Nobuyuki Yonekura

Fault mechanism and recurrence time of major earthquakes in southern Kanto district, Japan, as deduced from coastal terrace data

The southern Kanto region has had two shocks of magnitude 8 or greater during the past 1,000 years. They were the 1703 and 1923 earthquakes, which occurred along the Sagami trough, a northeastern boundary of the Philippine Sea crustal plate in contact with the Asian plate. Although they occurred in nearly the same region, the 1703 earthquake was significantly different from the 1923 earthquake in the distribution of coastal uplift and tsunami height. The 1703 earthquake deformation is described on the basis of the height of the marine terraces along the coast of the southern Kanto region. The 1703 earthquake is interpreted, as is the 1923 earthquake, as the result of low-angle right-lateral faulting with a thrust component at the plate boundary. However, the fault surface in 1703 was longer (about 200 km) and was located farther east than that of the 1923 earthquake. On the basis of the pattern of coastal uplift and the trend of the Sagami trough, the fault surface of the 1703 earthquake can be divided into three planes, which involve the eastern part of the source region of the 1923 earthquake to the west (plane A), the Kamogawa submarine cliff in the middle (plane B), and a segment near the source region of the 1953 Boso-Oki earthquake (M equals 8.0) to the east (plane C). The Boso and Miura Peninsulas in the sourthern Kanto region have been uplifted during at least the last 6,000 years, and major uplifts have been accompained by earthquakes like those of 1703 and 1923 many times. The recurrence time of similar uplifts is estimated at 800 to 1,500 years on the basis of the numbers of the uplifted Holocene terraces in the Boso Peninsula, the rate of upheaval during the last 6,000 years, and the present geodetic data. Thus, it is unlikely that major earthquakes such as the 1703 and 1923 earthquakes will occur in the same segments in the near future. The Oiso area, however, which is located west of the western end of the 1703 faulting, seems higher in seismic risk than the other parts of the Sagami trough fault, because the sum of the recent uplift in the 1703 and 1923 earthquakes in that area is significantly less than the average rate of uplift there during the past 6,000 years. /Author/

Late pleistocene and halocene sea-level changes in Japan: implications for tectonic histories and mantle rheology

Abstract It is very important to separate the tectonic and glacial-isostatic components in the observed Holocene and Late Pleistocene sea-level changes in tectonically active areas such as Japan for studying tectonic processes and for constraining mantle rheology. The separation can be achieved by considering the spatial dependence of the relative sea-level on the geometry of the coastline around the site where sea-level is evaluated. In fact, the relative sea-level caused by the last deglaciation at sites in the embayment such as Tokyo and Osaka has a sea-level curve with a high stand at mid-Holocene, and that site situated on the tip of peninsula has a sea-level curve culminating towards the present. The geometric effect also causes the regional difference of the sea-level variations in the late glacial phase. In the Japanese Islands, the regional difference of the predicted relative sea-level is about 5 m at 6000 years ago, 20 m at 10,000 years ago and 30 m at 18,000 years ago. Comparison between observations and predictions indicates that the observations at several sites in Japan are consistent with the predicted sea-level variations. More systematic data is required for the period 18,000 years ago to the present in order to examine tectonic processes and mantle rheology.

Anatomy of a Modern Coral Reef Flat: A Recorder of Storms and Uplift in the Late Holocene

ABSTRACT Cores from Kabira Reef, Ishigaki Island, southwest Japan, reveal the internal structure and temporal changes in sedimentary process of a complete coral reef flat. The reef crest caught up with sea level at about 4000 yr BP. Since then, it expanded oceanward and the reef pavement has grown landward. The backreef structure is composed of bioclasts derived from the reef framework. Of all the bioclasts, corals and coralline algae are most abundant. Coral fragments coarser than -3.0 o have dominated the backreef sediments since about 2000 yr BP. Benthic foraminiferal tests first occurred at about 4000 yr BP, and their abundance increased significantly starting around 2000 yr BP. Shallowing of the reef crest is also indicated by the presence of the shallow-water benthic Foraminifera Baculogypsina sphaerulata tests after 2000 yr BP. The date 2000 yr BP is coincident with tectonic uplift at Kabira Reef (Kawana 1989). We consider this uplift to have caused a relative sea-level fall that aided the deposition of coral fragments transported from the reef pavement by storms. The relative sea-level fall also caused subaerial exposure of the reef crest during low tides and the transition of reef-building organisms from corals to shallow-water species of benthic Foraminifera, resulting in a change noticeable in the constituents of backreef sediments. The results of this study suggest that the coral reef flat, and especially the backreef, can be a faithful recorder of relative sea-level changes.

