The availability of a large amount of seismic waveform data recorded by the Tabriz seismic network during the recent years has motivated this study to develop relations for the Empirical Distance Attenuation and the Local-Magnitude Scale with Joint Hypocentral Determination (JHD) technique for the northwest of Iran from digital horizontal components of the seismic waveform data records. M_L is a very useful scale for engineering. Many structures have natural periods close to that of a Wood-Anderson instrument (0.8 s), and the extent of earthquake damage is closely related to M_L. Therefore, any comprehensive seismic hazard analysis one needs a calibrated magnitude relationship as well as an earthquake catalog for the study region. It is a well-known fact that regional geology has a great influence on the magnitude relations. We considered a large data set of waveforms (68978 recordings for the horizontal components and 34489 for the vertical) to calibrate a local magnitude scale valid for northwestern Iran. The data refer to 6518 events occurring in the period from 1996 to 2009 in the area between 34 and 41 N and 43 and 52 E. We calculated the associated synthesized Wood Anderson seismogram for each SS-1 waveform, which is a velocity instrument. The procedure of calculating coefficients is based on the arithmetic mean of Horizontal component amplitudes. Current techniques for the development of a local magnitude scale involve the simultaneous determination of the following: the magnitudes of a number of earthquakes, site-specific correction terms for each of the recording stations, and the two constants in the equation engaged to represent the variation of amplitude as a function of distance. Therefore, we used the JHD technique for inversion. The resulting distance-correction function can be expressed as –log A₀=1.4050 log (r/100) + 0.0019 (r-100) +3.0, with amplitude A₀ in millimeters and hypocentral distance r in kilometers, showing a strong attenuation with distance. The distance-correction function can be expressed as –log A₀=1.4050 log (r/100) +0.0019 (r-100) +3.0, with amplitude A₀ in millimeters and hypocentral distance r in kilometers, showing a strong attenuation with distance. The Nuttli`s M_N magnitude for each event was determined and compared with M_L. M_L is related to M_N by M_N=0.9177M_L +0.6159 for earthquakes with 1≤M_L≤6.2 in this region. This equation indicates that M_L=M_N is a reasonable inference in a wide magnitude range of 3.5≤M_L≤6 in the northwest of Iran.