CRUST AND UPPER MANTLE STRUCTURES OF THE MAKRAN SUBDUCTION ZONE IN SOUTH-EAST IRAN BY SEISMIC AMBIENT NOISE TOMOGRAPHY

We applied seismic ambient noise surface wave tomography to estimate Rayleigh wave empirical Green’s functions from cross-correlations to study crust and uppermost mantle structure beneath Makran region in Iran. We analysed 12 months of continuous data from January 2009 through January 2010 recorded at broadband seismic stations from a variety of seismic sensors consist of digital broadband instrument (BH) recordings from the International Institute of Earthquake Engineering and Seismology (IIEES), Global Seismic Network(GSN) and the Virtual European Broadband Seismograph Network (VEBSN). Cross-correlations were computed in daily segments between all pairs of stations and then stacked over deployed duration. We obtain group velocity of the fundamental mode Rayleigh-wave dispersion curves from empirical Green’s functions between 10 and 50 s periods by multiple-filter analysis and invert for Rayleigh wave group velocity maps. Tomographic maps, with a grid spacing of 1° × 1° ,are computed at periods of 16, 20, 24, 30 and 40 s.

The final result demonstrate significant agreement with known geological and tectonic features. Our tomography maps display low velocity anomaly with south-western north-eastern trend, comparable with volcanic arc settings of the Makran region, which may be attributable to the geometry of Arabian plate subducting overriding lithosphere of Lut block. At short periods (<20s) there is a pattern of low to high velocity anomaly in northern Makran beneath the Sistan Suture Zone. These results are evidence that surface wave tomography based on cross correlations of long time-series of ambient noise yield higher resolution group speed maps in those area with low level of seismicity and few documented large or moderate earthquake than based on traditional earthquake-based measurements.