Hajime Naruse

Tsunami-generated turbidity current of the 2011 Tohoku-Oki earthquake

We show the first real-time record of a turbidity current associated with a great earthquake, the Mw 9.0, 2011 Tohoku-Oki event offshore Japan. Turbidity current deposits (turbidites) have been used to estimate earthquake recurrence intervals from geologic records. Until now, however, there has been no direct evidence for large-scale earthquakes in subduction plate margins. After the 2011 Tohoku-Oki earthquake and tsunami, an anomalous event on the seafloor consistent with a turbidity current was recorded by ocean-bottom pressure recorders and seismometers deployed off Sendai, Japan. Freshly emplaced turbidites were collected from a wide area of seafloor off the Tohoku coastal region. We analyzed these measurements and sedimentary records to determine conditions of the modern tsunamigenic turbidity current. We anticipate our discovery to be a starting point for more detailed characterization of modern tsunamigenic turbidites, and for the identification of tsunamigenic turbidites in geologic records.

THICKNESS AND GRAIN-SIZE DISTRIBUTION OF INDIAN OCEAN TSUNAMI DEPOSITS AT KHAO LAK AND PHRA THONG ISLAND, SOUTH-WESTERN THAILAND

Abstract The landward changes in grain size and thickness of the 26 December 2004 Indian Ocean tsunami deposit on Phra Thong Island and in the Khao Lak Area, south-western Thailand, are summarized in this chapter. The maximum heights of the tsunami on Phra Thong Island and Khao Lak were 7 and 10 m, respectively. The tsunami produced 1- to 4-m-high erosional scarps at the shoreline in these areas. The tsunami deposit is a sheet of sand ranging from a few centimetres to 22-cm thick over a distance of 1500 m inland. It thins near its distal margin, but the thickness is strongly affected by local topography and lacks a consistent trend. The deposit is composed of medium to very fine sand, and fines inland, reflecting loss of energy. It is graded or massive. Sets of graded units result from multiple waves.

Three-dimensional Morphology of the Ichnofossil Phycosiphon incertum and Its Implication for Paleoslope Inclination

Abstract Details of the three-dimensional morphology of the ichnofossil Phycosiphon incertum collected from deposits on submarine slopes are reconstructed by processing a series of images obtained from polished sections of the samples. Samples were collected from the mudstone around a slump scar in the Paleocene Shiomi Formation, northern Japan, which is characterized by the occurrence of slump scars. The reconstructed morphology of Phycosiphon incertum is a meandering tube with a flattened ellipse cross section. The tubes are flattened in a plane oblique to the bedding surfaces and aligned along the same direction at both the interior and exterior of the slump scar. Flattening of the tubes was likely caused by sediment compaction, and the tube flattens toward the horizontal plane that is oblique to the bedding plane because of the paleoslope inclination. The difference between the bedding and flattening planes of the tubes of Phycosiphon incertum may imply paleoslope inclination. When the inclination of the bedding plane of the Shiomi Formation is corrected using the flattened surfaces, the bedding plane dips by 9° toward the southeast, which conforms to the paleocurrent direction of the turbidites. The morphology of Phycosiphon incertum can, therefore, be used as a paleoslope indicator.

Large volume submarine ignimbrites in the Shikoku Basin: An example for explosive volcanism in the Western Pacific during the Late Miocene

