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Article ; Online: Geomagnetic relative paleointensity and direction during the last 40,000 years obtained from a sediment core in the Nankai Trough

Ryoya Goto / Toshitsugu Yamazaki / Natsumi Okutsu / Juichiro Ashi

Earth, Planets and Space, Vol 76, Iss 1, Pp 1-

2024 Volume 17

Abstract: Abstract Construction of regional geomagnetic secular variation curves for the last several tens of thousands of years is important for understanding the behavior of non-dipole fields and applications to geochronology. Around Japan, secular variation ... ...

Abstract	Abstract Construction of regional geomagnetic secular variation curves for the last several tens of thousands of years is important for understanding the behavior of non-dipole fields and applications to geochronology. Around Japan, secular variation records of older than 10 ka was scarce, in particular for relative paleointensity (RPI). Here, we conducted a paleomagnetic study of a sediment core covering the last ~ 40 kyr taken from a small basin in the Nankai Trough. The core consists of homogenous hemipelagic sediments except for turbidites and volcanic ashes. The age model was constructed based on seven 14C datings and two volcanic ashes. Turbidites and volcanic ashes were excluded from the construction of secular variation curves because of geologically instantaneous deposition. It was revealed that the magnetization of this core is carried largely by detrital magnetic minerals, although magnetofossils are also contained. Bulk magnetic properties show some temporal changes in magnetic concentration and grain size, but still homogeneous enough for reliable RPI estimations except for turbidites and volcanic ashes. The resultant RPI shows no correlation with the normalizer, anhysteretic remanent magnetization, of the RPI estimations or with a proxy for a magnetic grain size and/or the proportion of magnetofossils to detrital magnetic minerals. The obtained RPI record shows a long-term increasing trend since ~ 40 ka, which coincides with global stack curves. On the other hand, there are some differences in shorter timescale variations, which may reflect non-dipole fields. This study demonstrated that hemipelagic sediments in the Nankai Trough have potential for recovering high-quality RPI records when turbidites and volcanic ashes were excluded and are useful for accumulating records to construct a regional master curve. Graphical abstract
Keywords	Relative paleointensity ; Geomagnetic secular variation ; Paleomagnetism ; Rock magnetism ; Nankai Trough ; Geography. Anthropology. Recreation ; G ; Geodesy ; QB275-343 ; Geology ; QE1-996.5
Subject code	550
Language	English
Publishing date	2024-01-01T00:00:00Z
Publisher	SpringerOpen
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Relationship between depositional environments and preservabilities of Holocene tsunami deposits on the Hidaka coast, Hokkaido, Japan

Ryo Nakanishi / Juichiro Ashi / Yosuke Miyairi / Yusuke Yokoyama

Quaternary Science Advances, Vol 10, Iss , Pp 100081- (2023)

2023

Abstract: Rising sea levels and associated coastal topography changes are expected to increase coastal vulnerability to tsunamis. Reliable records of the periodicity of the most damaging tsunamis, with recurrence intervals of several centuries, are often ... ...

Abstract	Rising sea levels and associated coastal topography changes are expected to increase coastal vulnerability to tsunamis. Reliable records of the periodicity of the most damaging tsunamis, with recurrence intervals of several centuries, are often incomplete. To understand the likelihood of coastal disasters that would be worsened by sea-level rise, as well as their possible occurrence leading to the actual extent of inland inundation, it is useful to examine the geological record of the mid-Holocene highstand periods to supplement the written archives. However, the formation and preservation potential of event layers depends heavily on the depositional environment resulting from coastal evolutions. This study attempted to reconstruct the depositional environment changes using diatom and chemical analyses on the Hidaka coast of Hokkaido, Japan, to examine the preservation of event layer archives.Following field investigations, we found three sand layers formed by extreme waves from field investigations. The relationship between the depositional environment and the timing of the event layer formation over the past 7000 years along the central Hidaka coast indicates that supratidal to freshwater marsh environments are the most suitable for preserving tsunami deposits in this region. Since sand layers in the distant areas from wave sources were not well preserved in upland (as at elevations above marsh environment) and saltmarsh environments, the time windows of preservation were very limited depending on the depositional environments with relative sea-level changes. On the other hand, in areas closer to the Kuril Trench, sand layers were preserved not very sensitive to the depositional environment, suggesting that the event archive length is related to the magnitude of tsunamis.
Keywords	Sea level index points ; Tsunami deposits ; Diatom ; Depositional environment ; Preservation potential ; Geography. Anthropology. Recreation ; G ; Archaeology ; CC1-960
Subject code	550
Language	English
Publishing date	2023-04-01T00:00:00Z
Publisher	Elsevier
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: A dataset for distribution and characteristics of Holocene pyroclastic fall deposits along the Pacific coasts in western Hokkaido, Japan

