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Moment-magnitude relations based on strong-motion records in Greece

Institute of Engineering Seismology and Earthquake Engineering (ITSAK), P.O. Box 53 , GR 55 102, Finikas, Thessaloniki, Greece margaris{at}itsak.gr, costas{at}itsak.gr

Abstract

In this work, the variation of the local magnitude, M_LSM, derived from strong-motion records at short distances is examined, in terms of moment magnitude, M_W. Strong-motion data from Greek earthquakes are used to determine the strong-motion local magnitude, M_LSM, by performing an integration of the equation of motion of the Wood-Anderson (WA) seismograph subjected to an input acceleration. The most reliable strong-motion data are utilized for earthquakes with seismic moments log M₀ 22.0 dyne · cm and calculated local magnitudes, M_LSM 3.7. The correlation between the seismic moments, log M₀, and the calculated local magnitudes, M_LSM, using strong-motion records is given by log M₀ = 1.5*M_LSM + 16.07, which is very similar to that proposed by Hanks and Kanamori (1979). Moreover, it is shown that M_LSM is equal to moment magnitude, M_W, for a large M_LSM range (3.9 to 6.6). Comparison of the strong-motion local magnitude and the M_L magnitude estimated in Greece (M_LGR) and surrounding area shows a systematic bias of 0.4 to 0.5, similar to the difference that has been found between M_W and M_LGR for the same area. The contribution of the local site effects in the calculation of the local magnitude, M_LSM, is also considered by taking into account two indices of soil classification, namely, rock and alluvium or the shear-wave velocity, v³⁰_s, of the first 30 m, based on NEHRP (1994) and UBC (1997). An increase of M_LSM by 0.16 is observed for alluvium sites. Alternative relations showing the M_LSM variation with, v³⁰_s are also presented. Finally, examination of the WA amplitude attenuation, –log A₀, with distance shows that the Jennings and Kanamori (1983) relation for < 100 km is appropriate for Greece. The same results confirm earlier suggestions that the 0.4 to 0.5 bias between M_LGR and M_W (also M_LSM) should be attributed to a low static magnification (800) of the Athens WA instrument on which all other M_L relations in Greece have been calibrated.

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