Activity at the Soufrière Hills Volcano is low with no ash-venting since the episode on 23 March.
The seismic network recorded seven rockfalls, four volcano-tectonic (VT) , three hybrid and three long-period (LP) earthquakes. All the VT, hybrid and LP earthquakes occurred between 23 and 25 March, following the large swarms of VT earthquakes on 22 and 23 March. The magnitude of the largest earthquake in the swarms was 3.5, making this one of the largest VT earthquakes since the start of the eruption. This magnitude is consistent with reports of the earthquake being felt by some residents of Montserrat.
The average sulphur dioxide measurement this week was 1320 tonnes per day with a minimum of 264 tonnes per day and a maximum of 4594 - the third-highest value recorded in the last ten years. High values occurred between 24 and 26 March, averaging 2550 over the three days. The average for the rest of the week was around 400 tonnes per day.
The ash venting and fumarolic activity that started in the morning of 23 March peaked before noon and then slowly declined until it stopped in the late afternoon. The ash venting was pulsatory and sent ash up to about 6000 feet above sea level (3000 feet above the volcano). At its peak, black jets of ash were seen rising about five hundred feet above the floor of the collapse scar. There appears to have been no ash venting after 23 March and observations on 28 March showed only fumarolic activity from the vents inside the collapse scar.
The episode on 23 March is the most significant activity at the Soufriere Hills Volcano since the current pause began more than two years ago. The large VT earthquakes are likely to have been caused by increasing pressure below the volcano from uprising magma. The ash venting was probably phreatic in origin, generated by superheated rock interacting with the groundwater. This type of activity has occurred before at the Soufriere Hills Volcano up to several months prior to restarts in magma extrusion, for example in 2005 and in 2008.
The 23 March 2012 activity cannot yet be assumed to be a precursor to extrusion. However, it is a reminder that the volcano is still active. Pyroclastic flows can occur at any time without warning on any side of the volcano, including Gages from where they can travel rapidly into Plymouth. Lahars (mudflows) remain a hazard. Tracks across the Belham valley are frequently destroyed or heavily modified by lahars, and caution should be exercised crossing the valley during and after rainfall.
The Hazard Level is 2. As of 27 March, there is daytime access (8:00 am to 4:00 pm) to Zone C. Maritime zone W remains daytime transit only (boats may sail through the zone but must not stop).