Ryo Nakanishi / Juichiro Ashi / Satoshi Okamura

Data in Brief, Vol 33, Iss , Pp 106565- (2020)

2020

Abstract: The tephra layers known with eruption ages play an important role in an investigation of tsunami history and archaeology in addition to volcanic history in Hokkaido, Japan. We investigated the event and tephra layers of the Late Holocene in the Pacific ... ...

Abstract	The tephra layers known with eruption ages play an important role in an investigation of tsunami history and archaeology in addition to volcanic history in Hokkaido, Japan. We investigated the event and tephra layers of the Late Holocene in the Pacific coast of western Hokkaido, where the stratigraphy of the Late Holocene has not been clarified. Surveys in coastal peatlands, mostly undisturbed deposits, have allowed for the discovery of thin tephra layers. The newly discovered tephra layers at the unexplored site were used to describe facies, observation under a polarization microscope, refractive index measurement of volcanic glasses, and chemical analysis, and correlated with the reported widespread tephras. We conducted wide-area field surveys and succeeded in revealing a wider distribution of tephra layers than previously known. The distribution of volcanic ash in the coastal area will contribute to the investigations of future volcanic and coastal hazards.
Keywords	Pyroclastic fall deposit ; Tephra stratigraphy ; Usu volcano ; Tarumae volcano ; Komagatake volcano ; Hokkaido ; Computer applications to medicine. Medical informatics ; R858-859.7 ; Science (General) ; Q1-390
Subject code	930
Language	English
Publishing date	2020-12-01T00:00:00Z
Publisher	Elsevier
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Simple topographic parameter reveals the along-trench distribution of frictional properties on shallow plate boundary fault

Hiroaki Koge / Juichiro Ashi / Jin-Oh Park / Ayumu Miyakawa / Suguru Yabe

Earth, Planets and Space, Vol 74, Iss 1, Pp 1-

2022 Volume 11

Abstract: Abstract The critical taper model best describes the first-order mechanics of subduction zone wedges. The wedge geometry, which is conventionally defined by two parameters, slope angle and basal dip angle, accounts for the strength of megathrust. By ... ...

Abstract	Abstract The critical taper model best describes the first-order mechanics of subduction zone wedges. The wedge geometry, which is conventionally defined by two parameters, slope angle and basal dip angle, accounts for the strength of megathrust. By applying this theoretical model, fault frictional properties and earthquake occurrences can be compared among subduction zones, and within a single subduction zone, and the spatial distribution or temporal change of fault strength can be investigated. Slope angle can be accurately estimated from bathymetry data, but basal dip angle must be inferred from subsurface structure, which requires highly accurate depth-migrated seismic reflection profiles. Thus, application of the critical taper model is often limited by an insufficient number of highly accurate profiles, and the spatial distribution of frictional coefficients must be inferred from relatively few data. To improve this situation, we revisited the theoretical formula of the critical taper model. We found that the effect of basal dip angle on the critical taper model is small, and slope angle can be a proxy for the effective friction when the pore fluid pressure ratio is high, internal friction is small, or both. These conditions are met in many subduction zones. The validity of the approximation can be checked with a parameter newly introduced in this study. Therefore, this finding allows use of variations in slope angle, which could be obtained accurately from only the bathymetry as an approximation for relative variations in the effective coefficient of basal friction, if the targeted subduction meets the validity. We applied this approximation to the Japan Trench and estimated the variations in the friction coefficient distribution on the shallow plate boundary fault from 71 data points. We found that the area where the friction coefficient was smaller than the mean corresponded to a segment, where a large coseismic shallow rupture occurred during the 2011 Tohoku-Oki earthquake (Mw 9.0). Thus, by approximating tapered wedge geometry with a simple topographic parameter that can be obtained from existing global bathymetry, we can quickly estimate the distribution of frictional properties on a plate boundary fault along a trench and related seismic activity. Graphical Abstract
Keywords	Subduction zone ; Japan Trench ; Critical taper model ; Frictional variation ; Accretionary wedge ; Geography. Anthropology. Recreation ; G ; Geodesy ; QB275-343 ; Geology ; QE1-996.5
Subject code	550
Language	English
Publishing date	2022-04-01T00:00:00Z
Publisher	SpringerOpen
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Distribution and depth of bottom-simulating reflectors in the Nankai subduction margin

Akihiro Ohde / Hironori Otsuka / Arata Kioka / Juichiro Ashi

Earth, Planets and Space, Vol 70, Iss 1, Pp 1-

2018 Volume 20

Abstract: Abstract Surface heat flow has been observed to be highly variable in the Nankai subduction margin. This study presents an investigation of local anomalies in surface heat flows on the undulating seafloor in the Nankai subduction margin. We estimate the ... ...

