Although seismicity and volcanism were generally low during this reporting interval, mid-January to early June 2004, several episodes of elevated activity occurred. Weekly summaries for the early part of 2004 are presented in tables 54 and 55. The tables include a summary of seismicity, SO₂ emissions, and forward-looking infrared (FLIR) measurements of the HCl/SO₂ ratio.

Table 54. Summary of seismicity recorded at Soufrière Hills, 16 January to 4 June 2004. Courtesy of Montserrat Volcano Observatory.

On 18 January a low-amplitude swarm of long-period (LP) earthquakes comprised of 1000 separate events began and continued for ~36 hours. A similar swarm occurred on 30 January, lasting for ~30 hours. On 21 February a period of low-level tremor, including many small LP earthquakes, began at ~0600 and continued for ~36 hours.

A period of low-level tremor began on 2 March and continued until 1444 on 3 March when seismic activity increased significantly and an explosion and collapse event occurred. According to the Washington Volcanic Ash Advisory Center (VAAC), the ash clouds associated with the explosion reached an altitude of ~7 km. During 1445-1500 pyroclastic flows were observed in the Tar River, reaching the sea at the Tar River fan on at least two occasions. Seismicity returned to near background levels by 1525, but vigorous ash venting continued until ~0700 on 4 March. Visual observations reported that the explosion removed the small dome that had grown in the collapse scar in late July 2003, as well as a portion of the NW remnant of the 1995-1998 dome.

[A small amount of ash venting from the volcano's summit occurred on 2 May around 1815.] Episodes of tremor . . . continued until 7 May. During this period, tremor amplitude varied from low to moderate, and tremor duration varied from several days (continuous background) to a few seconds. Tremor peak frequencies were in the 1-10 Hz range. Subsequently, the activity level was low (table 54). The SO₂ flux level dropped to 146 metric tons/day on 13 May (table 55), the lowest value recorded since before the collapse event of 12-15 July 2003. For the remainder of the report period, activity remained at a low level. The seismic network recorded several hybrid earthquakes but also a number of 'mixed' events, characterized by emergent onsets and relatively short durations (~30 seconds) with broad frequency spectra (1-10 Hz), peaking at ~10 Hz.

Table 55. Summary of SO₂ emissions and the HCl/SO₂ ratio recorded at SoufriPre Hills, 16 January to 4 June 2004. Courtesy of Montserrat Volcano Observatory.

Geological Summary. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Gill Norton, Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/).