Tsunamigenic potential of the shallow subduction plate boundary inferred from slow seismic slip

Sugioka, Hiroko; Okamoto, Taro; Nakamura, Takeshi; Ishihara, Yasushi; Ito, Aki; Obana, Koichiro; Kinoshita, Masataka; Nakahigashi, Kazuo; Shinohara, Masanao; Fukao, Yoshio

doi:10.1038/ngeo1466

Cited by 154 publications

(254 citation statements)

References 25 publications

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“…We also found only a few earthquakes in the sedimentary wedge. Although very-lowfrequency (VLF) earthquakes are known to occur in the sedimentary wedge and at the plate boundary in this region Ito and Obara 2006;Sugioka et al 2012), their distribution has little overlap with that of the ordinary earthquakes (Fig. 2).…”

Section: Resultsmentioning

confidence: 99%

“…After the 2004 earthquakes, verylow-frequency (VLF) earthquake activity was triggered above the source region . Recent studies show that the VLF earthquakes are in the sedimentary wedge or at the plate boundary (Ito and Obara 2006;Sugioka et al 2012). Kitajima and Saffer (2012) considered that elevated pore pressures in the sediments play an important role in the generation of VLF earthquakes.…”

Section: Introductionmentioning

confidence: 99%

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Seismic activity beneath the Nankai trough revealed by DONET ocean-bottom observations

2013

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We conducted a detailed investigation of seismic activity from January 2011 to February 2013 along the Nankai trough off the Kii Peninsula, central Japan, by using data obtained from the DONET ocean-bottom observation network. The hypocenters are mostly within the subducting Philippine Sea (PHS) plate, although a few are along the plate boundary or in the sedimentary wedge below the Kumano forearc basin. The seismic activity can be separated into events above and below 20 km depth, which corresponds approximately to the Moho. The hypocenter distributions are distinctly different for these groups. The seismic activity in the oceanic crust can be further separated into three clusters. Most of the seismic activity recorded in our data represents aftershocks of the 2004 off the Kii Peninsula earthquakes (M JMA = 7.1, 7.4, and 6.5), which occurred in the PHS plate. The hypocenter distribution in the oceanic crust correlates well with the location of the Paleo-Zenisu ridge, which is formed by a chain of seamounts that is subducting beneath the forearc basin. The hypocenters in the uppermost mantle are aligned on a plane dipping to the southeast, consistent with the existence of a thrust fault cutting through the lithosphere of the oceanic plate. The focal mechanisms of the earthquakes show that the axis of compressive stress in the PHS plate is oriented N-S, almost perpendicular to the direction of plate convergence, indicating a complex tectonic regime in this region. These results suggest that intraplate shortening may be occurring in the subducting oceanic plate.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Seismic activity beneath the Nankai trough revealed by DONET ocean-bottom observations

2013

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“…The waves for the model with a seawater layer are less dispersive, and simultaneously arrive at the stations, resulting in large amplitudes at DONET stations. From analyses of pop-up type ocean bottom observations for very-low-frequency events off the Kii peninsula, Sugioka et al (2012) also showed that the seismic wave propagation in this ocean area is affected by the seawater layer and the sediment layers. Although the amplifications may Note that stations KMC10-12 had not been installed before the events, and therefore no data are plotted.…”

Section: Cause Of Anomalously Large Seismic Amplitudesmentioning

confidence: 99%

Anomalously large seismic amplifications in the seafloor area off the Kii peninsula

