Experimental Study on Near-Fault High-Frequency Strong Motion

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A stick-slip experiment is performed on a rock specimen with a pre-existing artificial fault to observe near-fault high-frequency elastic waves generated by stick-slip motion. The main results are as follows : 1) The observed near-fault high-frequency strong-motion duration is approximately proportional to the local breakdown time. 2) The highest peak acceleration observed amounts to a few tens of km/s^2, which is considered to correspond to about 1000gal in the case of natural earthquakes, when the differences is size and frequency are taken into account. Further we model the dynamic behavior of stick-slip motion observed in our laboratory experiment : A number of asperities circular in shape are assumed to distribute on a given fault surface. Each asperity on the fault is to be ruptured after the arrival of the macroscopic rupture front. We consider, however, that there exists some time lag between the arrival time of rupture front and the rupture initiation of the asperity. It is assumed that the time lags for individual asperities obey a certain probability density function. The synthesized strong motions reproduce well the characteristics of the observed strong motions.The above result based on the model implies that high-frequency strong motions observed during unstable sliding on a pre-existing fault are generated mainly by brittle fracture of asperities on the fault surfaces. It follows from this that acoustic emission (AE) due to brittle fracture of asperities should be observed even in the case of macroscopically stable sliding on a pre-existing fault. To examine this, an experiment of stable sliding is performed on a rock specimen with a pre-existing fault. A number of AEs are observed during macroscopically stable sliding to confirm our theoretical expectation. It is further found that the number of AEs per unit slip distance and the average magnitude of AEs are decreased with an increase in macroscopic sliding-speed.