Anisotropic properties of fracture zone in sandstone inferred from seismic moment tensors of acoustic emissions

We present an inversion for elastic anisotropy parameters of rocks using a large set of accurate moment tensors (MTs) derived from acoustic emissions (AEs). This method is demonstrated using AEs observed in a sandstone sample during a semi-circular bend test. We inverted 539 highly accurate MTs of AEs and retrieved both the orientation of anisotropy axes and elastic parameters defining the orthorhombic anisotropy within the fracture zone that developed in the sample during the test. The anisotropy results from the presence of aligned cracks in the fracture zone and the background transverse isotropy of the sandstone specimen. The observed anisotropy is moderate, with strengths of 18%, 14% and 21% for the P, S1 and S2 waves, respectively. We show that neglecting this anisotropy introduces a significant bias when estimating crack orientation and tensility from MTs. When anisotropy effects are accounted for by recalculating moment tensors into source tensors, the scatter in crack orientations is reduced, and the slope angle, which characterizes crack tensility, is systematically increased by approximately 10°. Our results confirm that the presented inversion method is a powerful and robust tool, capable of analyzing anisotropy in rocks, even in cases of low anisotropy symmetry.