Azores shook like it hadn't happened in years. Specialist explains earthquake

ANDram 18:07 Tuesday, October 20, when the land shook in the Azores. São Miguel was the most affected island, but the earthquake of magnitude 5.1 on the scale of Ritcher also shook the island of Santa Maria and Terceira.

Despite not to have caused victims or major damage, the intensity of the earthquake and the the fact that they were registered close to a hundred replicas it has raised some concerns.

In view of this, the News to Minutes she spoke with Rita Carmo, researcher and member of the director of CIVISA, the entity responsible for information and seismovolcanic surveillance of the Azores, to clarify some issues.

The Vulcanology expert first confirmed that “it was one of the largest earthquakes recorded in the region in recent years and therefore a relevant event from the seismological point of view.”.

“ A magnitude of 5.1 corresponds to an earthquake of moderate magnitude, but with the capacity to be strongly felt by the population and, depending on the location, depth, geological conditions and characteristics of the buildings, to cause damage”, he explained, recalling that “it is important, however, to distinguish the magnitude, which characterizes the dimension of the earthquake, the intensity, which describes the effects felt in a given location”.

“ In this case, the Centro de Informação e Vigilância Sismivulcanica dos Açores (CIVISA) estimated a maximum intensity of V/VI on the scale of Modified Mercalli, in the municipality of Ponta Delgada ", also recalled.

“The Azores are in a geodynamically complex area”

According to Rita Carmo, “the earthquake, with an epicenter about 39 km west of Ginetes, on the island of São Miguel, had a tectonic origin and the epicenter was located in the sea, in the region of Fossa Hirondelle, west of São Miguel”.

“The Azores region is in a geodynamically complex area, called the Triple Junction of the Azores, associated with the interaction between lithospheric plaques through tectonic fracture systems. The sudden release of energy accumulated in these structures can cause earthquakes of this nature”, he said.

On the other hand, the Hirondelle Fossa “corresponds to an underwater basin, whose configuration is controlled by tectonic structures. The earthquake was located in the failure that delimits the basin to the south”.

The seismic activity in this region is known to researchers and authorities and “object of monitoring and study, including through the seismic network and the work carried out by the competent scientific entities”.

However, she noted the geologist of the Institute for Research in Vulcanology and Risk Assessment (IVA), “the fact that the region is known and monitored does not mean that it is possible to predict when an earthquake will occur, what its magnitude or how a seismic sequence will evolve”.

Why was earthquake so felt? “There are several factors”

Regarding the “various” factors that help explain how the earthquake was felt, Rita Carmo listed: “First, the magnitude of 5.1 indicates that a significant amount of energy has been released. Secondly, the epicenter was relatively close to the island of São Miguel, about 39 km west of Ginetes”.

“Beyond the magnitude and location of the epicenter, the depth of the earthquake, local geological conditions and the characteristics of the buildings are also determinant in the way the vibration is felt and in their possible effects on the surface“, the researcher noted, adding that “seismic energy propagates through the rocks in all directions and can be felt at considerable distances”.

This is what happened in this case: the earthquake was felt in São Miguel and in the Terceira and Santa Maria islands, although with different intensities. The maximum intensity of V/VI was recorded in the municipality of Ponta Delgada, and the earthquake was felt in the other municipalities of São Miguel with less intensity. In the Third, the intensity was IV, while in Vila do Porto, in Santa Maria, it was III.

“ These differences in intensity result, among other factors, from distance to epicenter, from the propagation of seismic waves and geological characteristics of sites. As seismic waves propagate, the energy diminishes, although local conditions may influence the intensity of the effects felt, Rita Carmo clarified.

Almost a hundred replicas? “Redistribution of stress”

As for the approximately one hundred replicas that took place in the region after the earthquake of 5.1, the geologist stressed that “after a earthquake of significant magnitude, it is common for other smaller earthquakes to occur in the same region”.

“ The main earthquake causes a change in the stress state of the rocks and in the equilibrium conditions of the tectonic structures involved. This redistribution of tensions can trigger new movements not only in the structure where the main earthquake occurred, but also in other nearby failures, which seek to readjust themselves to the new equilibrium conditions. These movements release energy in the form of new earthquakes, called replicas”, clarified.

The expert recalled that most of the replicas recorded since Tuesday afternoon were “of low magnitude”, although some, “with magnitudes greater than 3, were also felt by the population”.

The evolution of the situation, however, points to a “trend of decreased activity, although the sequence can still continue and new earthquakes can occur felt by the population”.

“Number of replicas does not mean that danger is increasing”

According to Rita Carmo, “the number of replicas alone does not necessarily mean that danger is increasing”. To evaluate the evolution of the situation, it is necessary to analyze several parameters together, namely “the frequency of events and their evolution over time, the magnitude of replicas and the eventual occurrence of larger earthquakes, the spatial distribution of epicenters and the depth of events, as well as possible changes in the pattern of seismic activity”.

Regarding the question of whether the number of replicas is within the expected number, the researcher explained that “there is no fixed number of replicas expected for an earthquake of a certain magnitude. The number and duration of the sequence vary from case to case. Many replicas occur in the first hours and days, followed by a progressive decrease in frequency.”.

“The current trend points to a decrease and stabilization of the activity, but this does not allow to ensure that there are no new replicas felt by the population or, eventually, a greater earthquake. The message must therefore be serenity, without devaluing the phenomenon. CIVISA continues to monitor the situation and it is important for the population to remain informed through official channels and to comply with self-protection recommendations“, he concluded.