Showing papers by "Ken Ikehara published in 2012"

An overview of the role of long-term tectonics and incoming plate structure on segmentation of submarine mass wasting phenomena along the Middle America Trench

Abstract: We study mass wasting along the Middle America Trench (MAT), a subduction zone dominated by tectonic erosion, using a comprehensive data set of seafloor relief. We integrate previous studies of long-term tectonic processes to analyze how they influence the evolution of the slope structure and precondition the continental slope for mass wasting. We have used the distribution of an inventory of 147 slope failure structures along the MAT to discuss their relation to subduction erosion. We interpret that preconditioning of the slope by long-term tectonics, interacts in a shorter-term scale with features on the under-thrusting oceanic plate to modulate the abundance and types of mass wasting phenomena. The complex origin of the incoming oceanic plate has produced abrupt lateral changes in plate age, crustal thickness, relief, and response to bending deformation at the trench, leading to its partitioning into 6 segments. We found that the continental-slope failure style and abundance are partitioned into 6 segments that spatially match the ocean plate segments

Orbital- and millennial-scale paleoceanographic changes in the north-eastern Japan Basin, East Sea/Japan Sea during the late Quaternary

Abstract: Two gravity sediment cores (GH99-1239 and GH99-1246) obtained from the north-eastern Japan Basin in the East Sea/Japan Sea were analyzed for the orbital- and millennial-scale paleoceanographic changes. Chronostrati- graphically, core GH99-1239 represents a continuous sedimentary record since 32 ka, based on correlation of distinct lithological markers (i.e. dark layer or TL layer) with those in core GH98-1232 collected nearby. For core GH99-1246, the age model is constructed through correlation of lightness (L � ) values and tephra (Aso-4 and Toya) layers with those in the well-dated Oki Ridge core (MD01-2407), indicating about 134 ka of sedimentation since the latest Marine Isotope Stage (MIS) 6. New geochemical data from both cores corroborate orbital-scale paleoceanographic variation, such that surface-water productivity, represented by biogenic opal and total organic carbon (TOC) contents, increased during MIS 1 and MIS 5; CaCO3 contents do not show such distinct glacial-interglacial cycles, but were influenced by dissolution and preservation rather than foraminiferal production. During the glacial periods when sea ice was prevalent, surface- water productivity was low, and bottom-water conditions became anoxic, as indicated by high total sulfur (TS) contents and high Mo concentrations. The geochemical data further document millennial-scale paleoceanographic variability, corresponding to a series of thin TL layers in response to Dansgaard-Oeschger cycles but irrespective of the glacial or interglacial periods. In particular, thin TL layers formed during MIS 3 are characterized by less TOC (about 1%) and TS (about 0.4%) contents and lower Mo (about 5 p.p.m.) concentration, whereas those during MIS 4 and MIS 5 exhibit more TOC (up to 4%) and TS (up to 5%) contents and higher Mo (up to 120 p.p.m.) concentration. Such a discrepancy is attributed to different degree of surface-water productivity and of bottom-water oxygenation, which is closely related to the sea level position and extent of ventilation. Flux of the East China Sea Coastal Water controlled by millennial-scale paleoclimatic events is the most critical factor in deciding the properties of TL layers in the north-eastern Japan Basin. Our results strongly confirm that TL layers in the Japan Basin also validate the unique feature of basin-wide paleoceano- graphic signals in the East Sea/Japan Sea. Copyright # 2011 John Wiley & Sons, Ltd.

Offshore earthquake- and/or tsunami-induced sediment transports and their deposits: Importance of marine sediment study for understanding past earthquakes and tsunami

Abstract: Huge damages occurred along the Pacific coast of Tohoku by the 2011 off the Pacific coast of Tohoku earthquake. The preliminary results on submarine investigations after the earthquake indicate the wide distribution of the event deposits by the earthquake and/or its related tsunami. Characterization of the event deposits and consideration of their depositional processes is very important to understand what happen at sea floor by the particular earthquake, and to recognize the past earthquakeand/or tsunami-induced offshore deposits. Therefore, sedimentological and geological investigations at the offshore of Sanriku-Sendai coast from coastal shallow water to the Japan Trench should be conducted to clarify the depositional processes and temporal changes of the event deposits. Integration of results of the investigations with the simulation and experimental studies is also important.