During IODP Expedition 322, an interval of Late Miocene (7.6 to ∼9.1 Ma) tuffaceous and volcaniclastic sandstones was discovered in the Shikoku Basin (Site C0011B), Nankai region. This interval consists of bioturbated silty claystone including four 1–7 m thick interbeds of tuffaceous sandstones (TST) containing 57–82% (by volume) pyroclasts. We use major and trace element glass compositions, as well as radiogenic isotope compositions, to show that the tuffaceous sandstones beds derived from single eruptive events, and that the majority (TST 1, 2, 3a) came from different eruptions from a similar source region, which we have identified to be the Japanese mainland, 350 km away. In particular, diagnostic trace element ratios (e.g., Th/La, Sm/La, Rb/Hf, Th/Nb, and U/Th) and isotopic data indicate a marked contribution from a mantle source beneath continental crust, which is most consistent with a Japanese mainland source and likely excludes the Izu-Bonin island arc and back arc as a source region for the younger TST beds. Nevertheless, some of the chemical data measured on the oldest sandstone bed (TST 3b, Unit IIb) show affinity to or can clearly be attributed to an Izu-Bonin composition. While we cannot completely exclude the possibility that all TST beds derived from unknown and exotic Izu-Bonin source(s), the collected lines of evidence are most consistent with an origin from the paleo-Honshu arc for TST 1 through 3a. We therefore suggest the former collision zone between the Izu-Bonin arc and Honshu paleo-arc as the most likely region where the eruptive products entered the ocean, also concurrent with nearby (∼200 km) possible Miocene source areas for the tuffaceous sandstones at the paleo-NE-Honshu arc. Estimating the distribution area of the tuffaceous sandstones in the Miocene between this source region and the ∼350 km distant Expedition 322, using bathymetric constraints, we calculate that the sandstone beds represent minimum erupted magma volumes between ∼1 and 17 km3 (Dense Rock Equivalent (DRE)). We conclude that several large volume eruptions occurred during the Late Miocene time next to the collision zone of paleo-Honshu and Izu-Bonin arc and covered the entire Philippine Sea plate with meter thick, sheet-like pyroclastic deposits that are now subducted in the Nankai subduction zone.

Cretaceous to Paleocene depositional history of North-Pacific subduction zone: reconstruction from the Nemuro Group, eastern Hokkaido, northern Japan

Abstract The Campanian–Paleocene Nemuro Group comparing the oldest strata in the Kuril Arc, is distributed in the east of Hokkaido Island, northern Japan. Strata of the group in this region are sedimentologically classified into eight depositional facies, most of which are interpreted as sediment gravity flow deposits. These depositional facies comprise four facies associations. The distal and proximal basin plain facies associations are composed mainly of hemipelagic mudstones and sandstones that are interpreted as gravity flow deposits, and were deposited in a topographically flat environment inferred by good lateral continuity of lithofacies. The channel-levee complex and submarine slope facies associations, in contrast, are composed of hemipelagic mudstone, turbidites and debris flow deposits. These four facies associations stack in ascending order, and represent a regressive succession. Palaeocurrent data indicate that the deposits of the Nemuro Group were transported from the northwest. Hence, the group in the study area is concluded to record slope progradation away from the northern source area. Hiertherto, it has been known that the sea regressed from the Kuril Arc during the Eocene. This was attributed to ridge subduction beneath the Kuril Arc. My study has revealed that regression began as early as Maastrichtian. It may have been induced by volcanic activity in the Kuril Arc or by a significant eustatic fall in sea level.

Estimation of allometric shell growth by fragmentary specimens of Baculites tanakae Matsumoto and Obata (a Late Cretaceous heteromorph ammonoid)

Abstract We introduce a new biometric method to reconstruct ontogenetic shell development of Baculites species. In order to estimate original total shaft length from fragmentary specimens and to clarify their shell growth patterns, a large number of samples of Baculites tanakae Matsumoto and Obata, collected from the Upper Cretaceous deposits in Hokkaido, Japan, were examined. Biometric analysis revealed a characteristic allometric shell growth pattern of B. tanakae expressed by the formula L = 3.03H1.50, where L and H are original total shaft length and whorl height, respectively. The analysis gives a quantitative diagnosis of the morphology of this species and enables us to estimate L including the missing apical part. Reconstruction of the total shaft length reveals that the shell ornament of B. tanakae shifts ontogenetically from a smooth phase to a tuberculate phase via a ribbed phase. It also demonstrates wide intraspecific variation on switching timing of the shell ornament phases. The ontogenetic change and the intraspecific variation can be clearly discriminated from each other by our method.

Along‐strike variations in the Nankai shallow décollement properties and their implications for tsunami earthquake generation

Rupture of slow tsunami earthquakes at subduction zones propagates along a shallow plate-boundary fault (i.e., decollement) nearly all the way to the trench. Seismic reflection profiles reveal that the shallow decollements have variable reflection characteristics in the Nankai subduction zone, allowing us to divide the subduction zone into impedance-decreasing (inferred to be fluid-rich) and impedance-increasing (fluid-poor) decollement regions. The fluid-rich decollement regions with reverse-polarity reflections may play a role as conditionally stable patches because of elevated fluid pressures. In contrast, the fluid-poor decollement regions with normal-polarity reflections could be unstable seismogenic patches with no unusual fluid pressures. We propose that when megathrust earthquakes nucleate at shallow depth, the small unstable fluid-poor patches are prone to slip. They may also accelerate (velocity-weakening) adjacent large, conditionally stable patches, generating large shallow slip and large tsunamis. As a result, along-strike contrast of fault properties can involve large tsunami earthquakes along the Nankai shallow megathrust fault.

