Hiroyuki Kumagai’s research while affiliated with Nagoya University and other places

Long-period (LP) seismic events at active volcanoes are characterized by damped harmonic oscillations, which are thought to be generated by resonances in fluid-filled cracks. LP source properties (crack geometry and fluid properties) are generally estimated by analytically calculating the ratios of spectral peaks in crack resonance frequencies and empirically calculating a quality (Q) factor. However, because this method is applicable only to LP events with more than four spectral peaks, we cannot use it to analyze LP events with fewer spectral peaks. To bridge this gap, we developed a new method to estimate source properties using the frequency (f), Q factor and seismic moment (M0) of the lowest spectral peak of an LP event. We assumed misty gas (water vapor containing small water droplets) as the fluid in the crack and analytically derived the geometrical relationships of the crack. M0 was estimated from observed amplitudes of the lowest spectral peaks at different stations using an assumed crack mechanism. We applied this method to LP events observed from 1989 to 1993 at Kusatsu-Shirane volcano, Japan. We found that the crack length was quite variable, but the crack width was almost constant at around 100 m during our study period. We also found a strong correlation between the inverse of f and the total mass of water in the crack, which can be explained theoretically using the acoustic properties of misty gas and crack geometrical relationships. The total mass of water is proportional to the volume of water vapor in the crack, and the water vapor has been interpreted to be derived from magmatic degassing at depth. The oscillation frequency of the LP event is thus a useful metric for monitoring magma degassing into the shallow hydrothermal system, which is an important constraint on the occurrence of phreatic eruptions. Because our simple approach is widely applicable to LP events, it can enable improved monitoring of hydrothermal activity and evaluation of the risk of phreatic eruptions.

We investigated the relation between high-frequency seismic signals and eruption size and duration using seismic data of eruption tremor and explosion events generated during sub-Plinian and Vulcanian eruptions, respectively, at various volcanoes. We estimated source amplitude functions from seismic envelope seismograms in the 5−10 Hz band, in which S-waves are assumed to radiate isotropically. Because seismic data associated with explosive eruptions can be contaminated by infrasound signals, we confirmed that contamination did not significantly affect the source amplitude functions quantified from our analysed waveforms. We approximated the source amplitude functions of eruption tremor and explosion events by quadrilateral and triangular shapes. For eruption tremor, the durations of the source amplitude functions increased with decreasing slope of the initial phase, i.e. between onset and maximum amplitude. For explosion events, both the maximum and cumulative amplitudes of the source amplitude functions increased with increasing slope of the initial phase, but the overall durations clustered around a typical value. Moreover, the initial phase durations of eruption tremor were longer than those of explosion events. Based on eruption models proposed by previous studies, Vulcanian and sub-Plinian eruptions have been thought to be triggered by accumulation of magma at a shallow part in a conduit and mixing of cool mushy magma with hot fresh magma in a reservoir, respectively. The above differences between the source amplitude functions of eruption tremor and explosion events can be explained by the distinct eruption triggering processes of sub-Plinian and Vulcanian eruptions. Our results suggest that source amplitude functions are useful for investigating eruption processes and estimating eruption sizes and durations for seismic eruption monitoring.

Seismic source amplitudes determined by using the amplitudes of high-frequency (5−10 Hz) tremor signals generated by sustained explosive eruptions have been shown to be related to eruption plume height by power-law and exponential relations and to eruption volume flux by a proportional relation. We further examined these relations and extended this source quantification approach to investigate eruption duration by using the envelope width, defined by the ratio of the cumulative source amplitude to the source amplitude. We first confirmed that the relationship between source amplitude and plume height proposed by a previous study holds for small eruptions at Nevado del Ruiz (Colombia), although slight modifications were required. We then showed that the relations of envelope width with source amplitude and with cumulative source amplitude of eruption tremor associated with sub-Plinian eruptions at Kirishima (Japan) and Tungurahua (Ecuador) could be described by a power law. The source amplitude functions of these tremors were characterized by three periods and could be approximated by a trapezoidal shape. A power-law function fitted to the relation between eruption volume and eruption duration obtained from these relations was similar to that estimated by fitting a power-law function to previously reported eruption volume and duration data of well-documented silicic and andesitic eruptions. Our results suggest that eruption duration may systematically vary with eruption volume when the conduit is stably open during the second period of the trapezoidal source amplitude function. This study demonstrated that source amplitudes can be used for real-time predictions of both plume height and eruption duration, which in turn may be used to estimate ashfall distributions and tephra transport for local residents and aviation operations.

