Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Institute for Frontier Research on Earth Evolution (IFREE)
database, we conducted geological surveys and sampled
localities where magnetic dipole anomalies are
observed along the Chichibu Tectonic Zone (CTZ),
Shikoku Island, since , in order to clarify the formation
and evolution of serpentine diapirs. The distribution
of serpentines and ultramafic greenstones recognized
along the CTZ from Shikoku to the Kanto
area, is consistent with both magnetic and Bouguer
gravity anomalies. In , paleomagnetic measurements
were made of rock samples. The experiment
on such a large number of specimens will be the
first trial for serpentine research in the world.
The resulting paleomagnetic data show strong
anisotropy in rocks sampled in eastern Shikoku, where
large total magnetic intensity anomalies are characteristic.
A more than three times directional difference in
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(c)
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34
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(a) Geological map of SW Japan
(b) Bouguer anomaly map
mgal
292
185
146
119
95
73
51
27
0
-38
-58
nT
100
21
0
-17
-31
-43
-56
-70
-86
-109
the magnetic susceptibility was observed in some
specimens. Secondary magnetic minerals were also
recognized in thin sections of these rocks. By contrast,
serpentines sampled in central Shikoku are characterized
by small total magnetic intensity anomalies.
Ilmenite was clearly observed in the fine grained thin
sections there. Thus, the grade of serpentinization
differs in the eastern and central parts of the CTZ
in Shikoku, possibly suggesting progressive serpentinization
(Fig.).
4.2. Seismic Data Analysis
Prior to input to the database, the Seismic Data
Processing Division conducts data processing of
multi-channel seismic (MCS) reflection data obtained
by IFREE's cruises, and also developed a new
technique necessary for sophisticated processing. In
, we tried to process seismic data obtained by a
two-ship survey and applied a Median filter, which is
seldom used in MCS data processing but is sometimes
applied to VSP (Vertical Seismic Profiling) data. The
advantage of the two-ship survey is an increase in the
accuracy of the velocity analysis and in the Signal-to-
Noise ratio (S/N) in deep structural imaging, both of
which are the result of an increase in the sourcereceiver
distance. On the other hand, there is a serious
disadvantage in that multiples strongly interfere with
primary reflections, causing a deterioration in the data
quality. Application of the Median filter worked
effectively and increased the accuracy of the two-ship
MCS data (Fig.a and b). As a result, it is suggested
that poisson's ratio may be estimated by AVO
(Amplitude Variation with Offset) analysis (Fig.c).
132 133 134 135 136 137 138
(c) Magnetic anomaly map
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Fig.34 Regional geologic, gravity, and magnetic maps of SW
Japan. The top figure is modified from GSJ [1995], and the
middle and bottom figures are from GSJ [1996].
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