Yukinobu Okamura

Subducting seamounts and deformation of overriding forearc wedges around Japan

Abstract Two subducting seamounts under inner trench slopes have been identified around Japan on the basis of magnetic anomalies, morphology and geological structure. The first one is located under the foot of the inner trench slope at the junction between the Japan Trench and the Kuril Trench. Another one occurs beneath the slope slightly seaward of the Tosabae (the basement high at the trench slope break along the Nankai Trough off Shikoku). The magnetic anomalies of seamount origin are accompanied by the characteristic morphology of a forearc wedge i.e., a swell landward and a depression seaward. The seamounts beneath the inner trench slopes have preserved magnetization showing reasonably consistent directions, which suggests that the subducting seamounts have kept roughly their original shapes. The morphology of the forearc wedge can be explained by a subducting seamount on the oceanic crust pushing the forearc material forward and upward. Deformation of the forearc wedge by the subducting seamount extends to the forearc basin. The seamounts are stronger and less deformable than the inner slope material and are not offscraped onto inner trench slopes. Two other examples of deformed inner trench slopes around Japan which can be explained by subduction of topographic highs are presented. One example is a depression on the foot of the inner trench slope northeast of the junction between the Kyushu-Palau Ridge and the Nankai Trough. Another one is an area of complex morphology of the inner trench slope along the Japan Trench around the Daiichi-Kashima Seamount.

Marine incursions of the past 1500 years and evidence of tsunamis at Suijin-numa, a coastal lake facing the Japan Trench

Sandy deposits of marine origin underlie the floor of Suijin-numa, a coastal lake midway along the subduction zone marked by the Japan Trench. The deposits form three units that are interbedded with lacustrine peat and mud above a foundation of marine, probably littoral sand. Unlike the lacustrine deposits, all three sandy units contain marine and brackish diatoms. The middle unit (B) contains, in addition, graded beds suggestive of multiple waves of long wavelength and period. The uppermost unit (C) probably dates to a time in the areas written history when the lake was separated from the sea by a beach-ridge plain at least 0.5 km wide and several metres high. Units A and B postdate AD 540—870, and unit C postdates AD 1030—1640 as judged from radiocarbon dating of leaves and seeds. Unit B pre-dates AD 915 and unit C postdates that year as judged from a tephra within the peat that separates units B and C. The age constraints permit correlation of unit B with a tsunami in AD 869 that reportedly devastated at least 100 km of coast approximately centred on Sendai. Unit C may represent a later catastrophic tsunami in 1611, or perhaps a storm surge that inundated much of Sendai. The lake lacks obvious signs of tsunamis from the regions largest twentieth-century earthquakes, which were centred to the north in 1933 (M 8.1) and directly offshore in 1936 (M 7.5), and 1978 (Mw 7.6).

Channel-levee complexes, terminal deep-sea fan and sediment wave fields associated with the Toyama Deep-Sea channel system in the Japan Sea

Abstract The Toyama Deep-Sea Channel (TDSC) in the Japan Sea is one of the most prominent deep-sea channels in rifted margins. This study revealed the course and morphology of the entire TDSC system for the first time, based on high-resolution and air-gun seismic reflection profiles, and sediment cores. The channel starts from Toyama Bay and extends for 750 km through the Toyama Trough, a Miocene rift, to the Miocene Yamato and Japan back-arc basins. The TDSC system is fed with sediment from the Northern Japan Alps, through tributary canyons on the narrow shelf, which are directly connected with rivers, even during sea-level highstands. Levee complexes border the channel in the Toyama Trough and Yamato Basin, and in the Japan Basin, the channel feeds the terminal Toyama Fan. Levees in the confines of the Toyama Trough are accompanied by an over-bank turbidite plain, whereas levees in the Yamato Basin are characterized by prominent sediment waves. Toyama Fan consists of a channel-levee complex with sediment waves in the upstream part and of lobes at the channel end. The course and morphology of the channel-fan system are primarily controlled by basin morphology. Thick, sheet-like sediments, deposited from ponded turbidity currents, have accumulated in the narrow, elongate Toyama Trough, whereas extensive levees and the Toyama Fan have formed in the more open basins. Distribution of sediments and consequent morphology of the channel-levee complexes are also controlled by Coriolis and centrifugal forces. Preferential development of the levees on the right-hand side is attributed to Coriolis-force tilt effects in the Northern Hemisphere. Centrifugal forces at the meander loops or bends of the channel result in flow stripping, causing levees to build on the outer bends. The distribution, form and orientation of sediment waves are consistent with the extent and direction of inferred spill-over turbidity currents, and with consequent levee growth. Fluvial-like features such as meanders, terraces, levee slumps and a crevasse splay are developed along the TDSC. Unlike other submarine channels, sinuosity seems to be controlled by bedrock structures rather than by valley slopes. Channel avulsion has not been recorded in the TDSC system. Active clastic deposition on the uppermost lobe during the past 1 ka suggests recent active sediment transportation through the channel. Sediment transportation, however, may have ceased during the Holocene in the cut- and fill-tributaries developed in the Quaternary succession on the slope to the trough, where a relatively wide shelf separates canyons from rivers in the eastern margin of the drainage area.

