Passive Seismic Imaging of Active and Blind Faults in the North Tabriz, Iran Region Using Integrated MSM–HVSR Analysis

Seismic hazard assessment before earthquake occurrence is a fundamental component of risk mitigation in densely populated, tectonically active regions, such as Tabriz, Iran. A critical prerequisite for detailed seismic micro-zonation is the reliable identification and characterization of both exposed and concealed seismic sources, including active and blind faults buried beneath deposits. In this study, an integrated framework combining geotechnical constraints with passive geophysical observations is employed to delineate fault zones in the northern Tabriz area, with particular emphasis on the North Tabriz Fault (NTF) system. We apply the Microseismic Sounding Method (MSM) in conjunction with the Horizontal-to-Vertical Spectral Ratio (HVSR) technique to investigate subsurface structures along a high-resolution seismic profile. Microtremor data acquired in the north Tabriz region reveal a strong spatial correspondence between MSM spectral intensity anomalies, variations in HVSR resonance frequency, and independently mapped geological features. Systematic post-fracture increases in HVSR resonance frequency indicate transitions from fractured, low-velocity materials to more competent and shallow bedrock. Vertically persistent MSM anomalies associated with low HVSR frequencies are interpreted as fault-related damage zones. The results demonstrate that the combined MSM–HVSR approach provides a robust and sensitive tool for detecting both active and blind faults, as well as resolving subsurface structural boundaries in complex urban and tectonically active environments. Furthermore, the strong agreement between passive seismic results, geological observations, and complementary geomagnetic surveys validates the reliability of MSM for fault detection and seismic hazard assessment in heterogeneous subsurface conditions.