GEOLOGY (of Montserrat):
Based on: Young, Sparks, Aspinall, Lynch, Miller, Robertson, and Shepherd, Geophysical research Letters, vol.25, # 18, pp. 3389-3392, 1998:
In 1902, explosive eruptions at Mont Pelee, Martinique and Soufriere, St. Vincent killed about 30,000 people within a few days. These disasters led to Volcanology evolving into a modern, multidisciplinary science.
The Soufriere Hills Volcano (SHV) lies in the south-central part of the British overseas territorial island of Montserrat. It is located near the northern end of the Lesser Antilles volcanic arc. Montserrat comprises a number of volcanic centres up to c. 4.3 Ma old. The younger centres occur in the southern third of the island.
The SHV is made up of 4 andesitic lava domes and a fan of associated pyroclastic flow deposits.
Organic materials in these pyroclastic deposits have yielded radiocarbon dates (in 1988) between 24-16 Ka. Radiometric dates obtained in 1997 suggest that the summit crater was likely formed c. 4,000 years ago. It has been suggested that a small andesitic lava dome partially filled up the Castle Peak Dome Crater some c. 323 +/- 50 years BP.
Significant increase in seismicity and in hot spring activity occurred in 1897-98, 1933-37, and 1966-67 (Volcano-Seismic Crises). In 1933-37, earthquakes occurred at < 5km beneath the volcano and elsewhere.
In 1996-67, swarm of earthquake epicentres was scattered along an ESE-trending zone centred on SHV at estimated depths of 3-10 km. In both of these crises there was an increase of heat flow. Also, there were some changes in gas compositions at some or all of the soufrieres (Hot Springs) located on the flanks of the volcano. Water-tube tiltmetres in place during the 1966-67 crisis recorded minor changes on the lower flanks of the volcano.
A short-lived increase in volcano-related seismicity occurred in mid-1985 following a M 6.2 earthquake 30 km from SHV.
Some increase in seismicity was noted in 1992 and this activity increased markedly in November 1994, with a large number of relatively deep, 10-20 km, earthquakes. The hydrothermal system showed few changes during this period.
No, precursor seismic activity was noted in the days or weeks prior to the onset of phreatic activity on 18 July 1995.