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An M_L Scale in Northeastern Iran

Earthquake Research Center, Ferdowsi University of Mashad, Mashad, Iran jshoja{at}yahoo.com saeid.naserieh{at}gmail.com Aghafoorian{at}yahoo.com

Local-magnitude scales are derived for northeastern Iran (Khorasan province) from waveform data recorded at six stations from 205 local earthquakes, ranging in distance from 10 to 600 km. By averaging the horizontal components in a single measure, we used 1506 zero-to-peak amplitudes from synthetic Wood–Anderson seismograms to determine, in a least-squares sense, the appropriate -logA₀ attenuation functions, the event local magnitude, and the station corrections. Both a parametric and a nonparametric description of -logA₀ is considered while performing the inversions. In both cases, the constraint of 1-mm motion recorded at 100 km for M 3.0 earthquakes was used. To evaluate the distance correction curves in determining the local magnitude, M_L, in northeastern Iran we applied both linear and trilinear inversions to our datasets. The result of the linear inversion for distance correction is given by: -logA₀=(1.370±0.050)log(R/100)+(0.0020±0.0001)(R-100)+3. For trilinear inversion we have applied the Monte Carlo technique. The resulting coefficients evaluated for the area are R₁=106±5 km; R₂=347±49 km; n₁=1.380±0.045; n₂=0.597±0.132; n₃=0.415±0.236; k=0.0033±0.0003, where 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. The remarkable agreement between the parametric and nonparametric results confirms that both linear and trilinear attenuation functions that we made for deriving the parametric distance correction are equally reasonable. Moreover, inversion of bootstrap replications of our dataset furnished stable solutions. Station magnitude corrections range between -0.17 and 0.27, suggesting a variable and noticeable effect of station-site properties on recorded amplitudes.