Southeast Off Research Articles

Deep Flows on the Slope Inshore of the Kuril-Kamchatka Trench Southeast off Cape Erimo, Hokkaido

Deep flows on the slope inshore of the Kuril-Kamchatka Trench southeast off Cape Erimo, Hokkaido were observed for about five years from June 1989 to March 1995, using a mooring system with two current meters. In 1991 and 1993 directionally stable southwestward flows were observed at the upper layer (1000 m). These appear to be typical of the Oyashio because the characteristics of the flows were high mean kinetic energy, low eddy energy and high stability. However, the magnitudes of other mean flows at the upper layer, except for 1991 and 1993, were less than their standard deviations. This suggests that the Oyashio was observed for only a limited period of time. On the other hand, at the lower layer (3000 m) the magnitudes of the mean flows for 10–11 months were 1–3 cm s-1 and ellipses of their eddy kinetic energy were extremely flattened in the direction of the local isobath. The directions of the mean flows in 1990, 1991 and 1993 were southwestward along the local isobath. The relationships between the upper and the lower flows are discussed in terms of monthly change of kinetic energy, since the low-frequency fluctuations longer than 30-day are predominant from the eddy kinetic energy spectra. The results show that there are cases when the kinetic energy of the monthly mean flows at the lower layers are larger than those at the upper layers. This suggests the possibility that the lower flows are in part a southward deep western boundary current.

北海道南西沖地震に伴う被災者の個人領域の形成過程 : 自然災害に起因する環境移行研究 その2

Survivors' place making was investigated through critical environmental transition that accompanied Tsunami disaster caused by Hokkaido South East Off Earthquake in 1993. Each private spaces constructed following the disaster- temporary housing units, restoration housing units, self rebuilt houses- was traced by focusing on place makings of physical disposition. A comparative study was conducted about peoples cognition and evaluation of radical environmental transition. The findings are as follows. 1) Differences of restoration housings influence personalization by place making in private space. 2) People-environment model is shown that explains process of adaptation after environmental transition caused by natural disaster.

１９９５年奄美大島近海（喜界島近海）地震の地震活動とテクトニクス １９９５年奄美大島近海地震 活動と被害の概況

A shallow earthquake of MJMA 6.7 on October 18, 1995, at about 50km southeast off Kikai-jima in the Ryukyu Islands caused slope landslides, subsidence of concrete slabs at fishing ports and collapses of walls piled up with coral ragged stones. Several fishing boats moored to wharves were damaged by tsunami; four boats were turned over at ports in Amami-Oshima.From May 19 to 30, 1995, earthquakes of MJMA≤4.1 had occurred in the focal area after a long seismic quiescence since the 1911 Kikai-jima Earthquake of M=8.0. Some of them were felt in JMA intensity 1 or 2 at Kikai-jima. Numerous felt shocks also struck the island immediately after the main shock; 62 and 125 shocks on October 18 and 19, respectively. Numbers of aftershocks decreased rapidly, though they lasted through the end of 1996.The damage distribution of the 1911 event being compared with that of the 1995 event, their focal areas were considered to overlap with each other. It was, however, possibly an interplate earthquake of a thrust fault type in contrast with the 1995 event of a normal fault type in the subducting plate, because the first motions of P wave at Naze and of tsunami wave at Kikai-jima were reversed between the two events.

１９９５年奄美大島近海（喜界島近海）地震の地震活動とテクトニクス １９９５年１０月１８日，１９日の奄美大島近海の地震（Ｍ６．６，Ｍ６．５）の前震および余震活動について

An earthquake with M6.6 occurred about 50 km southeast off Kikai-jima which is located near Amami-Oshima in Kagoshima prefecture at 19 : 37 October 18, 1995. About 16 hours after the event, an another earthquake with M6.5 occurred in the same region. Seismic intensity of 5 in the JMA scale was recorded at Takigawa in Kikai-jima for the two earthquakes. Focal mechanisms of the two earthquakes were normal fault-type, and they were estimated to have had occurred within the Philippine Sea plate. Tsunamis were observed at Pacific coasts in southwest Japan. Tsunami waves were higher than usual at Kikai-jima, relative to seismic magnitude, at the two earthquakes. Characteristics of the seismic activity before and after the near Amami-Oshima earthquake are summarized as follows : 1. Preceding seismic activity and foreshocks were observed.2. Another earthquake with almost the same magnitude succeeded after a short interval.3. Following characteristic patterns were recognized in seismicity : Seismicity was decreased noticeably in the northeastern part of the aftershock area just before larger shocks over M5. While conspicuous decrease in seismicity was observed in the southwestern part of the aftershock area after the earthquake of M6.5.

