Water Circulations on La Soufriere Volcano Inferred by Self-Potential Surveys (Guadeloupe, Lesser Antilles). Renew of Volcanic Activity?

Abstract

Soufriere volcano is an andesite explosive volcano located in Guadeloupe in the lesser Antilles arc. Since the last magmatic eruption dated 1440 ± 40 AD several violent phreatic eruptions have occurred. The two last 1956 and 1976-1977 phreatic/phreato magmatic eruptions were preceded by several decades of quietness. During these quiet periods, hydrothermalization and argillization are expanding due to high tropical rainfalls (10 to 12 m per year). Sealing of fractures yields more and more difficult the release of thermal stresses developed by the magmatic body supposed to be at 6 km depth. Overpressures are increasing in confined superficial water tables. Renew of seismic activity started in May 1992. About 1400 earthquakes were recorded between May and December 1992. The onset of activity was preceded, one year before, by a slight increase of vapor emission inside South crater located on Soufriere dome at the upper end of August 30, 1976 fault, by the reactivation of the small Tarade hot spring dried up since the 1976-1977 eruption, and by the appearance of a new, low flow rate, hot spring called Pas du Roy (about 30°C). A Self Potential (S.P.) survey has been performed in July 1992 on Soufriere massif. The S.P. anomalies evidence structural heterogeneities and ground waters flowing within the massif. A large, smooth, positive S.P. anomaly, composed of two localized and pronounced anomalous zones, is spreading on the south-west-south-east sector of the dome basis. The larger anomalous zone is located on Morne Mitan in the axis of the regional Ty and August, 30, 1976 faults. This zone is extending along the contact zone between Soufriere dome and Carmichael cone. The 1992 survey is compared with 1987 survey. Soufriere dome is now submitted to a ring of positive S.P. anomalies which are lying along the inner part of Amic crater. The positive anomalous zone located on Morne Mitan has developed since 1987. The second anomalous zone, located on the south-west part of the dome, was not observed in 1987. The amplitude reaches 200 mV. The S.P. mapping points out that warm ground water beneath the south part of the volcano summit, along clayey beds, could generate the collapse of the south-west-south-east sector of Soufriere dome.