The November 2002 submarine low-energy explosion off Panarea island (Aeolian arc): submarine monitoring and crisis management

Abstract

The sudden unrest of submarine volcanic activity occurred off the island of Panarea (Aeolian arc) on November 2002 induced a submarine low-energy explosion that opened a “crater” of 20 by 10 meters wide and 7 meters deep. That event dramatically changed the geochemical features and the degassing rate of the submarine hydrothermal vents of the area and pushed the scientists to develop new methods to monitor the sea-floor venting activity. During the unrest period, the huge degassing activity increased the CO2 flow rate by some orders of magnitude. Apart from the former venting areas, degassing occurred from many new fractures opened at the seafloor along a N40°E trend and from the crater. Estimations before the event gave a degassing rate in the range of 107 litres/day of CO2, in contrast with the gas vented only by the crater, estimated to be in the order of 1-2 x109 l/d CO2. Such a large amount of toxic gas released to the atmosphere increased the volcanic risk due to the gas hazard. Coupling the information from geochemical investigations and data collected during the unrest of volcanic activity we were able to: a) provide the necessary information to the National Department of Civil Protection to manage the volcanic crisis and b) to develop theoretical models to gain a better insight on the submarine hydrothermal system and its relationships with the local tectonic structures. The measured helium isotopic ratios close to 4.3Ra (Ra = atmospheric 3He/4He ratio), in the range of the values given for the local magmatic signature, did not allow distinguishing if a fresh degassing magma or a cooling crystallized body was feeding the submarine emissions, however the recorded data were the same as measured on the island of Stromboli and underwent synchronous changes with the time. Besides the sampling activity (gases and hot waters collection by diving activity), a continuous monitoring has been carried out by a new sea-floor observatory developed to perform real-time data transmission from a sea-floor multidisciplinary observatory. Among the other sensors, the acoustic probe (hydrophone), installed for a long-term recording of the noise of the bubbling gases in a frequency range of 0.5-3 kHz, gave useful information for a tight link between the submarine volcanic activity of Panarea Island and the crater explosions of the nearby active volcanic island of Stromboli. The continuous monitoring of the submarine hydrothermal vents showing contemporaneous modifications of the vented fluids and the seismic activity of the nearby Stromboli, besides the contemporary variation of the 3He/4He ratios at both volcanoes, bear testimony to interconnections between the two volcanic islands. The almost contemporary start of the volcanic crisis at Panarea and Stromboli in 2002, together with the large number of submarine crater-shaped structures at Panarea, supports the above outcome too. As a matter of fact, our geochemical approach allowed us: 1) to gain a deeper insight for the management of such an unrest of the submarine volcanic activity results and 2) to recognize that the observed changes in the fluids geochemistry were caused by a magmatic input, as such, the nearby active volcanic system of Stromboli Island is somewhat involved in feeding magmatic fluids to Panarea, probably through the normal N40°E fault linking the two edifices.