The Double-Difference Relocation of Quchan earthquakes using a 3D velocity model

In the last 150 year, most destructive earthquakes of Kopeh Dagh occurred near Quchan. These earthquakes caused large damages to Quchan (Tchalenko, 1975; Ambraseys and Melville, 1982). The Kopeh Dagh zone accommodates a motion, by a combination of slip-partitioning in the NW, thrust faulting in the SE, and anticlockwise block rotation in the Central Kopeh Dagh (Hollingsworth et al. 2006, 2008). The system of NNW–SSE right-lateral strike-slip faults in the Bakharden–Quchan fault zone between Bojnurd and Quchan is one of the most prominent structural and topographic features of the central Kopeh Dagh (Hollingsworth et al., 2006). The Kopeh Dagh is made up of a sequence of mostly conformable and complete Mesozoic–Tertiary sedimentary rocks (Stocklin, 1968; Berberian, 1976). The Kopeh Dagh form a linear intracontinental fold and thrust belt trending NW–SE between the stable Turkmenistan platform and Central Iran (Hollingsworth et al., 2006). Shortening in Iran accommodates the northward motion of the Arabian shield into Eurasia. Recent GPS measurements (McClusky et al., 2003; Vernant et al., 2004) indicate that Arabia moves approximately northwards, with respect to Eurasia, at ∼ 23 mma−1 at the longitude of the Kopeh Dagh. The Kopeh Dagh fold belt as a part of Alpine-Himalayan mountain belt in western Asia, constitutes the north-eastern border of the Iranian plateau and lies on the south-western margin of the Turan (Turkmenistan) continental crust, forming its epi-Hercynian (Early Kimmerian) cover (Berberian, 1981; Nabavi, 1983).
In this study, the Double-Difference earthquake location algorithm was applied to the relocation of a large set of seismic events that occurred in Quchan region and recorded by Quchan and Mashhad seismic networks affiliated with the Iranian Seismological Center (IRSC) during the period from 1996 to 2012. The study area extends from 35.5°N to 39°N and 56°E to 60.5°E and is located in the Kopeh Dagh major seismotectonic province. The purpose of this study is to improve earthquakes location by using Double-Difference method developed by Felix Waldhauser and William Ellsworth (2000). Relative earthquake location methods can locate earthquakes with higher accuracy by removing effects due to unmodeled velocity structure. HypoDD program determines relative locations within clusters using the Double-Difference algorithm. In order to estimate capability of Double-Difference technique in the area, we performed synthetic tests by which four datasets, each including 10 synthetic earthquakes, were considered along the Kashafrud, Quchan, Binalud and Robat-e-Qarabil faults. The single event method by hypo71 program was applied to determine initial locations. Then, Double-Difference technique by hypoDD program was used for relocating the events. The results showed significant decrease in errors using Double-Difference technique. By using the synthetic tests, capability of Double-Difference algorithm was demonstrated. Then, by putting constraints on primary data, a number of 2516 earthquakes, recorded by Quchan and Mashhad's seismic networks from 1996 to 2012, were chosen to be relocated by the latest version of hypoDD program using the Double-Difference algorithm and Mehraban’s 3D velocity model (2012). The distribution of the events in the central part of the Kashafrud fault shows that the fault is dipping northeast and the occurrence of earthquakes at different depths can be the representation of a high-angle thrust fault and the activity in the entire fault plane of this reverse fault. According to the relocation of the earthquakes and the cross sections in the north of the Shandiz-Sangbast and the west of the Quchan faults, the existence of seismic activities can represent hidden fault activity. The linearity of earthquakes to the south of Baghan-Garmab fault can be also the representation of the continuation of seismic activity in this fault, though the surface trace of this activity is not visible in the geologic maps. In the present study, the average RMS is 0.27 s in the initial locating and reaches to 0.09 s in the relocation by hypoDD using the 3D velocity model. The average of the relative horizontal and vertical uncertainties stood at 686 m and 721 m for relative relocation. The relocation using a 3D model could improve the depth distribution of earthquakes, which is more accurate than initial location. This means that it reveals the concentration of the events between the depths of 5 to 23 km. As for the constraints imposed on the initial data, we considered a minimum depth of 3 km, but the Double-Difference relocation of earthquakes using the 3D model shows that 73 earthquakes occur at depth less than 3 km with least errors.