Deep Stratigraphy and Evolution of Baltimore Canyon Trough Based on Multifold Seismic Reflection, Refraction, Gravity, and Magnetic Data: ABSTRACT

A recent 48-channel seismic reflection profile (U.S. Geological Survey line 25) extends 330 km southeast off southern New Jersey and crosses the widest and deepest part of the Baltimore Canyon Trough (10 km southwest of the COST B-3 well). The profile has been migrated and converted to depth to reveal the deep sedimentary and basement structures across the ocean-continent transition zone. The sedimentary wedge thickens from 5 km nearshore to 17 km just landward of the East Coast Magnetic Anomaly (ECMA; 20 km landward of the shelf edge in this area). A strong, flat reflector about 10 km wide exists at a depth of 14 km, directly beneath ECMA. Acoustic basement becomes obscure and appears to rise to a depth of 5 km over the next 40 km to the southeast, beneath a Jurassic and lower Cretaceous carbonate shelf-edge complex which extends 20 km seaward of the present shelf edge. Landward-dipping continental rise sediments exist to a depth of at least 13 km on the seaward side of the Jurassic shelf edge. The top of oceanic basement is first seen as a set of prominent hyperbolic reflectors about 50 km seaward of the Jurassic shelf edge, where it occurs at 11 km depth and dips gently landward. It is obscured landward of this point by the prominent middle Jurassic (J3) horizon. A Jurassic and lower Cretaceous shelf-edge carbonate platform or reef complex prograded 40 km out over oceanic crust in this area. Greater differential subsidence and compaction of the basin west of the ECMA have produced back-tilted and arched horizons in the Jurassic and lower Cretaceous shelf edge units, creating a 20-km-wide with 500-m closure beneath the upper continental slope. Other lines to the southwest indicate the anticlinal arch extends at least 40 km to the southwest. Similar slope anticline structures have been reported off northwest Africa. End_of_Article - Last_Page 715------------

Coastal Terraces and their Deformations in Amami-Oshima, Ryukyu Islands, Southern Japan

Amami-Oshima is located at the central part of the Ryukyu Arc which extends from Kyushu to Taiwan. In Kikai-Jima at about 20 kilo-meters south-east off Kasari Peninsula of Amami-Oshima, rapid uplifting (1.5-2 mm/y) during the past 130, 000 years was revealed by the altitudes of its marine terraces and their absolute ages derived from the calcareous terrace deposits (KONISHI et al., 1970). As the Ryukyu Arc has similar geological and geophysical settings to the outer arc of Southwest Japan, the rapid uplifting in Kikai-Jima was interpreted as earthquake related deformation (YONEKURA, 1975) which has been critically studied in the outermost belt of Southwest Japan. The purpose of this paper is to clarify the mode of the vertical deformation in the late Quaternary period in Amami-Oshima with relation to that in Kikai-Jima, and to present some data for comparison with above mentioned seismic crustal movements.The geomorphic surfaces of Amami-Oshima are roughly classified as follows : low, middle and high coastal terraces and the uppermost low-relief erosion surface. The low terrace (L) consist of Holocene marine terrace, wave-cut bench and alluvial plain, altitude of which does not exceed several meters high. The middle terraces (Ml and M2) are mostly developed in Kasari Peninsula at the height of 10 to 60 meters above sea level and scarcely observable in the other part of Amami-Oshima, except a minor wave-cut bench of 8 meters high above sea level with subrounded gravel and red soil observed at Honohoshi Beach, east of Koniya. The profiles of the middle terraces are, in general, continuous without any cliff or knick point in them and gently inclined seaward. However, in the east coast of Kasari Peninsula they can be divided into two levels : the higher (M1) and the lower (M2). The terrace deposits of M1 terrace are generally thin marine gravels covered with red soil, but valley fill deposits more than 40 meters thick exist inside some small isthmus such as at Taira, Kawauchi and Ogachi. The upper parts of these deposits consist of marine sands and gravels, and the basal parts of them consist of clayey deposits with a lot of breccia and fragments of plants into which a white tuff layer is inserted. M2 terrace is abrasion platform cutting M1 terrace with thin terrace gravels covered with red soil. The high terraces (H) are mainly developed in the northeastern part of Amami-Oshima (mostly included in the area of Fig. 2), which are highly dissected by V-shaped valleys but still preserve very flat original terrace surfaces. They are now at the height of 180 to 340 meters above sea level and probably can be divided into two or three levels. Terrace deposits are not observed on them. The low-relief erosion surface is widely developed in the western half of Amami-Oshima and its dependant islets at the height of 250 to 400 meters above sea level.From the evidences for a large transgression more than 40 meters deep and from the sequence of the middle and low terraces, M1 terrace is inferred to have been formed by the transgression in the last interglacial period, which is called “Shimosueyoshi transgression” in South Kanto, Japan, and dated 120, 000-130, 000 years B. P. (MACHIDA and SUZUKI, 1971).The mode of the vertical deformation during the last 120, 000 or 130, 000 years inferred from the distribution of altitudes of former shoreline of Ml terrace is as follows : (1) Kasari peninsula has been uplifting and tilting north-westward (Fig. 2). Hyakunodai terrace in Kikai-Jima 20 kilo-meters south-east off Kasari Peninsula, which is contemporaneous with Ml terrace in Amami-Oshima, is at the height of 220 meters above sea level. Therefore the tilted area is seemed to extend from Kasari Peninsula to this island.

Turbulent diffusion of suspended particles near the ocean bottom

Vertical distributions of suspended matter were observed by EWING and THORNDIKE (1965) at three deep stations on the continental slope southeast off New York with a new nephelometer. Vertical profiles of eddy diffusivity are determined by use of a simplified Fickian equation from these distributions. A model of eddy diffusivity proposed by ROSSBY and MONTGOMERY (1935) for the ground layer of atmosphere seems to be applicable to the ocean bottom layer. This model represents vertical eddy diffusivity in three parts: a lowest part linearly increasing with distance from the bottom, a middle part quadratically decreasing with distance and an upper part of a constant value. Vertical velocity as a cause of transparent zones in determined.