Shallow structure of Faryab region using two-dimensional Love wave group velocity tomography

The Sanandaj-Sirjan Zone (SSZ) extends ~1500 km from the northwest (Sanandaj) to southeast (Sirjan) parallel to the Zagros Fold Thrust belt with average width of 150–200 km. This zone is a metamorphic–magmatic belt, associated with the Zagros Orogen and part of the Alpine-Himalayan orogenic system in Iran. Its limits on either side are marked with discontinuously preserved ophiolites including the following: (1) the Neyriz-Kermanshah ophiolite situated on the northern edge of the Zagros Mountains and (2) the Khoy and Nain-Baft ophiolite complexes to the northeast (Stöcklin, 1981).
The rocks in this zone are the most highly deformed of the Zagros belt and share the NW–SE trend of surrounding structures. The zone is dominated by Mesozoic rocks; Palaeozoic rocks are generally rare but are common in the southeast (Berberian, 1995). The SSZ is characterized by metamorphosed and complexly deformed rocks associated with abundant deformed and undeformed plutons, as well as widespread Mesozoic volcanic. The ophiolites are generally regarded as preserving a record of an ocean basin or basins that lay between these elements in Mesozoic through mid-Cenozoic time, as a whole referred to as the Neotethys Ocean, with the Eurasian continent to the north, and Gondwana-land to the south (e.g., Stöcklin, 1974; Sengör, 1979; Berberian and King, 1981; Stampfli and Borel, 2002; Agard et al., 2011).
Faryab region in the Sanandaj-Sirjan zone is located in a very tectonically active zone, materialized by highly deformed metamorphic rocks, colored melange and ultramafic-mafic complexes.
An earthquake with magnitude Mw 6.0 occurred on the Faryab region, on the southeastern part of Sanandaj-Sirjan, on February 28, 2006. Aftershocks of this earthquake were used to study Love wave's group velocity. Seismic surface wave tomography of short-period dispersion curves is a useful method for studying the shallow structures of the Earth.
The main aim of this study is to apply the group velocity dispersion to Faryab region, southeast of Sanandaj-Sirjan zone, to calculate the two-dimentional Love wave group velocity tomography.
We have analyzed surface wave dispersion curves of 2616 waveforms of 437 aftershocks, (figure 1). These aftershocks were recorded by a local temporary network including 9 short period station that were installed by International Institute of Earthquake Engineering and Seismology (IIEES) during 28 Feb. 2006 to 30 Mar. 2006. The temporary stations were equipped with Guralp CMG-6TD velocity seismometer with flat frequency response between 0.1 – 50 Hz. The epicentral distance and magnitude of earthquakes were less than 50 km and larger than 1.5, respectively. The dispersion curves were calculated in the period range between 0.1 seconds to 10 of seconds, which corresponded to the shallow structure of upper crust including sedimentary layers. Surface wave tomography was also performed to estimate the two-dimensional group velocity maps of Love waves in the Faryab region. The isolated surface wave fundamental modes (and group velocity dispersion curves) have been analyzed using linear inversion method for estimation of 2D tomography maps (Yanovskaya-Ditmar; 1990). Based on the ray coverage inside the 2 × 2 km cells in the region, the estimated minimum dimension of distinct heterogeneities was about 5 km.
There are numerous anomalies in tomography maps. The range of Love waves velocity has two part: in periods shorter than 3 seconds, the velocity ranges from 0.5 to 3 km/s, and in periods above 3 second, the velocity ranges are 0.2- 1.5 km/s. It seems that in this area we are faced with two different crusts: oceanic and continental crust. Waves with a period shorter than 3 seconds pass shallower part of the crust, seem to be related to the oceanic crust that confirmed by evidence of some rocks such as Gabbro, peridotite, and ophiolites, that are exposed on the surface (figure 4). Under this oceanic layer, there is some soft sediment of continental crust. Waves with a period more than 3 seconds travel through these soft materials.