Estimation of ML scale for earthquakes in Iran

1-Introduction
For the first time in the modern history of seismology, Richter, (1935) invented a local magnitude scale (M_L) for earthquakes for region of California. This scale is appropriate for estimation of magnitude of a wide range of earthquakes. Few investigations were performed about M_L scale for earthquakes in Iran in different areas and using different databases. Shoja-Taheri, et al. (2007), Askari et al. (2009) and Nemati et al. (2014) are the examples from M_L estimation in Iran. Shoja-Taheri, et al. (2007) used the strong motion data of NE Iran, Askari et al. (2009) used amplitude of short period seismograms in north of Iran and Nemati et al. (2014) used amplitude of waveforms of a local network data in eastern Alborz to calculate a local scale for magnitude of earthquakes in Iran.
 
2-Methodology
M_L of the earthquakes is calculated using averaging between the maximum amplitude of shear waves in the horizontal components of the waveforms of the earthquakes. Equation of M_L is a logarithmic and parametric relationship. M_L equation parameters should be determined for each seismic area. Estimation of a local magnitude scale is necessary for precisely estimation of magnitude of earthquakes in every seismic area. For calculation of M_L for the earthquake of a specific area, it is better that the parameters should be estimated using the earthquake data of the same region. In this research the M_L equation is determined for entire Iran using Iranian earthquakes, for the first time. Some researchers divide Iran into distinct seismotectonic regions, in which they calculate M_L for each area, separately. In this paper M_L is calibrated for entire Iran, because ray path of an earthquake in a specific region may cross the neighboring provinces. The chosen earthquakes for M_L determination should be the most precisely located earthquakes, because the epicentral distance is so important in the processing. For this purpose, 1409 synthetic Wood-Anderson amplitude of 229 earthquakes occurred in 24 – 42 °N and 43 – 65 °E with magnitude of 3.5 - 5.4, azimuthal gap less than 180°and RMS less than 0.5 were used. These earthquakes were recorded by seismological network of International Institute of Earthquake Engineering and Seismology of Iran between 2004 and 2016. The processing was done using Matlab software and arranging a big matrix composed of the epicentral distances, amplitudes and their functions. The amplitudes have been read using Seisan software on the horizontal components by automatic picking. In this stage, the amplitudes (velocity) are changed to the Wood-Anderson torsion seismograph scale (displacement). After reading, huge number of numbers (amplitudes, distances, earthquake coordinates,…) form the mentioned matrix equation.
 
3-Results and discussion
After the processing and inverting thematrix equation, using a parametric equation, in which geometrical spreading and inelastic attenuation were supposed, the attenuation equation for local magnitude in Iran were estimated:
 
Inelastic attenuation is related to microscopic incompletions in mineralogical structure of the minerals, existence of water or the other fluids in porosities of the rocks, discontinuities in earthcrust, friction and transforming the wave momentum energy into temperature in wave path. In this relation, 100 km for epicentral distance of the earthquakes and the constant number (3.0) for magnitude of earthquakes were put, because of Richter primary conditions for calibration of the M_L equation in 1935. The station corrections (S_j) were obtained for all of the stations representing overestimation and underestimation of M_L for correction the output magnitudes. The abovementioned numbers were put, because it is supposed that an earthquake with the magnitude of 3.0 produces 1 mm amplitude in a seismograph, which is installed in a distance of 100 km. This equation suggests more attenuation for wave amplitude for distances more than 150 km in comparison to the previously estimation of Hutton and Boore, (1987) equation.
 
4-Conclusion
1)      It could be concluded from this paper that wave amplitude attenuation decreases with distance in order of 1/r^1.0928 and geological site effects in Iran on magnitude estimation is in a broad range of 0.7.
2)      One of the preferences of this work (determination of M_L for the entire Iran) in comparison to the other works (determination of M_L for the seismotectonic provinces of Iran, individually) is that the ray path of a regional earthquake always cut more than two seismotectonic provinces.
3)      This relation is useful for the governmental institutions in Iran that calculates the M_L, like International Institute of Earthquake Engineering and Seismology of Iran.
4)      Obtained station corrections were estimated between -0.198-0.44 magnitude unit.
 
References
Askari.R., Ghods.A.R., Sobuti.F., 2009, Calibration of an ML Scale in the Alborz Region, Northern Iran, Bull. Seismol. Soc. Am., Vol: 99, p: 268-27.
Hutton. L.K., Boore.D.M., 1987, The ML scale in southern California, Bull. Seismol. Soc. Am., Vol: 77, p: 2074-2094.
Nemati, M., Ghods, A. and Ghasemi, M.R., 2014. A ML scale for the events in eastern-central Alborz. Quaterly Journal of Geoseinces, Geological Survey of Iran, 95, 115-122, (in Persian).
Richter.C.F., 1935. An instrumental earthquake magnitude scale. Bull. Seismol. Soc. Am., Vol: 25, p: 1-32.
Shoja-Taheri.J., Naserieh.S., Ghofrani.H., 2007, M_L and M_W Scales in the Iranian Plateau Based on the Strong-Motion Records, Bull. Seismol. Soc. Am., Vol: 97, p: 661-669.