HIGH-RESOLUTION DOUBLE-DIFFERENCE SEISMIC TOMOGRAPHY OF THE CRUST AND UPPER MANTLE BENEATH AZERBAIJAN: EVIDENCE FOR A SUBDUCTING OCEANIC SLAB
Keywords:
Seismic tomography; Double-difference method; Crustal structure; Upper mantle; Caucasus; Azerbaijan; Subduction; Seismicity; West Caspian FaultAbstract
This study presents new insights into the crustal and upper mantle structure of Azerbaijan based on high-resolution local seismic tomography using the double-difference (TomoDD) method. Travel-time data from approximately 7,600 local earthquakes (M ≥ 2.0), recorded by a regional network of broadband seismic stations between 2011 and 2024, were used to jointly relocate earthquake hypocenters and reconstruct three-dimensional P- and S-wave velocity models of the crust.
The results reveal pronounced lateral and vertical velocity heterogeneities associated with major tectonic units, including the Greater and Lesser Caucasus, the Kura–Araks Depression, and the South Caspian Basin. Low seismic velocities at shallow depths reflect thick sedimentary cover in the Kura Depression and Absheron Peninsula, whereas higher velocities at depths of 20–35 km indicate a high-velocity lower crust beneath the basin. These features are interpreted as evidence for the presence of oceanic-type crust beneath the Kura–Araks Depression. Relocated earthquake hypocenters delineate several seismically active structures, including the Main Caucasus Thrust and the West Caspian Fault, the latter expressed as a linear zone of deep-focus seismicity oriented NNW–SSE. The identification of two distinct seismogenic horizons in the eastern Greater Caucasus supports the hypothesis of ongoing subduction of a remnant oceanic lithosphere beneath the region. The results provide important constraints on regional geodynamics and have significant implications for seismic hazard assessment, particularly for densely populated areas such as Baku.
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