GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.2 GNGTS 2023 Newscasts from “Iddu”: gas jets excite rotational motions T. Braun, F. Bernauer, J. Wassermann Alexander von Humboldt described once in his writings: „...since the beginning of time, Stromboli ejects his fiery lava fountains at short time intervals into the sky”, and continued, “...Stromboli, a natural lighthouse of the Tyrrhenian Sea, indicated already to Greek and Roman seafarer the right way forward during night time … “. Nowadays, von Humboldt would probably extend his statement to: „ Stromboli, a natural field laboratory, indicates, not only to Greek and Roman, but also to French and German seismologists the way towards the volcano-seismic source (mainly during daytime) … ” . However, portable broadband seismometers, seismic arrays, tiltmeter, infrasound sensors and recently also rotational motion sensors have been first and foremost tested at Stromboli (Iddu), before installing them on less accessible active volcanoes. Periods of „normal strombolian“ activity in 2016, 2018 and 2022 have been utilised by the University of Munich, together with the company IXBlue and INGV, to install high sensitive Fibre Optical Gyroscopes (FOG), in combination with classical broadband seismometers, near the active craters. The resulting 6-component recordings show rotational motions around all three rotational axes, confirming the FOG’s excellent characteristics as a physical polarisation filter for seismic waves. During the 2018 field campaign we had the unprecedented chance to accidentally observe and record gas-jet eruptions, a rare and very spectacular acoustic phenomenon. These sudden onsets of degassing last some tens of seconds and can be compared with jet planes during take-off. The sound volume is ear-splitting, without any visible phenomenon at the craters. While the classical 3 component broadband seismometer seems to take no notice of the gas jets, the rotational components do. Comparison with synchronised video records show that the duration of the audio trace is coincident with the increase of the signal amplitude and the frequency changes on the rotational components. This is clear evidence and proof for the transmission of sound generated by the eruption into the ground and its continued propagation as „ ground-coupled air waves “. References: Braun T.; 2018. Sismologia “rotazionale”, nuovi sensori installati a Stromboli. https://ingvvulcani.com/2018/10/08/sismologia-rotazionale-nuovi-sensori-installati-a-stromboli/ Braun T.; 2020. Osservazioni dei movimenti terrestri mediante sensori rotazionali. https://ingvvulcani.com/2020/06/30/osservazioni-dei-movimenti-terrestri-mediante-sensori-rotazi onali/