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Experts concerned with abnormal seismic activity – Azores


Experts in the Azores have alerted the population of São Miguel Island for seismic activity which they considered to be above normal. The seismic events where located in the system of Fogo and Congro lakes, the central region of the island.

The situation is ongoing and the number of microseisms is slightly above reference values” said Wednesday João Luís Gaspar from the Centre for Volcanology and Geological Risks Assessment (CVARG) of the University of the Azores. He also said that the seismic activity was the result of “very low magnitude earthquakes” adding that “none of which have been felt by the population.”

Declining to comment on what the evolution of the crisis could be João Luís Gaspar recalled that “seismological activity is difficult to predict,” but that the possibility could not be eliminated of an earthquake occurrence which may be felt by the population. He said, “It does not mean it will happen; only that one cannot eliminate that possibility.”

According to the CVARG website, the Fogo-Congro system is one of the most critical seismologic areas of the archipelago where accumulated tensions exist resulting from the lithospheric interplay of the Eurasian, African and American plates.

The Azores are sitting on the so called “Azores triple junction” area where the North American, Eurasian and African plates meet. This area is responsible for frequent geological and volcanic activity on the islands. Periodic volcanic eruptions and earthquake events, with occasional tidal waves, can have devastating effects on the Azorean islands. (PAJ)

Magnitude 4.9 earthquake struck 162 km (100 miles) S of Santa Cruz das Flores and 244 km (151 miles) WSW of Horta, Azores (37.996°N, 31.344°W) at depth of  12.2 km (7.6 miles) on October 19 at 06:23:43 UTC.

About Azores archipelago – geology, seismicity and volcanism

The Archipelago of the Azores  is composed of nine volcanic islands situated in the middle of the North Atlantic Ocean, and is located about 1,500 km (930 mi) west from Lisbon and about 3,900 km (2,400 mi) east from the east coast of North America. The islands, and their economic exclusion zone, form the Autonomous Region of the Azores, one of the two autonomous regions of Portugal.

There are nine major Azorean islands and an islet cluster, in three main groups. These are Flores and Corvo, to the west; Graciosa, Terceira, São Jorge, Pico and Faial in the centre; and São Miguel, Santa Maria and the Formigas Reef to the east. They extend for more than 600 km (370 mi) and lie in a northwest-southeast direction. The vast extent of the islands defines an immense exclusive economic zone of 1,100,000 km2 (420,000 sq mi). The westernmost point of this area is 3,380 km (2,100 mi) from the North American continent.

All the islands have volcanic origins, although some, such as Santa Maria, have had no recorded activity since the islands were settled. Mount Pico, on the island of Pico, is the highest point in Portugal, at 2,351 m (7,713 ft). The Azores are actually some of the tallest mountains on the planet, measured from their base at the bottom of the ocean to their peaks, which thrust high above the surface of the Atlantic. The islands of the Azores emerged from what is called the Azores Platform, a 5.8 million km² region that is morphologically accented by a depth of 2000 meters.

From a geostructural perspective the Azores is located above an active triple junction between three of the world’s large tectonic plates (the North American Plate, the Eurasian Plate and the African Plate), a condition that has translated into the existence of many faults and fractures in this region of the Atlantic.

The principal tectonic structures that exist in the region of the Azores are the Mid-Atlantic Ridge, the Terceira Rift, the Azores Fracture Zone and the Glória Fault. The Mid-Atlantic Ridge is the main frontier between the American Plate and the African-Eurasian Plates that crosses the Azores Platform between the islands of Flores and Faial from north to south then to the southwest; it is an extensive form crossed by many transform faults running perpendicular to its north-south orientation, that is seismically active and susceptible to volcanism. The Terceira Rift is a system of fractures that extends from the Mid-Atlantic Ridge to the Glória Fault that represents the main frontier between the Eurasian and African Plates. It is defined by a line of submarine volcanoes and island mounts that extend northwest to southeast for about 550 km, from the area west of Graciosa until the islets of the Formigas, that includes the islands of Graciosa, Terceira and São Miguel. Its northwest limit connects to the Mid-Atlantic Ridge, while the southeast section intersects the Gloria Fault southeast of the island of Santa Maria. The Azores Fracture Zone is that extends from the Glória Fault and encompasses a relatively inactive area to the south of the islands of the Central and Eastern groups north to the Terceira Rift, along a 45º angle. The Glória Fault, for its part, extends 800 km along a linear line from the Azores to the Azores-Gibraltar Transform Fault.


