HR: 09:35h
AN: V51F-07    [Abstracts]
TI: Seismic Source Evaluation of Possible Magmatic Related Earthquakes in the Yellowstone Volcanic System
AU: * Taira, T
EM: taira@seis.utah.edu
AF: Department of Geology and Geophysics, University of Utah, 135 South 1460 East, Salt Lake City, UT 84112, United States
AU: Smith, R B
EM: rbsmith@mines.utah.edu
AF: Department of Geology and Geophysics, University of Utah, 135 South 1460 East, Salt Lake City, UT 84112, United States
AB: The accelerated uplift of the Yellowstone caldera, 2004 to present (Chang et al., 2007) with uplift rates up to 7 cm/yr encouraged us to investigate the possible occurrence of earthquakes with fluid interaction. We initially chose an event with the deforming area, 1 September 2004, a Mc 3.3 normal-faulting earthquake. The magnitude is one of the largest since the beginning of the current uplift in Yellowstone. We examined the source characteristics of the event for such properties as earthquake as static stress drop, apparent stress, and radiation efficiency. The stress drop was estimated from the observed P-wave spectra by employing an empirical Green's function technique. We used the stacked spectra of 12 small collocated earthquakes ( Mc ~1.4) as a proxy for empirical Green's functions to correct attenuation effects along the source-station paths. Using a bootstrap technique, we determined the reliable range of the corner frequency at each station. Our evaluation revealed a stress drop is 47 MPa and its 95% confidence level is 8 to 109 MPa. To minimize the error in the estimation of stress drop, we used a 3-D local seismic velocity structure in Yellowstone (Husen et al., 2004). Given an appropriate density model and assuming hydrostatic pore pressure, the predicted maximum shear stress for normal faults at the focal depth is 26 MPa which is about half of the estimated stress drop. One of the possible explanations for the stress drop is pore pressure reduction around the focal depth. The total seismic radiated energy and the apparent stress were measured, assuming the ratio of P- wave to S- wave corner frequencies because signals for the direct S-wave are clipped. We found that the total energy ranged from 9.0 × 10⁹ J to 9.6 × 10¹⁰ J, corresponding to apparent stress of 0.3 to 3 MPa, assuming the shear modulus of 32 GPa derived from local density and seismic velocity models. The estimated range of the apparent stress is consistent with similar earthquakes in the Lang Valley caldera. We also evaluated the radiation efficiency defined as the ratio of the apparent stress to the stress drop. Since estimates in stress drop and apparent stress are model dependent, we normalized the estimated radiation efficiency by the predicted one for a reference event with stress drop of 3 MPa. We found that the radiation efficiency for the Yellowstone earthquake is up to 30% less than one for the reference earthquake. Our result suggests that the rupture process of the initially evaluated earthquake involves relatively large energy dissipation. Additional analyses of several earthquakes occurring in close time and spatial proximity to the uplift area will also be shown.
DE: 7215 Earthquake source observations (1240)
DE: 7280 Volcano seismology (8419)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting