Japan Trench Research Articles

Meteoric 10Be Reveals a Young, Active Accretionary Prism and Structurally Complex Décollement in the Vicinity of the 2011 Tohoku Earthquake Rupture

AbstractWe present new meteoric 10Be concentration data from marine sediment cores recovered during Integrated Ocean Drilling Program Expedition 343 (JFAST) that help constrain the age, origin, and internal structure of the frontal prism at the Japan trench in the vicinity of the 2011 Tohoku‐oki M9 earthquake rupture. 10Be sediment ages from the lower portion of the frontal prism range from ~0–10 Ma, with >60% of analyzed samples above the décollement yielding young ages <2 Ma. Repetition and inversion of high‐over‐low 10Be‐concentration sediments indicate the presence of stratigraphic inversions that correspond to faulting and imbrication of late Miocene and Quaternary sediments. The density of faults inferred indicates that the frontal prism has a fault spacing on the order of 10 s of meters, and the identification of faults in the underthrust section suggests that the plate boundary décollement may be a zone with multiple slip surfaces. Comparison of 10Be concentrations in the frontal prism with those of the incoming and forearc slope sediments indicates that the majority of the prism is sourced from accretion of Pacific Plate sediments, rather than from reworked frontal prism or slope sediments. These data suggest that over at least the past ~1–2 Ma, the décollement preferentially has localized at or near the base of the incoming sediment section, with relatively efficient sediment accretion occurring even in the presence of subducted horst‐and‐graben topography.

Intra-trench variations in flexural bending of the subducting Pacific Plate along the Tonga-Kermadec Trench

We conducted a detailed analysis of along-trench variations in the flexural bending of the subducting Pacific Plate at the Tonga-Kermadec Trench. Inversions were conducted to obtain best-fitting solutions of trench-axis loadings and variations in the effective elastic plate thickness for the analyzed flexural bending profiles. Results of the analyses revealed significant along-trench variations in plate flexural bending: the trench relief (W0) of 1.9 to 5.1 km; trench-axis vertical loading (V0) of −0.5×1012 to 2.2×1012 N/m; axial bending moment (M0) of 0.1×1017 to 2.2×1017 N; effective elastic plate thickness seaward of the outer-rise region (\(T_{\rm{e}}^{\rm{M}}\)) of 20 to 65 km, trench-ward of the outer-rise (\(T_{\rm{e}}^{\rm{m}}\)) of 11 to 33 km, and the transition distance of 20 to 95 km. The Horizon Deep, the second greatest trench depth in the world, has the greatest trench relief (w0 of 5.1 km) and trench-axis loading (V0 of 2.2×1012 N/m); these values are only slightly smaller than that of the Challenger Deep (W0 of 5.7 km and V0 of 2.9×1012 N/m) and similar to that of the Sirena Deep (W0 of 5.2 km and V0 of 2.0×1012 N/m) of the Mariana Trench, suggesting that these deeps are linked to great flexural bending of the subducting plates. Analyses using three independent methods, i.e., the \(T_{\rm{e}}^{\rm{M}}\)/\(T_{\rm{e}}^{\rm{m}}\) inversion, the flexural curvature/yield strength envelope analysis, and the elasto-plastic bending model with normal faults, all yielded similar average Te reduction of 28%–36% and average Te reduction area \({S_{\Delta {T_{\rm{e}}}}}\) of 1 195–1 402 km2 near the trench axis. The calculated brittle yield zone depth from the flexural curvature/yield strength envelope analysis is also consistent with the distribution of the observed normal faulting earthquakes. Comparisons of the Manila, Philippine, Tonga-Kermadec, Japan, and Mariana Trenches revealed that the average values of \(T_{\rm{e}}^{\rm{M}}\) and \(T_{\rm{e}}^{\rm{m}}\) both in general increase with the subducting plate age.