Internal Stress Fields of a Large-Scale Submarine Debris Flow

Numerical experiments on subaqueous mass transport processes were conducted to understand the internal stress field associate with a natural example of a submarine debris flow, which was revealed by detailed analysis of a deposit exposed as a nearly 1.6 km continuous outcrop. Deposits of gravelly mudstone containing large deformed sedimentary blocks occur in the Upper Cretaceous to Paleogene Akkeshi Formation distributed in the Hokkaido Island, northern Japan. Application of the multiple inverse method to meso-scale faults observed in the blocks reveals possible internal paleostress fields that existed prior to deposition. This analysis suggested two different stress fields: (1) a uni-axial compressional stress field, where the maximum principal compression axis is normal to the bedding surface, and (2) a tri-axial compressional stress field, where the orientation of maximum principal compression axis is parallel to the paleocurrent direction. The results of numerical experiments imply that the first of these stress fields is generated by radial spreading of the flow during its downcurrent movement, while the second stress field results from compression during deposition on the basin plain. A horizontal compression paleo-stress field can be an indicator of the paleocurrent direction of the debris-flow. In addition, it is also suggested that existence of a horizontal compression paleo-stress field can provide a clue for the initial conditions of the submarine landslide.

Inverse Tsunami Flow Modeling Including Nonequilibrium Sediment Transport, With Application to Deposits From the 2011 Tohoku‐Oki Tsunami

Tsunami deposits provide important clues to understand ancient tsunami events. Several inverse models have been proposed to estimate the magnitude of paleotsunamis from their deposits. However, existing models consider neither non-uniform transport of suspended sediment nor turbulent mixing, which are essential factors governing sedimentation from suspension in tsunami flows. Here we propose a new inverse model of tsunami deposit emplacement, considering both transport of non-uniform suspended load and entrainment of basal sediments. This inversion model requires the spatial distribution of deposit thickness and the pattern of grain-size distributions of the tsunami deposit along a 1D shoreline-normal transect as input data. It produces as output run-up flow velocity, inundation depth and concentration of suspended sediment. To solve for advection of non-uniform suspended load, a transformed coordinate system is adopted, which increases computational efficiency. Tests of model inversions using artificial data successfully allow reconstruction of the original input values, suggesting the effectiveness of our optimization method. We apply our new inversion model to the 2011 Tohoku-Oki Tsunami deposit on Sendai Plain, Japan. The thickness and grain-size distribution of the tsunami deposit was measured along a 4 km long transect normal to the coastline. The result of our inversion fits well with the observations from aerial videos and field surveys. We conclude that this method is suitable for the analysis of ancient tsunami deposits, and that it has the advantage of requiring relatively little information about the condition of the emplacing paleotsunami for reconstruction.

Temporal changes in the internal stresses and pore pressures in a large-scale submarine mass transport deposit

AbstractWe examined the temporal changes in the internal stresses and pore fluid pressures of a submarine mass transport deposit (MTD) in the Akkeshi Formation of the Upper Cretaceous–Paleocene Nemuro Group, eastern Hokkaido Island, Japan. We first analyzed previous paleostress field results from meso-scale faults in the MTD blocks, which indicated two phases during the evolution of the debris flow: phase I, radial spreading of the flow body during downslope movement; phase II, the flow body underwent compression during deposition on the basin plain. We also estimated the pore fluid pressure ratio from the fault orientation distribution. There was a large increase in the pore fluid pressure ratio during the transition from phase I to phase II that continued to rise during the initial stage of phase II and then decreased in its latter stages, whereas the maximum horizontal compressive stress increased throughout phase II. This variation in pore fluid pressure relates to the dynamics and evolution of the debris flow, where the clasts in the central part of the flow were supported by the excess pore pressure due to the compression of the debris flow as the flow head decelerated. Although pore fluid pressure plays a critical role in the dynamics of debris flows, there was no previous methodology to quantify both the stress fields and pore fluid pressures in large debris flows and their resultant MTDs. Our results implemented for outcrop studies imply that meso-scale faults in MTDs can provide clues to better understand these paleoflow mechanisms.