The seismic crisis in São Jorge soon enters its third week. The number of earthquakes slightly diminished, but this Wednesday the island has been shaken by a 3.8 magnitude quake. The depth of the earthquakes is decreasing, people are leaving the are of Velas. A lot of information has been published in the past days making it difficult for a non-professional to follow. Where next in São Jorge?

Dr. Szabolcs Harangi, the Head of the Institute of Geography and Earth Sciences of ELTE University of Budapest and Head of the Volcanology Research Group published on March 30 a concise and understandable summary of what is going on in the Tűzhányó Blog on Facebook. If you want to understand what is happening right know, what do we know, what is the history and where next in São Jorge, take the time to read the full article.

Where next in São Jorge?…

On 19 March, the earth suddenly started to move on the island of São Jorge, 55 km long and 6.5 km wide, which is part of the Portuguese Azores. Overnight, hundreds of earthquakes occurred, many of which were felt by the island’s population of 8000. What is the cause? For several days there was debate among experts as to whether this was caused by a purely tectonic, i.e. structural movement of the earth, or by the pushing up of magma?

Continuous activity

As the days went by, increasing data, such as the analysis of satellite images of radar wave propagation (InSAR surveys), which show the vertical displacement of the earth’s surface to within millimetres, suggested that magma might be pushing up in the earth’s crust. The INVOLCAN team, trained on the volcanic activity of the Canary Islands, calculated that the upward pushing of some 20 million cubic metres of magma could be the cause of the small uplift.

Meanwhile, the number of earthquakes continues, with more than a hundred recorded daily and some >3M in magnitude. Last night, a 3.8M quake struck, just as I was telling my students to watch out for a major quake, after which the earthquake count would suddenly drop. Well, the earthquake count hasn’t gone down yet, and today’s seismogram is still showing a thick line of signals, which can now be followed live (links at the end of the article). The sequence of events is remarkably similar to what happened last year on the Reykjanes Peninsula in Iceland and La Palma in the Canary Islands. The case of São Jorge resembles more the first one.

The Azores, created by volcanoes

The Azores are located close to the Mid-Atlantic Ridge, along a so-called transform fault zone that cuts the ridge diagonally, but many experts also speculate that a deep mantle upwelling (‘plume’) may have contributed to the formation of the islands. The islands are aligned along a north-west-south-east zone, and the shape of São Jorge follows this exact direction. Nearby Faial, Pico, Terceira are all active volcanic areas. This arrangement, in my opinion, tends to confirm that volcanic activity is determined by the thrusting movements of the transform fault. Although volcanic activity in this area is not frequent, it is not rare.

The last volcanic eruption on the island of São Jorge was in 1808, and before that in 1580, but in 1964 the earth near the island opened up on the oceanic shelf and an underwater volcanic activity occurred. The most recent volcanic eruption occurred west of Terceira Island, also underwater (1998-2001). An eruption not just like any other, it was similar to the one 10 years later of El Hierro, the westernmost of the Canary Islands. During these eruptions, huge “lava rafts” were washed up above the water surface!

Earlier eruptions on São Jorge

In 1580, 1808, every 200 years or so, the island of São Jorge erupts? No, no, the maths doesn’t work here, nor can you say, well, now is the time for another volcanic event! In any case, there is a good chance. As I wrote above, it’s worth looking at the distribution of earthquakes, especially when a pause occurs after a major quake… So, where next in São Jorge? Two eminent researchers in the area, Vittorio Zanon and Fátima Viveiros, summarised the history of past volcanic activity on São Jorge three years ago. It was not easy for them, as nature is quick to hide witnesses. Heavy rains have made the vegetation lush, and in a few years even fresh lava fields are overgrown.

The eruption sites and crevices disappear. However, the meticulous work of the two researchers, who also used written records from the past, has revealed many important details. The eruptions are basically confined to the southern periphery of the island. Exactly where the earthquakes are currently occurring, along the settlements of Velas and Urzelina. The magma that surfaces is essentially basaltic, more specifically trachybasalt with a higher alkali content. These eruptions are characterised by lava flows and explosive volcanic activity after an earthquake swarm of 1-2 weeks. However, they are far from being without danger. 15 people died in 1580 and 30 in 1808! But that’s not all!

Take it serious

There is another very dangerous aspect of the island’s volcanic eruptions that not many people think of! This is the pyroclastic flows, or nuées ardentes. Although this type of eruption and the name itself are associated with Alfred Lacriox and the devastating eruption of Mt Pelée in the Caribbean in 1902. It was on São Jorge that the first such event was recorded and the name itself was first used! I have written about this at length in my book Volcanoes. But what does it mean? It’s a turbulent torrent of glowing gas and volcanic debris, moving turbulently, clinging to the surface, rushing sideways at high speed, destroying everything in its path. Both volcanic eruptions have occurred in this way, meaning that a volcanic event on the island poses a far greater threat than what happened in Iceland or La Palma.

According to IPMA, more than 8,000 earthquakes have been recorded since March 19, of which nearly 200 have been felt by the population, that’s a lot, about 20 a day, almost one every hour. The depth of the earthquakes’ cluster (where the earthquake-inducing rock fractures occur, i.e., the hypocenter) is in a depth between 9 and 13 km with an increasing tendency. The distribution of earthquakes follows the island’s trajectory, it’s no coincidence that the island is shaped like this, it has been created by volcanic activity. If we look at the epicentre distributions we can go even further and see that there are two main focal points, one in the Velas area from where the population is resettled, the other north of Urzelina. These are consistent with the locations of previous eruptions. But if we go further in our minds and try to localize, if a volcanic eruption starts, where it will happen, we can say that the ground could open up somewhere in this area, not necessarily where it did in the previous eruptions. In other words, volcanologically, it’s a volcanic field, where the magma always finds a new place to come to the surface and another volcanic cone is created during the eruption. We’ll see what happens with the ‘if’, because there is also a chance that this remains ‘just’ a seismic crisis and the magma doesn’t erupt. For now, however, there is a chance that there will be the next volcanic eruption after 1808 – that is the uncertainty of the forecast. Be prepared for everything, fewer surprises.

Where next in São Jorge?

Residents are constantly leaving the island, not knowing if and when they will be able to move back. But volcanologists such as Fátima Viveirosa are on the ground, measuring the amount of carbon dioxide gas escaping from the ground and trying to deduce the possible movement of magma. These are days full of anticipation, so you could say that anything could happen at any time. The question remains, where next in São Jorge.

Sources: CIVISA (earthquakes), Gaspar et al’s 2015 paper, Zanon and Viveiros’ 2019 publication, Wallenstein et al’s (2018) article and Vulkánok, Harnagi (2011).

Further literature on historical volcanic and seismic events in the Azores:

Please follow strictly the recommendations of the authorities, Civil Protection (SRPCBA), CIVISA and IPMA.

Dedicated site CIVISA

Life seismographic data

Civil Protection

Municipality of Velas

Thanks to Szabolcs Harangi for the permission to repost the article.