3D structure beneath Iranian plateau and Zagros using adjoint tomography

We perform an adjoint waveform tomography using a combined data set consisting of regional earthquake waveforms and Rayleigh wave ambient-noise Green's function to construct a new 3-D wave velocity model of the crust and uppermost mantle beneath the Iranian plateau. The earthquake waveforms come from 250 regional events with magnitudes of 4.5-6.5 recorded by 136 broadband seismic stations. The EGFs are derived from cross correlations of more than three years of continuous seismic noise. The inversion starts with an initial model derived from the global Crust1 model locally modified using information from previous studies. Adjoint tomography refines the initial model by iteratively minimizing the frequency-dependent travel-time misfits between real and synthetic earthquake data and EGFs and synthetic Green’s functions measured in different period bands. Our new model covers the known tectonic units such as the Central Iranian Block, Zagros fold-and-thrust belt, Sanandaj-Sirjan metamorphic zone and Urumieh-Dokhtar magmatic arc. Overall, the adjoint tomography provides images with more real resolutions and amplitudes due to the finite-frequency consideration. Using the numerical spectral-element solver in adjoint tomography provides accurate structural sensitivity kernels, which help generate more robust images rather than those generated by ray-theory tomography. Our study also demonstrates improvement of lateral resolution and depth sensitivity using combined data set instead of only earthquake data.