This study focuses on analyzing the crustal velocity structure at the Rudbar Dam located in Lorestan Province, using advanced earthquake tomography techniques. Local earthquake tomography is a powerful tool that provides crucial insights into various subsurface structures by generating a three-dimensional (3-D) velocity model of the region. Understanding crustal velocity structures and the anomalies that can arise within them is vital for improving our comprehension of fault geometries and other subsurface features, such as sediment layers, which are essential for regional seismic studies. By detecting these structural anomalies, we can significantly enhance our understanding of the area's tectonic behavior, contributing to better-informed assessments of seismic risks.
   The research included data from 920 initial earthquakes, which were meticulously recorded by a network of seven short-period seismic stations strategically positioned around the dam. To calculate the crustal structure in this area, we utilized the SimulPS12 software, a robust tool designed for processing and analyzing seismic data to produce accurate velocity models.
Initially, we developed and presented a one-dimensional (1-D) velocity model, which serves as a foundational step in the analysis. Following this, we employed the Joint Hypocenter Determination (JHD) method to relocate the earthquakes more precisely, ensuring the accuracy of the subsequent three-dimensional (3-D) velocity model. The resulting 3-D velocity model images clearly depict the region's geological features, particularly highlighting the anticlines characterized by high-velocity cores and the synclines with low-velocity cores. These findings align well with the existing geological evidence, further validating the accuracy and reliability of our model.
The study also incorporates a four-dimensional (4-D) tomography approach to analyze temporal changes in the velocity structure. In this context, two distinct time windows were considered for data analysis: one prior to dewater and another after the injection process. This temporal analysis allowed us to observe significant changes in seismic activity, with the seismicity cluster shifting either towards the northwest or southeast following dewater. This movement suggests a correlation between the injection process and the observed seismicity patterns, offering insights into how human activities can influence seismic behavior.
    By employing 4-D tomography, we concluded that fluctuations in pore pressure, induced by dewater, can have a substantial impact on the mechanical resistance of fault sections. This, in turn, influences seismic activity along the active faults in the region. These findings underscore the importance of monitoring subsurface pressure changes, as they can have direct implications for seismic hazard assessments, especially in regions with active fault systems like Lorestan.