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Kuo-Fong Ma@Ž
(Institute of Geophysics, National Central University, Taiwan, ROC)

2006.03.08

ƒ^ƒCƒgƒ‹FSlip Zone and Energetic of a large earthquake
@@@@@ from seismologicalmodeling and fault core of TCDP

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Taiwan Chelungpu-Fault Drilling Project (TCDP) drilled two holes 40 m
apart (a continuous coring of 500m to 2000m in hole-A, and 950m to
1300 m in hole-B) into the northern portion of the recently ruptured
large slip zone of the Chelungpu fault. The principal slip zone, PSZ,
was identified at the depths of 1111.23 to 1111.35 m in hole-A, and a
corresponding feature was identified in hole-B and side-track at the
the corresponding depths, consistent with geological observations at
the surface. The vertical extent of the damage zone relative to the
fault core is asymmetric, with a greater thickness above the fault
core. The PSZ is about 12 cm thick and located near the lower
boundary of the damage zone and contains subsidiary cataclastic
fracturing. A 2-cm less deformed layer was identified within the PSZ,
located in the bottom of the fracture zone. This zone is considered
to be the slip zone associated with the Chi-Chi earthquake. The
geophysical logging showed the measurements of low seismic velocities
and low electrical resistivity in the vicinity of the identified PSZ.
Along with the orientation of observed fractures, the geophysical
features support that the deformed zone at 1111m is the primary fault
zone.
The actual grain sizes from the gouge layer of the core were measured
to obtain a grain-size distribution for further estimation of surface
fracture energy. The seismic fracture energy calculated from the
dense strong motion waveforms for the fault block beneath the drill
site is 11.6 MJ/m2. From the comparison, it shows that the
contribution of the seismic fracture energy to the formation of fault
gouge is less than 45%. It yields the radiated efficiency of 0.65,
which is intermediately compared with the values from Punch bowl
fault and South African mine. Comparing to other observations in
various faults, our results suggest that the amount of energy needed
to form a fault zone probably depends on the maturity and style of
faulting.