Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
95 巻, 2 号
  • 平野 信一, 中田 高, 寒川 旭
    1986 年 95 巻 2 号 p. 71-93
    発行日: 1986/04/25
    公開日: 2009/11/12
    ジャーナル フリー
    The Philippine fault zone is one of the most remarkable active faults in the world. It streches on the Philippine archipelago and runs parallel with the Philippine trench. It can be traced for more than 1, 200 km from Luzon Island to the southwest, through Masbate Island, Leyte Island and Mindanao Island (Fig. 15). In central part of the Luzon Island ma ny geomorphic features of late Quaternary displacement are recognized along the fault zone.
    The aim of our study is to clarify the nature of the fault zone in central Luzon Island on the basis of the interpretation of topographic maps, vertical aerial photographs and detailed field investigations.
    The Philippine fault zone extends with high certainty in the direction of SW-NE more than 100 km long from Dingalan Bay to Lingayen Gulf. It runs across the Shiera Madre Mountains in the southern part. In the northern part of the fault zone, it separates the Cordillera Central Range on the north-eastern side from the Central Valley with the elevation less than 100 m on the southwestern side.
    The Philippine fault zone is divided into three fault systems based on the continuity of the fault traces and the strike of the faults. The first system extends from Dingalan Bay to Digdig (fault length ca. 90 km), the second is northeast of Lupao and Umingan (ca. 20km), and the third is San Manuel and its surrounding (ca. 30km) (Figs. 1 and 2).
    The main results obtaind follow ;
    1) In the study area, various fault topographic features develope along the Philippine fault zone. The southern end of it, from Dingalan Bay to Gabaldon, some lineaments and a number of fault sag develope along it (Fig. 3). The features in the southern part between Gabaldon and Rizal are characterized by displacements of alluvial fans, river terraces and a few offset streams along or near the foot of the Sierra Madre Mountains (Figs, 4, 7 and 8). In the northern part, between Lupao and San Manuel, fault displacements appear sharply just along the foot on the Cordillera Central Range. The fault features are offset streams and fault scarplets on alluvial fans and river terraces (Figs. 9, 10 and 12).
    2) The dip of fault plain is vertical (Fig. 6) or high as 60°E on fault outcrops.
    3) The sence of horizontal displacement is sinistral in the study area. Vertical one is variable in the southern part, but in the northern part it is downthrown to southwest side (Fig. 13).
    4) The ratio of horizontal and vertical displacements id deduced from clear breaks on geomorphic surfaces. While it is 1 : 1 to 3 : 2 in the southern part, but it becomes 8 : 1 in the northern part (Table 1).
    5) The average rate of horizontal displacements along the fault is estimated to be 1.5 5mm/year based on the relation between values of horizontal offsets and upstream lengths from the fault. The vertical one which is estimated from the proportion of the vertical displacement to horizontal displacement is calculated to be 0.190.63 mm/year (northern part), and 1.03. 3 to 1.5 5.0mm/year (southern part).
    6) It seems that the last earthquake along the fault in the study area took place in 1645 A.D. based on the 14C age data of plant fossil samples which were collected from terrace deposits in some places and the catalogue of historical earthquakes (REPETTI 1946). The magnitude of the earthquake is estimated to be a 8 class and the fault dislocation should appear along the fault.
    7) The recurrence interval of earthquakes is calculated as 1, 6005, 300 years from the average slip rate and the displacement of the last earthquake.
    8) The fault features and activity of the Philippine fault zone are remarkable, and its nature is similar to that of the faults which are located on other island arcs in the world.
  • 森脇 広, 町田 洋, 初見 祐一, 松島 義章
    1986 年 95 巻 2 号 p. 94-113
    発行日: 1986/04/25
    公開日: 2009/11/12
    ジャーナル フリー
    In the volcanic areas to the north of Aira caldera, southern Kyushu, two maars named Sumiyoshi-ike and Yonemaru were formed by phreatomagmatic eruptions in the early stage of Holocene. These maars were located in a lowland composed of coastal deposits. In this paper, we discuss first the characteristics of these eruptions on the basis of distribution, structure of pyroclastic deposits called Yonemaru and Sumiyoshi-ike tephra formations which were produced by these eruptions. And then the paleo-environment of the vents is discussed with special reference to coastal changes associated with Holocene sea level rise, because the nature of such eruptions was possibly affected by water condition around eruption plumes.
    Yonemaru tephra formation, 0.04km3 in volume, is characterised by many thin consolidated beds containing abundant scoriaceous ash with accidental lapilli and block. These thin beds show plane bedding forms, with intercalating wavy or lenticular beds. Such structural features and distribution highly affected by topography indicate that this eruption was phreatomagmatic and dominantly generated a base surge flowing towards the southeast to a distance of about 3 or 4km from the vent, with minor amount of tephra falls. Sumiyoshi-ike tephra formation, 0.005km3 in volume, also emblaces alternating thin ash and scoriaceous lapilli beds with accidental blocks, generally coarser than Yonemaru tephra. Structural features and distribution of this formation indicate that this eruption was also phreatomagmatic, and produced mainly tephra falls dispersed north of the vent.
    A chronological study of these two tephra formations and of Holocene coastal terraces and deposits around the maars strongly suggests that changes in water condition around the maars associated with the Holocene sea level rise were responsible for these maar-forming eruptions of phreatomagmatic type. Sumiyoshi-ike and Yonemaru maars were respectively formed around 7, 0006, 500 yBP, when sea level was rapidly rising and the sea invaded inland to the vicinity of the maars. As a results, the condition that rising magma most effectively contacted with water was produced in this area at these ages.
