Why the Sierras are so stressed-out: The effect of Long Valley Caldera inflation on faulting in the Eastern Sierra Nevada

Authors: Higbee, P., Bürgmann, R., Owen, S.E., Dreger, D.

Since 1978, an area located in the Sierra Nevada south of Long Valley Caldera, California, (here in called the Hilton Block) has been one of the most seismically active regions in California. The patterns of recent earthquake ruptures suggest previously unrecognized faults with north-south trending, sinistral strike-slip kinematics that are unusual with respect to the regional kinematics of the Walker Lane-Eastern California Shear Zone and the Basin and Range. Earthquake focal mechanisms, the spatial and temporal distribution of microseismicity, and preliminary investigation of surface lineations due to youthful crustal fracturing indicate that the Hilton Block is being faulted by a set of 3-5 NNE trending, sinistral strike-slip faults. These faults are bounded to the N by a NW trending dextral fault at the southern edge of Long Valley Caldera. The geometry of ruptures in the Hilton Block may be a consequence of the interaction between the inflation of the Long Valley magma chamber, regional Basin and Range strain, and pre-existing structural weaknesses. We evaluate the effects of these deformation sources on the Hilton Block faults with 3D boundary element models in an elastic half-space. We focus our studies on the unusual faulting patterns and high rate of seismicity, which may be attributed to the effects of induced stresses from recurring magmatic intrusions in the Long Valley Caldera, in conjunction with Basin and Range stresses acting on a pre-existing NNE-oriented fracture systems in the eastern Sierra Nevada.