AN EXPERIMENTAL TRIAL TO DETECT PRECURSORY SLIPS BY TRANSMISSION WAVES ACROSS A FAULT

It is known that there is a slow stage of precursory slip prior to a d ynamic rupture propagation. Thus it is expected that there should be s ome physical contrasts between before and after the precursory slip. T o detect the contrasts, we performed an observatory experiment in whic h P- and S-waves were transmitted across a fault in a double direct sh ear apparatus. Two types of experiments were made: first, Normal Stres s Holding Test (NSHT), in which we focused on the effect of stationary contact time on transmission waves. Second, Shear Stress Increasing T est (SSIT), in which we measured local shear stresses in addition to t ransmission waves. In NSHT we observed increases in amplitude of trans mission waves with the logarithm of stationary contact time. The chang e was several percent after about 3 hours contact. In SSIT we observed a significant increase in amplitude with a shear stress application u ntil a stick-slip occurred. It may be explained by a strengthening of asperity interlocking. However, the detailed mechanism of this phenome non is uncertain. Further we found a little reduction of increasing ra te of amplitude which corresponds to the local precursory slips. This reduction may be attributed to the reset of stationary contact time du e to local slips and replacement of asperity contacts with smaller con tact sizes.