WATER CIRCULATIONS ON LA SOUFRIERE VOLCANO INFERRED BY SELF-POTENTIALSURVEYS (GUADELOUPE, LESSER ANTILLES) - RENEW OF VOLCANIC ACTIVITY

Soufriere volcano is an andesite explosive volcano located in Guadelou pe in the lesser Antilles are. Since the last magmatic eruption dated 1440 +/- 40 AD several violent phreatic eruptions have occurred. The t wo last 1956 and 1976-1977 phreatic/phreato magmatic eruptions were pr eceded by several decades of quietness. During these quiet periods, hy drothermalization 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 magma tic 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 Dece mber 1992. The onset of activity was preceded, one year before, by a s light increase of vapor emission inside South crater located on Soufri ere dome at the upper end of August 30, 1976 fault, by the reactivatio n 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 degrees C). A Self Potential (S.P.) survey has been performed in July 1992 on Soufriere massif. The S.P. anomalies evidenc e structural heterogeneities and ground waters flowing within the mass if. A large, smooth, positive S.P. anomaly, composed of two localized and pronounced anomalous zones, is spreading on the south-west.south-e ast 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. So ufriere dome is now submitted to a ring of positive S.P. anomalies whi ch are lying along the inner part of Amic crater. The positive anomalo us zone located on Morne Mitan has developed since 1987. The second an omalous zone, located on the south-west part of the dome, was not obse rved in 1987. The amplitude reaches 200 mV. The S.P. mapping points ou t that warm ground water beneath the south part of the Volcano summit, along clayey beds, could generate the collapse of the south-west sout h-east sector of Soufriere dome.