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

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, MLSM, 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 M-0 greater than or equal to 22.0 dyne cm and calculated local magnitudes, M-LSM greater than or equal to 3.7. The correlation between the seismic moments, log M-0, and the calculated local magnitudes, M-LSM, using strong-motion records is given by log M-0 = 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 Mt 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, v(s)(30), 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, nu(s)(30) are also presented. Finally, examination of the WA amplitude attenuation, -log A(0), with distance shows that the Jennings and Kanamori (1983) relation for Delta < 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 (similar to 800) of the Athens WA instrument on which all other M-L, relations in Greece have been calibrated.