[SSS14-P06] Temporal variations of reflected and transmitted wave during a slide-hold-slide experiment on a biaxial friction testing machine
Keywords:slow slip event, biaxial friction experiment, gouge
The features of observed waveforms were classified clearly by a slide-hold-slide conditions into two categories: sliding and holing. We chose two materials as gouge powder, which were clay mineral (Ca- montmorillonite) and quartz. Regardless of the kinds of the gouge powder, the amplitude both of reflected and transmitted waves showed opposite temporal variation during the slide-hold-slide sequences; the amplitude of transmitted wave decreased during slides and increased during holds, respectively. This suggests that the amplitudes of reflected and transmitted waves indicate a change of an elastic-wave speed in the gouge layer.
The travel times of transmitted waves increased during slides and decrease during holds. According to Birch (1961), a propagation speed of elastic waves normally decreases as the density of media decreases, or as the porosity increases. Increasing travel times of transmitted waves in our experiments may indicate the change of the density, or porosity in the gouge layer. Although the travel times of reflected waves decreased during slides and increased during holds in the case of clay mineral, the travel time on quartz powder did not change through our experiment.
The temporal change on correlation of observed transmitted wave also shows significant changes from a slide to a hold. The change of travel time, the amplitude, and the cross-correlation coefficient of transmitted waves were more sensitive to a slide-hold-slide condition, such as slip rate on the gouge later than those of reflected wave. Those result suggests that a transmitted wave is more suitable for monitoring SSE although a reflected wave is also useful for monitoring, and that a beginning and an end of an SSE can be detected by reflected and transmitted waves.