Mechanism of the 2015 volcanic tsunami earthquake near Torishima, Japan

Fukao, Yoshio; Sandanbata, Osamu; Sugioka, Hiroko; Ito, Aki; Shiobara, Hajime; Watada, Shingo; Satake, Kenji

doi:10.1126/sciadv.aao0219

Cited by 36 publications

(59 citation statements)

References 46 publications

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“…Therefore, we performed low‐pass filtering of the recorded accelerograms and assumed that in the low‐frequency band, Love and Rayleigh waves are nondispersive. Within the framework of the nondispersive plane wave assumption, it is easy to calculate the slowness vector of seismic wave propagation through the array of observatories (Fukao et al, ). Note that Love and Rayleigh waves have different propagation velocities.…”

Section: Methodsmentioning

confidence: 99%

“…Note that Love and Rayleigh waves have different propagation velocities. Therefore, to apply the technique of Fukao et al (), it is necessary to separate the manifestations of the Love and Rayleigh waves in the horizontal OBS records. For this purpose, we must consider that the bottom particles move perpendicular to the wave propagation direction in a Love wave and in the wave propagation direction in a Rayleigh wave (Love, ).…”

Section: Methodsmentioning

confidence: 99%

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Free Gravity Waves in the Ocean Excited by Seismic Surface Waves: Observations and Numerical Simulations

et al. 2019

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The Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) is a submarine cabled real-time seafloor observatory network for precise earthquake and tsunami monitoring. Ten DONET observatories were in operation during the 2011 Tohoku-Oki event near the Pacific coast of Honshu Island. Each observatory was equipped with an ocean bottom pressure gauge (PG) and a threecomponent ocean bottom seismometer (OBS). A comparative analysis of the PG and OBS records revealed that shortly after seismic surface waves traversed the DONET region, free gravity waves were observed within the water layer. The period of these gravity waves was approximately 170 s, the peak-to-peak amplitude was approximately 3.5 cm, the length was on the order of 22 km, and the phase velocity was 134 m/s. We performed numerical simulations of the observed gravity waves using a combined 2D/3D numerical model. The ground motions required for the simulation were reconstructed from records provided by the DONET OBSs and the nearest ground-based GPS stations. The synthetic bottom pressure variations are in good agreement with the DONET PG records. The synthetic displacements of the ocean surface throughout the simulation domain showed that the observed gravity waves were excited directly above the submarine slopes. Theoretical estimates and numerical experiments revealed the generation mechanism of the observed gravity waves.Plain Language Summary During submarine earthquakes, the ocean floor shifts from its original position. Due to this displacement, a huge volume of water is moved, which can generate a destructive tsunami wave. However, earthquakes are accompanied not only by displacement of the ocean bottom in the epicentral area but also by the emission of seismic waves. These seismic waves are also capable of exciting waves in the ocean. For our study of this phenomenon, we used unique data from the Japanese Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) observatories that were recorded during the Great East Japan Earthquake on 11 March 2011. Based on the records of the DONET seismometers, we reconstructed the movement of the bottom during the passage of a surface seismic wave. Then, using the obtained information, we simulated the movement of the water in the entire DONET area. The results of our simulation are in good agreement with the records of the DONET pressure gauges. The results also reveal that waves are only generated directly above steep submarine slopes during the passage of seismic waves. This study can help to identify tsunami wave signals in the records of bottom pressure gauges.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Free Gravity Waves in the Ocean Excited by Seismic Surface Waves: Observations and Numerical Simulations

et al. 2019

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“…Deployment and recovery were conducted during cruises YK14‐07 and YK15‐08 of R/V Yokosuka (Japan Agency for Marine‐Earth Science and Technology, JAMSTEC), respectively. The coordinates of stations A01–A10 are given in a supplementary table of Fukao et al (). We refer to station A05 as the reference station at which the water depth is 1,762 m, corresponding to a barotropic long‐wave speed of 131 m/s.…”