Nakamura

Hayashimoto²,

Takahashi

et al. 2014

Mar Geophys Res

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Seismic wave amplifications were investigated using strong-motion data obtained from the ground's surface (K-net) on the Kii peninsula (southwestern Japan) and from the network of twenty seismic stations on the seafloor (DONET) located off the peninsula near the Nankai trough. Observed seismograms show that seismic signals at DONET stations are significantly larger than those at K-net stations, independent of epicentral distances. In order to investigate the cause of such amplifications, seismic wavefields for local events were simulated using the finitedifference method, in which a realistic 3D velocity structure in and around the peninsula was incorporated. Our simulation results demonstrate that seismic waves are significantly amplified at DONET stations in relation to the presence of underlying low-velocity sediment layers with a total thickness of up to 10 km. Our simulations also show considerable variations in the degree of amplification among DONET stations, which is attributed to differences in the thickness of the sediment layers. The degree of amplification is relatively low at stations above thin sediment layers near the trough axis, but seismic signals are much more amplified at stations closer to the Kii peninsula, where sediment layers are thicker than those at the trough axis. Simulation results are consistent with observations. This study, based on seafloor observations and simulations, indicates that because seismic signals are amplified due to the ocean-specific structures, the magnitude of earthquakes would be overestimated if procedures applied to data observed at land stations are used without corrections.

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“…In our 3D simulation case, we estimate a propagation speed of approximately 0.7 km/s in the Airy phase, based on analysis of the apparent velocity of the isolated later phase at DONET stations KMB05-KMB08 and KMD13-KMD16, where the thicknesses of the accretionary prism and the seawater layer are almost constant. Figure 7 illustrates the vertical component of the synthetic waveform at these stations in order of epicentral distance, indicating propagation with an apparent velocity of 0.7 km/ s. An Airy phase with similarly slow propagation has also been found in temporary ocean bottom observations of VLF events with source depths of 5-12 km near DONET stations (SUGIOKA et al 2012).…”

Section: Submarine Landslide Simulationmentioning

confidence: 89%

“…However, care must be taken when identifying seismic signals associated with submarine landslides in cases involving limited observation data obtained from a limited number of land stations that are located far from the associated epicenter. Some unusual suboceanic events such as submarine volcanic activity, very low-frequency (VLF) earthquakes, and tsunami earthquakes typically generate long-lasting low-frequency seismic signals (e.g., KANAMORI, 1972;SUGIOKA et al 2012), which may introduce difficulties in separating the seismic signals associated with submarine landslides from those related to unusual suboceanic events. However, use of seafloor broadband seismic stations near sources should improve detection of submarine landslide signals by allowing direct observation of suboceanic phenomena in oceanic areas.…”

Section: Introductionmentioning

confidence: 99%

Seismic Wavefields in the Deep Seafloor Area from a Submarine Landslide Source

Nakamura

Takenaka

Okamoto

et al. 2013

Pure Appl. Geophys.

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Abstract-We use the finite difference method to simulate seismic wavefields at broadband land and seafloor stations for a given terrestrial landslide source, where the seafloor stations are located at water depths of 1,900-4,300 m. Our simulation results for the landslide source explain observations well at the seafloor stations for a frequency range of 0.05-0.1 Hz. Assuming the epicenter to be located in the vicinity of a large submarine slump, we also model wavefields at the stations for a submarine landslide source. We detect propagation of the Airy phase with an apparent velocity of 0.7 km/s in association with the seawater layer and an accretionary prism for the vertical component of waveforms at the seafloor stations. This later phase is not detected when the structural model does not consider seawater. For the model incorporating the seawater, the amplitude of the vertical component at seafloor stations can be up to four times that for the model that excludes seawater; we attribute this to the effects of the seawater layer on the wavefields. We also find that the amplification of the waveform depends not only on the presence of the seawater layer but also on the thickness of the accretionary prism, indicating low amplitudes at the land stations and at seafloor stations located near the trough but high amplitudes at other stations, particularly those located above the thick prism off the trough. Ignoring these characteristic structures in the oceanic area and simply calculating the wavefields using the same structural model used for land areas would result in erroneous estimates of the size of the submarine landslide and the mechanisms underlying its generation. Our results highlight the importance of adopting a structural model that incorporates the 3D accretionary prism and seawater layer into the simulation in order to precisely evaluate seismic wavefields in seafloor areas.

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Tsunamigenic potential of the shallow subduction plate boundary inferred from slow seismic slip

Cited by 154 publications

References 25 publications

Seismic activity beneath the Nankai trough revealed by DONET ocean-bottom observations

Seismic activity beneath the Nankai trough revealed by DONET ocean-bottom observations

Anomalously large seismic amplifications in the seafloor area off the Kii peninsula

Seismic Wavefields in the Deep Seafloor Area from a Submarine Landslide Source

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