2023_programme: Energetic and short water-born signals from mid-oceanic ridges

• Session: 22. Underwater Seismoacoustics
  Organiser(s): Nathalie Favretto-Cristini and Paul Cristini
• Lecture: Energetic and short water-born signals from mid-oceanic ridges [invited]
  Paper ID: 1876
  Author(s): Ingale Vaibhav Vijay, Bazin Sara, Royer Jean-Yves
  Presenter: Ingale Vaibhav Vijay
  Abstract: The OHASISBIO network of autonomous hydrophones moored in the SOFAR channel of the southern Indian Ocean detected many short-duration and energetic acoustic events originating at mid-oceanic spreading ridge axes. Their average duration of 10 to 15s makes them “impulsive” relative to the 80 to 200s-long T-waves generated by submarine earthquakes. Their frequency goes up to 50-60 Hz and their source level ranges from 200 to 233 dB. Most of them are detected as far as 3700 km away from their source where received levels can reach up to 108 dB. They are associated with seismic swarms located on ridge axes where new seafloor is produced. Along the Southwest Indian Ridge (ultra-slow spreading rate at 14 mm/yr), such impulsive events (IMP) were found near Novara transform fault (58˚20’E), with 69 IMP among 1109 seismic events, near Melville transform fault (61˚00’E), with 118 IMP among 27624 events, and at 67˚45’E, with 58 IMP among 4880 events. Along the Central Indian Ridge (slow spreading rate at 40 mm/yr), 711 IMP among 2015 seismic events were found near an active hydrothermal field at 23˚52’S, 69˚30’E. The short duration, high energy and frequency content of IMP suggest that their source is punctual and generates water-borne waves, thus not resulting from the conversion of seismic waves on the seafloor. IMP sources are often located at the foot or slopes of bathymetric highs and occur in clusters after large-magnitude earthquakes nearby. All these clues point to a volcanic origin, perhaps to hot lava and seawater interactions during magmatic eruptions, after dike-intrusion triggered earthquakes. This hypothesis calls for in-situ inspections with a submarine vehicle to be confirmed. Except for their locations, these events could be mistaken for man-made underwater explosions. These narrow and energetic events could also provide useful sources to investigate the effects of long-distance propagation of acoustic waves in the ocean or to monitor sound-speed or temperature anomalies at long range.
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• Corresponding author: Mr Vaibhav Vijay Ingale
  Affiliation: Lab Geo-Ocean, University of Brest, CNRS, Ifremer, UMR6538, F-29280, Plouzané, France
  Country: France
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