2023_programme: Very low-frequency hydroacoustic waveforms associated with the passage of tsunamis over hydrophones at CTBT IMS hydroacoustic stations

• Session: 03. Comprehensive Nuclear-Test-Ban Treaty Monitoring and its Civil and Scientific Applications
  Organiser(s): Georgios Haralabus, Mario Zampolli and Peter Nielsen
• Lecture: Very low-frequency hydroacoustic waveforms associated with the passage of tsunamis over hydrophones at CTBT IMS hydroacoustic stations [invited]
  Paper ID: 1939
  Author(s): Zampolli Mario, Haralabus Georgios, Metz Dirk, Matsumoto Hiroyuki, Le Bras Ronan
  Presenter: Haralabus Georgios
  Abstract: Gravity waves associated with the passage of tsunamis can be detected at very low frequencies below 50 mHz (milli-Hertz) by the hydrophones of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) hydroacoustic stations at ocean basin scale distances from the source. These frequencies are three decades below the band 1 – 100 Hz in which IMS hydrophones are calibrated to provide data for Treaty monitoring purposes. In time-frequency spectrograms, the tsunami gravity wave signals are recognizable by a hyperbolic shape when the tsunami reaches the location of the hydrophone, followed by a linear up-sweep which can persist for several days. Examples are presented in chronological order for the 2004 Great Andaman earthquake tsunami (Boxing day tsunami) recorded at HA08 (Diego Garcia, Indian Ocean), the 2011 Tohoku earthquake tsunami recorded at HA11 (Wake Island, Pacific Ocean), the 2015 Illapel (Chile) earthquake tsunami recorded at HA03 (Juan Fernandez, Pacific Ocean) and the tsunami generated by the 2021 Hunga Tonga Hunga Ha’apai volcanic eruption recorded at HA11, HA03, HA04 (Crozet Islands, Southern Indian Ocean), and HA10 (Ascension Island, Atlantic Ocean), more than 16,000 km from the source. The arrival times of the waveform are compared to those obtained from tsunami propagation models. Secondary arrivals caused by reflections of the tsunami from continents or bathymetric features are also visible in the spectrograms. Detectability versus reported size of the tsunami is also briefly discussed, bearing in mind the caveat that there are necessarily uncertainties about the calibration response of the hydrophones at the very low frequencies considered here. This presentation highlights the potential of IMS hydroacoustic technology for providing complementary information about the passage of tsunamis in remote areas which may not be well covered by tsunami detection buoys or pressure gauges.
• Corresponding author: Dr Mario Zampolli
  Affiliation: Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), Vienna
  Country: Austria
  e-mail: