The instability of the MW and ML comparison for earthquakes in Greece for the period 1969 to 2007

Abstract

We use 576 earthquakes of magnitude, Mw, 3. 3 to 6. 8 that occurred within the region 33° N-42. 5° N, 19° E-30° E in the time period 1969 to 2007 to investigate the stability of the relation between moment magnitude, Mw, and local magnitude, ML, for earthquakes in Greece and the surrounding regions. We compare Mw to ML as reported in the monthly bulletins of the National Observatory of Athens (NOA) and to ML as reported in the bulletins of the Seismological Station of the Aristotle University of Thessaloniki. All earthquakes have been analyzed through regional or teleseismic waveform inversion, to obtain Mw, and have measured maximum trace amplitudes on the Wood-Anderson seismograph in Athens, which has been in operation since 1964. We show that the Athens Wood-Anderson seismograph performance has changed through time, affecting the computed by NOA ML by at least 0.1 magnitude units. Specifically, since the beginning of 1996, its east-west component has been recording systematically much larger amplitudes compared to the north-south component. From the comparison between Mw and ML reported by Thessaloniki, we also show that the performance of the sensors has changed several times through time, affecting the calculated ML's. We propose scaling relations to convert the ML values reported from the two centers to Mw. The procedures followed here can be applied to other regions as well to examine the stability of magnitude calculations through time. © 2009 Springer Science+Business Media B.V.