The island’s volcanism is associated with the rifting along the Azores Triple Junction; the spread of the crust along the existing faults and fractures has produced many of the active volcanic and seismic events,while supported by buoyant upwelling in the deeper mantle, some associate with an Azores hotspot. Most of the volcanic activity has centered, primarily, along the Terceira Rift. From the beginning of the island’s settlement, around the 15th Century, there have been 28 registered volcanic eruptions (15 terrestrial and 13 submarine). The last significant volcanic eruption, the Capelinhos volcano (Vulcão dos Capelinhos), occurred off the coast of the island of Faial (in 1957); the most recent volcanic activity occurred in the seamounts and submarine volcanoes off the coast of Serreta and in the Pico-Jão Jorge Channel.

The islands have many examples of volcano-built geomorphology, including: many of the caves and subterranean lava tubes (such as the Gruta das Torres, Algar do Carvão, Gruta do Natal, Gruta das Cinco Ribeiras), the coastal lava fields (like the coast of Feteiras, Faial, the Mistério of Prainha or São João on Pico Island) in addition to the currently inactive cones in central São Miguel Island, the aforementioned Capelinhos on Faial, the volcanic complexes of Terceira or Plinian caldeira of Corvo Island.

Although all islands have experienced volcanism during their geological history, within recorded “human settlement” history the islands of Santa Maria, Flores, Corvo and Graciosa have not experienced any volcanic eruptions; in addition to active fumaroles and hot-springs, the remaining islands have had sporadic eruptions since the 14th Century. Apart from the Capelinhos volcano in 1957–58, the last recorded instance of “island formation” occurred off the coast of São Miguel, when the island of Sabrina was briefly formed.

Owing to its geodynamic environment, the region has been center of intense seismic activity, particularly along its tectonic boundaries on the Mid-Atlantic Ridge and Terceira Rift. Seismic events although frequent, usually tectonic or vulco-tectonic in nature, but in general low to medium intensities, occasionally punctuated by events of level 5 or greater on the Richter scale. The most severe earthquake was registered in 1757, near Calheta on the island of São Jorge, which exceeded 7 on the Richter scale. In comparison, the 1522 earthquake, mentioned by Gaspar Frutuoso the historian, was only a magnitude 6.8, but responsible for the destruction of Vila Franca do Campo and landslides that may have killed less than 5000 of the inhabitants. (Wikipedia)



Centro de Vulcanologia e Avaliação de Riscos Geológicos, Universidade dos Açores

Centre for Volcanology and Geological Risks Assessment (CVARG)

Instituto de Meteorologia: Seismology (Seismic map)

  Volcanoes of Azores (GVP)


Azores Hazards – Earthquakes, Volcanoes, Landslides, Tsunamis

Destructive Earthquakes and Present-Day Seismicity

Since the settlement of the Azores more than 30 important earthquakes and countless seismic crises resulted in thousands of deaths and severe damages. The first reported destructive earthquake took place on the October 22nd, 1522, and reached intensity X (MM-56) on São Miguel Island. Vila Franca do Campo, the capital of the Azores at that time, was completely destroyed and more than 5,000 people died, many of which buried by major debris flows associated with two main landslides triggered by the earthquake.

The Azores strongest earthquake occurred on the July 9th, 1787, with epicentre close to São Jorge island north coast and caused over 1,000 deaths. The most recent devastating events were the January 1st, 1980, 7.2 magnitude earthquake which affected Terceira, São Jorge and Graciosa islands causing the dead of nearly 60 people, and the July 9th, 1998, 5.8 magnitude earthquake that hit Faial and Pico islands causing 8 casualities while 1,700 people were left homeless.

The instrumental data for the last thirty years clearly demonstrates that most of the seismic activity in the Azores region takes place along the TR and at the MAR. Both tectonic and volcanogenic seismic crises were frequent during this period.

Volcanic Eruptions, Unrest Episodes and Degassing Areas

The first noticed Azores historical eruption occurred at Furnas Volcano and its final stage coincided with the settlement of São Miguel Island, sometime between 1439 and 1443. Since then at least 28 volcanic eruptions of different nature and magnitude caused several human victims, killed animals and buried infrastructures and productive soils.

