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Introduction

The iscloc program is designed to replace procedures at the ISC that produce a definitive hypocentre for events reported by agencies around the world.  It is assumed that an event already has at least one hypocentre and that any reported phases that could contribute have already been associated with the event.   The need for a new program arose from requirements to test and implement improvements to the existing methodology.  The existing code is old and, in places, opaque making it difficult to maintain and adapt, for example additional programs are required to act as an interface between the ISC database and the current hypocentral iscloc program.  The new program is modular in design so that any particular method can be replaced by replacing the relevant function, making testing of new or alternative methods simple.  It also interfaces directly with the ISC database, making interactive editing of problem events potentially much simpler than it is at present.

The basic purpose of the program 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 processing at  the ISC is the ability to try different hypocentres as starting point if the first choice does not result in convergence to a solution.  The choice of which hypocentre to use first and the system for trying others if the first does not converge are important parts of the iscloc program.  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.

Input data will be read in from an ORACLE database at the ISC but ISF text files in bulletin format can be used as an alternative.  Likewise the resulting solution and associations can be output to either database, ISF file, or both.  An instruction line must be given to the program for each event to be located, containing at least a filename or database ID for the event.  By default these instruction lines are read from standard input but this, like the other input and output streams, can be changed by editing the configuration file .