Seismic interferometry ambient noise tomography—a study in the north-central part of Iran

Abstract

Seismic interferometry is one of the noteworthy new subsurface imaging methods that have been used in recent years in many tomographic studies. In this study, the interstation estimated Green functions extracted from the Rayleigh and Love wave ambient noise cross-correlations are used to obtain the Love and Rayleigh wave group velocity maps in different periods across the study area in order to determine the velocity anomalies and their corresponding geological structures within the crust. In the ambient noise method, the quality of the cross-correlations increases when longer time series of data are used, so we used 22 months of continuous data from the 17 broadband seismic stations as the primary database. After performing the initial corrections of the ambient noise data and obtaining the 3-month cross-correlation time series, the longer time series were obtained by stacking the 3-month time series with acceptable quality. In the next step, by using the time-frequency analysis and the estimated Green functions, the Love and Rayleigh wave dispersion curves were determined. Then, using the 2D tomography method, the Love and Rayleigh wave group velocity maps for the periods 10–25 s were produced. In the short periods, a low-velocity zone is observed in the Love and Rayleigh wave group velocity maps from the south Caspian basin toward central Iran, which is likely due to the effect of the thick alluvial deposits and the intermountain sediments in Alborz. At the longer periods, the Rayleigh and Love wave group velocity maps show a low-velocity zone in central Alborz, while in the eastern and western parts of Alborz, they show higher velocities, which is consistent with the prior studies. The dense path coverage permits to produce images that have substantially higher lateral resolution than is currently available from global and regional group velocity studies. Tomographic maps at high frequencies are well correlated with the upper crust structure and especially with sediment layer thickness.