The Montserrat Volcano Observatory recorded a “brief but intense” swarm of 93 earthquakes in the vicinity of Soufriere Hills volcano this morning, 19 December.
In an activity report published online just after 3pm, the observatory said the volcano-tectonic earthquakes started at 10.20am and lasted for about 90 minutes.
They were located at depths of two to three kilometres beneath the dome, which is usual for these types of events, and the two largest had a magnitude of 3.3 each.
The activity led the experts at the observatory to advise authorities to temporarily suspend activities in Zone V.
Access was restored after 12.40pm when activity was confirmed to have returned to background levels.
Director Graham Ryan said the swarm was “probably the most intense seen” since the pause in volcanic activity from February 2010.
He compared it to a volcano-tectonic earthquake swarm on 23 March, 2012, which lasted almost three hours and was accompanied by ash venting.
That swarm contained fewer earthquakes at 54 but its largest had a magnitude of 3.6, making it slightly more intense than the recent events.
There was no ash venting associated with the 19 December swarm and there have been no changes in fumarole or rockfall activity, Ryan explained.
The observatory recorded another spike in activity between 27 September and 4 October, with 43 volcano tectonic earthquakes and one hybrid earthquake.
Two thirds – 28 – of these earthquakes occurred in brief swarms on 28 and 29 September. They were relatively small, with the largest having a magnitude of 2.2.
This was the third highest weekly count since the current pause in volcanic activity.
On 5 December, the observatory published its Preliminary Statement SAC 29 in which it said there have been more volcano-tectonic earthquakes this past year, but volcanic activity remains low.
“The volcano, with its large lava dome, is still a potential source of hazards within Zone V, including explosions, pyroclastic flows and lahars,” it read.
“Resumption of lava extrusion or explosions is still possible, however, the chance that explosions or pyroclastic flows will occur within the next year remains low.
“Continuous monitoring of the volcano is essential.”
The observatory’s website explains that volcano-tectonic earthquakes are caused by the fracturing of the rock under the volcano as the magma forces its way to the surface.
They can occur in swarms lasting from a few minutes to several days in duration.