Petit-spot volcanoes on the oldest portion of the Pacific plate

The formation of a petit-spot volcano involves the ascent of an asthenospheric melt to the outboard part of a plate subduction zone where flexure of the plate has taken place in the outer trench swell. On the Pacific Plate, the only previously known examples of such volcanoes were in the vicinity of the Japan Trench. We describe here a newly discovered petit-spot submarine volcano that formed in one of the oldest parts of the Pacific Plate among a cluster of small conical knolls to the southeast of Minamitorishima (Marcus) Island. Geochronological data indicate that this petit-spot volcanic eruption occurred less than 3 million years ago. The volcano erupted on the eastern slope of the outer rise, and it is at an unusually large distance from the Mariana trench axis (~800 km) because this rise, near the older part of the Pacific Plate, is wider than those of other subduction systems.

Surface rupture and characteristics of a fault associated with the 2011 and 2016 earthquakes in the southern Abukuma Mountains, northeastern Japan, triggered by the Tohoku-Oki earthquake

The 2011 Tohoku-Oki offshore subduction earthquake (Mw 9.0) triggered many normal-type earthquakes inland in northeastern Japan. Among these were two very similar normal-faulting earthquakes in 2011 (Mw 5.8) and 2016 (Mw 5.9), which created surface ruptures along the newly named Mochiyama fault within the southern Abukuma Mountains, northeastern Japan, where no active faults had been previously mapped by interpretation of aerial photographs. We conducted field surveys in this area immediately after both earthquakes, and we performed trench excavations and observations of fault fracture zones after the 2016 event. These activities were complemented by an interferometric synthetic aperture radar analysis that mapped the areas of deformation and locations of surface discontinuities for both events. The combined results document the coseismic behavior of the Mochiyama fault during both events. Subtle tectonic geomorphic features associated with the fault were evident in a lidar digital elevation model of the area, and layered structures of gouge were documented in the field. These lines of evidence indicate repeated activity at shallow crustal levels and the possibility of Quaternary activity. In addition, our trench excavations revealed at least one faulting event before 2011. Our comparison of paleoseismic records on this and two other normal faults in the Abukuma Mountains suggests that great earthquakes in the Japan Trench supercycle of 500–700 years do not consistently trigger ruptures on these faults, and the case of 2011, in which the Tohoku-Oki megathrust earthquake triggered all three faults, is a rare occurrence.

2D spatial distribution of reflection intensity on the upper surface of the Philippine Sea plate off the Boso Peninsula, Japan

The region off the Boso Peninsula, Japan, is a tectonically complex area where the Pacific plate is subducting beneath both the landward plate and the Philippine Sea plate (PHS) from the Japan trench and the Izu-Bonin trench as the PHS is subducting under the landward plate from the Sagami trough. It is important to better determine the structure of this region to deepen our understanding of its seismicity. Previous seismic reflection studies have shown that reflections from the upper surface of the PHS vary with depth, being stronger in the main slip area of the slow slip events beneath the Boso Peninsula (Boso SSEs). However, the spatial relationship between the reflective area and the SSEs is poorly constrained. This study mapped the distribution of the reflective area using data recorded by ocean bottom seismometers during an active-source seismic experiment. We constructed a 3D P-wave velocity structure by using traveltimes of first arrivals from 18 ocean bottom seismometer records. We also adapted the traveltime mapping method to reflection traveltimes, projecting them to the depth–distance domain, to map the 2D distribution of strong reflections from the top of the PHS. These reflections were concentrated in two areas, one near the main slip area of the Boso SSEs and the other about 60km to the east. In the first area, the absence of strong velocity contrasts near the top of the PHS suggests that the reflections were generated by a thin low-velocity layer. In contrast, the structure of the second area has a convex shape of high velocity with a high velocity gradient near the top of the PHS. This structure may represent boninitic material of the outer-arc high, partially serpentinized peridotite, or gabbro displaced by intraoceanic reverse faults.

Dynamic Wedge Failure and Along‐Arc Variations of Tsunamigenesis in the Japan Trench Margin

AbstractElastic dislocation models require large near‐trench slip to explain large tsunamigenesis, which is probably best exemplified in the 2011 M9 Tohoku earthquake. However, it is puzzling that the largest Tohoku tsunami heights occurred about 100 km north of the largest slip zone, where bathymetric surveys indicate no large slip at the trench or submarine landslides. Here we show that coseismic yielding of plentiful sediments in the northern Japan Trench margin can induce large inelastic uplift landward from the trench and diminish slip near the trench. The scarcity of sediments in the south leads to nearly elastic response with large slip at the trench and mostly horizontal seafloor displacement. Thus, the variations of sediments along the Japan Trench and sediment yielding can explain the puzzling variations of tsunamigenesis and near‐trench slip in this earthquake. Inelastic wedge deformation can be an important mechanism of tsunamigenesis in accretionary and other sediment‐filled plate margins.

