Abstract
Infrasonic stations are sparse at many volcanoes, especially those on remote islands and those with less frequent eruptions. When only a single infrasound station is available, the seismic–infrasonic cross-correlation method has been used to extract infrasound from wind noise. However, it does not work with intense seismicity and sometimes mistakes ground-to-atmosphere signals as infrasound. This paper proposes a complementary method to identify the seismic component and the infrasonic component using a single microphone and a seismometer. We applied the method to estimate the surface activity on Ioto Island. We focused on volcanic tremors during the phreatic eruption on April 11, 2013, and during an unconfirmed event on September 12, 2018. We used the spectral amplitude ratios of the vertical ground motion to the pressure oscillation and compared those for the tremors with those for known signals generated by volcano-tectonic earthquakes and airplanes flying over the station. We were able to identify the infrasound component in the part of the seismic tremor with the 2013 eruption. On the other hand, the tremor with the unconfirmed 2018 event was accompanied by no apparent infrasound. We interpreted the results that the infrasound with the 2013 event was excited by the vent opening or the ejection of ballistic rocks, and the 2018 event was not an explosive eruption either on the ground or in the shallow water. If there was any gas (and ash) emission, it might have occurred gently undersea. As the method uses the relative values of on-site records instead of the absolute values, it is available even if the instrument sensitivity and the station site effects are poorly calibrated.
Highlights
Eruption in isolated volcanic islands are becoming the focus of attention for their significant growth, as in the case of Nishinoshima (Maeno et al 2016; Kaneko et al 2019), and for the hazardous nature, as in Anak Krakatau (Williams et al 2019; Perttu et al 2020) and White Island more recently (Dempsey et al 2020)
If the ground velocity associated with the seismic wave is significantly large, it generates pressure perturbation that is noticeable in the infrasound data (Kim et al 2004; Watada et al 2006)
This study aims to identify volcanic infrasound using a single pair of seismometer and microphone at Ioto Island
Summary
Eruption in isolated volcanic islands are becoming the focus of attention for their significant growth, as in the case of Nishinoshima (Maeno et al 2016; Kaneko et al 2019), and for the hazardous nature, as in Anak Krakatau (Williams et al 2019; Perttu et al 2020) and White Island more recently (Dempsey et al 2020). When infrasound data during an eruption is available only from a single station, it is difficult to distinguish the eruption signals from wind noise. McKee et al (2018) extended the method by incorporating the phase shift and the seismic particle motion to estimate the infrasound back azimuth. If the ground velocity associated with the seismic wave is significantly large, it generates pressure perturbation that is noticeable in the infrasound data (Kim et al 2004; Watada et al 2006). Such a ground-to-atmosphere signal can be mistaken as an infrasound signal when only a single infrasound station exists
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