Determination of Empirical Distance Attenuation and the Local-Magnitude Scale for Northwest Iran with JHD Technique

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 digital horizontal components of the seismic waveform data records. ML 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 ML. 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 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 MN magnitude for each event was determined and compared with ML. ML is related to MN by MN=0.9177ML 0.6159 for earthquakes with 1≤ML≤6.2 in this region. This equation indicates that ML=MN is a reasonable inference in a wide magnitude range of 3.5≤ML≤6 in the northwest of Iran.