First insight of bathymetric patterns among deep-sea harpacticoid diversity and composition on landward slopes of subduction zones along the Japanese island arc

Universal classification schemes have been proposed for the vertical zonation of sessile benthos on intertidal rocky shores; however, generalized patterns among deep-sea meiofauna of different regions remain rare. We conducted a preliminary, family-level study of deep-sea harpacticoid diversity and community composition off Tosa Bay, Shikoku island, Japan, along a transect on a landward continental slope of the Nankai Trough. We then compared these data to those of other subduction zones along the Japanese island arc, including the Kuril, Japan, and Ryukyu trenches. Common bathymetric trends in harpacticoid composition were observed among these subduction zones from upper bathyal to abyssal depths (400–5800 m). Ectinosomatidae was the most abundant family within this depth interval; however, its frequency decreased with increasing depth, whereas the abundance of Pseudotachidiidae was highest at abyssal depths. Harpacticoid composition differed significantly between landward slopes of the Ryukyu Trench and those of the Japan and Kuril trenches. However, harpacticoid composition on the landward slope of the Nankai Trough, between the Ryukyu and Japan trenches, was similar to that around the neighboring subduction zones. These findings imply that patterns of deep-sea harpacticoid community composition are similar along the continental slope of the Japanese island arc, and that the Izu–Ogasawara arc, which subdivides the subduction zones, has not functioned as a physical barrier to harpacticoid expansion between zones.

Sources, Degradation, and Transport of Organic Matter in the New Britain Shelf‐Trench Continuum, Papua New Guinea

AbstractHadal trenches are considered as depocenters for organic matter and hotspots for microbial diagenetic activity. Here, we explore the sources, degradation, and transport of organic matter in the shelf‐trench continuum using seven short sediment cores collected along two transects with water depths ranging between 1,553 and 8,901 m in the New Britain Trench area, Papua New Guinea. Carbon isotopic compositions (δ13C) and radiocarbon contents (Δ14C) of sedimentary organic matter accompanied by total organic carbon/total nitrogen ratios suggest an important contribution from the preaged soil organic matter mixed by the marine algae and terrigenous C3 vascular plants. In addition, the trench axis sites are characterized by elevated accumulation of terrigenous organic materials. Rates of organic matter mineralization approximated by dissolved inorganic carbon fluxes at the sediment‐water interface reveal an approximately threefold higher rate at the trench axis sites than the abyssal sites. 210Pbxs profiles and burial of carbonate (up to 50%) at both trench axis sites reflect recent occurrence of mass‐wasting events possibly induced by earthquakes, which is responsible for the transport of preaged, terrigenous organic matter to the trench bottom. This tectonically triggered deposition event, which was also shown to occur in the Japan Trench, the Tonga Trench, and probably in many other trenches, is likely to efficiently transport terrigenous organic matter to hadal trenches, thereby underpinning the importance of terrigenous organic matter burial in hadal trenches for the ocean organic carbon budget. Furthermore, we hypothesize that hadal trenches may host a distinct microbial community that is capable of feeding on old, refractory terrigenous organic matter.

Double seismic zone and seismicity in the mantle wedge beneath the Ogasawara Islands identified by an ocean bottom seismometer observation

Around the Ogasawara Islands, only a few seismic stations in the area can be used to determine the hypocenters of regional earthquakes; thus, hypocenter location precision tends to be low. To more precisely determine hypocenter locations, we deployed a temporary seismic observation network of pop-up ocean bottom seismometers around the Ogasawara Islands from July to October 2015. We identified a double seismic zone in the 70–200 km depth range associated with the subducting Pacific slab. The slab-normal distance between the two planes of the double seismic zone is about 30–35 km, similar to such distances observed along the Japan and Mariana trenches. Furthermore, we found unusual seismicity in the mantle wedge at 20–50 km depth beneath the Ogasawara trough that might be related to structure formed at the onset of the oceanic slab subduction. The hypocenters determined from the ocean bottom seismometer observation were horizontally separated by a few tens of kilometers from hypocenters published by the Seismological Bulletin of Japan. USGS locations (Preliminary Determination of Epicenters) seem to be offset westward about 30 km compared with the locations determined in this study.

