THE DESIGN OF OPTIMUM NETWORKS FOR AFTERSHOCK RECORDINGS

The analysis of aftershocks is more and more recognized by the seismological community as being an essential tool to improve the understanding of the earthquake faulting process, as well as of the various factors contributing to the generation of earthquake damage. In this context, the success or failure of aftershock monitoring programs depends to a large degree on the performance of the mobile seismic network deployed. In comparison with pure location networks, for aftershock monitoring networks the situation is complicated by the fact that they have to perform well for additional tasks such as determination of focal mechanisms, source-parameter estimation, tomographic experiments etc. We have developed a method to design optimum networks for aftershock recordings based on the technique of simulated annealing. If aftershock data are not yet available for the design of a mobile network, they are simulated for the optimization process from the mainshock-source parameters. Different objective functions to quantify network performance for each intended task are used. For the location problem it is based on the average confidence volume of hypocentral locations while for the focal mechanism determination it is derived from the distribution of rays on the focal sphere. For the tomography problem, the resolution matrix of the inverse problem is used. Furthermore, for each station individual noise levels can be assumed. Optimization for multiple tasks is performed in a sequential way with highest priority given to the location problem. The method has been applied to the Roermond earthquake of 1992 April 13. Optimum subsets of the existing ad hoc multinational temporary network for aftershock recordings of this event have been evaluated.