Abstract	Abstract Surface heat flow has been observed to be highly variable in the Nankai subduction margin. This study presents an investigation of local anomalies in surface heat flows on the undulating seafloor in the Nankai subduction margin. We estimate the heat flows from bottom-simulating reflectors (BSRs) marking the lower boundaries of the methane hydrate stability zone and evaluate topographic effects on heat flow via two-dimensional thermal modeling. BSRs have been used to estimate heat flows based on the known stability characteristics of methane hydrates under low-temperature and high-pressure conditions. First, we generate an extensive map of the distribution and subseafloor depths of the BSRs in the Nankai subduction margin. We confirm that BSRs exist at the toe of the accretionary prism and the trough floor of the offshore Tokai region, where BSRs had previously been thought to be absent. Second, we calculate the BSR-derived heat flow and evaluate the associated errors. We conclude that the total uncertainty of the BSR-derived heat flow should be within 25%, considering allowable ranges in the P-wave velocity, which influences the time-to-depth conversion of the BSR position in seismic images, the resultant geothermal gradient, and thermal resistance. Finally, we model a two-dimensional thermal structure by comparing the temperatures at the observed BSR depths with the calculated temperatures at the same depths. The thermal modeling reveals that most local variations in BSR depth over the undulating seafloor can be explained by topographic effects. Those areas that cannot be explained by topographic effects can be mainly attributed to advective fluid flow, regional rapid sedimentation, or erosion. Our spatial distribution of heat flow data provides indispensable basic data for numerical studies of subduction zone modeling to evaluate margin parallel age dependencies of subducting plates.
Keywords	Surface heat flow ; Bottom-simulating reflector ; Methane hydrate ; Shallow thermal structure ; Nankai Trough ; Geography. Anthropology. Recreation ; G ; Geodesy ; QB275-343 ; Geology ; QE1-996.5
Subject code	550
Language	English
Publishing date	2018-04-01T00:00:00Z
Publisher	SpringerOpen
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Dynamic formation process of thick deformation zone on the shallow plate boundary fault of the Japan Trench

Hiroaki Koge / Yasuhiro Yamada / Akihiro Ohde / Arthur Bauville / Asuka Yamaguchi / Juichiro Ashi

Progress in Earth and Planetary Science, Vol 5, Iss 1, Pp 1-

insight from analog experiments of half-graben subduction

2018 Volume 6

Abstract: Abstract The 2011 Tohoku-oki earthquake unexpectedly ruptured to the shallowest portion of the plate boundary fault and triggered a large tsunami. The shallow portion had generally been regarded as a seismically stable zone until this event, but its ... ...

Abstract	Abstract The 2011 Tohoku-oki earthquake unexpectedly ruptured to the shallowest portion of the plate boundary fault and triggered a large tsunami. The shallow portion had generally been regarded as a seismically stable zone until this event, but its significance has now been dramatically revealed for future disaster mitigation. This research approaches the shallow portion, especially the formation process of its structure and plate boundary faults. Scientific drilling conducted near the Japan Trench after the earthquake reported a thin plate boundary fault (~ 7 m) and thick deformation zone (~ 100 m). This thin fault would be expected given the relatively small displacement near the trench (~ 3.2 km), but the deformation zone thickness is anomalously wide given this small magnitude of slip. To understand the dynamic deformation processes that lead to the development of a thick deformation zone surrounding a thin fault core, we conducted forward modeling of an analog experiment with the technique to visualize fault activity. Sandbox experiments are effective for the approximation of the geological phenomenon and structure. The seismic profile of the largest slip region in the 2011 earthquake shows that a half-graben structure has been subducted underneath the frontal wedge, thus we focused on this structural architecture. As a result, we found a new fault formation pattern, i.e., the frontal thrust (the most frontal part of décollement) periodically partitioned into pieces, which connect again to form a large-continuous fault. The fault also oscillates up and down during this process, which we call “dancing,” and a thick shear zone is formed in a relatively short time where this occurs, even though the fault only has a small displacement. By analogy, the thick deformation zone observed at the Japan Trench could be formed by such fault dancing. The energy of the fault activity is commonly estimated from the fault displacement derived from the thickness of the shear zones. Applying the thickness-displacement law ...
Keywords	Analog model ; Digital image correlation ; Japan Trench ; Sandbox ; Thrust formation ; Geography. Anthropology. Recreation ; G ; Geology ; QE1-996.5
Subject code	550
Language	English
Publishing date	2018-11-01T00:00:00Z
Publisher	SpringerOpen
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Mantle-derived helium released through the Japan trench bend-faults