Seismic velocity structure and its implications for oceanic mantle hydration in the trench–outer rise of the Japan Trench

Seismic velocity structure and its implications for oceanic mantle hydration in the trench–outer rise of the Japan Trench

Megathrust earthquake drives drastic organic carbon supply to the hadal trench

The giant 2011 Tohoku-oki earthquake has been inferred to remobilise fine-grained, young surface sediment enriched in organic matter from the slope into the >7 km deep Japan Trench. Yet, this hypothesis and assessment of its significance for the carbon cycle has been hindered by limited data density and resolution in the hadal zone. Here we combine new high-resolution bathymetry data with sub-bottom profiler images and sediment cores taken during 2012–2016 in order to map for the first time the spatial extent of the earthquake-triggered event deposit along the hadal Japan Trench. We quantify a sediment volume of ~0.2 km3 deposited from spatially-widespread remobilisation of young surficial seafloor slope sediments triggered by the 2011 earthquake and its aftershock sequence. The mapped volume and organic carbon content in sediment cores encompassing the 2011 event reveals that this single tectonic event delivered >1 Tg of organic carbon to the hadal trench. This carbon supply is comparable to high carbon fluxes described for other Earth system processes, shedding new light on the impact of large earthquakes on long-term carbon cycling in the deep-sea.

The Thermal Effects of Plate‐Bending‐Related Thickening of the Oceanic Crustal Aquifer in the Nankai Trough and Japan Trench Subduction Zones

AbstractFor the Nankai Trough and Japan Trench subduction zones, we examine the combined thermal effects of lateral heat exchange by fluid circulation in an oceanic crustal aquifer and the thickening of that aquifer due to plate‐bending‐related faulting. Faults induced by the bending of a plate entering a subduction zone are hypothesized to increase the depth over which vigorous hydrothermal circulation can redistribute heat in the oceanic crust. Previous 1‐D (vertical) thermal models have examined how aquifer thickening can mine heat from deep in the crust seaward of the trench. Here we construct 2‐D thermal models that include aquifer thickening and lateral heat exchange in the aquifer. We vary the maximum aquifer thickness and the landward extent to which hydrothermal circulation persists within the subducted crust. For the Nankai margin, models most consistent with heat flux data require vigorous fluid circulation extending up to 150 km landward of the trench; aquifer thickening is permitted but not required for models to be consistent with the heat flux observations. Conversely, for the Japan Trench, preferred models include substantial aquifer thickening (maximum aquifer thickness of 1.8–5 km); vigorous circulation extending landward of the trench is permitted but is not required. For hot subduction zones, the thermal effects of aquifer thickening are modest relative to the large lateral advective heat redistribution. For cold subduction zones, where small lateral temperature gradients limit the amount of lateral heat redistribution, aquifer thickening can be the dominant process generating thermal anomalies.

Time lapse seismic analysis of the Tohoku-Oki 2011 earthquake

A detailed time lapse seismic analysis of subsurface movements is presented using seismic 2D lines acquired prior to and after the Tohoku-Oki earthquake offshore Japan in 2011. Estimated movements of the seabed from the time lapse seismic data corresponds well with estimations using bathymetric data. On the shelf, we find seabed subsidence of up to 8–9 m, and closer to the Japan trench, we find a seabed uplift of more than 10 m. Along the 100 km long 2D seismic line, we find alternating subsidence and uplift. We find horizontal displacements at the seabed that are significantly larger, up to 40–50 m, however these estimates are more uncertain. Close to the Japan trench, these horizontal displacements are practically opposite in direction (pointing towards the trench from both sides) and large (˜15 m). At the sediment-basement interface, we estimate vertical subsidence of two independent large blocks (each 4 km wide) to be up to 14 m. This means that the sediment package in this region has been stretched by 5–6 m. This type of overburden stretching is similar to hydrocarbon reservoirs that compact due to production and produce corresponding overburden stretching. Several examples of new faults, new layering and orientation of sedimentary layers are found. Implications for subsurface storage of CO2 in areas close to subduction zones are also discussed.

