HR: 1340h
AN: V53B-1319    [Abstracts]
TI: Shear-Wave Splitting From Local Earthquakes as an Indicator of Crustal Stress at Yellowstone
AU: * Waite, G P
EM: gpwaite@mtu.edu
AF: Dept. Geol. and Mining Eng. and Sciences, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931, United States
AU: Chang, W
EM: wchang@earth.utah.edu
AF: Dept. Geol. and Geophysics, University of Utah, 135 S. 1460 E., Salt Lake City, UT 84112, United States
AB: Our analysis of split shear waves from local earthquakes on the Yellowstone Plateau is consistent with the complex stress and strain fields determined from previous studies. Splitting fast directions (φ) and delay times were determined using a cross-correlation algorithm for a subset of data from well-located earthquakes recorded from 1993-2006. Seismic anisotropy in the crust can be caused by aligned platy minerals, layering of bedding or foliation, and aligned microcracks. Stress in the crust aligns microcracks such that cracks with faces normal to the maximum compressive stress (σ₁) close, while cracks with faces oriented normal to the minimum compressive stress (σ₃) may open. This creates a stress-dependent anisotropy where φ is perpendicular to σ₃. Previous studies, which used earthquake focal mechanism inversion, GPS, and InSAR, showed that crustal stresses and strains at Yellowstone are influenced by regional extension as well as transient deformation. The σ₁ direction is generally near vertical, but σ₃, which is nearly horizontal, varies from roughly N-S near the surface rupture of the 1959 M7.5 Hebgen Lake earthquake to NE-SW near the rim of the Yellowstone Caldera 35 km east of the fault. The agreement between our shear-wave splitting results and previous stress and strain field studies suggests that the inferred anisotropy is due to stress-oriented microcracks in the upper crust. The spatial variation in the σ₃ direction has been modeled as due to postseismic viscoelastic relaxation in the lower crust-upper mantle following the Hebgen Lake normal-faulting earthquake combined with regional NE-SW extension. Changes in the directions of anisotropy observed at Ruapehu Volcano in New Zealand over a few years were attributed to changes in the stress field associated with pressure changes in the magmatic system. But despite well-documented temporal deformation on similar time scales at Yellowstone there are no corresponding temporal trends in the shear-wave splitting data. As the number of three-component stations in Yellowstone increases, we will be more likely to detect possible temporal changes in anisotropy.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 7203 Body waves
DE: 7205 Continental crust (1219)
DE: 7218 Lithosphere (1236)
DE: 8164 Stresses: crust and lithosphere
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting