||Houchiali fault is located along the eastern border of Tainan tableland, southern Taiwan. The fault extends approximately in the north-south direction with a length of 11 km. It takes an S shape and a part of it is parallel to the highway. On the ground, the fault has a clear fault scarp and the Tawan lowland is at its eastern side. Previous studies indicated that Houchiali fault was formed by the uplifting of Tainan anticline which causes a fault shifting along the side. The research used the shallow seismic reflection method to detect the Houchiali fault as well as its nearby structures. By using three long reflection seismic lines, we are able to examine this fault. The obtained fault images are extremely clear on the seismic profiles. By carefully comparing the reflection layers on each side, we find that the Houchiali fault is a normal fault. Tainan formation is covered on the Houchiali fault steadily. It seems that Houchiali fault may not activate after the deposition of Tainan formation.|
In addition to the fault detection, we also used fourteen short-range seismic lines arranged from the Tainan coast to the Kuanmiao area to investigate the background geology structure near the fault. After carefully reserving the characteristics of each reflector, we are able to correlate different layers on different seismic profiles and obtain a gross picture of structural variation along the east-west profile. The related structures from west to east are Tainan anticline, Tawan syncline and Chungchou anticline.
The research obtained the following results:
1. The fault images are extremely clear on the seismic
profiles. The reflection layers in the eastern part of the
fault are unbelievably flat, but the layers in the western
part of the fault are tilted, updipping to the west. By
carefully comparing the reflection layers on each side, we
find that the Houchiali fault is a normal fault.
2. Tainan formation is covered upon the Houchiali fault
steadily. It seems that Houchiali fault may not activate
after the deposition of Tainan formation. Because the layers
are deformed in a consistent way, Houchiali fault probably
occurred in a one-time process.
3. Tainan anticline is believed due to the mud diapiric lifting
from the Gutingkeng formation. The Erhchungchi formation
above it formed an anticline structure after mud diapiric
intrusion. The axis of the anticline is located west of that
proposed by Sun (1964). Tainan anticline has an
unsymmetrical shape, the east limb is dipper than the west
4. Houchiali fault is formed by the mud diapiric lifting
together with the plate tectonic forth. The tectonic
movement twisted the eastern limb of Tainan anticline and
trigged the Houchiali fault along its eastern boundary.
5. The subsurface structure in Tawan lowland is located at the
deeper places. The shape of Tawan syncline is not obvious,
and the Chungchou anticline is gentler than Tainan
anticline. The Liushuang formation has entered the lowland
and unconformably seated above the Erhchungchi formation.
6. The research proposed an effective seismic reflection survey
technique to detect the activity fault. The procedures
included (1) use several seismic lines (three at least) and
overlap (do it twice for one seismic line), to define the
fault images carefully, (2) use areal seismic line spread to
find the structure near the fault which may provide the
basic structural information related to the fault. These
procedures are proved useful to detect the fault as well as
to find the geology setting that may induce the fault.
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