crustal velocity structure in fin region (zagros - iran)

the nw-se trending zagros fold and thrust belt extends for about 1,800 km from a location some 300 km se of the east anatolian fault in ne turkey to the strait of hormuz where the north-south trending zendan-minab-palami fault system (zmp) separates the zagros belt from the makran accretionary prism. the ne limit of the zagros belt is marked by the main zagros reverse fault which is rotated about a horizontal axis to form a steeply ne-dipping to sub-vertical reverse fault with a right-lateral component of movement of unknown magnitude (wellman, 1966; st?cklin, 1974 berberian, 1995). the extension of the main zagros reverse fault to the nw of latitude ~33o is referred to as the main recent fault (tchalenko and braud, 1974), and is a right-lateral strike-slip fault as indicated by earthquake focal mechanism solutions and field evidence (talebian and jackson, 2002; bachmanov et al., 2004). there is no clear surface boundary to the frontal edge of the zagros fold and thrust belt where folding is gentle both on land and beneath the persian gulf. however, the southern edge of the zagros deformation front can be defined at different levels by the shape in map view of the oil- and gas fields (talbot and alavi, 1996), and also by the seismicity and topography (jackson and mckenzie, 1984). the deformation within the zagros fold and thrust belt is due to the relative convergence between arabia and eurasia since the middle-late cretaceous period (falcon, 1974; st?cklin, 1974; koop and stoneley, 1982). however, the zagros fold and thrust belt were formed during the main phase of the zagros orogeny in the late miocene epoch to recent times (st?cklin, 1968; stoneley, 1981; hessami et al., 2001). current shortening at a rate of about 7 mm/yr (tatar et al., 2002; vernant et al., 2004; hessami et al., 2006) as well as active seismicity indicate that this deformation is still active. a moderate earthquake (mw=5.9) struck the fin region in hormozgan province on march 25, 2006 (07:29 gmt) with a little damage. the main-shock was followed by 3 aftershocks with magnitude greater than 5. after reading seismic phases and initial location of the aftershocks and using 90 selected aftershocks which had reliable accuracy, we obtained a velocity model for the upper crust beneath the studied region. calculated velocity model for the fin region showed two discontinuities at 12 and 18 kilometer depths. p wave velocity obtained was 5.65 km/s, 5.9 km/s and 6.25 km/s for the first layer, second layer and half space respectively.