Wide-scale sensing of natural and human-made events is critical for protecting against environmental disasters and reducing the monetary losses associated with telecommunication service downtime. However, achieving dense sensing coverage is difficult given the high deployment overhead of modern sensor networks. In this work, we offer an in-depth exploration of state of polarization (SOP) sensing over fiber-optic networks using unmodified optical transceivers, establishing a strong correlation with ground truth distributed acoustic sensing (DAS). To validate our sensing methodology, we collect 85 days of SOP and DAS measurements along two colocated, 50km fiber-optic cables in Southern California. We then examine how SOP sensing can improve network reliability by accurately modeling overall network health and preemptively detecting loss of traffic. Finally, we explore the feasibility of wide-scale seismic monitoring with SOP sensing, showcasing the SOP perturbations following low-intensity earthquakes, and the potential to more than double seismic monitoring coverage in Southern California alone.