Jin-Oh Park / Naoto Takahata / Ehsan Jamali Hondori / Asuka Yamaguchi / Takanori Kagoshima / Tetsuro Tsuru / Gou Fujie / Yue Sun / Juichiro Ashi / Makoto Yamano / Yuji Sano

Scientific Reports, Vol 11, Iss 1, Pp 1-

2021 Volume 9

Abstract: Abstract Plate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical studies and numerical simulations suggest that bend-faults play a key role by ... ...

Abstract	Abstract Plate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical studies and numerical simulations suggest that bend-faults play a key role by providing pathways for seawater to flow into the oceanic crust and the upper mantle, thereby promoting hydration of the oceanic plate. However, deep penetration of seawater along bend-faults remains controversial because fluids that have percolated down into the mantle are difficult to detect. This report presents anomalously high helium isotope (3He/4He) ratios in sediment pore water and seismic reflection data which suggest fluid infiltration into the upper mantle and subsequent outflow through bend-faults across the outer slope of the Japan trench. The 3He/4He and 4He/20Ne ratios at sites near-trench bend-faults, which are close to the isotopic ratios of bottom seawater, are almost constant with depth, supporting local seawater inflow. Our findings provide the first reported evidence for a potentially large-scale active hydrothermal circulation system through bend-faults across the Moho (crust-mantle boundary) in and out of the oceanic lithospheric mantle.
Keywords	Medicine ; R ; Science ; Q
Subject code	550
Language	English
Publishing date	2021-06-01T00:00:00Z
Publisher	Nature Portfolio
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article ; Online: Postseismic fluid discharge chemically recorded in altered pseudotachylyte discovered from an ancient megasplay fault

Ryota Hasegawa / Asuka Yamaguchi / Rina Fukuchi / Yohei Hamada / Nobuhiro Ogawa / Yujin Kitamura / Gaku Kimura / Juichiro Ashi / Tsuyoshi Ishikawa

Progress in Earth and Planetary Science, Vol 6, Iss 1, Pp 1-

an example from the Nobeoka Thrust in the Shimanto accretionary complex, SW Japan

2019 Volume 16

Abstract: Abstract Megasplay fault branching from plate boundaries of subduction zones is thought to be important sources of earthquakes and tsunamis. In this study, we performed structural and geochemical analyses on a fossilized megasplay fault (the Nobeoka ... ...

Abstract	Abstract Megasplay fault branching from plate boundaries of subduction zones is thought to be important sources of earthquakes and tsunamis. In this study, we performed structural and geochemical analyses on a fossilized megasplay fault (the Nobeoka Thrust of the Shimanto accretionary complex) to understand fluid-rock interaction and how the splay fault plays a role in fluid flow in the seismogenic zone. As a result of structural observations, we report that the principal slip zone (PSZ) of the Nobeoka Thrust is composed of foliated cataclasite originating from a sandstone-shale mélange and includes a thin (~ 1.5 mm thick) pseudotachylyte layer. Major and trace element composition analysis and EPMA element mapping revealed that the pseudotachylyte is enriched in Li and Cs within the PSZ, as well as in the slip zone of a minor fault in the footwall. Li and Cs enrichment in pseudotachylyte is interpreted as a result of fluid-rock interaction in the postseismic stage, because such an anomaly only results from a large fluid/rock ratio (R > 512–24 at 250–350 °C) under the influence of Li- and Cs-enriched fluids. The amount of fluid that reacted with the pseudotachylyte is estimated to be 1.78 × 100 to 7.61 × 103 m3.
Keywords	Megasplay fault ; Fluid-rock interaction ; Pseudotachylyte ; Hydrothermal alteration ; Geography. Anthropology. Recreation ; G ; Geology ; QE1-996.5
Subject code	550
Language	English
Publishing date	2019-04-01T00:00:00Z
Publisher	SpringerOpen
Document type	Article ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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