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Taiwan is located at the junction between the Philippine
Sea plateand the Eurasia plate, where plenty of earthquakes are
accumulated. Theearthquakes bring in numerous information of the
earth which enable usto explore the underground structure in
detail. During 1995 and 1997, a30 station seismic network (PANDA
array) was developed in the Kaoping plain, southwestern Taiwan,
which the purpose to collect the seismic dataand to study the
underground structure of the area. Active seismicity was
observed along the Chaochou fault, These earthquakes provide
ususeful data to examine the wave propagation inthe Kaoping
area. A trend of structural variation toward the southwestwas
found for the whole area. The P arrival residues are larger in
thesouthwestern part, which means thicker sediments there. The
Sp to S arrivaltime difference also indicates the same trend
with the larger values towardthe Pingtung coast, Tainan, and
Kaohsiung areas, Qp and Qs distribution shows large variation,
however, a low Q pattern in the southwestern part of Pingtung
plain and western Tainan and Kaohsiung areas is still obvious.A
coda Q study and high frequency attenuation also reveal a
similar trendof variation toward the southwest and the west.
After matching the seismicphases by the 2D ray tracing, we built
up the underground structure alongseveral profiles across the
area. Westward tilt layers are included in themodel which even
has a 0.5-0.6 km/sec velocity lower in the western partthan in
the eastern part. A conclusion can be drawn that the
structure under the Kaoping area,southwestern Taiwan, tilts
toward the southwest. One possible explanationto this
observation is that the tectonic movement has uplifted the
easternside and makes the area slant westward. The structure
block on the east ofChaochou fault may thrust upward due to
Philippine Sea plate insertion,which can be identified by large
earthquake activities. The pushing forcetwists the South China
Sea plate under the Kaoping area and makes its structure tilt.
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