The ISC is making available source code for a flexible earthquake location program. This is the program used in day-to-day processing at the ISC. The program was written to allow these methods to be changed easily and make the testing of new or alternative methods simple. As provided, the program implements the algorithms used at the ISC for phase reidentification, residual calculation using Jeffreys-Bullen travel time tables, and the iterative revision of focal parameters using the Method of Uniform Reduction. It includes routines that calculate depth phase depth and body-wave and surface-wave magnitudes in the way that they have been calculated at the ISC. In an addition to current ISC procedures a function is provided for applying a crustal correction to travel times using the 2x2 degree global crustal model made available by Gabi Laski on the REM web site. All of this functionality is implemented in a modular way so that to change methods it is only necessary to replace the discrete subprograms concerned.
The basic purpose of iscloc is to give the best possible source time and location for a seismic event given the arrival data available. This is done using Geiger's method, taking an existing hypocentre as a starting point and iterating to reduce travel time residuals for as many phases as possible. At the moment P and S residuals are used but other phases could be added if desired. A feature of this program is the ability to automatically try different hypocentres as a starting point if the first choice does not result in convergence to a solution. The hypocentres input for an event are ranked in order of their suitability as starting points by a simple algorithm and will be tried in this order unless the operator specifies which starting point to use. If a solution is found for an event body wave and surface wave magnitudes are calculated if suitable amplitude data is available and a depth phase depth is calculated if enough pP phases are reported. Data is input and output from the program as ISF bulletin format text files. The program is fully configurable with all required constants stored in a non-compiled parameter file.
Full documentation for iscloc can be consulted online here.
The C source code and makefile can be downloaded as a gzipped tar file after filling in a very brief form.
Also downloadable is a tar file of the HTML documentation.