Reinvestigation of the 1989 Mammoth Mountain, California seismic swarm and dike intrusion

During the period of unrest and inflation beneath Long Valley, east central California, between 1988 and 1992, a swarm of seismic activity occurred beneath Mammoth Mountain, on the caldera rim in 1989 followed by the onset of diffuse CO2 degassing. These events, combined with elevation changes along a leveling line north of Mammoth Mountain suggested a dike intrusion. An early inversion of deformation data resulted in a dike model with an unrealistically small amount of opening (13 cm) for the proposed dimensions. The current reinvestigation of the 1989 seismic event at Mammoth Mountain, CA incorporates relocated earthquakes (Prejean et al., 2003) and ellipsoidal inflation sources beneath the caldera to the east (Langbein, 2003) to better constrain the location and dimensions of a proposed dike intrusion beneath Mammoth Mountain. Assuming that the dike coincides with a “keel” of seismicity at 7–9 km depth, most of the dislocation parameters can be constrained. Fixing all dike parameters except opening to coincide with the keel, inversion of the local leveling data yields a dike with 3.3 m of opening. Allowing the dike height to vary as well improves the fit to the data significantly and results in a dike 1 m wide and nearly 7 m high. These results indicate that the intruded dike had significantly more opening than previously modeled. Magma volumes for reasonable models center around 0.015 km3, suggesting a small, likely basaltic, intrusion.