Radon mapping, automatic measurements and extremely high 222Rn emissions during the 2002–2007 eruptive scenarios at Stromboli volcano

Abstract

We report additional radon data collected at Stromboli during 2002–2007. The whole data set of periodic measurements has been systematically analyzed to retrieve the values of background, threshold and anomaly for all the stations of the network. Maps of radon concentrations in space and time correlate with changes in volcanic activity. Higher radon emissions are essentially concentrated at summit stations and may also affect some stations located onto the summit-eastern sector of the cone (along the N60°E fracture zone), as well as two stations placed at the NE edge of Sciara del Fuoco. Most of these stations are well above their threshold values during effusive eruptions (namely 2002–2003 and 2007) due to the progressive structural adjustment of the volcano edifice coupled with the opening and resetting of the fracture network.A two-year-long timeseries of automatic 222Rn measurements (2005–2007) was analyzed together with local environmental parameters and selected geophysical data (on seismic tremor and infrasonic puffing). The data show good correlation between radon and the latter parameters particularly during periods of high Strombolian activity (March–April 2006), and before the onset of the February 27, 2007 lava effusion (with radon emissions being well above 20,000Bq/m3).Extremely high radon emissions (up to ~1.7×106Bq/m3) were recorded along the fractures of the SW crater during the first two months of the 2002–2003 effusive eruption. Very high radon emissions (up to ~470,000Bq/m3 for 222Rn, and ~780,000Bq/m3 for 220Rn, respectively) were also detected by the automatic measurements at the summit station during June 20–early July 2007: i.e., just prior to the resuming of the Strombolian activity at summit craters. These data give us the opportunity to estimate the relative depths of the summit fractures during fracturing (being constrained between 200 and 310m). This is in good agreement with the hypocentral depths of the so called “hybrid events” and the source of the very long period seismic signals (VLP) localized at a depth of about 500ma.s.l. below the upper–central part of the Sciara del Fuoco.