Structural development of Sumisu Rift, Izu‐Bonin Arc

Geophysical swath mapping, multichannel seismic profiling, and ocean drilling data are used to document the structural evolution of Sumisu Rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc. The ∼120-km-long, 30–50-km-wide Sumisu Rift is bounded to the north and south by structural and volcanic highs west of the Sumisu and Torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dip slopes. The zig-zag pattern of normal faults (average strikes 337° and 355°) indicates extension oriented 076°±10°, orthogonal to the volcanic arc. Three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift/subsidence patterns. Differential strain across the transfer zones is accommodated by interdigitating, rift-parallel faults and sometimes by cross-rift volcanism, rather than by strike- or oblique-slip faults. From estimates of extension (2–5 km), the age of the rift (∼2 Ma), and the accelerating subsidence, we infer that Sumisu Rift is in the early synrift stage of back arc basin formation. Following an early sag phase, half graben formed with synthetically faulted, structural rollovers facing large-offset (2–2.5 km throw) border fault zones. In the three northern rift segments the largest faults are on the arc side and dip 60°–75°W, whereas in the southern segment they are on the west side and dip 25°–50°E. The present “full graben” stage is dominated by hanging wall antithetic faulting, basin widening by footwall collapse, and a concentration of subsidence in an inner rift. The hanging wall collapses, but not necessarily as a result of border fault propagation from adjacent rift segments. Whereas the border faults may penetrate the Theologically weak lithosphere (Te ≈ 3 km), many of the hanging wall and footwall collapse structures are detached only a few kilometers below the seafloor. Back arc volcanism, usually erupted along faults, occurs in the rift and along the protoremnant arc during both stages. Where drilled, the arc margin has been uplifted 1.1±0.5 km concurrently with ∼1.1 km of rift basin subsidence. Extremely high sedimentation rates, up to 6 m/kyr in the inner rift, have kept pace with synrift faulting, created a smooth basin floor, and resulted in sediment thicknesses that mimic the differential basin subsidence. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. No obvious correlations are observed between the rift structures and preexisting cross-arc trends.

Tsunami heights and damage along the Myanmar coast from the December 2004 Sumatra-Andaman earthquake

The tsunami heights from the 2004 Sumatra-Andaman earthquake were between 0.4 and 2.9 m along the Myanmar coast, according to our post tsunami survey at 22 sites in Ayeyarwaddy Delta and the Taninthayi coast. Interviews to coastal residents indicate that the tsunami heights were lower than high tide level in rainy season, probably by storm surge. They also testified that the arrival times were between 2 and 5.5 hours after the earthquake but the reliability may be low because nobody felt ground shaking. Much smaller tsunami than the neighboring Thai coast, where the tsunami heights were 5 to 20 m, explains relatively slighter tsunami damage in Myanmar; the casualties were reported as 71, compared to about 8300 in Thailand. The smaller tsunami was probably due to the fact that the main tsunami source did not extend to Andaman Islands. The tsunami travel times and maximum heights computed from a 700 km long source are basically consistent with the observations. For a nearby tsunami source, the tsunami hazard would be more significant in Myanmar, because coastal houses are unprotected for tsunamis and no infrastructure exists to disseminate tsunami warning information.

GEOLOGIC EVIDENCE FOR THREE GREAT EARTHQUAKES IN THE PAST 3400 YEARS OFF MYANMAR

Tectonic environments, recent stress and crustal strain observations, and historical descriptions of geomorphological changes and eyewitness accounts of the 1762 Bengal earthquake suggest that great earthquakes (M 8.0 or larger) can occur along the northward continuation of the 2004 Sumatra-Andaman earthquake. We describe marine terraces along the Rakhine coast of Myanmar as evidence for three great earthquakes in the past 3400 years. Radiocarbon dating of coral remains suggests that the oldest terrace emerged three times, during 1395–740 BC, AD 805–1220 and AD 1585–1810. We assign the youngest age to the 1762 earthquake, which reportedly raised parts of the Burmese coast by 3–7 m. These indicate that the great subduction-zone earthquakes have repeatedly occurred west off Myanmar with an average recurrence interval of about 1000–2000 years. The time since the last earthquake, ~ 250 years, is much shorter than the average interval, hence the chance of next earthquakes in the near future may be considered as low. However, the variability in both uplift amounts and recurrence intervals suggests the next great earthquake could happen sooner or later than would be expected from the average interval.

