Northern Tehran is situated within a tectonically active domain and ranks among the most seismically vulnerable regions of the country. The Mosha-Fasham Fault constitutes one of the principal seismogenic sources affecting this area. The study area of this research extends between longitudes 49°–54° E and latitudes 35°–37° N, within the Central Alborz Mountains. It is located in the southern part of the Alborz range, north of Tehran Province, and is characterized by mountainous topography, steep slopes, and deeply incised valleys. Owing to its particular tectonic setting, this area is among the most critical zones for seismotectonic and geomorphological investigations. Its geotectonic setting, structural architecture, and documented history of activity play a fundamental role in seismic hazard assessment for the capital. The high population density of Tehran further amplifies its seismic risk and potential socio-economic consequences. Accordingly, investigating temporal variations in seismicity, together with the geometric, kinematic, and present-day structural relationships of active seismogenic sources—particularly along the central and eastern segments of the fault system—is essential for deciphering the active tectonic regime and refining probabilistic seismic hazard evaluations. Given the considerable seismic potential of the Mosha-Fasham Fault, this study focuses on characterizing its seismic behavior and the prevailing stress regime. Seismicity analysis was conducted through estimation of maximum expected magnitude (M_max) and examination of spatiotemporal variations in the parameters of the Gutenberg-Richter frequency-magnitude relationship. The results indicate that temporal fluctuations of the b-value between 1990 and 2023 along the Mosha Fault zone reveal a systematic decrease in the b-value preceding moderate to strong earthquakes. Temporal variations of the b-value indicate that this parameter reached one of its relative minima by the end of 2023, which may reflect increased stress accumulation and a higher probability of larger-magnitude earthquakes. Such reductions in the slope of the Gutenberg-Richter distribution are interpreted as indicative of stress accumulation along the fault plane and increasing likelihood of forthcoming moderate to large seismic events. Spatial distribution maps of the b-value further demonstrate clustering of seismic events within zones characterized by reduced b-values. Estimation of maximum magnitude shows that the Gibowicz-Kijko-Bayes, Kijko-Sellevoll, Tate-Pisarenko, and non-parametric Gaussian approaches consistently yield M_max ≈ 7.6, whereas the Gaussian non-parametric method provides a slightly lower estimate of M_max ≈ 7.1. Overall, these findings provide critical insights into the seismotectonic behavior of the Mosha Fault system, highlighting the diagnostic significance of b-value variations and maximum magnitude estimates for assessing the probability of large seismic events and improving seismic hazard appraisal in northern Tehran; Therefore, incorporating these results into policy-making, urban planning, and seismic risk reduction strategies in the surrounding areas of this fault is essential.