Focal Mechanism Inversion of Microseismic Events at the First EGS Collab Testbed

Continuous microseismic monitoring was performed to monitor the fracture growth in the repeated fracture stimulation experiments at the first EGS Collab testbed. Previously, the detection and location of microseismic events was achieved using automatic triggering, manual picking, and double-difference location. The focal mechanism inversion is challenging because of the high-frequency (~several kilohertz) signature of the meso-scale monitoring. To determine focal mechanisms of meso-scale microseismic events, we first use the continuous active source seismic monitoring (CASSM) data to determine the orientation of accelerometers using the hodogram analysis. We then use 3-D tomography velocity models obtained from the campaign cross-borehole seismic data and CASSM data, and the event location information to calculate 3D Green’s functions using finite-difference modeling. We develop a hybrid full-waveform moment-tensor inversion method to determine the moment tensors of those microseismic events. We first grid-search the strike, dip, and rake, and then search for the full moment tensor using the simulated annealing method.