The generalized earthquake-location (GENLOC) package: an earthquake-location library

We describe a library and associated set of applications for locating seismic events. The library is called the GENeralized LOCation (GENLOC) library because it is a general library that implements most methods commonly used for single event locations. The library has a flexible implementation of the standard Gauss-Newton method with many options for weighting schemes, inversion methods, and algorithms for choosing an initial location estimate. GENLOC also has a grid-search algorithm that makes no assumptions about the geometry of the grid it is searching returning only the point with a best fit solution for the specified residual norm. GENLOC supports both arrival time and array slowness vector measurements. A unique feature is the strong separation between the travel time/earth model problem and the location estimations. GENLOC can utilize data from any seismic phase for which the user can supply an earth model and method to compute theoretical travel times and/or slowness values. The GENLOC library has been used in five different working applications: (1) a simple command line program, (2) an interactive graphical user interface version used in an analyst information system, (3) a database-driven relocation program, (4) a recent implementation of the progressive multiple event location method, and (5) a real-time location program. We ran a validation test against LOCSAT and found reasonable consistency in estimated locations. We attribute observed differences in the solutions to roundoff errors in different calculators used by the two programs. (C) 2004 Elsevier Ltd. All rights reserved.