Marine Ecosystems Research Department - jamstec japan agency ...

Japan Marine Science and Technology Center
Institute for Frontier Research on Earth Evolution (IFREE)
evolution after shear failure. The permeability of the
basalt is estimated to range from - to - m under
the environmental conditions corresponding to the
seismogenic zone. The permeability showed a strong
reduction with increasing effective confining pressure
and temperature. Following shear failure of the basalt,
rapid sealing at elevated temperatures was observed
during hold experiments: a three orders of magnitude
decrease in permeability after about hours holding.
This result indicates that the permeability of the
subduction megathrust fault would rapidly reduce
due to the precipitation of clay-like minerals and
other minerals, and shows the potential of high fluid
pressure in the fault zone. Further, the maximum
slip-weakening rate of the basalt during the shear
failure process has nearly the same value as that of
granite in the brittle regime, which suggests the
possibility that unstable slip occurred along the fault.
() Shallow portion of a splay fault
The Chi-Chi, Taiwan, earthquake (Mw.)
Shear Resistance (MPa)
0.15
0.1
0.05
Normal Stress: 0.15 MPa
Peak Slip Velocity: 2 m/s
Porosity = 47 %
0.15
0.05
0
0
0 1 2 3 4 5 6 7
Time (sec)
Shear Resistance
Pore Pressure
Fig.23 Result of high-velocity ring shear experiment on simulated
fault gouge showing fluidization during slipping.
0.1
Pore Pressure (MPa)
produced a spectacular surface rupture along the east
dipping Chelungpu thrust fault and provided new
important near-field strong motion data. The analysis
of the possible rupture zone and a high-velocity ring
shear experiment using simulated material were performed
to clarify what dynamic processes in the shallow
portion of the splay fault control the large slip and
slip velocity with a low level of high-frequency seismic
radiation. The results clearly indicate that fluidization
occurred during co-seismic slip (Fig.).
When the undrained condition was maintained during
the earthquake rupture process, the fault composed of
loosely packed, granular material can lose frictional
resistance due to fluidization and enhance rupture
propagation even in a stable frictional slip regime.
() Deep portion of subduction zone
In order to understand fluid flow processes at the
transition between the seismogenic zone and the creep
zone, sealed cracks developed in the past plate boundary
rocks of the Shimanto accretionary prism and
Sambagawa metamorphic rocks were studied in detail.
Three types of sealed cracks were classified for the
Sambagawa metamorphic rocks in terms of geometry,
distribution patterns, and spacio-temporal relationships
between the host rocks. Mineral composition
and microstructures suggests that each type corresponds
with the tectonic stages of subduction, underplating
and exhumation. Regional differences that are
probably due to the opening interval and frequency or
fluid flux were inferred from the relationship between
the length and width of the same sealed crack types in
the Kanto mountain and Central Shikoku.
Sealed cracks in the Shimanto accretionary prism
revealed that two opening directions, trench-parallel
and trench-vertical, are common for the plate boundary
cracks. Stress fields suggested by the structural
relationships show vertical maximum stress and
a horizontal conversion between minimum and intermediate
stress.
A precise inverse method was used to evaluate the
temperature-pressure-fluid flux path. Results show
that P-T conditions during subduction are nearly equal
to that of exhumation, and that each path is composed
of substatic a low-P segment and a rapid high-P segment.
High fluid flux and deformation correspond
with the latter segment.
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