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
.