Journal of Geography (Chigaku Zasshi)
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
Volume 125 , Issue 5
Showing 1-7 articles out of 7 articles from the selected issue
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Review Articles
  • Hiroyuki TANAKA
    2016 Volume 125 Issue 5 Pages 647-659
    Published: October 25, 2016
    Released: November 18, 2016
    JOURNALS FREE ACCESS
     Atmospheric and anti-electron neutrinos generated inside the solid Earth (geoneutrinos) are potentially powerful tools for imaging the Earth's interior, in order to visualize the spatial distribution of the density of uranium and thorium concentrations. This review is limited to neutrino imaging techniques. Observations of atmospheric neutrinos and geoneutrinos have been reviewed previously and are not discussed here. An elementary introduction to neutrino generation on the Earth and propagation through matter opens the review. After reviewing neutrino tracking methods in the context of today's views of technological developments, the current experimental limits on neutrino imaging are presented. A technique to confront the standard Earth model is discussed in the conclusion. Neutrino imaging of the Earth has been pursued at IceCube. It is fair to mention that it has opened the possibilities of this new elementary particle technique for the first time.
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  • Tetsuya KOMATSU
    2016 Volume 125 Issue 5 Pages 661-698
    Published: October 25, 2016
    Released: November 18, 2016
    JOURNALS FREE ACCESS
     The Pamir is a mountain region in the westernmost part of the Himalayan–Tibetan orogen. It extends 〜300 km from north to south and 〜300–400 km from west to east. The Pamir lies on a double subduction zone, where two buoyant continental plates have subducted several hundred kilometers deep into the asthenosphere. The southern plate is the northward-dipping Hindu Kush slab, and the northern plate is the southward-dipping Pamir slab. The subduction of the Hindu Kush slab began at 〜8 Ma. This narrow part of the Indian plate subducts to a depth of 〜600 km and has a neck at a depth of between 250 and 300 km, causing lithospheric thinning. The Pamir slab, a part of the Asian plate, probably started subduction at 〜25 Ma accompanied by slab rollback (possibly with the back-arc extension), subduction erosion, subduction accretion, and marginal slab-tear faulting. This slab, subducting to depths of 〜400–450 km, forms a broad arcuate shape split at the center due to a vertical tear extending from a depth of 〜200 to 〜400 km. The current geometry of both slabs implies the possible occurrence of tectonic events, such as slab break-off, in the near geological future. The metamorphic and exhumation history of the crystalline basement domes in the Pamir reveal regional tectonic evolution since the Cenozoic India–Asia collision. Prograde metamorphism of the mid–lower crust, driven by crustal shortening/thickening, continued from 45 to 25 Ma. Subsequently, retrograde metamorphism and exhumation of the mid–lower crust began during the period 25–15 Ma. This change suggests a transition from crustal shortening/thickening to crustal extension. A possible cause of this transition is that the Pamir slab obtained sufficient gravitational potential to enable the gravitational collapse of the crust, mainly due to the isostatic uplift of the orogen, which was triggered by the break-off of the Indian plate during 25–15 Ma. Subsequently, activation of extensional exhumation in the southwest and eastern Pamir was synchronous with magmatism in the eastern Pamir at 〜10 Ma. At present, the Pamir region is characterized by an active E–W extension of the Kongur Shan footwall to the east and a westward lateral extension of the western Pamir into the Tajik–Afghan Basin.
