ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

st Glacial vegetation rather than the degree of temperature
depression.
Reconstruction of the vegetation map of Japan at the Last
Glacial Maximum has been attempted by several researchers.
They yielded different results, since data and criteria
for map compilation were not identical. One of the maps
was compiled based on the distribution of existing life, as
some kinds of life closely related to vegetation types have
scarcely widened their territories since the Last Glacial
Maximum. The map demonstrates that treeless and/or
sparse forest areas broadly occurred in the lowlands of western
Japan and south Korea. These areas agree well with
distribution of fossil periglacial landforms. In addition,
these areas are thought to have low potential for tree
growth because of two reasons. First, half of the areas correspond
to the distribution of recent tree less mountains
(Hageyama in Japanese) which were human-triggered but
have been maintained owing to an unfavorable physical
settings such as slope form, geology, and climate. Second,
the other half of the areas are in the proximity of active
volcanoes, and thus were subjected to frequent fall of
tephras during the Last Glacial. These correlations validate
the applicability of the vegetation map. Moreover, most of
the treeless and/or sparse forest areas have small amounts
of snow in winter, giving weak protection against freezethaw
action. The condition of little snow also applied to
the Last Glacial, because the winter atmospheric pressure
pattern ofJapan has been basically unchanged since then.
In summary, factors including slope form, geology, climate
and frequent fall of volcanic ash worked together to lower
the land potential for tree growth in the lowlands. This
caused recession of thick forestation in spite of temperatures
high enough for tree growth, resulting in strong freezethaw
action during cold seasons. Thin snow cover in winter
also facilitated the occurrence of periglacial landforms.
The landforms are located far below the Last Glacial treeline
determined by temperature, and thus can be called
«extrazonal». Such extrazonallandforms, however, developed
in broad zones of western Japan and Korea.
FIG. 1. Distribution of Last Glacial periglacial features in the lowlands of
western Japan and south Korea. Square: erosional crest slopes, Diamond:
depositional foot slopes, Circle: block streams, Triangle: asymmetric valleys,
Reversed triangle: deposits with angular gravel. Hatched zone: Last
Glacial treeless and/or sparse forest areas (1: caused by slope form, geology
and climate unfavorable for the recovery of disturbed forest, 2: caused
by frequent fall of volcanic ash, 3: caused by low temperature).
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HIROO OHMORI
Morphotectonic evolution and relief forming processes
in Japan
Department of Geography, University of Tokyo,
Bunkyo- ku, Tokyo 113, Japan
The Japanese Islands are of a part of inter-plate island arcs
with violent crustal movement and located in a warm-humid
climatic region with heavy rainfall causing active mass
movements. Under the natural conditions, the landforms
of Japan have been quickly and continuously changed due
to the active tectonics and intense erosion. The geomorphological
processes have caused serious damage by
frequent occurrence of earthquakes, volcanisms, landslides,
debris flows and many other kinds of natural hazards.
In order to predict and prevent the hazards, contemporary
crustal movements and hazardous events have been quantitatively
surveyed especially these three decades. Digital
altitude data were published and analyzed to clarify the regional
characteristics of landforms over the Japanese
Islands. Vertical displacement during the Quaternary was
also investigated over the Japanese Islands. Most of the active
faults and folds in the Japanese Islands were precisely
surveyed to clarify their activities in connection with earthquake
prediction programme. Levelling surveys have been
also carried out at short intervals especially along the Pacific
coast of Japan. Damage caused by mass movements such
as landslides, slope avalanches and debris flows has intensively
surveyed over Japan every year since 1975. From
these observations, contemporary or recent rates, frequency
and magnitude of both tectonics and mass movements
have been evaluated. Through analyses of their spatial
and temporal characteristics together with study of
geological structures of Cenozoic, the study of Quaternary
tectonics and geomorphological processes has markedly
advanced, revealing historical complexity and regional characteristics
of morphotectonics and relief forming processes
in Japan.
The morphotectonics have been progressed in relation to
the Quaternary crustal movements characterized by the recent
plate tectonics which are caused by the relative movements
among four plates. Due to regional variation in the
stress caused by plate motion, Japan is divided into 5
rnorphotectonic terrains each of which is divided into two
sub-terrains by a great tectonic line or a volcanic front. Each
sub-terrain shows its own tectonic type and rate of faulting,
folding and warping, and its own landform features.
The resultant rates of contemporary inter-seismic and coseismic
movements revealed by analysis of precise leveling
surveys, those calculated from altitude of the raised shorelines
of late Pleistocene marine terraces and those estimated
from Quaternary vertical displacement show a good
correspondence in mode and rate to each other. Thus, it is
inferred that the landforms of Japan have been developed
by the presently progressing tectonic processes.