Optimization of earthquake locations in northwest of Iran during the period 2006-2013 using azimuth and ray parameters

An important aim of seismology is to determine focuses of earthquakes. The more precisely the process is carried out, the better results will be achieved in various studies, for example, in vestigation of the crustal structure and seismogenic zone or the determination of the plane of faults causing earthquakes. The classic methods of locating are based on Geiger's equations (1910). The relation between the arrival time and coordination of the earthquake
is linearized in this method using the first term of the Taylor's expansion and a velocity model available for the study area. The optimum responses in the linearized methods are obtained by minimizing the differences between the observed and calculated data using iterative Least Squares (L.S.) method.
A considerable error is usually observed in determination of earthquake focus because of the absence of a proper azimuthal bearing, as well as the mere use of the travel time data. With the purpose of reduction of the errors, researchers have used azimuth and ray path parameters for optimization of their performance.
The study area is located within 44-50^o E and northern latitude 36-40^oN, which is located in northwestern Iran, forming part of the central Alborz-Azerbaijan tectonic region. In this research the earthquakes of a magnitude equal or greater than 1.4 in the body wave scale occurred in the area within the period from 2006 to 2013, included 11,200 events, first were located using only P wave traveltimes and then process was repeated by adding ray path and P wave azimuthal parameters data. To this effect, the aforesaid earthquakes were classified into two general classes, consisting of earthquakes of magnitudes 1.4-3.4 and earthquakes of magnitudes exceeding 3.4. Concerning the first class of earthquakes, processing was made only for the data obtained from the stations in the local seismic network of the Institute of Geophysics, University of Tehran. The velocity model used in this part of the work was local Velocity Model (Bayramnejad, 2008) that is specific to the study area. Because of a large number of earthquakes, each of the aforesaid classes of earthquakes were subdivided into two subclasses consisting of the earthquakes with azimathal gaps less than 180 degrees and those with azimuthal gaps greater than 180 degrees. The results obtained by the earthquakes locating are represented by histograms which indicate that utilization of ray parameter and azimuthal parameter considerably reduce the horizontal error in relocating of earthquakes, specially for the earthquakes with the azimuthal gaps exceeding 180 degrees compared to utilization of mere travel time data. For the second class of earthquakes, we have used all the data from the stations located within the whole seismic range of Iran. It is evident that locating of earthquakes is carried out with a greater precision where more data is available. The velocity model used in this part was similar to the one used for the whole. Optimization of relocating of earthquakesare indicated by histograms for the magnitude of 3.4-4.6earthquakes.Results indicated reduction in the horizontal error when the azimuthal and ray path parameters have been used. For the magnitude 4.6-6-2 earthquakes, it is proved, by drawing a certainty ellipse, that the use of azimuthal and ray parameters may optimize locating of earthquakes,and lower the dimensions of the certainty ellipse.