Investigating a tsunamigenic megathrust earthquake in the Japan Trench

doi:10.1126/science.abe1169

GSTDTAP > 气候变化

DOI	10.1126/science.abe1169
	Investigating a tsunamigenic megathrust earthquake in the Japan Trench
	Shuichi Kodaira; Takeshi Iinuma; Kentaro Imai
	2021-03-12
发表期刊	Science
出版年	2021
英文摘要	Ten years ago, the magnitude 9 Tohoku-oki earthquake rocked Japan and caused massive damage. The earthquake also generated a destructive tsunami, the impacts of which are still being managed today. Kodaira et al. review what was learned from the tremendous number of observations from the great earthquake that unexpectedly ruptured into a shallow part of the megathrust fault. Postseismic deformation is ongoing, as is the risk of another very large normal fault earthquake seaward of the Japan Trench. Science , this issue p. [eabe1169][1] ### BACKGROUND Ten years have passed since the 2011 Tohoku-oki earthquake occurred in the Japan Trench, where the Pacific plate subducts beneath the continental plate. The earthquake and tsunami caused enormous damage along the coast of northeast Japan in the Tohoku region, and local communities are still recovering. Tsunami traces more than 10 m above sea level were observed along 530 km of coastline in central and northeast Japan, and runups higher than 20 m were observed over about 200 km of the Tohoku coast. The tsunami inundated an area of 561 km2, and its runup reached a maximum of 40 m in northern Tohoku. These statistics made it one of the largest tsunamis ever recorded in historical literature as well as in geological records. The earthquake occurred in the vicinity of the world’s most densely instrumented seismic, geodetic, and tsunami observation networks, which clearly recorded the dramatic geodynamic effects of the earthquake. In addition, geophysical and geological data were acquired offshore, in the rupture zone, before and after the earthquake. These marine observations are decisive ground-truth data showing that coseismic slip exceeded 50 m in places and that the rupture reached the shallowest parts of the megathrust fault in the subduction zone. ### ADVANCES Geodetic data from the seafloor before and after the earthquake, from the Global Navigation Satellite System–Acoustic combination technique, show that the seafloor near the epicenter underwent coseismic displacement of 31 m toward the southeast and had an uplift of 3 m. Differential bathymetric mapping, comparing the seafloor before and after the earthquake, inferred a coseismic seafloor displacement of more than 50 m at the trench axis. A rapid-response deep-sea drilling project successfully collected material from the earthquake’s rupture zone on the plate boundary fault near the trench and measured the thermal anomaly due to frictional heating of the fault during coseismic slip. This showed that the plate boundary fault is rich in weak layers of clay and suggested that thermal pressurization within the clay layers promoted the exceptionally large coseismic fault slip. These ground-truth data provided evidence to constrain a slip behavior in a shallow part of the subduction zone. Crucial progress in determining the coseismic slip character is represented by the evidence showing lateral variations of coseismic slip near the trench and possible structural factors to control the lateral variation. For example, no resolvable coseismic seafloor displacement from differential bathymetry was observed at the north and south of the main rupture zone, and seismic images showed lateral discontinuity of a pelagic clay layer due to subduction of petite-spots at the north of the main rupture zone. ### OUTLOOK Crustal deformation, including afterslip and viscoelastic relaxation of the mantle, continues in the Japan Trench 10 years after the earthquake. Viscoelastic relaxation is predominant in the central part of the trench, where the large coseismic slip extended to the trench. The relaxation displaces this seafloor westward, whereas afterslip displaces it eastward around the main rupture zone. These observations, along with aftershock activity from normal fault earthquakes in the incoming oceanic plate, indicate that trench-normal extension remains in the oceanic plate and seaward of the main rupture zone. The recurrance probability of a great earthquake (magnitude = ~9) in the Japan Trench in the near future is very low, but even 10 years after the Tohoku-oki earthquake, seismic activities in east Japan, including the trench outer-slope and surrounding areas of the main rupture zone, are still higher than those before the earthquake. Because past observations show that large normal fault earthquakes in the incoming oceanic plate can occur after great plate-boundary earthquakes, further investigations into the temporal changes in the stress state around the Japan Trench are necessary to evaluate the possibility of such a “follow-up” earthquake in the incoming Pacific plate. ![Figure][2] Coseismic displacement and tsunami height of the 2011 Tohoku-oki earthquake. Oblique view of northern Japan and the Japan Trench showing coseismic displacements (black and white arrows), tsunami heights (colored bars), and differential bathymetry profiles across the trench (multicolored lines). The scale of the onland displacements is five times as large as those of the seafloor displacements. Dashed areas indicate approximate areas where a set of three turbidite layers attributed to the Tohoku-oki earthquake and past large events are observed (ellipse) and where a deformed and altered pelagic clay layer by a petit-spot volcanism is inferred from seismic images (half-rectangle), respectively. A yellow circle indicates the location of the Japan Trench Fast Drilling Project (JFAST) site. The 2011 Tohoku-oki earthquake occurred in the Japan Trench 10 years ago, where devastating earthquakes and tsunamis have repeatedly resulted from subduction of the Pacific plate. Densely instrumented seismic, geodetic, and tsunami observation networks precisely recorded the event, including seafloor observations. A large coseismic fault slip that unexpectedly extended to a shallow part of megathrust fault was documented. Strong lateral variations of the coseismic slip near the trench were recorded from marine geophysical studies, along with a possible cause of these variations. The seismic activities in east Japan are still higher than those before the earthquake, and crustal deformation is still occurring. Although the recurrence probability of a great earthquake (magnitude = ~9) in the Japan Trench in the near future is very low, a large normal fault earthquake seaward of the Japan Trench is a concerning possibility. [1]: /lookup/doi/10.1126/science.abe1169 [2]: pending:yes
领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/318688
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Shuichi Kodaira,Takeshi Iinuma,Kentaro Imai. Investigating a tsunamigenic megathrust earthquake in the Japan Trench[J]. Science,2021.
APA	Shuichi Kodaira,Takeshi Iinuma,&Kentaro Imai.(2021).Investigating a tsunamigenic megathrust earthquake in the Japan Trench.Science.
MLA	Shuichi Kodaira,et al."Investigating a tsunamigenic megathrust earthquake in the Japan Trench".Science (2021).