Section: Pressure Measurement and Analysismentioning

confidence: 99%

Detection of Ocean Internal Tide Source Oscillations on the Slope of Aogashima Island, Japan

et al. 2019

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Barotropic tidal currents over bottom topography force density surfaces to oscillate vertically and thereby to act as quasi‐stationary internal tide sources. Deploying a seafloor pressure gauge array with an aperture of 30 km for a year (2014–2015), we detected the low‐mode semidiurnal internal tidal waves propagating with a horizontal phase speed of ~1 m/s in the onshore and offshore directions over the array along the eastern slope of Aogashima Island, south of Japan. The amplitudes of the offshore propagating waves were greater than those of the onshore propagating waves, and both were positively correlated with the amplitudes of the local semidiurnal tide, which peaked in September and March. A tide‐resolving ocean circulation model (JCOPE‐T) well reproduced the observed onshore and offshore internal tidal wave propagation. The model indicated a standing wave region on the slope, where offshore propagating waves interact with standing waves locally pinned to the slope. Along the same profile over a distance of 100 km, we conducted seismic‐oceanographic analysis of the legacy multichannel seismic reflection data to retrieve vertical cross sections of the reflecting layers, which indicated sharp temperature changes in the ocean. Many of the slant reflecting layers were subparallel to the contour lines of the semidiurnal internal‐tide‐associated temperature anomalies in the JCOPE‐T model, suggesting a causal link between the fine reflection layering structure and the semidiurnal low‐mode internal tidal field.

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“…All tsunami waveforms with amplitudes of approximately 2 cm are similar to each other (Figure ). A tsunami earthquake with a surface wave magnitude of Ms 5.6 in the same area occurred on 13 June 1984 (Kanamori et al, ; Satake & Kanamori, ); their focal mechanisms suggest magma injection with the submarine volcano (Fukao et al, ; Kanamori et al, ).…”

Section: Comparison With Observationsmentioning

confidence: 99%

“…A compensated‐linear‐vector‐dipole‐type earthquake occurred on 2 May 2015, near Torishima island, Izu‐Bonin arc, Japan (Figure ), generating an abnormally large tsunami (e.g., 0.5 m at Hachijozima 180 km north of the epicenter) for the moment magnitude of M w 5.7, determined by the U.S. Geological Survey. The tsunami was caused by large deformation in a shallow part of a submarine volcanic body (Fukao et al, ). A triangular array of OBP gauges recorded an offshore tsunami (Sandanbata et al, ).…”

Section: Comparison With Observationsmentioning

confidence: 99%

Seismic Observation of Tsunami at Island Broadband Stations

2019

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Previous studies have reported seismic observations of tsunami recorded at island broadband stations. Coastal loading by the tsunami can explain them. For further quantification, we model tsunami propagation assuming an axisymmetric structure: a conical island with a flat ocean floor. The total tsunami wavefield can be represented by superposition between an incident tsunami wave and the scattering. The ground deformation due to the total tsunami wavefield at the center is calculated using static Green's functions for elastic half‐space with a first‐order correction for bathymetry. By fitting the modeled displacement to observed seismic data, we can infer the incident tsunami wave, which can be interpreted as the virtual tsunami amplitude without the conical island. First, we apply this new method to three components of seismic data at a volcano island, Aogashima, for the 2015 Torishima‐Oki tsunami earthquake. The estimated tsunami amplitude from the vertical component is consistent with the offshore array observation of absolute pressure gauges close to the island (1.5–20 mHz). The estimated incident azimuth from the three components is also consistent with ray theory. Second, we apply this method to seismic data at four island broadband stations in the Indian ocean for the 2010 Mentawai tsunami earthquake in Indonesia. Despite the limited observed frequency range from 0.5–2.0 mHz, the amplitudes and incident azimuths are consistent with past studies. These observations can complement offshore tsunami observations. Moreover, this method is applicable not only for a tsunami but also for background ocean infragravity wave activity.

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Mechanism of the 2015 volcanic tsunami earthquake near Torishima, Japan

Abstract: Anomalously large tsunamis generated by a submarine volcanic earthquake were recorded by an ocean bottom pressure gauge array.

Cited by 36 publications

References 46 publications

Free Gravity Waves in the Ocean Excited by Seismic Surface Waves: Observations and Numerical Simulations

Free Gravity Waves in the Ocean Excited by Seismic Surface Waves: Observations and Numerical Simulations

Detection of Ocean Internal Tide Source Oscillations on the Slope of Aogashima Island, Japan

Seismic Observation of Tsunami at Island Broadband Stations

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