HR: 1330h
AN: V52B-0441    [PDF]
TI: Seismic Character of an Active Silicic Volcanic System: Long Valley Caldera, California
AU: * John, B E
EM: bjohn@uwyo.edu
AF: Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071
AU: Fountain, D M
EM: dfountai@nsf.gov
AF: NSF, 4201 Wilson Blvd., Arlington, VA 22230
AB: Knowledge of crustal structure in the Long Valley Caldera region is key for interpretation of active tectonic and magmatic processes, and for volcano hazard prediction. Measured Vp and Vs from large diameter core collected in drill holes within the caldera to 1.2 km depth provide a reference section through intra-caldera units; core collected from basement sampled in LVEWIII provides data beneath the resurgent dome. Together these samples provide insight into the physical properties of an active, silicic volcanic system. Seven samples of basement (banded, meta-pelite) from the lower part of LVEW III (2220-2853m) were measured at confining pressures from 0-400 MPa, and ambient temperature. No measurements were made of highly fractured rocks recovered from the hole due to experimental limitations. Sample density varies from 2.60-2.76 g/cc, all with porosity $< $0.5%. P-wave velocity at 100 MPa varies from 5.28-5.72 km/sec; Vs 3.22-3.63 km/sec, with a strong linear correlation between velocity and density. Vp/Vs varies between 1.52-1.72. Measured values are consistent with logging-based estimates of Vp to within 4%. Anisotropy in Vs (based on orientation of compositional layering) ranges from 0.5-9%. Intra-caldera volcanic deposits have heterogeneous seismic velocities that vary linearly with density (1.20-2.76 g/cc). P-wave velocities at 100 MPa for rhyolites (including early rhyolite, Bishop Tuff, and moat rhyolite) range from 2.52-5.24 km/sec, and show complete overlap with experimental results for rhyolite melt; Vs for the same samples range from 1.51 to 3.34 km/sec. Late intra-caldera basalt, have Vp of 2.59-5.72 km/sec, and Vs 1.59 to 3.08 km/sec. A detailed seismic velocity study of the Bishop Tuff (up to 250 meter thick) from both intra-caldera and outflow samples, suggests that porosity, density, and therefore velocity are all functions of compaction and welding. Densely welded samples with porosities between 2 and 4%, have Vp $>$ 4.65-4.88 km/sec and Vs $>$2.8 km/sec, whereas those showing little welding (up to 30% porosity), have Vp from 2.64-4.46 km/sec and Vs 1.53-2.79 km/sec. Vp/Vs varies from 1.58 to 1.82. P-wave anisotropy varies systematically relative to compaction foliation, and ranges up to 4% (Vs %<%2.5%). These relations suggest that the high-velocity base of a densely welded tuff (up to tens of meters thick) could act as a wave-guide for seismic waves if deposited directly above low velocity sediments, or thick unwelded air fall deposits.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 7280 Volcano seismology (8419)
DE: 8015 Local crustal structure
DE: 8145 Physics of magma and magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting