Tuesday evening quake

On Tuesday, 20 October, at 18:07 local time (19:07 in Lisbon), a magnitude 5.2 earthquake was felt on the Azores islands of São Miguel, Santa Maria and Terceira, according to the Portuguese Institute for the Sea and Atmosphere (IPMA). IPMA placed the epicentre about 40 kilometres west of Ginetes, in the municipality of Ponta Delgada on São Miguel. The Euro-Mediterranean Seismological Centre gave a depth of five kilometres. The Azores seismic and volcanic monitoring centre, CIVISA, put the magnitude at 5.1 in its own statement, while IPMA's figure of 5.2 was repeated in several outlets. According to CIVISA, the maximum felt intensity was V/VI on the Modified Mercalli scale in Ponta Delgada, and IPMA reported IV/V in the Ponta Delgada and Relva parishes.

No victims or damage reported

The regional civil protection service, SRPCBA, said in a statement issued at 19:37 local time that no victims or material damage had been reported so far. The agency said it was monitoring the situation together with other civil protection bodies and urged residents to stay calm. It also pointed people to official information and to self-protection measures published on its website. Early reports carried by Lusa did not include an official assessment of intensity in each locality. A SAPO report noted that the tremor was felt in several parts of São Miguel, according to accounts shared on social media.

Second and third tremors, and aftershocks

At 18:30 local time, a magnitude 3.4 quake was recorded about 37 kilometres west of Ginetes, and CIVISA said it was felt with a maximum intensity of IV in Ponta Delgada. Just after midnight, at 00:12 local time on Wednesday, 21 October, a magnitude 3.2 tremor struck about 41 kilometres west of Ginetes and was felt in Santo António, Ponta Delgada, with a maximum intensity of III/IV. By about 10:00 local time on Wednesday, CIVISA had recorded almost 100 aftershocks, all of smaller magnitude, although some above 3 were felt by residents. Rita Carmo, a CIVISA researcher and board member, told Lusa that seismic activity had decreased during the night. She cautioned that the process could take some time:

“The situation is tending to stabilise, but it may still take time to reach a new equilibrium.”

Where the quake struck and what may follow

The epicentre lies in an area known as Fossa Hirondelle, a submarine depression in the Atlantic between São Miguel and Terceira, bounded by tectonic faults. Carmo said the earthquakes are located on one of the faults that limit this depression. She said it is natural for the system to respond to new conditions as plate dynamics evolve. She added that aftershocks of a magnitude similar to, or nearly equal to, the main shock are not unusual, and cited examples from around the world. She stressed the limits of forecasting:

“We cannot predict that an earthquake with the same magnitude as the main one will occur, but it is a situation we cannot neglect.”

She urged residents to keep the self-protection measures for earthquakes in mind.