Program Hypotable

This information is also located on Arthur Snokes web site.

Chapter 85.7 of the IASPEI International Handbook of Earthquake and Engineering Seismology 

Earthquake location programs for events recorded at regional to teleseismic distances have traditionally used traveltime tables, such as the Jeffreys-Bullen tables which are based on spherical-earth ray tracing in a laterally-homogeneous earth. Such traveltime tables may not give accurate locations for events located with stations at local to near-regional distances because of regional differences in the crustal and upper-mantle velocity structure — the regions of the earth in which the first arrival P and S waves spend most of their time for such events. Until recently, it was a non-trivial task to create traveltime tables for different spherical-earth velocity structures. Accordingly, for the past 30+ years locations for local and near-regionally recorded events have used programs which used flat-earth ray tracing through constant-velocity layers. Examples of such programs are Hypo71, Hypoinverse, Fasthypo, and Hypoellipse.

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 a reprint.

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.


Arthur Snoke
Department of Geological Sciences, 4044 Derring Hall (0420)
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061 USA
Phone: 540-231-6028; Fax: 540-231-3386; e-mail: snoke@vt.edu
Webpage: http://www.geol.vt.edu/profs/jas/
Last updated: 21 May 2002