Hellenic Unified Seismological Network: an evaluation of its performance through SNES method

Abstract

In this paper we analyze the location performance of the Hellenic (Greek) Unified Seismological Network (HUSN) by SNES method (Seismic Network Evaluation through Simulation). This method gives, as a function of magnitude, hypocentral depth and confidence level, the spatial distribution of the: number of active stations in the location procedure and their relative azimuthal gaps and confidence intervals in hypocentral parameters regarding both the geometry of the seismic network and the use of an inadequate velocity model. Greece is located on a tectonically active plate boundary at the convergence of the Eurasian and African lithospheric plates and exhibits a high level of seismicity. The HUSN monitors the seismicity in Greek territory from 2007. At present it is composed by 88 seismic stations appropriately distribute in the area of Greece. The application of the SNES method permitted us to evaluate the background noise levels recorded by the network stations and estimate an empirical law that links the variance of P and S travel time residuals to hypocentral distance. This latter permitted us to assess the appropriateness of the velocity model used by the HUSN in the location routine process. We constructed SNES maps for magnitudes (M_L) of 2, 2.5 and 3, fixing the hypocentral depth to 10 km and the confidence level to 95%. We also investigated, by two different vertical sections, the behavior of the errors in hypocentral parameters estimates as function of depth. Finally we also evaluated, fixing the hypocentral depth to 10 km and the confidence level to 95%, the Magnitude of Completeness. Through the application of the SNES method, it is demonstrated that the HUSN provides the best monitoring coverage in western Greece with errors, that for M_L=2.5, are less than 2 and 5 km for epicenter and hypocentral depth respectively. At magnitude 2.5, this seismic network is capable of constraining earthquake hypocenters to depths of about 160 km and more, and provides a threshold of completeness down to magnitude 2 for most of Greek territory. Some seismogenic areas of southern Greece, that probably are not adequately covered by HUSN were delineated. The upgrading of the network in these areas could be optimized using the SNES technique.