With the advent of digital recording and advances in telemetry, there is an increasing number of datasets which include arrival times for small-to-medium-sized events at both local and regional distances. In 1991, I (Arthur Snoke) and my colleagues had such a data set, so I worked with John Lahr to introduce spherical-earth ray tracing as an option to his program Hypoellipse. Lahr subsequently included this option in his official Hypoellipse package, but until now the program for generating the traveltime tables to go with that option had not been made available. This write-up accompanies the public release of program Hypotable.
Many seismologists are surprised at how small a distance one begins to see the effect of the earth's curvature when calculating traveltimes. A plot of differential traveltimes vs epicentral distance for three focal depths is instructive. One sees that for shallow-focus events, systematic errors of at least 0.1 s are found for distances beyond the Pg-Pn crossover distance, while for deep-focus events the errors become significant at smaller epicentral distances. John Lahr and I have published a paper which expands on this theme:
Snoke, J. A. and J. C. Lahr, 2001. Locating earthquakes: At what distance can the earth no longer be treated as flat?, Seism. Res. Lett., 72, 538-541.
Click here for an HTML file which gives further details about the Hypotable package including downloading instructions.
Program Hypotable is a front-end for program Hypoellipse. Click here for the top page of the Hypoellipse distribution.
Department of Geological Sciences, 4044 Derring Hall (0420)
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061 USA; E-mail
This Page: http://www.geol.vt.edu/outreach/vtso/hypotable/
Last updated: 12 September 2003