by B. Roach
Activity at the Soufrière Hills Volcano is low, although seismicity has increased.
The seismic network recorded nine rockfalls, one hundred and five volcano-tectonic (VT) and four hybrid earthquakes. Two swarms of VT earthquakes have occurred, one between 16:04 and 16:51 hrs on the 22 March with 49 events and another between 03:10 and 05:27 hrs on 23 March with 54 events. Earthquakes in the second swarm were markedly larger than those in the first.
The average sulphur dioxide measurement this week was 433 tonnes per day with a maximum of 654 and a minimum of 282 tonnes per day.
Observations on the morning of 23 March showed several changes on the volcano. Steam venting (Fumarolic) activity on the volcano has increased and a new steam vent (fumarole) has appeared on the northwest face of the lava dome behind Gages Mountain. A pulsing steam vent containing a small amount of ash has formed in the back of the February 2010 collapse scar. Very light ashfall is occurring on the western flank of the volcano. Audible roaring associated with the venting can be heard intermittently from MVO, 5.75 km NW of the volcano.
Pyroclastic flows can still occur at any time without warning. Lahars (mudflows) remain a hazard. Tracks across the Belham valley are frequently destroyed or heavily modified by lahars, therefore caution should be exercised crossing the valley during and after rainfall.
The Hazard Level is 2, however until further notice there is no access to Zone C. Maritime zone W remains daytime transit only (boats may sail through the zone but must not stop).
The MVO weekly report is sent out every Friday by mid-day.
But, it has been the habit for a long time now for the MVO to send to The Montserrat Reporter (TMR), nothing else in connection with the volcano except for the weekly report. It was on this background we received the following release at mid-afternoon which reads as follows:
“A noticeable increase in seismic activity at the Soufriere Hills Volcano resulted in a closure of Zone C, which had been reopened for daytime entry in December 2011.
Director of the Montserrat Volcano Observatory (MVO) Dr. Paul Cole said on ZJB Radio Montserrat on Friday morning that it was a precautionary measure as they continued to monitor the volcano, which has been relatively quiet since early 2010.
By early afternoon the MVO released a statement noting that mild ash venting had begun and was emanating from the 11 February 2010 collapse scar.
MVO photo taken from Jack Boy Hill of the ash vents today March 23, 2012 (supplied with release)
“Following two volcano-tectonic (VT) swarms on the 22 and 23 March 2012 involving 49 and 54 events respectively, mild ash venting began at Soufriere Hills Volcano at around 8:00 am local time on 23 March. The venting was sourced from the floor of the 11 February 2010 collapse scar, immediately south of the old English’s crater wall and to the west of the long-lived hottest fumarole previously identified. Fumarolic activity on the volcano on the morning of 23 March increased markedly and a new steam fumarole was identified on the NW side of the dome immediately behind Gages Mountain,” the statement read.
“The ash venting was clearly pulsatory and sent ash to approximately 6000 feet above sea level (3000 feet above the volcano). At its peak, black jets of ash were seen rising a few hundred metres above the floor of the collapse scar. This type of activity is probably ‘phreatic’ in origin and is formed where superheated rock meets groundwater causing the rocks to fragment, generating ash.
“VT earthquakes are related to fracturing rocks probably as a result of increases in pressure. It is likely that these pressure increases and the resulting earthquakes are related to uprising magma below the volcano. Similar types of activity have occurred at Soufriere Hills Volcano up to several months prior to restarts in magma extrusion, for example in 2005 and 2008,” the MVO statement ended.
Zone C which comprises of Cork Hill, Weekes, Foxes Bay, Richmond Hill, Delvins and extending 500 m out to sea is part of the Exclusion Zone. It was reopened in December 2011 after the hazard level was lowered from 3 to 2, which made daytime access possible in those areas. Dr. Cole says there are no plans to raise the hazard level but that the area would be closed off and the gates locked.”