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Original Articles
  • Naoko NAGUMO, Hisaya SAWANO
    2016 Volume 125 Issue 5 Pages 699-716
    Published: October 25, 2016
    Released: November 18, 2016
    JOURNALS FREE ACCESS
     The Pampanga River basin, which is the second largest drainage basin on Luzon Island (Republic of the Philippines), frequently suffers from severe flood events, caused by monsoon rainfall and typhoon strikes. Therefore, the aim of this study is to determine local flood characteristics and potential flood vulnerability, based on the basin's geography (e.g., distribution of topography, land use and past flood records). Our land classification shows that the basin consists of three major topographic regions: mountain and hill, volcano, and alluvial plain. The mountain and hill region is further divided into topographic units of mountain and hill, and volcano region is subdivided into volcanic slope, volcanic piedmont gentle slope, and volcanic fan. On the other hand, the alluvial plain is divided into fan, terrace, back marsh, swamp, delta, valley plain, natural levee, meander scroll, and former channel. In the upper alluvial plain, the supply of sediments, triggered by the Philippine Fault activities, contributes to a southwestern fan I, fan II and terrace II development on the western side of the Pampanga River. Terrace I and terrace II on the other hand, develop in western direction on the eastern side of the Pampanga River. Owing to the mountains, hills, and volcanoes that surround the alluvial plain, its width is reduced to 20 km at Arayat. As a consequence, floodwaters easily concentrate and stagnate here, creating the two swamps of San Antonio and Candaba. In the lower portion of the alluvial plain, low gradient bed slopes, land subsidence, and tidal intrusion of sea water enhance poor drainage situations. As a result, floodwaters from Candaba Swamp cannot drain efficiently, causing severe floods. The alluvial plain was divided into four zones (I–IV), based on the flood patterns that were identified by the geographical conditions of the basin. Floods in zone I and zone II located on the western and eastern side of the Pampanga River are smoothly drained. On the other hand, people in the most downstream flood-prone area (zone III and zone IV) receive much benefit from the cyclic floods by stimulating the agricultural and fishery production, but the deep inundation also damages their houses. Nonetheless, some coexist with the floods seemingly without fear, and did not even evacuate during the largest recent flood event of 2011. The population of the basin is still increasing, and flood risks should be reduced through government-initiated actions. Furthermore, in order for the region's inhabitants to take effective measures to fight flooding and sustain flood-adaptive lifestyles, they should be required to understand the local geographical characteristics and regional flood vulnerability.
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  • Hiroki NAKAHATA, Yukio ISOZAKI, Yukiyasu TSUTSUMI
    2016 Volume 125 Issue 5 Pages 717-745
    Published: October 25, 2016
    Released: November 18, 2016
    JOURNALS FREE ACCESS
     To reconstruct the detailed paleogeographic configuration of the Cretaceous arc-trench system in East Asia, shallow marine sandstones from the Ryoke, Sanbagawa, and Chichibu belts in western Shikoku are investigated with age spectra by U–Pb dating detrital zircons with LA-ICPMS. The mid-Cretaceous Shuki and Nigyu formations, unconformably covering the pre-Cretaceous accretionary complex of the Chichibu belt, contain abundant detrital zircons from the Jurassic to Early Cretaceous ages, with small quantities of Permian and Triassic detrital zircons. These age spectra are almost identical to those previously obtained from other coeval formations elsewhere in the Chichibu belt (the Monobegawa Group), which represent the Cretaceous fore-arc setting. The common age spectra suggest that the provenance of the Cretaceous fore-arc domain had ubiquitous compositions of rocks for nearly 1,000 km along the arc, and that Jurassic to Early Cretaceous granitoids were predominant, with associated small quantities of older pre-Jurassic granitoids. From the Ryoke belt, the Campanian (Late Cretaceous) Yamanouchi Formation of the Izumi Group, unconformably covering the mid-Cretaceous Ryoke granitoids, also has a similar age spectrum. This confirms that the provenance of the fore-arc remained more or less the same at least until the Late Cretaceous. The most intriguing age spectrum was obtained from the Maana Formation, which occurs as a klippe sitting on top of the southern Sanbagawa belt. As well as other dated sandstones, Maana sandstone contains Early Cretaceous to Jurassic zircons; nonetheless it accompanies not only Permo-Triassic zircons but also abundant Paleoproterozoic (2400–1600 Ma) grains. This unique age spectrum is correlative solely with those from the Tetori/Jinzu groups in the Hida belt, whose depositional setting has featured provenance with the Precambrian basement. This suggests that the Maana Fm was primarily deposited at the continent side of all the Paleozoic accretionary complexes and their high-pressure metamorphosed equivalents of SW Japan; i.e., the back-arc side of the Cretaceous arc-trench system in East Asia. As to the age spectrum of detrital zircons, in addition to lithofacies, deformation style, and occurrence as klippe, Maana Fm is almost identical to the Atogura Fm located in the northern Kanto Mtn., ca. 800 km to the east. These klippe-forming Cretaceous strata were originally located at the continent-side of the Cretaceous arc, and they were moved toward the ocean side by Cenozoic tectonics, possibly related to the tectonics of the Miocene Japan Sea (back-arc) opening. The present study succeeds first in estimating the displacement of crustal rocks, i.e., over 200 km in the across-arc direction, contemporaneous with the inferred Cenozoic tectonics.
