VDAverage
Horizontal Crustal Strain Rates in Japan Period
Deduced from Geodetic Surveys during Interseismic
(Part 2) NORIHIKO ISHIKAWA
Geographical Survey Institute, Kitasato 1, Tsukuba,
Ibaraki 305-0811, Japan
MANABU HASHIMOTO
Research Center for Earthquake Prediction,
Disaster Prevention Research Institute, Kyoto
University,
Gokasho. Uji, Kyoto 611-0011, Japan
Journal of Seismological Society of Japan,
Vol.52, P.299-315, 1999
This is a revision of Hashimoto's (1990)
study on average horizontal crustal strain
rates
in Japan derived from geodetic data collected
during
the past 100 years. This new study includes
Hokkaido since the completion of the latest
nationwide
trilateration enables us to apply the methods
of the previous study and obtain strain rates
there. In this method, side lengths of triangulation
networks are calculated using adjusted coordinates
for different epochs and then the rate of change
of each side length is estimated by regression.
Finally the principal strain rate in each triangular
region is obtained. In order to avoid the effects
of large earthquakes or volcanic eruptions,
data before or after these events are simply
discarded.
Unfortunately, due to frequent occurrence of
large events in northern Japan, we can use
data from only two epochs for most of Hokkaido,
which
causes large uncertainties in the estimated
strain rates. Hokkaido appears to be divided
into four
provinces according to the characteristics
of the horizontal strain rates, although the
uncertainties
are too large to be definitive. Eastern Hokkaido
is characterized by large WNW-ESE contraction.
In northern Hokkaido E-W contraction is prevailing,
while extension is dominant in southwestern
Hokkaido. In other regions the directions of
principal
axes of strain rates are not noticeably different
from the of the previous study. In the Tohoku
region, tensile strain rates in a N-S direction
are prevailing. A NNW-SSE or NW-SE trending
contraction is dominant along the coast from
southern Kanto
to Shikoku, though it is smaller in the Tokai
area and Kii peninsula than in southern Kanto
and Shikoku In the same Shikoku region, there
is a remarkable region of NE-SW extension that
is as large as contraction. In central Japan,
there is a NE-SW trending region of contraction.
An E-W oriented contraction is dominant west
or northwest of this region, such as in the
Kinki and eastern Chugoku districts, while
a NW-SE
trending contraction is prevailing in the southeast.
Kyushu is under an extensional regime with
tensional directions of N-S and NW-SE in central
and southern
Kyushu, respectively. Magnitude of strain rates
obtained in this study ranges from 1 to 3 x
10^-7/yr in most regions except in the Shikoku,
southern
Kanto and Fukui areas where it reaches 6 x
10^-7/yr. Over all, magnitude of strain rates
are as large as or slightly larger than those
derived from continuous GPS observations, but
are larger than
geological or seismological strain rates by
a factor of 10.
Key words: Geodetic survey,
Triangulation, Trilateration, Horizontal
strain rates, Interseismic deformation.
_ΆκΦίι
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