Volcano-seismic crisis in Montserrat,West Indies, 1966–67 |
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| Authors: | J B Shepherd J F Tomblin D A Woo |
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| Institution: | 1. Seismic Research Unit, University of the West Indies, St. Augustine, Trinidad
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| Abstract: | The Soufriere Hills volcano in the south-eastern part of Montserrat erupted pyroclast flows as recently as A. D. 1646 ± 54 years and must therefore be considered dormant, not extinct. The highly destructive nature of pyroclast flow eruptions makes it imperative that such activity should be predicted and, if the threat of eruption becomes sufficiently large, the population should be moved to a sale distance from the volcano. Sharp increases in seismic and solfataric activity occurred in 1966 and these events indicated the abnormally high risk of an eruption in the near future. A network of four short period seismographs was established in the island in May 1966 and between this date and the end of 1967, 723 local earthquakes were recorded, of which 32 were reported felt in the island. Hypocentres were determined for 189 of these earthquakes, and most of these lay in a WNW to ESE belt beneath the Soufriere Hills, at depths of less than 15 km. The average rate of seismic energy release decreased with time throughout the series but there was a strong seasonal variation with maxima in May and November–December of each year. The average depth of the earthquakes decreased from 5.2 km in April and May 1966 to a minimum of 2.8 km from July to September 1966. After September the mean focal depths increased again and by the end of the crisis in November 1967 the mean depth was 9.7 km. Measurements carried out using water-tube tiltmeters showed that the region 2–3 km south-east of the Soufriere Hills was doming upwards until January 1967, then subsided between January–March 1967 and finally rose again at a slower rate between March and September 1967. Heat flow from Galway’s Soufriere which was 3 × 105 cal/sec in 1954 increased to a maximum of 2 × 105 cal/sec in October 1966, then declined to 5 × 105 cal/sec in September 1967. The earthquake series was not the aftershock sequence of any major tectonic earthquake, and only two hypocentres were determined at depths greater than 15 km. It is concluded that magma was intruded into the upper crust beneath the Soufriere Hills volcano and that the earthquakes and other phenomena resulted from the upward migration of this magma. |
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