The Dolores-Guayaquil Megashear is a broad right-lateral fault system running through the Inter-Andean valley of Ecuador, extending from the Gulf of Guayaquil in the south toward Colombia in the north. It marks the eastern boundary of the North Andean Sliver, a crustal block being pushed northeast relative to stable South America.
The system results from the oblique subduction of the Nazca Plate beneath South America, which drives the Andean block northward at roughly 8 to 10 millimetres per year. Rather than a single clean break, it comprises several linked faults and fold structures threading between the volcanoes of the Ecuadorian highlands.
This fault zone is associated with numerous damaging historical earthquakes, including events near Riobamba and Ambato that repeatedly devastated Andean towns. The catastrophic 1949 Ambato earthquake (magnitude ~6.8) killed over 5,000 people, and the megashear remains a primary source of seismic hazard for Ecuador's densely populated highland corridor.