Long-period (LP) seismic events have occurred repeatedly at Galeras volcano, Colombia, during the transition from effusive dome formation to explosive Vulcanian eruptions. Since 1989, two types of LP events have been observed there: one characterized by long-lasting, decaying harmonic oscillations (NLP events) and the other by non-harmonic oscillatory features (BLP events). NLP events are attributed to resonances of a dusty gas-filled crack in the magma plugging the eruptive conduit. Sixteen episodes of NLP events occurred at Galeras during 1992–2010, each characterized by systematic temporal variations in the frequencies and quality factors of NLP events. Our and previous estimates of crack model parameters during three of those NLP episodes indicate that the similar temporal variations in crack geometry and fluid properties can be explained by an increase in the ash content within the crack and a decrease in crack volume. We found that NLP events, associated with low SO2 fluxes, are anti-correlated with BLP events, which are accompanied by high SO2 emissions. From our observations and analytical results, we inferred that BLP events are generated by resonances of open cracks in the uppermost magma plug, corresponding to tuffisite veins, that efficiently transfer volcanic gases. After sufficient degassing and densification, the magma plug effectively seals the conduit. The growing overpressure in the deeper magma is then released through a shear fracture along the conduit margin. The intrusion of deeper, vesiculated magma into the shear fracture depressurizes and fragments the magma, producing a dusty gas and triggering the crack resonances that generate NLP events. Our results thus indicate that the evolution of the properties of the magma plug controls the occurrences of BLP and NLP events at Galeras. Although NLP events do not always precede explosive eruptions, they indicate that an important overpressure is building in the shallow conduit.

Seismic scattering and attenuation at volcanoes, thought to be strongest on the Earth, can be used to map volcanic feeding systems. We systematically analyzed high‐frequency (5–10 Hz) seismograms of volcano‐tectonic earthquakes at Galeras volcano (Colombia) and active sources at Kirishima, Unzen, Bandai, and Iwate volcanoes (Japan) to investigate their scattering and attenuation characteristics. The envelope widths estimated from these seismograms were compared with those calculated by Monte Carlo envelope simulations for 1D layered models parameterized by the scattering mean free path and the quality factor of medium attenuation for S waves. Our results indicated a surficial, highly heterogeneous, and attenuative layer up to around 1 km thickness at all studied volcanoes. The strongest heterogeneities at volcanoes thus exist in a thin surface layer, likely comprising unconsolidated and/or highly fractured materials. Using the space‐weighting function for diffusive wavefields, we mapped the residuals between observed envelope widths and those calculated with our estimated 1D models at Kirishima, Unzen, Bandai, and Iwate. These maps showed spatial distributions of the envelope‐width residuals were unique to each volcano and correlated with P wave velocity tomographic images. Areas of positive residuals correspond to low‐velocity anomalies and thus to heterogeneous, strongly scattering rocks, whereas areas of negative residuals correspond to high‐velocity anomalies and thus less heterogeneous volcanic or basement rocks. Our results demonstrate that envelope widths can improve the characterization of scattering and attenuation structures beneath volcanoes.

We performed repeated three-dimensional seismic tomography in 2009–2018 for the area influenced by the intersection of the Silvia–Pijao (Buesaco) and Cauca–Almaguer fault systems beneath Galeras volcano, Colombia. We analyzed two periods, 2009–2012 and 2013–2018, which were characterized by explosive eruptive activity and relative quiescence at Galeras, respectively. We used P- and S-wave arrival times of 2848 (2009–2012) and 2289 (2013–2018) earthquakes that occurred beneath the volcano. P- and S-wave velocity (Vp and Vs, respectively) anomalies and their changes between the two periods were interpreted to be associated with magma movement in the volcanic plumbing system, which was controlled by the intersection of the faults. Two possible pathways of magma ascent and accumulation were identified in the fault systems. One, located SW of the active crater, consisted of areas with high Vp/Vs values (>1.9) at depths of −2 to 4 km below sea level, and the other located NE of the active crater with high and low Vp/Vs values. The low-Vp/Vs (<1.7) area at depths of 0 to 2 km may represent vesiculated magma that was partly erupted during explosive eruptions in 2009–2010. These anomalies, found in 2009–2012, displayed clear changes in 2013–2018. Large surface deformation observed between January 2013 and February 2014 may be associated with magma movement from the SW pathway. The largest volcano-tectonic (VT) earthquake, with a magnitude of 5.1, ever recorded at Galeras occurred on 12 June 2018 and was followed by a swarm of VT earthquakes. This VT activity may have been caused by magma intrusion along the NE pathway. On the basis of our tomographic images and geodetic, petrological, and geochemical results, we propose a model of the plumbing system beneath Galeras during 2009–2018.