HR: 0800h
AN: T21A-0442    [Abstracts]
TI: Testing new Empirical Relations Between Elastic Wavespeeds in the Earth's Crust Using SAFOD Main Hole Logs
AU: * Brocher, T M
EM: brocher@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025 United States
AB: I test new empirical relations between compressional-wave (Vp) and shear-wave (Vs) velocity for the Earth's crust using SAFOD Main Hole logs from Phase 1 and 2 drilling. In a recent paper, Brocher (BSSA, Dec. 2005) compiled thousands of observations of Vp and Vs for a wide variety of common rock types to define new empirical relations between these properties. The new nonlinear relations are intended to provide a useful average fit between Vs and Vp, much the way that the Nafe-Drake curve provides an average fit between density and Vp. One of the most useful of these relations is Vs (km/s) = 0.786 - 1.2344Vp + 0.7949Vp² - 0.1238Vp³ + 0.0064Vp⁴, for Vp between 1.5 and 7.5 km/s. This relation yields a strongly non-linear Poisson's ratio versus Vp function that can be approximated as a linear decrease in Poisson's ratio from 0.50 to 0.24 for Vp between 1.5 and 4.1 km/s, and a linear increase in Poisson's ratio from 0.24 to 0.27 for Vp between 4.1 and 8.5 km/s. The empirical relation is a regression of data from Vs and Vp borehole logs in Quaternary alluvium and sedimentary rocks in California, Salinian granites from the SAFOD Pilot Hole, as well as global averages of laboratory measurements of crystalline rocks over a 6-km/s-wide range in Vp. SAFOD Main Hole logs sampling Salinian granite in the upper 2/3 of as borehole, as well as sedimentary rocks (Great Valley Sequence?) in the lower 1/3 of the borehole, provide an independent test of the new empirical relation. Using the new empirical relation to calculate Vs from the Vp log over predicts the observed Vs log for the Great Valley Sequence (?), having an average Vp of 4.46 km/s, by an average of 0.10 km/s, and over predicts the entire SAFOD Main Hole log by an average of 0.12 km/s. This close fit indicates that the new empirical relation provides a useful approximation for Vs in sedimentary rocks as well as for a wide-variety of other common rock types. The fact that Vs observations match the empirical relation in the SAFOD Main Hole, which penetrates at least two active strands of the San Andreas fault, implies that there is nothing particularly exceptional about Vs and Vp relations near or within active fault zones.
DE: 0905 Continental structures (8109, 8110)
DE: 0915 Downhole methods
DE: 5102 Acoustic properties
DE: 7205 Continental crust (1219)
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: Fall Meeting 2005