1964 Rosais earthquake
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| Date | February 15, 1964 |
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| Origin time | 7:00 a.m. |
| Epicenter | 38°42′N 28°12′W / 38.7°N 28.2°W |
| Max. intensity | VII-VIII (Very strong–Severe) |
| Foreshocks | August 1962 |
The 1964 Rosais earthquake (Portuguese: Crise sísmica dos Rosais) was a series of seismic swarms occurring in February 1964, leading to two major earthquakes on the 15 and 21 of February, resulting in the destruction of most of the habitable dwellings on the western part of the island of São Jorge, in the archipelago of the Azores. The event resulted in the evacuation of approximately 5,000 residents from that island to Terceira, Pico and Faial.
The island of São Jorge is divided into two tectonic regions by tectonic fractures oriented north-northwest to south-southeast, located along an axis between Norte Pequeno and Calheta (the Ribeira Seca fault).
The northwest portion of the island is the most modern, consisting of basalt and intermediary rock (andesites) of the Rosais Volcanic Complex. This region was also covered swaths of the Manadas complex and debris from the 1580 and 1808 volcanic eruptions. The first eruption (1580) resulted in lava flows in three areas northwest to southeast: along the Ribeira da Almeida, in Queimada (south of Santo Amaro) and between Ribeira do Nabo and the beach of Cruzes (west of Urzelina). The second eruption (1808) resulted in andesite-basaltic lava flows in the southwest of Pico do Pedro, that extended to Urzelina.
The São Jorge-Pico Channel that separates the islands of Pico and São Jorge is divided by fractures and faults extending north-northwest to south-southwest. The northernmost group of faults on the island of São Jorge extend from Cerrado das Almas to the peak of Loiçano. But, the island is almost bisected by a fault that extends from the promontory of Rosais to just north of Calheta (in the southeast). The Rosais promontory, at a height of 250–400 metres (820–1,310 ft) above sea level, an area dominated by an area of scoria cones, the highest being Monte Trigo at 503 metres (1,650 ft) altitude. A final group of fractures aligned to scoria cones northwest of Velas: to the southeast there are faults towards Pico do Carvão, while to the northest of Velas are fractures responsible for the submarine eruption in 1964.
São Jorge has had a history of tectonic movements associated with the submarine fractures between the island and Pico. The most violent of these events occurred on the night of 9 July 1757, with its epicenter in Fajã dos Vimes. This event resulted in landslides, opening fractures and resulting in the destruction of dwellings in Manadas (killing one fifth of the population, 1000 people).
The February 1964 event was a phenomenon related to a submarine volcanic eruption 6.5 kilometres (4.0 mi) west-northwest of the village of Velas, 1,000 metres (3,300 ft) below sea level.
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