Abstract <p>4-D seismic tomography study was conducted in the northwestern region of Iran using travel time tomography during 1996 to 2016. The study area is situated within the active tectonic and seismic zones of the collision between the Eurasian and Arabian plates. This region has experienced multiple significant and destructive earthquakes in the past. Furthermore, it includes Lake Urmia, which has undergone a substantial decrease in water level by about 8 meters during the study period. To minimize the artifacts caused by uneven distribution of seismic rays, similar data sets including about 700 earthquakes in each period were selected for five periods. Subsequent experiments utilizing synthetic models demonstrated that the selected subsets of data for different time periods have the potential to identify anomalies in different periods. 3-D tomography was performed for each selected data sets, and the results were compared across different time periods. The results, focused on two depths of 8 and 23 kilometers and receiving significant attention, exhibit certain similarities in the observed anomalies across different time periods. These anomalies correspond to the tectonic structures of the region. Some differences are also observed among the various time periods. The trends of most anomalies and the differences in velocity models across different time periods are consistent with the general trends of the active faults in the area. The majority of the differences in velocity models for different time periods are observed at a depth of 8 km, along northwest-southeast-trending faults.</p>