Pseudotachylytes and seismic fault slip

Abstract

Earthquakes occur due to the slip of faults, and thus studies on the materials of fault slip zones are crucial to understanding earthquake generation mechanisms. When an earthquake occurs, a fault slides so fast (similar to 1 m/s) that temperature rise due to frictional heating may be large enough to induce the melting of minerals in a slip zone. The fault rock generated by frictional melting during seismic fault slip is called 'pseudotachylyte'. To identify the pseudotachylyte correctly, detailed analysis of structural and material characteristics at the outcrop and microscopic scales is necessary. By conducting an integrated study of field observation, materials analysis and shear tests, some important information about the past seismic fault slip (e.g., temperature rise in the slip zone, coseismic slip direction, fault displacement, coseismic shear stress, slip rate, and the depth of seismic slip) may be drawn from the pseudotachylyte. For better understanding of frictional melting during seismic fault slip, some important issues, including frictional melting under fluid (or water) pressure, fundamental characteristics of frictional melting, physico-chemical processes that occur simultaneously with frictional melting, and frictional melting in volcano-tectonic faults should be explored.