Sao Jorge Isl explained devs

 

The seismic swarm under the northern part of the island continues, but at somewhat lower intensity today.

In total, about 100 quakes above magnitude 2.0 occurred in the past 24 hours, the largest a felt quake of magnitude 3.3 at 6.53 a.m. local time this morning. If to include tiny quakes below magnitude 2.0, the number is approx. 10 times higher.

The locations of the quakes under the island remain concentrated in an elongated area underneath the central northern rift zone in a layer between 10-15 km depth. There is no clear trend in strength and occurrence visible, except that quakes seem to become slightly less frequent over the past 12-24 hours.

While there is no official update from the volcano observatory of the Azores as to any other indications of possible volcanic origin, such as ground deformation, degassing activity etc, it can only be guessed at the moment that these earthquakes could reflect magma intruding into an elongated, approx. 15 km long area at around 10-15 km depth underneath the central ridge of the island, which would corresponds to the lower boundary of the oceanic crust with the uppermost mantle.

If indeed magma intrusion is causing the quakes, the quakes observed are the expression of rock fracturing at depth as magma moves into and forms new sheet-like structures known as dikes.

While volcanic eruptions on ocean islands such as the Azores or the Canaries are almost always preceded by such earthquake swarms, most such earthquake swarms are actually not followed by an eruption as magma often remains stalled at depth; over several years, such swarms then might re-occur as the volcanic system gradually becomes more active, until the intruded magma at depth is "ready" to actually breach the surface and produce an eruption.

Whether the current seismic swarm will end with a new volcanic eruption cannot therefore be said; chances that it will NOT are probably much bigger for the moment than that it will, at least at this point. With the absence of other data, all of this however is to some extent speculation.

Update 3/26:

São Jorge Island (Azores): volcano-seismic crisis continues as scientists conclude magmatic origin, raise alert level

The volcano-seismic crisis on the island continues. Quakes decreased a bit after the first 48 hours since the swarm started on 19 March, but picked up again lately to almost match the initial intensity.
Over the past 24 hours, São Jorge Island was shaken by 2 quakes of magnitudes 3.0 and 3.2 as well as 57 quakes between 2.0 and 3.0. The location of the quakes remains clustered in an NW-SE elongated area at 10-15 km depth beneath the central-northern ridge of the island, under the sector between Velas and Fajã do Ouvidor.
The Azores Seismovolcanic Information and Surveillance Center (CIVISA) reported that by 24 March, there had been more than 2300 quakes in total, including almost 200 quakes so far felt by the population.

In its latest statement, scientists state that the quakes are "tectonic" in origin, but at the same time also admit that they consider that the quakes could indicate a reactivation of the Manadas Fissural Volcanic System and might very well be a precursor of renewed volcanic activity in the future although it is not believed to be "imminent". In any case, the alert level had been raised to "V4" during the past days.
In the proposed scenario, the current earthquakes are result of deep magmatic intrusions at the mantle-crust boundary below the island into the same areas that fed the historical historical eruptions of 1580 and 1808. A similar seismic crisis was mentioned to have taken place in 1964, presumably in the same area. All of this confirms the likelihood that magmatic processes are indeed the cause of the current events, and as it's going on, a volcanic eruption might become more and more likely.

Campaigns to measure gases and temperature in the soil have been undertaken since the beginning of this crisis in the epicentral area, have not shown any anomaly so far, according to CIVISA, but field surveys will continue in the coming days.
Using satellite-based radar ground measurements as well as ground-based monitoring, scientists admit that there is "some deformation in the epicentral area", another indication that magma has been accumulating at depth.
"The integration of available information allows us to conclude that the
CIVISA warns of the possibility of earthquakes that can reach magnitudes higher than those recorded so far, as well as the danger of landslides potentiated by seismic activity and adverse weather conditions that affect the archipelago. There is a real possibility that a volcanic eruption could occur, but there is no evidence that this is imminent.", CIVISA concluded.

As a result, CIVISA has been on ALERT V4 since 3:30 pm on March 20th. CIVISA also issued an alert to the Santa Maria Air Control Center (ACC Santa Maria), the Toulouse Volcanic Ash Advisory Center (VAAC) and the Portuguese Institute of the Sea and Atmosphere (IPMA).

Update 3/28:

The volcano-seismic crisis on the island continues.
The seismic instruments recorded about 839 earthquakes during midnight and 22:00 local time today even though the seismic activity reduced in the past few days. All detected quakes are characterized by low magnitude levels and tectonic origin so far.
Source: Center For Seismovolcanic Infromation And Surveillance Of The Azores volcano activity update 27 March 2022

 

Update 3/29:

Preliminary satellite-based radar data confirm that a section of island's northern part, roughly corresponding to where the recent quakes have been occurring, has experienced uplift.
This almost certainly confirms that magma intrusion at depth is the cause of the seismic swarm.

The earthquake swarm itself continues, but has been decreasing in intensity. In the past 24 hours, São Jorge volcano had 1 quake of magnitude 3.0 and 25 quakes between 2.0 and 3.0. On the other hand, the seismic plot has been showing an episode of what could be volcanic tremor a short time ago this afternoon; it could also be a local disturbance (some machines running near the seismic station etc); we cannot judge this without other data available. However, it if IS volcanic origin, it could be sign that magma has been moving inside some of the newly created conduits.

According to latest calculations, the Volcanological Institute of the Canary Islands (INVOLCAN) estimated that as much as 20 million cubic meters of magma have intruded under the island of São Jorge since last March 19.
If the figures are correct, the likelihood of a volcanic eruption in the Azores is no longer a small one, and we therefore assign our color code "orange", as a warning of potential activity in the near future.
For comparison, the initial magma intrusion leading up to last year's eruption on La Palma Island in the Canary Islands was estimated to be only 11 million cubic meters only, while it erupted a total of approx. 300 cu m of magma in the course of the 3-month-long eruption from September-December 2021. The preceding seismic swarm lasted from 13-19 Sep 2021 and is in many ways quite similar to what is being observed now on São Jorge.