Northeastern Kyushu Research Articles

大分県中部，由布・鶴見火山群の地質と岩石

Yufu-Tsurumi volcano group of northeastern Kyushu yields very porphyritic andesite lavas. These lavas show interesting disequilibrium mineral assemblages and contain characteristic dark inclusions. Analyses of phenocryst minerals revealed that basic mineral group have similar compositions of those of dark inclusions. The volcano group consists of three closely associated volcanoes, which we define as Takahira group, Tsurumi group and Yufu group. Volcanic activity of Takahira group was older than those of Tsurumi and Yufu groups. Volcanic activities of both Tsurumi and Yufu groups continued until some time after 6, 000 y. B. P. Pyroclastic flow deposits dominated older volcanic products, whereas lava flows and lava domes dominated younger products. Hornblende andesite to dacite are abundant among the volcanic rocks of Yufu-Tsurumi volcano group. Andesites contain phenocrysts of plagioclase (An>80 and An<50), clinopyroxene (Mg/(Mg+Fe)=86-71), orthopyroxene (Mg/(Mg+Fe)<70), amphiboles (pargasite and magnesio-hornblende) and opaque minerals with rare olivine, biotite and quartz. In contrast, dacites contain phenocrysts of Mg-poor orthopyroxene, magnesio-hornblende and An-poor plagioclase. Dark inclusions are nearly equigranular and consist of Mg-rich clinopyroxene, pargasite and An-rich plagioclase, with ± olivine, ± orthopyroxene and ± quartz. Some inclusions have chilled margin at the circumference, which implies that they were incorporated into the host magma as liquid state. The analyzed samples from Yufu-Tsurumi volcano group have a wide SiO2 range of 56-65 wt. %, but a nearly constant FeO*/MgO ratio. Thus they show a typical calc-alkaline character. The SiO2 content of the dark inclusions ranges from 48 to 55 wt. %. In the variation diagrams, these inclusions and other volcanic rocks form a single trend with the inclusions being the basic end. Both chemistry and mineralogy of these lavas changes systematically with time. Lavas became more basic toward the younger stage. Modal composition of clinopyroxene increases, and the percentage of dark inclusions becomes progressively greater, as the volcanic products became younger.

Geomorphic Development of the Yukuhashi Alluvial Plain, Northeastern Kyushu

The Yukuhashi Plain, about 13km and 10km across from east to west and north to south, respectively, is located at the northeastern part of Kyushu and faces western Seto Inland Sea. The plain is surrounded by the mountains composed of Paleozoic limestone, metamorphics, Mesozoic granites and Quaternary volcanics except in its eastern margin. The plain is divided into two parts in consideration of its geomorphic features. The northern part of the plain consists of alluvial lowlands along the Obase and the Nagao Rivers flowing from west to east. The southern part of the plain is characterized by alluvial fans formed by the Harai and the Ima Rivers flowing south to north.The sequence of Recent deposits along the shoreline is shown in Fig. 4. The thickness of the deposits exceeds 16.5m at the hurried valley. From their textural characteristics, the deposits are divided into five layers as follows:1) Lowermost Sand and Gravel layer mainly consists of rounded gravels which are 0.5-4cm in diameter. Along the Harai rever, the layer is distributed to near surface.2) Lower Sand layer containing marine shells is considered as foreset bed of the deltaic sediment. At the lower part of the layer is dated 7, 710±210 14C years B. P. Marine diatom species are prominent at the upper part of the layer.3) Middle Mud layer containing marine shells (e.g. Tegillarca granosa) and humic materials is made up of silt or sandy silt. The layer also contains a lot of marine and brackish diatom species (e. g. Cyclotella striata, Melosira sulcata etc.), therefore the layer is considered to have been formed as bottomset bed sediments. At the period of the Middle Mud deposition, Postglacial transgression was reached at its maximum stage, and the inner bay widely spread in the plain. The period is estimated around 4, 800 years B. P.4) Upper Sand layer holds shell-fragments. This layer is considered to indicate the sedimentary environment of delta front. After the deposition of Middle Mud layer, deltaic sedimentation has been continuing.5) Uppermost Mud layer consists of organic silt deposited at backmarsh or lagoonal environment.In the plain, marine and brackish diatom species are appeared up to 1.8m above sea level. It denotes the highest sea level of the Postglacial transgression in the Yukuhashi Plain, but its age is unknown. The present high tide which exceeds 2m above mean sea level in this area may suggest that the present sea level is the highest during the Postglacial transgression.

