Ken Ikehara

Submarine Mass Movements and Their Consequences

Submarine mass movements represent major offshore geohazards due to their destructive and tsunami-generation potential. This potential poses a threat to human life as well as to coastal, near shore and offshore engineering structures. Recent examples of catastrophic submarine landslide events that affected human populations (including tsunamis) are numerous; e.g., Nice airport in 1979 (Dan et al. 2007), Finneidfjord in 1996 (e.g., L’Heureux et al., this volume, Steiner et al., this volume), Papua-New Guinea in 1998 (Tappin et al. 2001), Stromboli in 2002 (Chiocci et al. 2008), and the 2006 and 2009 failures in the submarine cable network around Taiwan (Hsu et al. 2008). The Great East Japan Earthquake of March 2011 also generated submarine landslides that may have amplified effects of the devastating tsunami as shown in Fryer et al. (2004). Given that 30% of the World’s population lives within 60 km of the coast, the hazard posed by submarine landslides is expected to grow as global sea level rises. In addition, the deposits resulting from such processes provide-various types of constraints to offshore development (Shipp et al. 2004), and have significant implications for non-renewable energy resource exploration and production (Weimer and Shipp 2004; Beaubouef and Abreu 2010).

Transgressive and highstand systems tracts and post-glacial transgression, the East China Sea

Abstract The Late Pleistocene–Holocene depositional sequence on the shelf in the East China Sea (ECS) is interpreted on the basis of the analyses of four sediment cores and high-resolution seismic reflection sub-bottom profiler records along a NE–SW across-shelf transect. Sedimentary strata deposited above a lowermost planar erosional surface that was formed during sea-level fall of the last glacial are divided into two units along a NE–SW across-shelf transect (Changjiang–Okinawa). The Lower Unit is characterized by seaward-dipping tangential clinoforms with a thickness of 30–40 m at mid-shelf depths and less than 30 m beneath the outer shelf. Prograding clinoforms are more obvious in mid-shelf environments. The Upper Unit is characterized by an upper transparent layer that is formed into ridge-and-swale topography. The boundary between the Upper and Lower Units is sharp and erosional. Surficial sediments taken by cores from the central ECS shelf are also divided into two facies: a sandy facies consisting of sand or sandy gravel with moderately abundant molluscan shell fragments, and a muddy facies comprising mud intercalated with thin sand layers. The sandy facies is widely distributed on the middle to outer shelf seafloor and has a measured thickness of 30 cm, up to several metres. Radiocarbon ages of molluscan shells in this sediment are less than approximately 10 ka BP except for ages from basal shell lag deposits, which are >10–40 ka BP. The muddy facies underlies the sandy facies and has radiocarbon ages of 2.7–2.9 ka BP (previous work) and 20–38 ka BP (this study) and TL ages of 27–50 ka BP (previous work). The boundary between these lithologic units is an erosional sharp contact. The sandy and muddy facies are correlated with the Upper and Lower Seismic Units, respectively. The Lower Unit is characterized by prograding clinoforms and is interpreted to represent deltaic or nearshore tidal ridge sediments of the paleo-Changjiang River that were deposited during the last glacial lowstand of sea-level as a lowstand systems tract. The underlying erosional surface is interpreted to be a sequence boundary formed as a regressive marine erosional surface during the fall of sea-level. The erosional boundary between the Lower and Upper Units is interpreted as a transgressive surface of erosion formed during the subsequent rise of sea-level. The Upper Unit has a modern sand ridge topography and is interpreted to represent offshore shelf sand ridges of the transgressive to highstand systems tract.

