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The purpose of this study is to detect some active faults in western Taiwan using the newly developed shallow seismic reflection method. Most of these faults are in urban areas or close to the populous regions, hence, it is important with respect to earthquake hazard mitigation. The position, fracture attitude, faulting mechanism and even activity of the fault are under investigation. The revealed fault images from some seismic sections are so sharp that many details can be measured. It is found that every fault has its own characteristics. They all behave differently. The following conclusions are obtained: (1) Hsinchuang fault (Taipei basin)is obviously a normal fault with a vertical displacement of 50m within the 150m distance range. The fault may have slid down the basement rock in a sequence of stages. The seismic section obtained here reveals only a part of it. Many small back-thrust faults are found on the hanging wall, which has cut through the young sediment. This fault could be potentially active. (2) Hsintien fault (Taipei basin) is an old thrust fault with a large offset on Tertiary rocks. Obvious reflections are obtained on the hanging wall side, but not on the other. This is due to lithological variations across the fault. The apparent difference in reflection signals can be easily used to delineate the fault. (3) Shihtan fault (1935 earthquake) is an interesting fault. The breakage on the surface is attributed to the effect of squeezing on the vertically dipping unconsolidated sandstone layer (Shangfuchi Sandstone) by the compressional pressure which induced the earthquake. This coarse-grained sandstone 'flowed' out, protruded the overlaying alluvial deposit, and caused the surface rupture. The fault resulted in a reverse type. (4) Tunzuchiao fault (1935 earthquake) shows a complicated sense of movement along its fault during the earthquake. A twisted near-surface curved layer caused by the earthquake is used to explain this fault. The rupture happened at a place where the layer had large curvature. The position and the direction of rupture are random within a narrow zone surrounding the fault depending on where the laver is distorted most. (5) Hsinhua fault (1946 earthquake) has developing fracture type. Two fracture zones were found, which are separated more toward the east. A large disturbance occurred within a 60m range where the two fracture zones came together in the west. The reflection signals in this 60m zone were totally lost due to elimination of layering by mass mixture on the fault. Outside the fault zone, however, the layer is quite flat and free of disturbance.

The Detection of Active Faults on Taiwan Using Shallow Reflection Seismics