According to the 2023 Azores Seismicity Chart, published on the website of the Azores Seismovolcanic Information and Surveillance Centre (CIVISA), the tremors felt were justified by the seismic activity recorded in the Santa Bárbara volcano, on Terceira Island.

According to the document, the most energetic earthquake felt in the region, in September, was recorded in this volcanic system.

“This earthquake was felt with maximum intensity V/VI in the parishes of Santa Bárbara, São Bartolomeu, Terra Chã and São Mateus”, CIVISA stated in a statement.

In terms of geographical distribution, the Azores Seismicity Map indicates that “most of the locatable seismicity” was centered on the Santa Bárbara volcano - Terceira (seismogenic zone SZ24), west of Faial (seismogenic zone SZ43) and on the island of São Jorge (seismogenic zone SZ36).

The largest number of earthquakes was recorded in July (22), “with the seismovolcanic crisis at the Santa Bárbara volcano contributing in large part to this number”, it reads.

The Charter was published by CIVISA as part of the VOLRISKMAC II project, funded by the INTERREG V-A Spain-Portugal MAC Cooperation Programme (Madeira-Azores-Canary Islands) 2014-2020, and in collaboration with the Institute for Research in Volcanology and Risk Assessment of the University of the Azores.

The publication follows the commitment made by the Information Centre in 2018 to make the document available annually “on an accessible platform and free of charge, with the aim of promoting greater public awareness of the seismic activity occurring in the region”.

In addition to the annual seismicity map of the archipelago, the publication presents monthly seismicity maps, the map showing the configuration of CIVISA's Permanent Seismic Monitoring Network, as well as the map showing the seismogenic areas defined for the archipelago and which are subject to monitoring.

The daily and monthly distribution of the number of recorded and felt events, as well as the seismic energy released, is also indicated.

Regarding the seismicity felt, according to the same report consulted today by the Lusa news agency, it occurred predominantly in the months of July (22 felt earthquakes) and September (19), followed by May (16), December (13) and October (11).

The months with the lowest number of recorded events were January (three), February, March and November (six each), April and June (seven each) and August (eight).

According to the Richter scale, earthquakes are classified according to their magnitude as micro (less than 2.0), very small (2.0-2.9), small (3.0-3.9), slight (4.0-4.9), moderate (5.0-5.9), strong (6.0-6.9), large (7.0-7.9), important (8.0-8.9), exceptional (9.0-9.9) and extreme (when greater than 10).