5:15 PM - 6:30 PM
[SCG45-P09] What happened to the seafloor around Nishinoshima volcano during deployment of OBEMs between September 2018 and December 2020?
Keywords:Nishinoshima volcano, OBEM, VTM, bathymetric feature change
We have conducted geomagnetic and electric fields observation using OBEMs and VTMs (hereafter “OBEM” is used to express both instruments) on the seafloor around Nishinoshima volcano belonging to Bonin Islands since 2016 in order to find magma reservoirs and estimate an electrical conductivity structure beneath Nishinoshima. We totally deployed 22 instruments by January 2021, but five OBEMs, two of six OBEMs deployed in September 2018 and three of five OBEMs deployed in July 2019, were not able to be recovered.
Two OBEMs deployed in September 2018 were first tried to recover by self-pop-up in July 2019. Although they normally received the release commands of weights through underwater acoustic communication, they could not float up to the sea surface. We determined the position of the OBEMs both in September 2018 and in July 2019, which indicates both OBEMs shifted a few tens meters in about ten months. It might be possible that the OBEMs were partially buried in sediments by sector collapse or something like that. Two OBEMs were deployed in July 2019 and were safely recovered by self-pop-up in December 2020. Both of them were moved about 3 km in a direction away from Nishinoshima island. We tried to search and rescue three of five OBEMs which would be still on the seafloor by using KM-ROV (KM20-11 cruise in December 2020) and HPD (KS-21-2 cruise in January 2021), but could not find.
According to videos during the dives of KM-ROV and HPD, volcanic activities of Nishinoshima, especially violent Strombolian eruptions after December 2019 (Yanagisawa et al., 2020) might have some influence on the displacements and disappearances of OBEMs. We are analyzing bathymetric data obtained before May 2019 and since December 2020 to evaluate the effect of changing of bathymetric feature on the OBEMs quantitatively.
Two OBEMs deployed in September 2018 were first tried to recover by self-pop-up in July 2019. Although they normally received the release commands of weights through underwater acoustic communication, they could not float up to the sea surface. We determined the position of the OBEMs both in September 2018 and in July 2019, which indicates both OBEMs shifted a few tens meters in about ten months. It might be possible that the OBEMs were partially buried in sediments by sector collapse or something like that. Two OBEMs were deployed in July 2019 and were safely recovered by self-pop-up in December 2020. Both of them were moved about 3 km in a direction away from Nishinoshima island. We tried to search and rescue three of five OBEMs which would be still on the seafloor by using KM-ROV (KM20-11 cruise in December 2020) and HPD (KS-21-2 cruise in January 2021), but could not find.
According to videos during the dives of KM-ROV and HPD, volcanic activities of Nishinoshima, especially violent Strombolian eruptions after December 2019 (Yanagisawa et al., 2020) might have some influence on the displacements and disappearances of OBEMs. We are analyzing bathymetric data obtained before May 2019 and since December 2020 to evaluate the effect of changing of bathymetric feature on the OBEMs quantitatively.