influence of chemistry and distribution of pore fluids and their movement during seismic and inter-seismic events. 3. Fault healing 4. Aseismic creep versus seismic rupture. 5. Scale effects between laboratory and field studies. 6. Experimental differences between studies at low-velocity rates and the present high-velocity friction experiments. 7. Inferences of mechanical parameters such as pore pressure, ambient and frictional temperatures, fluid chemistry, etc from field data.