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São Jorge Island (Azores): seismic swarm continues with no clear trend

Wed, 23 Mar 2022, 19:09
19:09 PM | BY: T
Quakes under Sao Jorge Island in the past 24 hours as well as distribution of depths over time
Quakes under Sao Jorge Island in the past 24 hours as well as distribution of depths over time
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.

Previous news

Mon, 21 Mar 2022, 08:37

Latest quakes in or near São Jorge Island, Azores

More than 100 quakes of magnitudes between 2.0 and 2.9 occurred during the last 24 hours beneath the island. Therefore, we have set up a new page to monitor earthquakes under São Jorge Island, Azores. ... Read all
Sun, 20 Mar 2022, 22:35

São Jorge Island (Azores): seismic swarm could be from magma intrusion, possible precursor of volcanic eruption?

Quakes under Sao Jorge Island in the past 24 hours
A swarm of earthquakes has started yesterday under the northwestern half of São Jorge Island (Azores), with 19 quakes of magnitudes between 2.0-3.5 and many more smaller ones. ... Read all
São Jorge volcano
fissure vents 1053 m
Azores Islands, Portugal, 38.66°N / -28.11°W
Current status: restless (2 out of 5) São Jorge volcano eruptions:
1580, 1808
Typical eruption style
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