    This region has been so affected by active uplifts that paleoshores at several stages are found considerably higher than the present sea level as follows : c. 6m at c. 7, 000 yBP, c. 10m at c. 6, 500 yBP and c. 15m at 6, 0006, 300 yBP.
  • (I) 風化殻形成の機構
    徳山 明, 湊 秀雄
    1986 年 95 巻 2 号 p. 114-125
    発行日: 1986/04/25
    公開日: 2009/11/12
    ジャーナル フリー
    Wide and flat planes found on top of mountains are often related to deep weathering crusts of the Miocene and Pliocene Ages. Because surfaces of the crusts are porous and mechanically weak, they are easy to suffer from aeolian and frost actions, and flattened during glacial ages.
    Process of weathering is significant to the morphogeny and therefore to the recent morphology in Japan. In this respect the mechanism of deep weathering is discussed. In aquifer or waterholding parts of the crust, rocks as well as water change chemical properties in course of weathering reactions from surface to bottom. Near the surface rocks are leached under acidic conditions with CO2 and water. Beside silica, alkali and alkali-earth are romoved in ground water to react and replace mother rocks beneath. Further removed materials are deposited as interstitial clay or opal under nutral or slightly alkaline conditions. Therefore by removal of weathering products the reaction continues in the same direction to change chemical conditions, and form an almost closed system between rocks and ground water in the aquifer. The weathering crust includes a whole pack of reacted, replaced and deposited materials in the aquifer.
    Taking granitic rocks as typical components of the earth's crust, layered structure of weathering crusts are described. They are made up of 3 layers of horizontal zones as follows :
    In the top of the granitic crust completely leached rocks are found as saprolites, in which original textures of rocks are presurved, although most minerals except quartz are converted into kaolinite, halloysite or gibbsite. Density of original rock (2.68g/cm3 in a fresh granodiorite) is reduced to 1.0g/cm3 in dry condition. Here almost 60% of materials are leached. The upper part of the layer is red coloured by ferric oxides, while lower part is white by halloysites. Because rocks are porous, lightened and weakened, the upper most part of the crust is often removed by wind and the crust is truncated by wide aeolian surfaces.
    In the second layer, rocks are moderately weathered manganese and ferrous hydroxides are deposited beside removed clay minerals, tuch as halloysite and vermiculite. Quartz and most feldspars are not decomposed, but micas are changed ingo vermiculite. The layer is 1.8-2.0g/cm3 in density. It is represented by the montmorillonite zone in another kind of weathering crust, containing dacite, dacite tuff and another volcanics.
    In the third layer, rocks are not changed chemically nor minerallogicaly, but they are fractured or fragmented by open clacks along grain boundaries or cleavages. Density of the rock is reduced to 2.4-2.0g/cm3 in this layer by leaching reaction of percolating water.
    Change in density by weathering is closely related to the geomorphology especially to the inclination of slopes along valleys.
  • 山口 寿之
    1986 年 95 巻 2 号 p. 126-142
    発行日: 1986/04/25
    公開日: 2010/04/23
    ジャーナル フリー
    Cirripeds are of either male, female, and hermaphrodite sex, Male is dwarf (or complimental) and usually attaches to female or hermaphrodite. Four sex combinations are found among cirripeds : (1) female and dwarf male ; (2) female, dwarf male and hermaphrodite ; (3) dwarf (complimental) male and hermaphrodite ; and (4) only hermaphrodite.
    BROCH (1922) pointed out that of cirripeds separate sex is a characteristic they share with most of the other Crustacea. Hermaphrodite sex in cirripeds has evolved from separate sex, which is more primitive than hermaphrodite.
    Sex in cirripeds does not intimately correspond to their evolutionary stage. It is, however, closely related to ecological conditions they live in.
    Sex combinations with a male, such as (1), (2) and (3) are found in taxa (or groups) having low population density or living as parasites on coelenterates, echinoderms, or on other arthropods. The existence of a dwarf male guarantees efficient reproduction under these ecological conditions. Sex combination (4) of only hermaphrodite is characteristic in taxa (or groups) with high population density. Their gregariousness guarantees hermaphrodites efficient reproduction by mutual fertilization.
    Gregariousness was observed already in the oldest cirriped fossil Priscansemarinus from the Middle Cambrian Burgess Shale and Cyprilepas from the Silurian of Estonia. NEWMAN (NEWMAN et al. 1969) concluded that two different size classes found in Cyprilepas indicate either sexual dimorphism or different stage of growth. Evidence of gregariousness found in Cyprilepas indicates that they are hermaphroditic, and that the size difference is not sexual dimorphism, but is different stage of growth.
    Sexual polymorphism in these cirripeds resulted from sexual selection for aquisition of the most efficient reproduction strategy under the ecological conditions mentioned above, in addition to sessile mode of life and copulative mode of reproduction. Sexual polymorphism originated as a result of change from free living to sessile mode of life, perhaps in the early Cambrian or latest Precambrian.
    Existence of dwarf males is known in some aschelminthes, molluscs, annelids, arthropods, and vertebrates. The same ecological factors produced sexual polymorphism in these groups.
  • 小林 貞一
    1986 年 95 巻 2 号 p. 143-144
    発行日: 1986/04/25
    公開日: 2009/11/12
    ジャーナル フリー
  • 村上 政嗣
    1986 年 95 巻 2 号 p. 145
    発行日: 1986/04/25
    公開日: 2009/11/12
    ジャーナル フリー
  • 山本 正三, 丸山 浩明, 山本 充
    1986 年 95 巻 2 号 p. 146-149
    発行日: 1986/04/25
    公開日: 2009/11/12
    ジャーナル フリー
  • 徳山 明
    1986 年 95 巻 2 号 p. Plate1-Plate2
    発行日: 1986/04/25
    公開日: 2009/11/12
    ジャーナル フリー