HOLOCENE FRINGING REEFS AND SEA-LEVEL CHANGE IN MANGAIA ISLAND, SOUTHERN COOK ISLANDS*

Abstract Holocene relative sea levels in oceanic islands that are situated sufficiently far from glaciated regions provide basic information for the study of the melting histories of the continental ice sheets and of the rheological structure of the Earth. We have studied Mangaia Island, South Cooks, as one such oceanic island located in the middle Pacific Ocean. In addition to the usual geological and geomorphological observations of geomorphic features associated with former shorelines, we have used a portable drilling sampler for shallow borings of coral reefs. Geological and geomorphological studies of the coast of Mangaia Island have revealed that there was a higher sea level than the present in the mid-Holocene. The heights and ages of emerged microatolls on the emerged bench indicate that the sea reached a maximum level of +1.7 m around 4000-3400 yr B.P., and then emergence is considered to have occurred between 3400 and 2900 yr B.P. At the same time, the reef crest formed in the period from 5000 to 3400 yr B.P. emerged above the sea and suffered erosion. As a result of this change in sea level, upward reef growth from the fore reef slope began to form a new reef crest by 2000 yr B.P. to seaward of the old reef crest. The reef margin of Mangaia grew outward intermittently with seaward jump of the reef front, in accordance with a fall of sea level, in the late Holocene. The presence of fossil reef crests or eroded algal ridges on modern reef flats is a rather common feature in the South Pacific. The geomorphic development of such features on reef flats can be explained by the effect of the late Holocene fall in sea level.

Water circulation in a fringing reef located in a monsoon area: Kabira Reef, Ishigaki Island, southwest Japan

Abstract Kabira Reef is a well-developed fringing reef situated in a monsoon area where the dominant wind direction changes seasonally: south in summer and north in winter. Circulation in this reef shows a marked wind influence. The circulation pattern under calm wind conditions is characterized by an inflow of ocean waters into the moat over the reef crest and an outflow through a prominent channel. Current vectors change according to wind conditions, and this pattern is weakened and strengthened under southern and northern wind conditions, respectively. We establish a simple model to explain these circulation patterns with two factors: wind and a fundamental circulation pattern under calm conditions. We estimate the ratios of the component of wind to that of the fundamental circulation. The ratios reach 3 and 10 in absolute values under southern and northern wind conditions, respectively. These results can be applied to water circulation throughout the year, with the southern wind-driven circulation dominant in the summer, and the northern wind-driven circulation dominant in the winter. While trade wind conditions often result in a constant circulation pattern, monsoonal wind conditions make the circulation pattern vary according to the seasons.

Influence of wave energy on Holocene coral reef development: an example from Ishigaki Island, Ryukyu Islands, Japan

Abstract Wave energy induced by ocean swell and local winds is an important factor controlling the coral reef development. Here we compare and contrast facies, ages, and accretion rates of reefs that developed independently along windward and leeward margins of the same island. As both reefs face the open ocean, swells provide a background of low- to medium-energy condition with additional energy along windward margins due to prevailing winds. This similarity of the wave-energy regimes (medium to high energy) produced a similar framework facies that was constructed by robust-branching Acropora. However, much greater lateral accretion rates were observed in the high-energy reef than in the medium-energy reef. Also, coral cobble facies were only found in the medium-energy reef, indicating that the reef framework was more susceptible to storm breakage than that in the high-energy reef. These differences are attributable to the differing biological and ecological responses of corals to wave energy and water motion.

Restraining step behaviour along the Nobi fault system, central Japan

Abstract The Nobi fault system consists of en-echelon left-lateral strike-slip faults. Two restraining steps, the Midori and Hakusan upwarps, have formed between the en-echelon faults, and have demonstrated different failure behaviour. The Midori upwarp has developed into a secondary reverse fault while the Hakusan one has not and is still a fold. Structurally, the Midori upwarp is more evolved than the Hakusan upwarp. Recent faulting has had a shorter recurrence interval since c. 14 ka B.P. This change may have been associated with the development of the secondary reverse fault through the Midori upwarp, as an effective linkage in the Nobi fault system.

Recent and present-day subduction and collision along the northern tip of the Philippine sea plate

Abstract The northernmost converging boundary of the Philippine Sea (PHS) plate crosses the base of the Izu Peninsula about 60 km southwest of Tokyo, subducting beneath Japan, a part of the Eurasian (EUR) plate. The 1923 Kanto earthquake (Ms = 8.2) occurred along the portion east of the peninsula and another underthrust event (M ~ 8) is expected in the foreseeable future along the portion west of the peninsula. Consequently, a vast amount of data has been and is being accumulated in various fields, including geodesy, seismology, submarine geology and geophysics. This paper tries to give a unified interpretation of the above observations across and along the 200 km long plate boundary in terms of a plate tectonic model in which the PHS plate is subducting beneath and partially colliding with the EUR plate at an annual rate of 3 ~ 5 cm in a 310° ± 5° direction. Because of the resolving power being greater than usual both in time and space, discrimination between material and mechanical boundaries is necessary for the converging plate boundary.