The interaction of tectonic and magmatic processes in the Long Valley caldera, California

The Long Valley caldera of eastern California is a hazardous province where tectonic and magmatic processes interact to drive on-going seismicity and deformation. In this dissertation, I investigate the interaction between tectonic and magmatic processes in the Long Valley caldera over a range of scales. To gain a first order understanding of the mechanics by which Long Valley deforms, I relocated seismicity in Long Valley to establish fault locations and orientations. From the focal mechanisms associated with individual faults, I developed a kinematic model for seismic deformation in the area. Seismicity within the caldera occurs primarily on a set of east/west-trending right-lateral faults in the caldera's south moat. Since the south moat is located in a left step of the Sierra Nevada range-bounding faults, the south moat shear zone forms a “transform” zone between loci of regional extension.; To understand better the mechanical interaction of tectonic and magmatic processes at the regional scale, I performed a series of focal mechanism stress inversions in the caldera area. The inversions show that around the caldera the minimum compressive stress is perturbed from the more regional WNW-ESE direction to a NE-SW orientation. Dislocation modeling of the mapped stress field reveals that the stress perturbation may reflect the large-scale left-stepping offset in the Sierran range-bounding normal faults. Thus, the direction of fault slip seems to be controlled by regional tectonic processes rather than local magmatic processes.; To understand the relationship between tectonic and magmatic processes at the scale of the earthquake source, I examine the source processes of earthquake Although most earthquakes in the area are typical “tectonic” events, a small number of earthquakes show magmatic signatures. A close examination of a seismicity swarm on November 22, 1997 in the caldera reveals that the swarm was triggered directly by magmatically derived fluids. Thus, although earthquakes slip in accordance with the regional tectonic stress field, magmatic activity can trigger seismicity by decreasing the effective normal stress across faults.