The Septentrional Fault is a left-lateral strike-slip fault running east to west across the northern Dominican Republic, threading through the fertile Cibao Valley near Santiago, the country's second-largest city. It is a principal strand of the wider North American-Caribbean plate boundary that slices across the island of Hispaniola, part of the broader system sometimes called the Caribbean megafault.
The Caribbean Plate moves eastward past North America here, and the Septentrional accommodates a large share of that motion at roughly one to a couple of centimetres per year. Paleoseismic trenching shows it is capable of great earthquakes but has been ominously quiet for centuries, accumulating strain.
The fault is associated with the catastrophic 1842 Cap-Haïtien earthquake that devastated northern Hispaniola, and its long seismic silence since worries scientists, who estimate it could produce a magnitude 7.5 or larger event. Together with the Enriquillo Fault to the south, which caused the deadly 2010 Haiti earthquake, it makes Hispaniola one of the highest-risk islands in the Caribbean.