The Mt Etna 2001-2003 eruptive period: the seismological point of view

Abstract

During 2001 and 2002-2003 Mount Etna produced two significant flank eruptions. Many similarities between the two eruptions have been recognized by several authors regarding the high degree of explosivity, the location of the eruptive centres and the emission of different compositionally distinct magma types. A new tomographic study concerning the 2001-2003 interval time including the two lateral eruptions is here presented, which take into account precise earthquake locations. The used algorithm was the double-difference seismic tomography method (TomoDD), which uses both absolute and differential arrival times to simultaneously determine event locations and Vp and Vp/Vs velocity models. Firstly, we performed a tomographic inversion on the whole dataset consisting of 1,035 local earthquakes occurred from July 2001 to January 2003, combining 14,205 P-wave and 3,337 S-wave arrival time differences. After, we separately analysed the 2001, 2002-2003 and intra eruptive datasets to obtain the tomographic images in the three distinct periods. The tomographic inversion during the eruptive periods revealed the presence of anomalous volumes with very low Vp/Vs (values as small as 1.64) located in the same place of the 2001 and the 2002-2003 dike intrusions also evidenced by attenuation tomographies and modeled by geodetic data. We suggest that these anomalies trace the intrusion of volatile-rich magma leading to these peculiar explosive eruptions. In the tomograms related to the intra-eruptive period some variations in the elastic parameters have also been observed. The analysis of the geodetic and seismic deformation pattern in the intra eruptive period suggested that during 2001 –2003 period the active deformation at Mt Etna has been continuous, excluding a brief period of deflation following the 2001 eruption. In fact, after this period, the seismicity rate remained high, the geodetic data showed a renewal of the areal dilatation, suggesting a new magma upraising from depth causing an overpressure in the shallow reservoir which triggered the 2002-2003 eruption.