Why The Sierras Are So Stressed-Out: 
The Effect of Long Valley Caldera 
Inflation On Faulting in the Sierra Nevada
Authors: Higbee, P.*, Bürgmann, R.*, Owen, S.E.**, Dreger, D.*
phigbee@seismo.berkeley.edu, burgmann@seismo.berkeley.edu, owen@terra.usc.edu, dreger@seismo.berkeley.edu
* Geology and Geophysics Dept, University of California, Berkeley, CA 94720 
** Earth Sciences Department, University of Southern California, Los Angeles, CA 90089
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


What is the story?

Temporal and spatial distribution of seismic events south of the Long Valley Caldera indicate a correlation between caldera inflation and seismic activity in the Hilton Block.

Spatial distributions of seismic events within the Hilton Block indicate at least 3 distinct NNE trending fault structures and 2 WNW trending lineations.

Focal mechanisms indicate sinistral motions on the NNE-striking seismic lineaments, whereas the regional tectonics would suggest Basin and Range normal faulting and/or right-lateral faulting along the Eastern California Shear Zone

Hilton Block seismic events and aftershocks do not occur on the Hilton Creek Fault nor the Laurel-Convict Fault, the major NW trending normal fault structures in this region of the Sierras.

The 1997 inflationary period within the caldera appears to have distributed strain to the Hilton Block via a right-lateral lineation and then further south along the Whisky Creek Fault, producing normal and left-lateral motions.