Rock-magnetic changes with reduction diagenesis in Japan Sea sediments and preservation of geomagnetic secular variation in inclination during the last 30,000 years

A rock-magnetic and paleomagnetic study was conducted on a sediment core of about 4.4 m long taken from the northeastern part of the Japan Sea. The core covers the last about 30 kyrs, which was dated by nineteen radiocarbon (14C) ages. Remanent magnetization is carried dominantly by magnetite. Reductive dissolution of magnetic minerals occurs between 1.2 and 1.6 m in depth (about 5–8 ka in age). A rapid downcore decrease of anhysteretic remanent magnetization (ARM) begins at the shallowest depth. Saturation isothermal remanent magnetization (SIRM) follows, and a decrease of magnetic susceptibility (k) takes place at the deepest. Within this zone, coercivity of natural remanent magnetization (NRM) and the ratios of ARM to k and SIRM to k also decreases with depth. These observations indicate that finer magnetic grains were lost earlier than larger grains. A decrease of S ratios, wasp-waisted hysteresis curves, and a deviation from a mixing trend of single-domain and multi-domain grains in a Day plot occur as the dissolution proceeds, which suggests that high coercivity minerals like hematite are more resistive to dissolution than low coercivity minerals like magnetite. The start of the dissolution at 1.2 m in depth is synchronous with increases in organic-carbon and total-sulfur contents, but the horizon does not coincide with the present Fe-redox boundary at about 0.02 m below the sediment-water interface. From low-temperature magnetometry, it is estimated that magnetites with maghemite skin are reduced to pure magnetites prior to dissolution. There is no evidence for precipitation of secondary magnetic phases and acquisition of chemical remanent magnetization (CRM). Neither pyrrhotite nor greigite was detected. Information of paleomagnetic directions have survived the reductive dissolution. Inclination variations of this core resembles closely to the secular variation records available around Japan. Well-dated records older than 10 ka are still very rare, and hence our new record could be useful for establishing regional secular variations.

Paleomagnetic record from Academician Ridge, Lake Baikal: a reversal excursion at the base of marine oxygen isotope stage 6

Abstract Paleomagnetic and rock-magnetic studies on a hydraulic piston core (Ver98-1, St.6) from Academician Ridge, Lake Baikal showed the occurrence of a reversal excursion at 670–696 cm depth, which is at the base of marine oxygen isotope stage 6. A correlation of X-ray CT values, as a proxy of relative density, to the marine oxygen isotope record provides an age of 177–183 ka for this reversal excursion. It can be correlated with other excursion records from Lake Baikal, found in Core 287-K2 from Academician Ridge [King et al., Russ. Geol. Geophys. 34 (1993) 148–162] and in core BDP93-1 drilled on the Buguldeika saddle [BDP-93, Quat. Int. 37 (1997) 3–17]. We correlate the Lake Baikal reversal excursion with a well documented excursion in the Brunhes Chron, the Iceland Basin event (186–189 ka) from ODP Sites 983 and 984 in the North Atlantic [Channell, J. Geophys. Res. 104 (1999) 22937–22951]. Also the relative paleointensity record agrees well with that from ODP Site 983 [Channell, J. Geophys. Res. 104 (1999) 22937–22951]. The Lake Baikal excursion and the Iceland Basin event correspond to the minimum of relative intensity at 188 ka in Sint-800 [Guyodo and Valet, Nature 399 (1999) 249–252]. We argue that it is distinct from the Jamaica/Pringle Falls excursion, estimated at 205–215 ka [Langereis et al., Geophys. J. Int. 129 (1997) 75–94]. This is supported by the recalibration of the age of another excursion found in Core St.16 in Lake Baikal [Sakai et al., Bull. Nagoya Univ. Furukawa Mus. 13 (1997) 11–22] with an age of ∼223 ka, which is close to the age of the Jamaica/Pringle Falls excursion, as suggested earlier [King et al., Russ. Geol. Geophys. 34 (1993) 148–162]. The VGP path of the reversal excursion (177–183 ka) consists of a southward swing through the North Atlantic, followed by a loop through Africa and the Indian Ocean. The path morphology is similar to that of the Iceland Basin event from the North Atlantic [Channell, J. Geophys. Res. 104 (1999) 22937–22951].

