Source mechanism is an attribute of interest in acoustic emission (AE) analysis which has been used to determine the nature of failure during laboratory experiments. In this abstract, we present the results of the moment tensor inversion of AEs recorded during the hydraulic fracturing of a Colton sandstone block at triaxial stress conditions. The source geometries given by the biaxial decomposition of the moment tensor solutions contain a family of auxiliary planes lying parallel to the orientation of the main hydraulic fracture's plane. During fracture propagation, most AEs were concentrated near the edge of the fracture, reflecting a tensile stress regime with opening angles mostly around 30 degrees. After breakdown, the pumps were reversed to withdraw fluid from the borehole. At this point, the stress field became compressive with AEs distributed across the complete face of the hydraulic fracture showing source mechanisms that reflected the closing of the fracture's plane. Closing angles were in general lower in magnitude than opening angles.