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  • Kiyoyuki SHIGENO, Futoshi NANAYAMA
    2016 Volume 125 Issue 5 Pages 747-762
    Published: October 25, 2016
    Released: November 18, 2016
    JOURNALS FREE ACCESS
     Features and depositional processes are revealed of sandy event deposits (SED) caused by a storm surge and high waves during the 1959 Miyakojima typhoon around the Hirahama coastal lowland, along the western coast of the Oshima Peninsula, southwestern Hokkaido. Three new trenches were excavated in the lowland to study sedimentary features and grain size. Sedimentary features imply that the 1959 SED was deposited from an unidirectional run-up flow. The deposits can be subdivided into three units: T, S, and F in ascending order. Unit T shows 3D dunes. Unit S shows bedform transition from 2D dunes to ripples. Unit D consists of a mud layer including suspended plants and pieces of wood. Grain-size analysis shows that Units T and S peak at around 2.0 Phi (P-2 population), which is the same as beach sand from the Hirahama Coast, and the wide grain-size distribution is over 0–4 Phi due to mixing with the fluvial bed of Yumiyama River (P-1 and P-3 populations). According to Dmax, Unit T shows coarsening upward from -0.25 to 0.25. On the other hand, Unit S shows finning upward to 0.75 from 0.25. Therefore, Unit T recorded the amplification process of the storm surge and high wave energy due to the typhoon after 09:00 on September 18. Unit S recorded the decay process of high waves and storm surge energy associated with the movement of the typhoon from 13:00 to 14:00 or later. After the period 00:00 to 01:00 on September 19, suspended solids and wood fragments in stagnant water covered Unit S, then deposited Unit D because the typhoon had passed.
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Letter
  • Hidetsugu YOSHIDA, Tomoki KASAHARA
    2016 Volume 125 Issue 5 Pages 763-773
    Published: October 25, 2016
    Released: November 18, 2016
    JOURNALS FREE ACCESS
     There are two geomorphic surfaces (terraces) in the northwestern Kanto Plain: Takasaki Upland (Takasaki Surface) and Inokawa Lowland (Ino Surface). It is considered that about 11,000 years ago the Inokawa Mudflow traveled down from the west, forming the Takasaki Upland as a depositional surface. However, opinions are divided and another theory is that the Inokawa Lowland is also the depositional surface of the Inokawa Mudflow. Therefore, this study examines the subsurface geology around the Inokawa Lowland, based on an expanded dataset of existing borehole data obtained from our portable hand-operated drilling survey. The results clearly show that the Inokawa Lowland is basically a depositional surface of the Inokawa Mudflow. Reflecting upon this conclusion, how the terrace scarp between the two surfaces was formed becomes problematic. There are two possible causes to be clarified by future investigations:
     (1) Mantle bedding of the Inokawa Mudflow on the pre-existing fluvial scarp,
     (2) As-yet-unrecognized vertical faulting after full deposition of Inokawa Mudflow deposits in this region, probably related to the Fukaya fault system.
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