Damage to ShihKang Dam Inflicted by Faulting in the Sept. 1999 Chi ...

Fig. 11
Chelungpu fault
Figure 1 Tectonic Structure around Taiwan
(after Seno, 1999)
Figure 3 Faults in Taiwan
(Central Geological Survey, 1998)
Figure 2 Plate tectonic of Taiwan
(after Seno, 1999)
longitudinal north-south direction. The island is
located right upon the wedge of the Philippine Sea
(PS) plate, the wedge cutting deep into the Eurasian
(EU) plate. The northern side of the wedge subducts
along the Ryukyu Trench beneath the EU plate,
while the southwestern side of the wedge overrides
the EU plate (Fig. 1). The collision of the two plates
has led to the formation of the backbone ridge of the
island exceeding 3000m elevation with a number of
faults. The activated fault runs along the western
edge of the accretionary prism accumulated along
the subduction zone boundary (decollement) as
illustrated in Figure 2 (after T. Seno, 1999). The
earthquake therefore may be categorized as a
subduction zone earthquake. Figure 3 is taken from
“An Introduction to the Active Faults of Taiwan”,
Central Geological Survey (CGS), Ministry of
Economic Affairs, Taiwan. The CGS has confirmed
that 51 active and/or suspicious faults longer than 5
km exist, and are classified as follows:
(1) 1 st category active faults (9):
a) activated at least once in the past 10,000 years,
b) having caused some damage to existing
structures,
c) responsible for earthquake occurrences,
d) having deformed alluvial soil deposits, and
e) whose presences are clearly recognized from
surface geological configurations.
(2) 2 nd category active faults (15):
a) activated at least once in the past 10,000 years,
and
b) having deformed diluvial terraces.
(3) Suspects (27):
a) with ambiguous features in Quaternary
configurations,
b) with ambiguous features in laterite soils, and
c) looking alike but without any clear evidences.
The earthquake, with a magnitude of 7.3 (Central
Weather Bureau, CWB) took place at 1:47 AM,
local time on 21, Sept., 1999. The epicenter was
located at latitude 23.85°N and longitude 120.81°E.
Figure 4 shows the main shock and aftershocks.
The earthquake that originated at a shallow focal
depth of 6.99 km produced spectacular reverse
faulting. The activated fault seems to have appeared
exactly along the Chelungpu Fault, whose presence
had been already recognized and classified into the
2 nd category of active faults. Hence, the activated
fault, in this chapter hereafter, will be referred to as
the Chelungpu Fault. The activated fault, however,
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