Unstable forearc stress in the eastern Nankai subduction zone for the last 2 million years

Abstract Transpressional tectonics characterizes the SW Japan arc. However, we will show in this article that offshore seismic profiles and onshore mesoscale faults indicate that the eastern part of the forearc was subject to transtensional tectonics since ca. 2.0 Ma. Offshore normal faults imaged on the profiles run parallel to the Nankai Trough, and started activity at 1.0 Ma, but transtensional tectonics commenced the onshore area earlier. In order to understand the stress history in the forearc region, we collected fault-slip data from onshore mesoscale faults in Plio-Pleistocene sedimentary rocks in the Kakegawa area at the northeastern extension of the offshore normal faults. Most of the mesoscale faults are oblique-normal, indicating that the area was subject to transtensional tectonics. The faults suggest that the compressional tectonic regime was followed by the transtensional one at 2.0 Ma, in agreement with regional tectonostratigraphic data, which indicate that folding ceased at that time. Present compressional stress followed the transtensional tectonic regime sometime in the late Pleistocene. Transtensional or extensional tectonic zone shifted from the Kakegawa area to the offshore region. These observations indicate that the state of stress just behind the accretionary prism of the eastern Nankai subduction zone has been unstable in the last ∼2 million years, suggesting that the forearc wedge has been at critical state in that gravitational force and basal shear traction on the wedge have been balanced, but the forearc tectonics has been susceptible to small perturbations. Possible factors compatible with the observed stress history include the change of subduction direction of the plate at 1.0 Ma, and the rapid uplift of Central Japan thereafter.

Myanmar Coastal Area Field Survey after the December 2004 Indian Ocean Tsunami

A post-tsunami survey was conducted along the Myanmar coast two months after the 2004 Great Sumatra earthquake ( Mw =9.0) that occurred off the west coast of Sumatra and generated a devastating tsunami around the Indian Ocean. Visual observations, measurements, and a survey of local peoples experiences with the tsunami indicated some reasons why less damage and fewer casualties occurred in Myanmar than in other countries around the Indian Ocean. The tide level at the measured sites was calibrated with reference to a real-time tsunami datum, and the tsunami tide level range was 2–3 m for 22 localities in Myanmar. The tsunami arrived three to four hours after the earthquake.

Holocene ages and inland source of wood blocks that emerged onto the seafloor during the 2007 Chuetsu-oki, central Japan, earthquake

At least 300 tons of subrounded to well-rounded wood blocks emerged onto the seafloor at a water depth of 70–100 m during the 2007 Mw 6.6 Chuetsu-oki, central Japan, earthquake. Radiocarbon dating and taxonomic identification of eight of those wood blocks suggest that they were transported from inland during the middle to late Holocene, buried by subsequent sedimentation, and brought up onto the seafloor in 2007, most likely by submarine liquefaction induced by strong shaking. In particular, all eight blocks gave ages older than 2500 cal yr BP, implying the possibility that the 2007 earthquake was the first earthquake during the last two millennia to have caused shaking strong enough to induce submarine liquefaction in the 2007 meizoseismal area. However, we cannot rule out the possibility of multiple large earthquakes after approximately 2 ka, if the buried wood sources cannot be emptied by a single earthquake. Further studies are required to examine paleoseismic implications of the emergence of these wood blocks in 2007.

Accretionary prism collapse: a new hypothesis on the source of the 1771 giant tsunami in the Ryukyu Arc, SW Japan

The giant 1771 Yaeyama tsunami occurred in the southwestern part of the Ryukyu Arc, a region on an obliquely subducting plate boundary, which shows no direct evidence of inter-plate coupling. Studies of tsunami boulders and deposits suggest that the recurrence interval of comparably giant tsunamis is roughly 500 to 1000 years. Tsunami source models, which include either slip on a shallow plate boundary or active faulting plus a landslide on the overriding plate, are controversial because of inconsistencies in the geophysical and geological data. We discovered a seafloor depression that is approximately 30 km wide and 80 km long extending in the ESE-WNW direction. This depression is accompanied by a seaward bulge on the accretionary prism along the Ryukyu Trench, which is based on detailed bathymetric data and interpreted to be the result of accretionary prism collapse and seaward displacement by rotational slide. A simple tsunami simulation shows that the slide is a plausible source of the 1771 tsunami. We propose a collapse model, in which the accretionary prism remained over-steepened as strike-slip faulting removed the prism toe. Our model indicates that some oblique subduction zones are capable of generating giant tsunamis regardless of weak or strong coupling.