Strombolian Activity of Mount Etna in July 2001: Insights From Doppler Radar Measurements

Ground-based Doppler radar measurements of the Strombolian activity of Etna's SE crater were performed on July 4, 7 and 13, 2001, from 1 km to the SSW. Gradually more violent Strombolian eruptive episodes occurred at intervals of a few days from June, including dense/dilute lava jets and lava bubble outbursts. This culminated in lava fountains up to 700 m high, followed by a period of quiet degassing and lava outpourings. Fifteen such paroxysms occurred between 7 June and the main flank eruption, which lasted from 17 July to 9 August 2001. Maximum radar velocities measured at SE crater were 70-80 m/s on July 4, 90m/s on July 7, and 90-100 m/s on July 13, confirming that this episode was one of the most violent of the series. Sudden changes in activity intensity were recorded by the radar on July 13, and are clearly correlated to some of the numerous earthquakes felt. Moreover, trends of radar power and radial velocity data follow the tremor intensity level, suggesting the shallow control of the volcanic tremor by the dynamics of gas bubbling in the magmatic conduit. Additional radar measurements were carried out on July 29 and 31 during intense Strombolian activity of the new Laghetto cone, north of La Montagnola. This magmatic phase was preceded and followed by phreatomagmatic phases and lasted about 2 weeks. Maximum jet velocities recorded by the radar at Laghetto during intense Strombolian activity exceeded 130 m/s, emphasizing the strongly explosive dynamics. Video analyses corroborate these values for ejecta velocities. A range of initial gas velocities is inferred from maximum particle velocities for all episodes and corresponding gas flux estimates will be presented. The radar reflectivity factor characterizing the size and number of ejecta is relatively constant around 54 dBZ during explosions of 4, 13, and 29 July. Assuming, as a first step, Rayleigh scattering, this suggests an average particle size around 10 cm.