Identification of infrasonic and seismic components of tremors in single-station records: application to the 2013 and 2018 events at Ioto Island, Japan

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.

Seismo-acoustic characterisation of the 2018 Ambae (Manaro Voui) eruption, Vanuatu

“…Observations and modelling to date indicate that air-to-ground coupling (propagating at acoustic velocity) is more common given volcanic source and velocity structure configurations than groundto-air coupling (propagating at seismic, e.g., Rayleigh wave velocity Matoza et al (2009)), although both ground-air and air-ground coupling have been documented in seismoacoustic publications (Blom et al 2020). Air-to-ground conversions or ground-coupled airwaves have been observed in local to remote volcano monitoring systems (Hagerty et al 2000;Johnson and Malone 2007;De Angelis et al 2012;Ichihara et al 2012;Matoza and Fee 2014;Fee et al 2016;Ichihara 2016;Nishida and Ichihara 2016;Smith et al 2016;Matoza et al 2018;Mckee et al 2018;Haney et al 2020;Kurokawa and Ichihara 2020;Sanderson et al 2020).…”

“…To check for air-to-ground-coupled waves in seismograms, (1) cross-correlation (Ichihara et al 2012), (2) coherence (Matoza and Fee 2014), (3) response function (Ichihara, 2016;Kurokawa and Ichihara, 2020), and (4) reverse-time migration (RTM) have been applied. Variations on the RTM method include back projection, stacking, source scanning, and time-reversal (e.g., Walker et al 2011;De Angelis et al 2012;Jolly et al 2014;Sanderson et al 2020).…”

Special issue “Understanding phreatic eruptions - recent observations of Kusatsu-Shirane volcano and equivalents -”

“…They identified the intense eruption with low-frequency infrasonic signals several hours after the onset of the phreatic eruption. Kurokawa and Ichihara (2020) measured infrasound and seismic tremor at a single station during the 2013 and 2018 events at the Ioto island, an active volcano located 1200 km south of Tokyo in the Izu-Bonin arc. They could successfully identify the phreatic eruption of the 2013 events by using spectral amplitude ratios of the vertical ground motion to the pressure oscillation.…”

A simple method to evaluate the seismic/infrasonic energy partitioning during an eruption using data contaminated by air-to-ground signals

“…The generation efficiency of the air-to-ground signal is ∼ 0.1 − 10µm/s/Pa [Ichihara 2016;Novoselov et al, 2020], which is much larger than that of the ground-to-air signal (∼ 0.0003 Pa/(µ m/s)) [e.g., Kim et al, 2004;Watada et al, 2006;Ichihara et al, 2012;Kurokawa and Ichihara, 2020], the former is usually more significant in the seismo-acoustic observation during volcanic eruptions. The air-to-ground signals recorded by seismometers are used to investigate infrasound when few infrasound sensors are available [Johnson and Malone, 2007;Ichihara et al, 2012;De Angelis et al, 2012;McKee et al, 2018].…”

“…It is achieved by the propagation velocity when seismic arrays are available [Nakamichi et al, 2013, Heck et al, 2019Marchetti et al, 2020]. Kurokawa and Ichihara [2020] used known seismic waves and acoustic waves recorded by an infrasound sensor and a collocated seismometer as references. They calculated the spectral power ratios between them to evaluate the ground-to-air coupling and air-to-ground coupling efficiency.…”