Source mechanisms of subcrustal and upper mantle earthquakes around the northeastern Kyushu region, southwestern Japan, and their tectonic implications.

The source mechanisms of subcrustal and upper mantle earthquakes with magnitudes from 6.0 to 6.8 that occurred around the northeastern Kyushu region have been closely investigated to clarify its tectonic features in relation to the subducting Philippine Sea plate with a laterally bending configuration.Two subcrustal earthquakes that occurred in Suonada and the Bungo channel, which are located close to the leading edge of the subducting Philippine Sea plate, show the mechanism of normal faulting type. These events may have been generated by lateral bending of the oceanic plate. The Kunisaki peninsula earthquake of August 26, 1983 (M=6.8, h=116 km), which was the largest upper-mantle earthquake in this region, shows a reverse fault type mechanism. Comparisons between the observed and synthetic seismograms suggest that the southeastward dipping nodal plane may be the fault plane, and that the rupture propagated northwest-upwards. The seismic moment was estimated to be MO=-1.13×1026 dyn·cm. The focal mechanism of three upper mantle earthquakes inland of Kyushu, including the Kunisaki peninsula earthquake, suggests that they may have been caused under the stress regime of down-dip extension. The tensional stress working southwest-downwards at intermediate depths in the Suonada and inland Kyushu regions may be explained by the gravitational pull acting on the Philippine Sea plate subducting deeper southwestwards.

Distribution of C. V. of Rice Yield per TAN in Japan

In 1953, Section of Statistical Researches in the Ministry of Agriculture and Forestry published the account tables of annual variation of paddy rice & wheat yield per TAN of cities, towns and villages in Japan, which contain the mean yield per TAN, standard deviation and the C. C. V. (coefficient of variation.) of rice and wheat. Those were accounted by the statistics from 1926 to 1940. Based on these tables the writer has drawn up a map of distribution of C. V. of paddy rice yield per TAN (Fig. 1). The map points out the stability and the unstability of agricultural production in Japan, for daddy rice is the most important in every prefecture. Eastern Japan in comparison with western Japan, has a high unstability, almost all parts have the C. V. of 20 or more. Western Japan has a high stability, and its C. V. is less than 20 in most of the region. Centers of the unstable areas in eastern Japan are on the mountain-lands, plateaus of Kitakami and Abukuma, mountains of the central chain of Ou, and some mountains of Chubu. Generally, eastern Ou, which lies to the east of the central chain of Ou, is much less stable, where the low land facing the sea without shelter to the north-eastern or eastern cold wind, e. g. the coastal plain of Sanbongi-hara, and marginal Jow land of Soma, are also unstable. But western Ou, the western side of the central chain of Ou, is comparatively stable. Especially, basins and plains which lie on the western side of the mountains and plateaus are stable. The cold wind which blows from the east or north-east in summer brings a poor harvest in these regions. That wind is named YAMASE in the nothern part of eastern Ou, and disfavoured by the farmers. It is an interesting fact that the centers of the unstable areas in Ou correspond to the grazing regions. In western Japan, mountain-lands are rather stable, the plateau of Chugoku, is very stable. And some low lands are unstable. Especially in the so-called Setouchi, which includes the low lands of Sanyo, the islands in the inland-sea of Seto, north-eastern Kyushu, and northern Shikoku. In that region poor harvests by low temperature seldom occur, but droughts can cause them there. Some regions suffer from floods by typhoons. In north-western Kyushu, Hizen Peninsula, Iki Island Tsushima etc. are unstable. The farther westward the region is from the district of Kinki, and the farther northward it is from the same district, the greater is its C. V. accounted for each prefecture. It seems to me that the nearer it is to the dry limit of the rice culture region on south-eastern Asia, the more frequent and severe damages of drought occur, while the nearer to the poler limit of it, the more frequent and severe damages of low temperature occur in summer.