Changes in AE similarity track fault kinematics during laboratory earthquakes

It is now recognized that acoustic emissions (AEs) generated during stick-slip frictional sliding (i.e., lab earthquakes) can be considered as microearthquakes. Over the past decades, many laboratory AE studies have addressed issues related to the physics of earthquakes such as fault nucleation and growth in brittle rocks, frequency magnitude statistics of earthquakes, laboratory earthquake precursors and, more recently, laboratory earthquake prediction based on machine learning techniques. Here, we conduct double direct shear experiments on samples of Westerly granite under applied normal loads of 5-15 MPa and with shearing rates of 1-100 μm/s. We use template matching and other cross correlation techniques to study the evolution of AE similarity during the laboratory seismic cycle. The aim of this study is to connect changes in AE similarity to fault stress-loading and kinematics. AE similarity is derived from the correlation matrices of AE catalogs and is found to vary primarily with fault slip velocity. AE similarity is, on average, constant at slow speed (fault slip velocity <= 10 mm/s) and drops as fault slip velocity increases. Our observations show that AE similarity follows a power law of fault slip velocity. Based on previous experimental and theoretical works, we suggest that AE similarity reflects the evolution of fault contact area. One interpretation of our results is that a simple metric such as AE similarity carries relevant information about fault kinematics and fault structural properties that may be used for forecasting and prediction of failure.