Spatio-temporal changes in the seismic velocity induced by the 2011 Tohoku-Oki earthquake and slow slip event revealed from seismic interferometry, using ocean bottom seismometer\u2019s records

Seismic interferometry is one of the most effective techniques for detecting temporal variations in seismic velocity caused by large earthquakes. Before the 2011 Tohoku-Oki earthquake (Mw9.0) near the Japan Trench, a slow slip event (SSE, Mw7.0) and low-frequency tremors were observed near the trench. Here, we applied a seismic interferometry technique using ambient noise to data from 17 ocean bottom seismometers (OBSs) installed above the focal region before the main shock. We used our technique to detect temporal variations in seismic velocity caused by the main shock, SSE, and low-frequency tremors. In the region above the large coseismic slip area, we detected a 1–2% seismic velocity decrease after the main shock. In addition, we observed very small temporal increases in seismic velocity near the SSE fault during the initial SSE stage. Moreover, for most of the OBSs, we observed temporal variations in the autocorrelation functions (ACFs) during the low-frequency tremors. These may have been caused by temporal variations in the ambient noise source distributions, resulting from low-frequency tremors. These results suggest the possibility of detecting low-frequency tremors using ACF monitoring.

Seismic Structure of the Oceanic Crust Around Petit‐Spot Volcanoes in the Outer‐Rise Region of the Japan Trench

AbstractThe structure of an incoming oceanic plate in a subduction zone systematically changes toward the trench because of the flexure of the plate. Changes in seismic velocity from the outer rise toward the trench are widely considered to be caused by the development of bending‐related faults. However, few studies have focused on the effects of igneous activities preceding the subduction, which lead to structural changes of the incoming plate. Here we present active‐source seismic data acquired around petit‐spot volcanoes situated in the outer‐rise region of the Japan Trench. The resultant velocity model showed low seismic velocities (Vp < 4.0 km/s) at the top of the oceanic crust under the petit‐spot volcanoes. Moreover, the velocity reduction was significantly larger than that caused only by bending‐related faults. Our results suggest that petit‐spot volcanism is a contributing factor affecting structural variation and modification of the incoming oceanic plate in this subduction zone.

Stress change before and after the 2011 M9 Tohoku-oki earthquake

Megathrust systems hold important clues for our understanding of long- and short-term plate boundary dynamics, and the 2011 M9 Tohoku-oki earthquake provides a data-rich case in point. Here, we show that the F-net moment tensor catalog indicates systematic changes in crustal stress in the years leading up to the M9, due to the co-seismic effect, and for the last few years due to viscous relaxation. We explore the match between imaged stress change and the perturbations that are expected from 3-D, mechanical models of the visco-elastic relaxation and afterslip effects of the M9. While these models were constructed based on geodetic and structural seismology constraints alone, they match many characteristics of the seismicity-inferred stress change. This provides additional confidence in the modeling approach, and new clues for our understanding of plate boundary dynamics for the Japan trench. The success of deterministic approaches for exploring crustal stress change also implies that joint inversions using stress from focal mechanisms and geodetic constraints may be feasible. Such future efforts should provide key insights into time-dependent seismic hazard including earthquake triggering scenarios.

