The collision between the Eurasian Continental Plate and the Pacific Plate along the eastern margin of Asian Continent produced a series of apron-frill-shaped island-arcs. The Taiwan Island-arc is only one of them. Its activity follows the general regularities of the plate motion but has regional characteristics. The Taiwan Islandarc is different from its adjacent Ryukyu and Luzon island-arcs and has transformlike fault occurring in it.

Fujian, Taiwan Strait, and Taiwan Island proper, located in a zone west of the Huadong Longitudinal Valley,are a part of the Eurasian continental crust and joined up as a whole body,dipping slightly to ward east at an angle of 29°～30°. The eastward flow of the asthenosphere led the body to strongly collide with the Philippine Sea Plate at the Huadong Longitudinal Valley and caused the Mesozoic clastic sediments and volcanic rocks in the upper continental crust to be strongly folded. They have westward overthrust and uplifted on the base of Euarasian Plate and hence a double-layer structure of Jurassic type was formed. The activity age of this series of NNE-trending westward overthrust faults is younger from east to west . ,

The western Taiwan Plain and the Taiwan strait represent tht Pliocene-Quaternary foreland basins in the arc-continent collision zone. In the basins the Quaternary deep-water fine-grained sediments were accumulated and the deposits are evenly distributed. The lower part of these deposits unconformably overlie the Pliocene deposits,in which a series of mainly N-S-trending stepped faults is developed. It reflects a trend of weakening tectonic movement in the zone since the Pliocene.

Under the control of the described above general tectonic conditions, the seismicity in the strait is in some concert with that on the Taiwan Island proper. But crustal stress is largely released in the high seismic zone on the western margin of the Central Mountain Range, which has screen and prevention effects on the straits area. However,the zones in outer sea areas off both ends of the Taiwan Strait and Taiwan Island proper controlled by NW-trending transform-like faults are prone to strong earthquakes when the NE-trending seashore faults intersect the transform-like faults.

The Taiwan Strait are located on the Eurasian continental crust and its seismicity is of intraplate type. Large historic earthquakes of magnitude around 7 occurred on both northern and southern ends of the Taiwan Straits. Moreover,the Cenozoic deposits are evenly distributed in the intended engineering project area,where no active fault zone was found. It is estimated that the probability of occurrence of future M≥6 earthquake is quite lower.Thus,construction of the tunnel across the straits is feasible in seismotectonic aspect.