Submarine sediments in Ariake Bay, Kyushu Japan, were examined to assess geoenvironmental changes in the area. 6 core samples from representative stations (Stas. 1-6) were analyzed for 16 minor elements (S, Cl, Ba, Rb, Th, Nb, Zr, Y, Ga, V, Zn, Cu, Ni, Co, Cr, and Pb) using XRF. Based on measurements of 210Pb radioactivity, sedimentation rates at 2 stations, one off the Chikugo River (Sta. 2) and the other at the mouth of Isahaya Bay (Sta. 4), were calculated, respectively, at 0.24 cm/year and a maximum of 0.15 cm/year. The concentrations of heavy metals, such as Cr, Ni, Zn, and Pb, in the marine sediments are about one-third of those of the highly polluted marine sediments recorded in Tokyo Bay around 1970. The vertical profile of a minor element can be divided into an upper section and a lower section. The upper section indicates various gradual changes, but the lower section is almost constant. The boundaries between the sections roughly correspond to the beginning of the 20th century, based on the sedimentation rate. Zn and Pb in the submarine sediments can be useful indicators to trace suspended mud derived from the Chikugo River. The complementary relationship of the vertical profiles in the spatial distribution suggests that the continuous migration of the sedimentary province may be governed by the geometrical arrangement of the estuaries of major rivers and by the tidal circulation pattern in Ariake Bay. Probably, large dams built on the major rivers have hampered the sedimentation process dominated by traction flow and have decreased the maximum drainage speed. As a result, such processes have promoted the expansion of the muddy area in Ariake Bay. Planktonic dinoflagellata (Sta. 2) and diatom (Sta. 4) fossils were also analyzed. Assuming such sedimentation rates at the stations, marine phytoplanktons (Brigantedinium spp., Rhizosorenia spp. and Skeletonema costatum), which indicate eutrophication of the sea water, have increased in relative abundance during the past 10 years.
The Sekidosan fault is a northeast-trending, 33-km-long active reverse fault located along the southeastern margin of the Ouchigata Plain, an elongated tectonic lowland whose northwestern and southeastern sides are bounded by reverse faults. We re-examined precise locations of the surface traces of the Sekidosan fault by interpreting large-scale aerial photographs, and then carried out topographic profiling across the tectonic scarps. Among the 34 profiles, 15 profiles are interpreted to represent the vertical offset associated with the most recent surface-faulting event, which enable us to reconstruct the vertical slip distribution during the most recent surface-faulting event along the middle and northeastern parts of the Sekidosan fault. The amount of vertical offset along the middle part is fairly constant (23 m) along the trace. In contrast, the amount of vertical offset along the northeastern part is smaller (12 m) and tapers off toward the northeastern end. In addition, the amount of vertical offset along other 10 profiles nearly equals to twice the amount of vertical offset during the most recent surface-faulting event, indicating that the amount of offset at individual sites along the middle and northeastern parts of the Sekidosan fault was almost the same during the latest and penultimate surface-faulting events. Subsurface geologic structures and radiocarbon ages obtained by a drilling survey at Nakagawa-Udono site, Hakui City, suggest that a surface-faulting event occurred between 6000 and 4000 years B.P. and this event probably occurred at approximately 4000 years B.P.
Sea urchins graze on limestone and at the same time, they also produce sediments by discharging various intakes into the sea bottom of the coral reefs. This study was done on the Shiraho and Arakawashita reefs, the Ishigaki Island, Ryukyus. There are some differences in reef topographies and sediment composition between the 2 reefs. In this study, the author mainly analyzed the gut contents of Echinometra mathaei distributed predominantly within the 2 study reefs. The result are as follows; 1) E. mathaei produces the fine sediment of less than 1 mm in diameter into the sea bottom of the coral reef. The grain size distribution of the gut contents of E. mathaei is different from that of bottom sediments near the area inhabited by E. matheaei. It is thought that the fine sediment ingested by E. mathaei moved from the inhabited area to the other locations by wave and tidal currents. This indicates that E. matehaei discharges calcium carbonate accumulated by reef builders and moves to another place. 2) Comparing the results obtained from 2 coral reefsites, the contents ingested by E. mathaei change according to geographical zone. The intake of E. mathaei contains not only fragments eroding coral limestone but also sediments from the nearby sea floor. 3) There has been no research on the characteristics of intake materials to quantify erosion by E. mathaei. The erosion rates reported in the previous research seem to be over estimats than the actual rate. 4) E. mathaei yields fine sediments by crushing and discharging coral limestone produced by reef builders. Moreover, it is thought that the sediments from E. mathaei have a little influence on the total amount of sediment produced by other organisms in the whole coral reef.
This paper describes the Holocene development of landslides in the northern area of Mt. Shiroumadake in the Hida Mountains of central Japan. Late Quaternary landforms caused by glaciers or landslides are common in the study area, where previous studies have provided basic information on glacial landforms with only few details on landslides. This study classifies landslide topography into three types : I, II, and III, based on air photo interpretation and field investigation, to reveal morphological characteristics. Type I comprises up-hill facing scarps or linear depressions without a deforming landslide body. However both type II and type III have a visible landslide body. Only type III features frontal debris accumulation. Type I is distributed almost throught the study area. Type II and type III are distributed on slopes underlain mainly by serpentinite and stratified rocks comprising of shale, sandstone, felsic tuff, and tuff breccia. Gravitational rock creep and resulting rock deformation seem to be important in the formation of type I and type II. Type I landslide seems to be transformed into type II landslide with rock deformation on the slopes in the order : (1) gravitational creep, (2) deformation with a discontinuous slip surface, and (3) slide with a continuous slip surface. Radiocarbon dating and tephrostratigraphical evidence reveal that landslide activities began in the early Holocene epoch, and have continued until the present.
The 2004 off-Sumatra earthquake (Mw=9.0) was the largest subduction-zone earthquake in the world since the 1964 Alaska earthquake. The 2004 earthquake generated large tsunamis, which propagated across the Indian Ocean and caused the catastrophic disaster. Our international tsunami survey team investigated in the province of Nanggroe Aceh Darussalam, northern Indonesia, in order to clarify the magnitude of tsunami and distribution of tsunami deposits caused by this earthquake.