Global Vs30 Map Server
Wald et al. (2004) first, and Wald and Allen (BSSA, 2007, in press), more fully, describe a methodology for deriving maps of seismic site conditions using topographic slope as a proxy. Vs30 measurements (the average shear-velocity down to 30 m) are correlated against topographic slope to develop two sets of coefficients for deriving Vs30: one for active tectonic regions that possess dynamic topographic relief, and one for stable continental regions where changes in topography are more subdued. These coefficients have been applied to continental U.S. by Wald and Allen, and in other regions around the world by Allen and Wald (USGS Open File Report, 2007, in review). They also compared topographic slope-based Vs30 maps to existing site condition maps based on geology and observed Vs30 measurements, where they were available, and found favorable results.
Having a first-order assessment of seismic site conditions anywhere in the world provides a valuable tool in the prediction of ground-motions following a global earthquake, the primary motivation for this research. These Vs30 maps will enable us to better quantify possible ground-shaking and rapidly deliver these predictions to emergency managers and responders. However, the Vs30 maps for the Globe will also have practical applications for numerous related probabilistic- and scenario-based studies, hence this Web delivery service allows users to download maps and grids of seismic site conditions for specified regions.
The Global Vs30 Server allows a user to select from a map or input a rectangular region of interest. It then provides (optionally) a Vs30 grid in ASCII or GMT grid format, and a JPEG Vs30 map.
The underlying computation is a PERL Generic Mapping Tools (GMT) script that pastes appropriate sheets of the global SRTM30 database (30 arc-sec global topography, Farr and Kobrick, 2000), performs slope calculations and converts slope to Vs30 values based on the input coefficients relating Vs30 to slope. The grid can also be customized by the user if they choose to modify the predefined correlations determined by Wald and Allen.
- Allen, T. I. and D. J. Wald (2009). On the Use of High-Resolution Topographic Data as a Proxy for Seismic Site Conditions (VS30), Bull. Seism. Soc. Am., Vol. 99, No. 2A, pp. 935-943.
- Allen, T. I., and Wald, D. J. (2007). Topographic Slope as a Proxy for Seismic Site Conditions (Vs30) and Amplification around the Globe, U.S. Geological Survey, Open File Report 2007-1357, 69 pp.
- Farr, T. G., and M. Kobrick (2000). Shuttle Radar Topography Mission produces a wealth of data, EOS Trans. 81, 583-585.
- Wald, D. J., P. S. Earle, and V. Quitoriano (2004). Topographic Slope as a Proxy for Seismic Site Correction and Amplification, EOS. Trans. AGU, 85(47), F1424.
- Wald, D. J., and T. I. Allen (2007). Topographic slope as a proxy for seismic site conditions and amplification, Bull., Seism. Soc. Am, Vol. 97, No. 5, 1379-1395.
The Vs30 maps and grid provided herein are based a simplified approach and should not be considered definitive for any specific location or region. The maps and grids are provided for general purpose use, and are not intended to supplant or supercede existing, detailed Vs30 maps or measurements. Wald and Allen (2007) note significant limitations to this simplified approach. Users should be aware of these limitations and should exercise caution in using this approach for anything other than regional scale Vs30-based site amplification estimates. As always, site-specific Vs30 values should be used at finer scales or at particular locations.
The Vs30 map server also provides programatic access via a simple web-based API. Read the documentation
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