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
log A0 attenuation functions, the event local magnitude, and the station corrections. Both a parametric and a nonparametric description of
log A0 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, ML, 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:
log A0  1:370  0:050 logR=100  0:0020 0:0001R
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. Introduction The Khorasan province in northeastern Iran (the area under study) extends from 30° to 40° north latitude and from 52° to 62° east longitude (Fig. 1). The major tectonic provinces of the region are the Kopeh Dogh folded belt (Tchalenko, 1975), the eastern Alborz, and the central and eastern Iran province, an area of complex block movement. Near its southwest corner, the region approaches the Zagros main thrust line. Along its eastern edge, the region is bound by the north-south trending Harirud fault that, despite its current aseismic character and pre-Jurassic age (Stöcklin, 1974), serves as a boundary between the aseismic zone of western Afghanistan and the highly seismic region of northeastern Iran (Shoja-Taheri and Niazi, 1981). The Khorasan province, as a region with a high level of seismic activity, has suffered numerous destructive earthquakes throughout its long history, including four major ones in 1968, 1978, 1979, and 1997. Local magnitude, ML, was the first magnitude scale to describe

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