Holocene paleoseismic history of the Yunodake fault ruptured by the 2011 Fukushima-ken Hamadori earthquake, Fukushima Prefecture, Japan

The 11 March 2011 Mw 9.0 Tohoku earthquake, which occurred along the Japan Trench, induced an unusual shallow normal-faulting earthquake: the 11 April 2011 Fukushima-ken Hamadori earthquake (Mw 6.6) near the Pacific coast in southeastern Fukushima Prefecture. Remarkable surface ruptures appeared along the NW-trending Yunodake fault, the NNW-trending Itozawa fault (west) (also known as the Shionohira fault). Although the Yunodake fault is mapped with a normal down-to-the-SW sense of slip, its paleoseismic history is not clear. A trenching survey on the northwestern part of the Yunodake fault is reported here that reveals its Holocene paleoearthquake history. The trench wall beneath the 2011 ruptures exposed fissures, open cracks, stratigraphic offsets, and a succession of cumulatively inclined Holocene sediment layers. Detailed observations of the trench walls and radiocarbon dating reveal that the two latest previous surface-rupturing earthquakes occurred between 800 and 6200 years ago and between 5900 and 6700 years ago respectively. The estimated recurrence interval of the Yunodake fault is 800 to 5900 years at this location.

Along‐Arc Heterogeneity of the Seismic Structure Around a Large Coseismic Shallow Slip Area of the 2011 Tohoku‐Oki Earthquake: 2‐D Vp Structural Estimation Through an Air Gun‐Ocean Bottom Seismometer Experiment in the Japan Trench Subduction Zone

AbstractThe 2011 Tohoku‐oki earthquake occurred with a rupture length of 500 km along the Japan Trench, causing a large slip (>30 m) at the shallowest portion of the plate boundary fault south of 39°N off Miyagi and a smaller slip (~10 m) north of 39°N off Sanriku, the northern part of the source area. We estimated the P wave velocity (Vp) structure around the shallowest portion of the plate boundary along the trench to investigate the spatial correlation between the structural variation and coseismic shallow slip distribution of the 2011 earthquake. For this purpose, we analyzed data from an air gun‐ocean bottom seismometer survey on an along‐arc line on the lower part of the landward slope of the Japan Trench. We detected a high Vp zone in the hanging wall of the plate interface off Miyagi, which corresponds to the Cretaceous backstop. The Vp model also showed the absence of the high Vp zone off Sanriku. This suggests that the low‐Vp deformed prism is wider off Sanriku than off Miyagi. Therefore, the backstop is close to the trench particularly off Miyagi. We suggest a correlation between this along‐arc variation of the hanging wall side structure and the extent of the coseismic shallow slip in the trench axial region; the large slip reached the trench off Miyagi but not off Sanriku.

Tectonically-triggered sediment and carbon export to the Hadal zone

Sediments in deep ocean trenches may contain crucial information on past earthquake history and constitute important sites of carbon burial. Here we present 14C data on bulk organic carbon (OC) and its thermal decomposition fractions produced by ramped pyrolysis/oxidation for a core retrieved from the >7.5 km-deep Japan Trench. High-resolution 14C measurements, coupled with distinctive thermogram characteristics of OC, reveal hemipelagic sedimentation interrupted by episodic deposition of pre-aged OC in the trench. Low δ13C values and diverse 14C ages of thermal fractions imply that the latter material originates from the adjacent margin, and the co-occurrence of pre-aged OC with intervals corresponding to known earthquake events implies tectonically triggered, gravity-flow-driven supply. We show that 14C ages of thermal fractions can yield valuable chronological constraints on sedimentary sequences. Our findings shed new light on links between tectonically driven sedimentological processes and marine carbon cycling, with implications for carbon dynamics in hadal environments.

Seafloor Displacement After the 2011 Tohoku‐oki Earthquake in the Northern Japan Trench Examined by Repeated Bathymetric Surveys

AbstractWe conducted multibeam bathymetric surveys crossing the northern Japan Trench offshore Sanriku and examined seafloor displacement by means of the difference in bathymetry before and after the 11 March 2011 Tohoku‐oki earthquake. The seafloor displacements were less than several meters in vertical displacement and less than 20 m in trench‐normal horizontal displacement; consequently, lower amplitude seafloor displacements were evaluated. Comparatively, the landward lower slope near the trench was uplifted, the middle slope was subsided, and the midslope terrace was depressed after the earthquake. Therefore, localized very large fault slip or very large submarine landslide is unlikely. Instead, smaller uplift of the lower slope near the trench extending broader along the trench seems to be plausible explanation for the devastating tsunami source in the northern Japan Trench area.