The instability of the M (W) and M (L) comparison for earthquakes in Greece for the period 1969 to 2007

• Published in: Journal of seismology Vol. 14; no. 2; pp. 309 - 337
• Main Authors: Roumelioti, Zafeiria, Kiratzi, Anastasia, Benetatos, Christoforos
• Format: Journal Article
• Language: English
• Published: 01.04.2010
• ISSN: 1383-4649
• DOI: 10.1007/s10950-009-9167-x

We use 576 earthquakes of magnitude, M (w), 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, M (w), and local magnitude, M (L), for earthquakes in Greece and the surrounding regions. We compare M (w) to M (L) as reported in the monthly bulletins of the National Observatory of Athens (NOA) and to M (L) 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 M (w), 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 M (L) 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 M (w) and M (L) reported by Thessaloniki, we also show that the performance of the sensors has changed several times through time, affecting the calculated M (L)'s. We propose scaling relations to convert the M (L) values reported from the two centers to M (w). The procedures followed here can be applied to other regions as well to examine the stability of magnitude calculations through time.