Electricity networks are increasingly exposed to seismic events, with substations–critical connection points–being among the most vulnerable components. Their failure can cause extensive disruptions and trigger cascading outages across the system. Thus, resilience assessments must incorporate models that not only evaluate substation outage risk but also capture the propagation of failures triggered by seismic disturbances. In this context, this paper presents a methodological framework for assessing the seismic resilience of power transmission systems, integrating detailed component vulnerability modeling and cascading outage simulation. The proposed framework consists of four key stages: seismic hazard characterization, bay-level substation fragility analysis, system response simulation under cascading failures, and system restoration modeling. To demonstrate its applicability, the framework is applied to evaluate the impact of earthquakes on two test systems: the IEEE 24-bus test network and the Chilean transmission network. A comparative analysis is conducted between conventional models without cascading failures and the proposed cascading failure model. The results show that (a) detailed substation modeling significantly affects the extent and propagation of outages, (b) resilience metrics such as unserved energy and recovery time are underestimated when cascading effects are ignored, and (c) the proposed approach is computationally scalable and well-suited for resilience assessment of power systems exposed to natural hazards.