Iranian Journal of Geophysics

Iranian Journal of Geophysics

Imaging strong lateral heterogeneities across the Caspian basin and northern Iran using body-to-surface wave scattering

Document Type : Research Article

Authors
1 Ph.D. Student, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran
2 Associate Professor, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran
3 Ph.D., Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran
Abstract
Seismic scattering is commonly observed and results from wave propagation in heterogeneous medium. Body-to-surface wave scattering, originated from strong lateral heterogeneity. In this study, we present the results of an analysis on body-to-surface wave scattering in the region of Iran, performed on the teleseismic recordings of 82 broadband stations of temporary and permanent networks in Iran. The type of scattering we consider is the conversion of the SH wave to Love wave from geological scatterers. For earthquake sources, we use more than 275 events with magnitude ≥ 5.5 and we limit the epicentral distances of the events to 40°and 100°. We apply phase coherence analysis to broadband waveforms to detect coherent signals associated with body-to-surface wave scattering. The best result was obtained for  the 23 June 2014 earthquake (Mw=7.9) in the Pacific Ocean, whose scattered Love waves in our region of study traveled at a dominant period of ~25 s and a velocity of ~3.7 km/s. We utilize the converted surface waves to map strong lateral heterogeneities in the Caspian basin and the regions to its south and north.  We found 4 other earthquakes with similar results. We located the sources of scattering by back-propagating the beamformed energy using straight-ray approximation for the 5 earthquakes. Back projections suggest that strong scattering is associated with several regions; 1) the Apsheron ridge in the interior of the Caspian basin, 2) along the right-lateral strike-slip shear zone between the Caspian Basin and the Kopet Dag Mountains of NE Iran, 3) in the Alborz Mountains of northern Iran, and 4) along the ancient Paleozoic suture zone of the Ural Mountains in Kazakhstan. In the Apsheron ridge, the steep variations in the structure of the sedimentary cover of the basin is the likely source of scatterers. In the Alborz, steep topographic slope on the northern and southern limit of the mountain range might be the cause of scattering. Also, the major fault zones of the Alborz both on its northern and southern flanks could play a role. In Kazakhstan, the location of the scatterers might indicate the existence of structural steps such as those of the sedimentary sequences and the Moho in the crust. Because of the lack of stations, we could not find scatterers in other regions. The results show strong lateral heterogeneities in seismic wave velocity or density, such as fold and thrust belts, basin edges, and fault zone structures, are potentially strong scatterers.
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