A slump in the trench: Tracking the impact of the 2011 Tohoku-Oki earthquake

We present differential bathymetry and sediment core data from the Japan Trench, sampled after the 2011 Tohoku-Oki (offshore Japan) earthquake to document that prominent bathymetric and structural changes along the trench axis relate to a large (∼27.7 km 2 ) slump in the trench. Transient geochemical signals in the slump deposit and analysis of diffusive re-equilibration of disturbed SO 4 2– profiles over time constrain the triggering of the slump to the 2011 earthquake. We propose a causal link between earthquake slip to the trench and rotational slumping above a subducting horst structure. We conclude that the earthquake-triggered slump is a leading agent for accretion of trench sediments into the forearc and hypothesize that forward growth of the prism and seaward advance of the deformation front by more than 2 km can occur, episodically, during a single-event, large mega-thrust earthquake.

Distribution and origin of subaqueous dunes on the shelf of Japan

Abstract Subaqueous dunes are found on the shelf around the Japanese Islands, mainly in the straits between the islands. Some of the dunes are formed by ocean currents, and others by tidal currents. Combined flows also produce shoreparallel coarse-grained bands and medium dunes on the inner shelf. The dynamics of these bedforms was examined using repeated observations of bedforms and by comparing the hydraulic conditions required to form the bedforms with the existing oceanographic conditions. The study indicates that most of bedforms are active but the volumes of sediment transported are too small to form the sand bodies which underlay the large dunes. Seismic records show these sand bodies occur on the erosional surfaces formed after the last glacial maximum. It is suggested that most of the sand bodies were formed during the postglacial transgression.

Diagenetic effect on magnetic properties of marine core sediments from the southern Okhotsk Sea

It is well known that magnetic records of marine sediments are affected to a certain degree by early diagenesis, the extent and mode of which are controlled by depositional environments. In order to investigate variations in the effects of early diagenesis, we have analyzed 11 gravity cores estimated to be Holocene in age that constitute a depth transect off the southern coast of the Okhotsk Sea. The rock magnetic parameters have peaks produced by the presence of volcanic ashes, basaltic granules and/or relatively coarser-grained layers. The concentration-dependent parameters display characteristic trends of a rapid decrease within the homogeneous silty clay zone in the middle parts of the cores, assuming that the total amount of magnetic minerals decreases through early diagenesis at this interval. This decrease is found in all of the cores from the transect (from both near-shore and offshore sites). In near-shore sites where larger volumes of organic materials are supplied compared to offshore sites, the depth of the decrease in magnetic minerals is relatively shallow. These results imply that the presence of organic materials accelerates the pyritization through the bacterial reduction of ferric ion, thereby causing the decreases in magnetic minerals to become shallower.

A 6600 year earthquake history in the region of the 2004 Sumatra-Andaman subduction zone earthquake