Fluctuations in the late Quaternary East Asian winter monsoon recorded in sediment records of surface water cooling in the northern Japan Sea

Abstract: The East Asian monsoon climate system involves interactions between land and ocean and is composed of summer and winter monsoons. Cooling of surface water by the East Asian winter monsoon (EAWM) promotes the formation of sea ice and deep water in the northern Japan Sea. Modern observations suggest close relationships between intensification of the EAWM and both sea ice formation and sea-surface temperature (SST) decrease in the northeastern Japan Sea. Records of ice-rafted debris (IRD) and alkenone-based SST in the northeastern Japan Sea are examined to clarify the temporal variability of the EAWM and its controlling factors. Close relationships with oxygen isotope records of Chinese stalagmites and Northern Hemisphere summer insolation are observed in the records of IRD occurrence and SST during Marine Isotope Stages (MIS) 3–5. The co-occurrence of maxima in the abundance of IRD and minima in SST in the northeastern Japan Sea indicates that the intensity of the EAWM was enhanced during periods of low summer insolation and high oxygen isotope ratios in Chinese stalagmites. Decoupling between the IRD and SST records during MIS 2 and MIS 6 might be the result of differences in surface water conditions caused by low sea levels and the resultant closure of the Tsushima Strait. During MIS 3 and 4, some small but clear maxima in IRD and minima in SST occurred at relatively low summer insolation periods. Some of these peaks co-occur with North Atlantic Heinrich events, suggesting a possible climate linkage between the North Atlantic and the Asian monsoon. The presence of finer fluctuations in the occurrence of IRD than in summer insolation suggests finer millennial–centurial timescale fluctuations in the intensity of the EAWM. Furthermore, some IRD peaks occurred during periods of high insolation, such as 39, 86, 103, 128, 145, and 153 ka. Some of these IRD peaks correlate with minima in reconstructed SST and maxima in oxygen isotope ratios of Chinese stalagmites. This result suggests that extremely cold winters occur even during periods of enhanced East Asian summer monsoon intensity, and indicates that solar insolation is not the only factor that controls variations in the intensity of the EAWM at millennial–centurial timescales. Copyright © 2012 John Wiley & Sons, Ltd.

Settling of Earthquake-Induced Turbidity on the Accretionary Prism Slope of the Central Nankai Subduction Zone

Abstract: Studied of turbidite deposits can provide valuable paleoseismological information, such as earthquake recurrence intervals. However, there are few studies based on in situ observations of gravity flows during or soon after earthquakes. We used a remotely operated vehicle in dive surveys near the epicenters of the 2004 off the Kii Peninsula earthquakes (maximum magnitude 7.4) soon after and 6 years after the events. Video observations 20 days after the main shock revealed two turbid layers exhibiting different degrees of turbidity: a dilute suspension layer (DSL) and a bottom turbid layer (BTL). Pervasive distributions of DSL on the prism slope suggest that clouds of suspended sediments were induced by sliding, slumping, seafloor shaking, or sediment gravity flow at multiple locations during the earthquakes. BTL is a highly turbid layer of around 2.4 m thickness, the upper surface of which is sharply defined. That BTL was observed only in a slope basin implies that it was formed by a sediment gravity flow dumping muddy deposits in a depression. Our dive observations revealed that seafloor disturbance by seismic shaking plays a significant role in sediment redistribution in subduction zones.

Submarine Slope Response to Earthquake Shaking Within Western Sagami Bay, Central Japan

Abstract: Earthquakes are a major trigger of submarine landslides. Strong ground shaking of the sea floor by an earthquake increases the sediment pore pressure and induces slope failure. As submarine landslides may generate tsunamis, it is important to understand the submarine slope response to earthquake ground shaking. Slope response may change spatially according to the strength of ground shaking and slope characteristics, such as submarine topography, gradient, sea bed materials, and sedimentation rate. For a better understanding of slope response, it is necessary to obtain data on changes to the sea floor following an earthquake. In 1997, 1998, and 2006, the JAMSTEC Hatsushima Deep-Sea Observatory recorded turbidity currents triggered by the Izu-toho-oki earthquake swarm (magnitude ∼6) off Hatsushima Island in western Sagami Bay, Central Japan. In 2004, cores of undisturbed surface sediment were collected using a multiple corer at the foot of the submarine slope near the Observatory. No clear gravity-flow deposits (e.g., turbidites and debrites) were observed near the tops of the cores, indicating that earthquakes of magnitude ∼6 have insufficient energy to generate clearly defined sandy turbidites on this slope. In contrast, several sandy turbidite layers are present in a piston core collected from a nearby site, suggesting that this site is affected by a larger magnitude of ground shaking than that produced by the Izu-toho-oki earthquakes. The most likely origin of this greater ground shaking is the Kanto earthquakes (magnitude ∼8), the epicenter of which is located in the northwestern Sagami Trough.