CIVISA, the Azores Seismovolcanic Information and Monitoring Centre (CIVISA) has recorded nearly a hundred aftershocks following the magnitude 5.2 earthquake on the Richter scale that occurred in the region yesterday. But the situation does appear to have stabilised.
The quake was felt on the islands of São Miguel, Santa Maria and Terceira, causing no casualties or material damage.
Speaking to the Lusa news agency, Rita Carmo, a researcher and member of CIVISA’s board, said that since the main earthquake, “almost a hundred aftershocks, all of lower magnitude, with some above magnitude 3 on the Richter scale being felt by the population” had been recorded up to around 10am local time today (11am in Lisbon).
“The situation is showing a tendency to stabilise, but it may still take some time to reach a new equilibrium”, she stressed.
According to Rita Carmo, “during the night the trend was towards a decrease in seismic activity, as is natural in these circumstances”, but she warned that “this situation may still take some time until the system finds its stability”, so “earthquakes felt by the population may occur again”.
Earthquakes are common events in the region. This situation occurring in an area known as the Hirondelle Trench, an underwater depression in the Atlantic Ocean, located in the Azores between the islands of São Miguel and Terceira.
“It is a depressed basin that is bounded by tectonic faults and the location of the earthquakes is precisely on one of these tectonic structures that bounds this Hirondelle Trench”, the CIVISA researcher explained, noting that “it is natural for systems to tend to respond to the new conditions arising from the normal evolution of plate dynamics”.
Rita Carmo also said that “it is not unusual in an earthquake situation, even one of tectonic origin, for aftershocks to reach magnitudes similar, or almost similar, to those of the main earthquake”, noting that “there are several examples worldwide”.
“We cannot predict that an earthquake with the same magnitude as the main earthquake will occur, but it is a situation we cannot overlook”, so “people should keep self-protection measures related to seismic phenomena firmly in mind”, she advised.
The main earthquake had its epicentre about 40 kilometers west of Ginetes, in the municipality of Ponta Delgada, on the island of São Miguel, according to Portuguese Institute for Sea and Atmosphere (IPMA).
CIVISA has said that the tremor “was felt with a maximum intensity of V/VI (Modified Mercalli scale) in the municipality of Ponta Delgada”.
“The event was also felt with intensity V in the municipalities of Ribeira Grande, Lagoa, Vila Franca do Campo and Povoação, with intensity IV/V in the municipality of Nordeste, with intensity IV in the municipalities of Angra do Heroísmo and Praia da Vitória (Terceira island) and with intensity III in Vila do Porto (Santa Maria island)”.
According to the Richter scale, earthquakes are classified by magnitude as micro (less than 2,0), very small (2,0-2,9), small (3,0-3,9), light (4,0-4,9), moderate (5,0-5,9), strong (6,0-6,9), major (7,0-7,9), significant (8,0-8,9), exceptional (9,0-9,9) and extreme (when greater than 10).
The Modified Mercalli scale measures the “degrees of intensity and their respective descriptions”.
At intensity VI, considered fairly strong, the earthquake is felt by everyone and many people become frightened and run into the street, according to the description on the IPMA website.
At intensity V, considered strong, earthquakes are felt outdoors, people are awakened, liquids slosh and some spill over, small objects in unstable positions move or are knocked over, doors swing, close or open, while blinds and pictures move and clock pendulums stop or change their swinging motion.
In cases of earthquakes with an intensity of IV, considered moderate, “hanging objects swing, the vibration is similar to that caused by the passage of heavy vehicles or to the sensation of a heavy ball striking the walls, parked cars rock, windows, doors and dishes shake, glassware and dishes knock together or rattle and at the upper end of this degree walls and wooden structures creak”, explains IPMA.
Source material: LUSA/ noticiasaominuto