In order to investigate the possibility of a long-term paleoseismic history from offshore sedimentary records in Sumatra, we collected 144 deep-sea sediment cores in the trench and in lower slope piggyback basins of the Sumatra accretionary prism. We used multibeam bathymetry and seismic reflection data to develop an understanding of catchment basins, turbidity current pathways, and depositional styles, as well as to precisely locate our gravity cores, piston cores, Kasten cores, and multicores. We use detailed physical property data, including computed tomographic X-ray, gamma density, magnetic susceptibility, grain-size analysis, faunal analysis, and smear slides, to evaluate the turbidite stratigraphy and sedimentology at each site. We use radiocarbon age control for piggyback basin sites above the carbonate compensation depth, and use 210 Pb and 137 Cs to evaluate the timing of the most recent sedimentary deposits. Using well-log correlation methods and radiometric age control, we test for potential correlations between isolated sites in piggyback basins and the trench. We find evidence for very young surface turbidites along the northern Sumatra margin, most likely emplaced within the past few decades at the seafloor in both the 2004 and 2005 earthquake rupture zones, with no overlying hemipelagic sediment. Based on the young soupy deposits, lack of oxidation, and 210 Pb and 14 C age determinations, we interpret the uppermost turbidite in 21 cores within the 2004 rupture area to have been deposited within a few years of collection in 2007, and most likely as a result of the 2004 moment magnitude (M w ) ∼9.2 earthquake. The likely 2004 turbidite has a distinctive stacked structure of three major fining-upward sequences observed at several basin and trench sites, similar to the pattern of moment release in the 2004 earthquake. We observe rapid die out of the 2004 and 2005 deposits with distance from the slip zones, from local sources of sediment supply, and in the segment boundary between the slip zones. Many individual turbidites show strong similarities between isolated sites, as well as having similar emplacement times. Based upon radiocarbon age control and lithostratigraphic correlations between isolated basin and trench core sites, we interpret that 43 turbidites can be linked spatially over a distance of ∼230 km within the southern portion of the 2004 rupture zone. Sampling at deep-water sites isolated from terrestrial and shallow-water sediment sources, as well as potential storm or tsunami wave triggers, limits potential mechanisms for initiating turbidity currents to plate boundary, crustal, or slab earthquakes. Other potential triggers, such as tectonic oversteepening, random self-failures, gas hydrate destabilization, are unlikely to be correlative between any two isolated sites. The most probable explanation for the similarity of timing, turbidite sequences, and individual turbidite structure in isolated basin and trench stratigraphic sequences is a seismogenic origin. The mean emplacement time for turbidites (likely triggered by Great earthquakes, magnitude > ∼8) in the 2004 rupture region for the past 6.6 ± 0.14 k.y. is 160 yr for 43 turbidites. The ages of 8 of the 10 uppermost turbidite deposits, spanning the past ∼1500 yr, are largely consistent with the terrestrial paleoseismic and/or tsunami records in Thailand, Sumatra, India, and the Andaman Islands, suggesting either coincidence or a common origin. The mean interseismic time from the turbidite record for this same period is 170 yr, comparable to the ∼210 yr recurrence for regional tsunami. The turbidite record, at 180 yr (6 events), compares reasonably well to the average for all events on northern Simeulue of 220 yr, and is identical to the tsunami interval of 180 yr for the same time period (6 events). Of the 43 correlated turbidites in the 2004 earthquake region, 13 are well correlated in our cores along strike lengths of 150 km or greater, and satisfy criteria for robustness; 24 turbidites correlated along a shorter strike distance may represent other plate boundary earthquakes of shorter spatial extent and may include turbidite beds sourced from crustal and slab earthquakes.

Radiocarbon Marine Reservoir Ages in the Northwestern Pacific off Hokkaido Island, Japan, During the Last Deglacial Period

We measured radiocarbon ages of planktic foraminifera in 4 sediment cores from the northwestern Pacific region off northern Japan in order to estimate marine reservoir ages during the Blling-Allerd period. The ages of deglacial tephra markers from 2 Japanese source volcanoes identified in these sediment cores had been previously estimated from 14C ages of terrestrial charcoal and buried forests. By comparing the foraminiferal and tephra ages, we estimated the surface water reservoir age during the Blling-Allerd period to be ~1000 yr or more in the region off northern Japan. The deglacial reservoir ages were more than 200 yr higher than the Holocene values of ~800 yr. The older deglacial ages may have been caused by active upwelling of deep water during the last deglaciation and the consequent mixing of older deep water with younger surface waters.

Ocean current generated sedimentary facies in the Osumi Strait, South of Kyushu, Japan

Abstract An investigation of the surface sediments and bedforms in the Osumi Strait, located between Kyushu and Tanegashima, south of Kyushu, Japan, was carried out. The distribution of some characteristics of the surface sediments and bedforms is clarified. In the Osumi Strait, the surface sediments tend to become finer in size, better sorted and lower in specific gravity from southwest to northeast. The bedform distribution shows a systematic change in the same direction. This direction is the same as the direction of sediment transportation and of the current flowing through the Strait. It is considered that these changes in bedforms and sediment properties are formed by the decrease in the energy of the current. The current generating bedforms and sediment distribution is the Osumi Branch Current, one of the branches of the Kuroshio. The sediment transportation is active under the present hydraulic conditions. The sedimentary facies developed in the Osumi Strait is controlled by a unidirectional ocean current. The ocean current is one of the important factors for sedimentary processes where strong ocean current prevails along or near the continental shelf such as around the Japanese Islands.