The Crustal and Upper Mantle Structure beneath NW Iran: An Integrated Analysis of Surface Waves and Gravity Data

Summary Understanding the crustal seismic characteristics of tectonically active regions is crucial for seismic hazard assessment. The study conducted in NW Iran utilized surface wave tomography, radial anisotropy, and density information to analyze the complex crustal structure of the region, which is outstanding because of diverse tectonic features, sedimentary basins, and volcanic formations. By selecting a dataset of 1243 events out of over 3,500 earthquakes with M>4, and employing strict data selection criteria (such as SNR, M, Δ), the researchers calculated Rayleigh and Love wave group velocity dispersion curves using Gaussian multiple filters and phase-matched filtering. The tomographic procedure was initiated by excluding data with residuals > 2σ for enhanced stability. Individual inversions were then carried out for local Rayleigh and Love wave dispersion measurements to obtain 1D VSV and VSH models. Radial anisotropy and VS iso were determined through a discrepancy and averaging of the obtained VSH and VSV, respectively. Gravity modeling was also employed alongside surface wave analysis to understand the region's complex geology, revealing insights into upper-middle-lower crust boundaries, subsurface structures, and Moho depths. The study's velocity maps reveal significant findings related to geological units and tectonic features in various regions based on the provided results. Low velocities in the South Caspian Basin (SCB) and Kura Depression (KD) regions are attributed to substantial sedimentary layers, while low velocities, and depth of VS in NW Iran and Eastern Anatolian Accretionary Complex (EAAC) regions suggest the presence of partially molten materials in the upper and middle crust. The Sanandaj-Sirjan Zone (SSZ) region shows a low velocity anomaly in longer periods and greater depths of VS, surrounded by normal to high velocities, indicating a thick middle crust. Analyzing radial anisotropy and VS iso profiles offers insights into upper-middle-lower crust boundaries, subsurface structures, and Moho depths, highlighting middle crust thickening and lower crust thinning beneath the SSZ. The study confirms the gentle subduction of the SCB oceanic-like lower crust beneath NW Iran in the Talesh (TAL) region, with a rigid middle crust. Additionally, cross-sections reveal igneous laccoliths underplate with a VS iso of 3.7 km/s in the volcanic region. The difference observed by subtracting the velocity models at two adjacent depths, combined with parametric test results, indicates that the Sahand volcanic system is clearly identifiable, while the influence of subtle subduction on the Sabalan volcano at depths up to 30 km remains less distinct. The magma chamber beneath Sahand is situated at depths ranging from 18 to 25 km.