oa 1-D Velocity Model for use by the SANSN in Earthquake Location

Knowledge of the velocity model of an area is essential both for earthquake location and tectonic implication. Locating earthquakes using an unreliable model contributes in part to the uncertainties of active fault mapping and unexplained scatter of seismic locations. Given that we strive to continually improve our location abilities, it is necessary to always improve on the model used in the location process. The travel time inversion method was used to estimate a 1 - D velocity model that can be utilised by the South African National Seismic Network (SANSN) in seismic data analysis. It should be noted that the velocities obtained are approximately equal to the average velocity of the 3D structure within the same depth range that has been sampled by the data. In order to test the new model, it was used to relocate a sample of well recorded data from the SANSN database and results compared to previous data analyses. The new model VM1 was found to provide improved locations compared to the previously published CGS locations especially when considering the clustering of events in the mining areas as well as the observed reduction in location errors. Station corrections were calculated and showed strong lateral variations across the region. The velocity model will continue to be improved with time as more seismic stations are installed throughout the country especially in the southern part and thus more data are collected.