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M_L and M_W Scales in the Iranian Plateau Based on the Strong-Motion Records

¹ Earthquake Research Center
Ferdowsi University, Mashad, Iran
P.O. Box 9177948974
j-shoja{at}seismo.um.ac.ir
s-naserieh{at}seismo.um.ac.ir
ghofrani{at}seismo.um.ac.ir

^* Present address: Building and Housing Research Center, Sheikh Fazlollah Expressway, P.O. Box 13145-1696, Tehran, Iran.

The availability of a large amount of strong-motion data recorded by the National Strong Motion Network of Iran (NSMNI) has motivated this study to develop relations for routine determination of M_L and M_W from digital horizontal components of the strong-motion records. The dataset comprises 861 two-component horizontal acceleration time series recorded for 125 earthquakes with magnitudes of 4.5 and larger. The M_L scale is based on the horizontal synthesized Wood–Anderson seismograms. We have applied the Monte Carlo technique to evaluate distance correction curves for use in determining the local magnitude, M_L, in Iran and in its northern, eastern, and Zagros subregions. Results indicate that the distance correction curves show trilinear behavior for geometrical spreading. The resulting coefficients evaluated for Iran as a whole are: R₁ = 96 ± 5 km; R₂ = 131 ± 5 km; n₁ = 1.01 ± 0.02; n₂ = –0.14 ± 0.1; n₃ = 0.14 ± 0.03; k = 0.00020 ± 0.00008. The distances less than R₁ correspond to attenuation of the direct waves. Between R₁ and R₂ is the distance where the multiply reflected and refracted shear waves from Moho dominate the arrivals. n₁, n₂, and n₃ are the coefficients of geometrical spreading for distances from the source to R₁, R₁ to R₂, and beyond R₂. k is the coefficient of inelastic attenuation. For estimating the M_W scale from the strong-motion data, we used the method proposed by Andrews (1986). To find the best correlation between the moment magnitudes measured from the strong-motion data and those measured from teleseismic data, we examined several time windows (e.g., whole trace, S-wave coda, and source time durations). The regressions show that the M_W estimates from different time windows are all equally well correlated with the corresponding reported values with nearly identical standard deviations. Finally, relations between the estimates of local and moment magnitudes for the regions show that for earthquakes with magnitudes larger than about 6.0, the M_L scale gradually becomes saturated and, therefore, it gives smaller values than those obtained by the M_W scale. However, for smaller earthquakes, the M_L scale overestimates the M_W scale. This discrepancy occurs mainly because the frequency contents of the waveforms employed in these scales are different.

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				J. Shoja-Taheri, S. Naserieh, and A. H. Ghafoorian-Nasab An ML Scale in Northeastern Iran Bulletin of the Seismological Society of America, August 1, 2008; 98(4): 1975 - 1982. [Abstract] [Full Text] [PDF]

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