Owing to its geodynamic environment, the region has been a center of intense seismic activity, particularly along its tectonic boundaries on the Mid-Atlantic Ridge and Terceira Rift. Seismic events although frequent, are usually tectonic or vulco-tectonic in nature, but in general are of low to medium intensities, occasionally punctuated by events of level 5 or greater on the Richter magnitude scale. The most severe earthquake was registered in 1757, near Calheta on the island of São Jorge, which exceeded 7 on the Richter magnitude scale. In comparison, the 1522 earthquake that was mentioned by historian Gaspar Frutuoso measured 6.8, but its effects were judged to be X ("Extreme") on the Mercalli intensity scale, and was responsible for the destruction of Vila Franca do Campo and landslides that may have killed more than 5,000 of the inhabitants.