Experts at the Montserrat Volcano Observatory (MVO) have recorded an increase in seismic events this past year, but overall activity at Soufriere Hills remains low.
According to the latest annual report, there have been more volcano-tectonic earthquakes, however this is not an indication of imminent eruptions or explosive events.
The Preliminary Statement SAC 29 was published on 5 December and refers to the findings from the 29th meeting of the MVO’s Scientific Advisory Committee from 25 to 29 November.
“The volcano, with its large lava dome, is still a potential source of hazards within Zone V, including explosions, pyroclastic flows and lahars,” it reads.
“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.”
Volcano-tectonic earthquakes
According to the report, the volcano has produced higher rates of volcano-tectonic earthquakes since 2019, and this has been sustained over the past 12 months.
“A small number of these are more widely distributed around the volcano than has been observed in recent years,” it says
The MVO’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.
Between 27 September and 4 October this year, the observatory recorded a spike in activity with 43 volcano-tectonic earthquakes.
This was the third highest amount in one week since the current pause in volcanic activity began in February 2010.
Low frequency activity
In addition, the observatory has observed an increase in low frequency seismicity since June 2024 to levels not seen since 2012.
This includes long period earthquakes, thought to be caused by resonance in the gas or magma inside a volcanic conduit, the MVO website says.
It also includes hybrid earthquakes, which are a mix of long period and volcano-tectonic earthquakes, and are thought to represent magma making its way to the surface at shallow depths.
“Based on previous restarts, we would expect a further increase in unrest prior to the resumption of any significant eruptive activity,” the report explains.
“The best overall explanation for this unrest is pressurisation of the magmatic reservoir at depth, possibly driven by a continued deep supply of magma and gas.”
Ground deformation
The report adds that monitoring of ground deformation shows a continuation of the pattern of inflation observed over the last 14 years but the rate of inflation continues to gradually decrease.
According to the MVO website, monitoring the deformation of the surface of the volcano such as bulging, sinking and fractures gives information on the magmatic system architecture.
It also reflects movements of magma, gas or other fluids, or movements along faults, within the earth crust.
While the surface displacements can sometimes reach metres and be observed with the naked eye, they are most of the time subtle and can be just millimetres a year.
The recent statement says that the slowing inflation since 2010 suggests a modest reduction in the deeper processes experienced at the volcano.
SO2 fluxes
The observatory team regularly carries out measurements of the gas emissions at the Soufriere Hills volcano including sulphur dioxide (SO2) and hydrogen chloride (HCl).
In the past six months, measured SO2 fluxes have averaged at about 250 tonnes per day – a decrease from the previous year and slightly below the long-term average.
Thermal camera images of fumaroles on the dome show a temperature range between 200 and 580 degrees celsius. Some fumaroles show a slight decline in temperature, the report adds.
“SO2 flux has shown a decreasing trend in the last two to three years and there are indications of cooling in some fumarole temperatures,” it says.
“In other volcanic systems, cooling and sealing of shallow degassing pathways has sometimes led to non-magmatic explosions.”
