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Collaborative Study of Determination of Intermediate (100 m) and Deep (300 m) Shear Wave Velocity Profiles for the Community Velocity Model

Principal Investigator: Kenneth H Stokoe II, University of Texas at Austin

The objectives of this project are to contribute to the development of a community velocity model of the Salt Lake Valley basin in three ways:

  1. extend Vs profiles to greater depths than previously existed
  2. identify depth to R1 interface in many locations
  3. identify velocities of characteristic rock

The development of a community velocity model was the first research goal of a consensus plan developed by the Utah Ground-Shaking Working Group (February, 2004) and is cited as a priority in the USGS NEHRP FY2006 Announcement. Current studies by other investigators (Bay, 2004 and 2005) have focused on building the velocity model to a depth of 30 to 50 m. The testing proposed in this study will produce Vs profiles:

Therefore, a total of 9 to 12 intermediate (100 m) and deep (300 m) Vs profiles will be determined in the Salt Lake Valley Basin. These measurements, combined with the measurements of previous investigators, will be used to build the community velocity model of the basin that will be used by both researchers and practitioners. The testing sites will be located by the Utah Geological Survey such that they complement the information derived from previous and on-going studies. Some sites will be located near ANSS strong-motion stations.

The proposed testing approach involves actively generating and measuring surface wave motions, especially lower-frequency (1 to 4 Hz) motions. The use of surface waves minimizes the need for deep boreholes, thus making the testing approach very cost effective and efficient. The specific methodology that will be employed is the Spectral-Analysis-of-Surface-Waves (SASW) method (Stokoe, et al., 1994). The SASW method has been validated at numerous sites in a blind study comparison to conventional downhole testing (Brown et al., 2002). The application of the SASW method to very deep Vs profiling (300 m and greater) is possible due to the construction of a one-of-a-kind, low-frequency shaker under a National Science Foundation – Network for Earthquake Engineering Simulations (NSF-NEES) grant to the University of Texas. This shaker has been designed to perform in the frequency range of 0.5 to 4 Hz to permit deep Vs profiling. In the past year, deep profiling (to 350+ m) has been performed at the proposed high-level radioactive waste site at Yucca Mountain, Nevada.

This study will provide shear stiffness information about the deep sediments underlying the Salt Lake Valley. This information is currently unknown. These Vs profiles will provide a set of important data that will be used in advanced modeling of the Salt Lake Valley. Therefore, the results from this study will fill an important void in the current knowledge of the Salt Lake Valley during an earthquake. To assure the project’s contributions, it is being planned and conducted with the help and active participation of the Utah Geological Survey and the United States Geological Survey.