Seismicity in the western coast of the South Caspian Basin and the Talesh Mountains

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Zanjani, Asiye Aziz ; Ghods, Abdolreza ; Sobouti, Farhad ; Bergman, Eric ; Mortezanejad, Gholamreza ; Priestley, Keith ; Madanipour, Saeed ; Rezaeian, Mahnaz

We have studied the seismicity of the western margin of the South Caspian Basin (SCB) and the neighbouring Talesh fold and thrust belt. We have used the hypocentroidal decomposition multiple-event location technique to obtain accurate location of events recorded during 2 yr of observation. Data from a temporary seismic network in northwest Iran and other national and regional networks were combined to make an accurate assessment of seismicity in the region. Significant offshore seismicity is observed in a 50-km wide margin of the SCB. East of the Talesh Fault along the Caspian coastline, the depth of seismicity varies from 20 to 47 km. This pattern extends inland about 20–25 km west of the North Talesh Fault. This pattern of seismicity indicates that the basement slab of the South Caspian is undergoing intense seismic deformation as it is underthrusting beneath the northern Talesh, whereas the sedimentary cover deforms aseismically. The seismicity, depths, and previous focal mechanisms of the larger offshore events are consistent with low-angle underthrusting of the South Caspian floor. Within the Talesh, seismicity is mostly concentrated in the northern and southern structural arcs of the range, where deformation is more intense and complicated. Shallow crustal seismicity in the eastern flank of the Talesh is much less intense than in the western flank, where it signifies the deformation of the upper continental crust. One major observation is the lack of any significant N–S alignment of shallow epicentres inside the central Talesh to match the observed right-lateral shear deformation there. This suggests that shear deformation inside the Talesh may have a distributed nature, rather than being concentrated on a single thorough-going fault zone, as the Talesh moves northward relative to the South Caspian. We have determined a new moment tensor solution in the southwestern Talesh, with a dominant N–S trending right-lateral motion, the only solution so far confirming along-strike shear deformation in the Talesh.
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