A seismic swarm at São Jorge began at 1605 on 19 March along a WNW-ESE fissure system stretching from Ponta dos Rosais to Norte Pequeno - Silveira. The seismic data as well as deformation data indicated a magmatic intrusion, though by early April no significant deformation was detected. The frequency of earthquakes had decreased in late May-early June, leading CIVISA to lower the Alert Level to V3 (on a scale of V0-V6) on 8 June. Seismicity continued to be elevated; by 18 July a total of 43,410 low-magnitude tectonic events had been recorded.

Geological Summary. The dominantly basaltic São Jorge Island is 55 km long and 6.5 km wide. It was formed by fissure eruptions beginning in the eastern part of the island. The western two-thirds of the island contains youthful, fissure-fed lava flows resembling those on neighboring Pico Island. Lava effused from three locations above the south-central coast during 1580, producing flows that reached the ocean. In 1808 a series of explosions took place from vents along the south-central crest of the island; one of the vents produced a lava flow that reached the southern coast. Submarine eruptions have also been reported on several occasions from a submarine ridge to the SE. The 1964 event offshore W of Velas was considered "probable" by Madiera and Brum da Silveira (2003), who also provided 14C dates for several other Holocene eruptions.

Source: Centro de Informação e Vigilância Sismovulcânica dos Açores (CIVISA)