The majority of the subaerial events was dominated by strombolian and hawaiian eruptive styles producing basaltic scoria and lava flows. Eruptions of this nature occurred at São Miguel (1563 and 1652), Terceira (1761), São Jorge (1580 and 1808), Pico (1562, 1718, 1720) and Faial (1672). More explosive eruptions took place at São Miguel Island central volcanoes involving magmatic and hydromagmatic phases (1563, 1630). In the 1630 Furnas Volcano subplinian hydromagmatic eruption approximately 100 people lost their lives due to pyroclastic surges. During this event important landslides changed the morphology of the nearby coastal zone while pumice and ash fall covered almost all the island reaching as far as Santa Maria island about 80 km to the south.

Many of the recorded historical eruptions in the Azores region were of submarine origin (Chaves, 1960). In all the Azoreans memory stays the 1957-58 volcanic eruption of Capelinhos Volcano, close to the western coast of Faial Island (Machado, 1959). The event had surtseyan characteristics during the first stages of activity and changed to strombolian and hawaiian styles when the accumulation of ashes around the vent prevented the entrance of seawater into the crater. A major consequence of this eruption was the immigration of more than 4,000 people to the United States.

On December 1998, another submarine volcanic eruption started 10 km NW offshore Terceira Island, being characterized by the production of the so-called basaltic lava balloons. This Azorean type of submarine volcanic activity is believed to be related with very fluid and gas-rich basaltic magmas. The reinterpretation of several historical records of submarine volcanic events shows that this type of eruption is not so rare all over the world.

More recently, in 2005, a volcanic unrest episode occurred in São Miguel Island, in the Fogo-Congro volcanic system, involving magma rising to shallow reservoirs. The phenomenon was characterized by an intense seismic activity and ground deformation that caused surface ruptures and triggered more than 250 landslides in the central zone of the island.

Present-day volcanic activity is expressed by hydrothermal manifestations characterized by low temperature fumaroles, steaming grounds, soil diffuse degassing areas and thermal and cold CO2 springs. The spatial distribution, intensity and physicochemical characteristics of such phenomena are depending from the seismic and volcanic activity and can be influenced by the meteorological conditions. The presence of degassing phenomena in (1) inhabited areas, (2) visited lava structures and even (3) open-air depressed zones constitutes an important public health issue (Baxter et al., 1999; Viveiros et al., 2009). In 1992 two people died at Graciosa Island when visiting the Furna do Enxofre lava cave and more recently in São Miguel and Faial islands some people were evacuated from their homes due to the presence of high indoor CO2 concentration.


The Azores are a region particularly vulnerable to slope instability due to geological, geomorphologic and meteorological factors. The volcanic nature and morphology of the islands, namely the existence of steep slopes developed on incoherent materials, condition the occurrence of landslides, which are typically triggered by earthquakes, volcanic eruptions or extreme meteorological events.

The previously mentioned events of 1522 and 1630 typify landslides related with earthquakes and eruptions, respectively. However, the meteorological conditions are the most common cause of this hazard. One of the most recent events occurred on October 31st, 1997, and was triggered by an extreme rainfall episode. Nearly 1000 shallow slope movements, mainly translational slides and debris-flow, were generated and one of them caused 29 casualties at the Ribeira Quente village in São Miguel Island. During this event 114 residents were left homeless, 36 houses were destroyed, several roads were cut, water and energy supply systems suffered serious damages and areas of fertile land became covered by mud and debris.


In the last 500 years at least 12 tsunamis generated by earthquakes or landslides reached the Azores archipelago. Recent studies revealed that the higher magnitude event was the teletsunami generated by the 1755 Lisbon earthquake, with epicentre in the North Atlantic, SW of Portugal. The tsunami attained the Azores with a maximum run-up of about 15 metres in Terceira Island causing the death of 6 people. Local earthquakes, like the ones of 1757 or 1980, generated small tsunamis that didn’t cause any victims.

Rather important in the Azores are the tsunamis caused by landslides. Flank volcano collapses and high magnitude sea cliff landslides are known to have occurred in pre-historical times in several islands and some historical reports allow reconstructing more recent events. The major reported landslide-triggered tsunami took place in 1847, when a rotational slide occurred in a sea cliff with more than 350 metres high at the NW coast of Flores Island. The entrance of material into the sea originated a wave that affected both Flores and Corvo islands, being responsible by the death of 10 people and more than 100 injured. (Earthzine – Geological Hazards and Monitoring at the Azores)


Fogo Volcano (São Miguel, Azores): a hazardous edifice (GEOmorphologie)


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  1. Excellent article: well wrtten, packed with research data & understandable illustrations.

    Thank you for taking the time to do good justice to this important subject. I really appreciate what you’ve taught me.


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