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

SUMMARY In this paper, we analyse the location performance of the Hellenic (Greek) Unified Seismological Network (HUSN) by Seismic Network Evaluation through Simulation method (SNES). 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 traveltime residuals to hypocentral distance. The statistical analysis of the P and S traveltime residuals allowed us to assess the appropriateness of the velocity model used by the HUSN in the location routine process. We constructed SNES maps for magnitudes () of 2, 2.5 and 3, fixing the hypocentral depth to 10 km and the confidence level to 95 per cent. We also investigated, by two different vertical sections, the behaviour 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 per cent, the Magnitude of Completeness. Through the application of the SNES method, we demonstrate that the HUSN provides the best monitoring coverage in western Greece with errors, that for  = 2.5, are less than 2 and 5 km for epicentre and hypocentral depth, respectively. At magnitude 2.5, this seismic network is capable of constraining earthquake hypocentres to depths of about 160 km and more, and provides a threshold of completeness down to magnitude 2 for most of Greek territory. We delineate some seismogenic areas of southern Greece that probably are not adequately covered by HUSN. Use of the SNES technique could optimize upgrades of the network in these areas.