Evolution of Rupture Style with Accumulation of Fault Displacement during Large-scale Biaxial Friction Experiments

Abstract:

We report results with Indian Gabbro (Vs=3.62km/s) that are obtained from a series of large-scale biaxial friction experiments conducted at NIED. We focus on strain gage array data of stick-slip events loaded with 0.01mm/s and under 6.7MPa normal stress, and find the following: (1) During early stage when the contact surface is relatively intact, ruptures mainly behave as slow-slip events, with a transition from extremely slow slip (~ 10 m/s) to normal slow slip (~ 100 m/s). (2) With the accumulation of total fault displacement, grooves indicative of locally high normal-stress patches (i.e. asperities) are generated along the sliding surface, which are primarily elongated along the loading direction and are associated with gouge formation. On the other hand, the rest part of the surface continues being polished, indicated by a contrast in light reflectivity with respect to the initial level. At this stage, rupture speeds start to increase but are still well below the shear wave speed (~ 1/4Vs). (3) After long enough total fault displacement (> 500mm), grooves and gouges of a sufficient amount are generated. The following ruptures then show a classic behavior as documented by Ohnaka (2000), which composes of a quasi-static phase, an accelerating phase, and an unstable propagation phase. Although the terminal propagation speed usually reaches a level comparable to the shear wave speed, there is a significant variability for the earlier phases among different events, suggesting that those earlier phases are more sensitive to the evolving local fault structure and/or stress heterogeneity. Further investigation reveals that fault properties (e.g. grooves and gouges) as a function of the accumulated displacement can influence both the macroscopic and the local strain drop, which are most-likely responsible for the evolution of rupture behavior under the same macroscopic loading conditions. We aim to quantify this relation in a continued study.