Thanks to the destructive earthquakes of 1906, 1941, 1946, 1994 and 1999, we have obtained an up-to-date movement picture to understand the seismotectonics of the Tainan basin more fully. This picture is simply a new version of the surface trace of the dextrally mobile Ichu fault, which is of unparalleled importance in the present-day tectonization of southwestern Taiwan. We have learned from the picture that the well-known Meishan fault and Hsinhwa fault are not independent seismogenic fault themselves but particularly active segment of the Ichu fault.The source location of the 16 September 1994 Taiwan Strait earthquake is near the western boundary of Tainan basin. The source mechanism of this event consistently shows a pure normal faulting with striking approximately E-W. It indicated that normal faulting processes induced from the shear of dextral strike-slip faults are active in the historically subsiding Tainan basin. Also found in the same picture is a fault-framed earthquake swarm in which the epicenter of the main shock and many aftershocks of the 1941 and 1999 event were concentrated. Plus, this swarm of the parallelogram shape is structurally a typical contractional strike-slip duplex freshly built up at a double restraining bend of the Ichu fault. All of the above and some other details support our longtime believe that the Ichu fault has been the powerful tectonic motor responsible for the sideways crustal movements in, and the resultant high seismicity of, southwestern Taiwan.
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