Exploring the Role of Mixed-Mechanism Fracturing and Fluid-Faulting Interactions During the 2014 Long Valley Caldera, California, Earthquake Swarm

Several sequences of intense earthquake swarm activity occurred beneath the Long Valley Caldera between May and November 2014. At the height of swarm on September 26, three magnitude 3.5 events occurred within a matter of hours. The swarm has been proposed to result from an interaction between aqueous fluid, a product of the underlying volcanic system, and dominantly tectonic stress. To explore this hypothesis, we performed a stress inversion based on a high-resolution catalog of earthquake locations and focal mechanisms. We determined that the orientation of the minimum principal stress was well-constrained to be subhorizontal at an azimuth of roughly 225° to 245°. The principal coordinate system is oriented close to the vertical and horizontal directions. Assuming a vertical stress gradient of 25 MPa/km, the minimum and maximum horizontal stress magnitudes were estimated to range from 15.1 to 18.3 MPa/km and from 26.0 to 36.0 MPa/km, respectively. The mixture of both strike-slip and normal faulting focal mechanism solutions suggests that the magnitude of intermediate and maximum principal stresses are similar to each other, indicating that the maximum horizontal stress is likely to be toward the lower end of the range estimated from the stress inversion. We integrated the results of the stress inversion with the fault structure geometry inferred from the relocated seismicity and focal mechanism catalogs to develop a three-dimensional hydromechanical model of the Long Valley Caldera site. Our numerical simulations were aimed at identifying fluid-faulting interactions that may have controlled the swarm activity. In particular, we investigated the hypothesis that fluid overpressure events caused the formation of hydraulic splay fractures to occur in bursts such that fluid migrated through both preexisting and newly formed fractures in a mixed-mechanism process. We found that our modeling results were consistent with the observed earthquake sequence behavior, suggesting that mixed-mechanism fracturing may have been the process controlling fluid-faulting interactions during the swarm.