Onshore/ offshore wide-angle deep seismic profiling in Taiwan

In summer 1995, in collaboration with the deep multi-channel seismic profiling project around the island of Taiwan, anonshore-offshore wide-angle deep seismic profiling experiment was conducted in Taiwan. The results are expected to provide the first complete seismic images of the deep crustal structure for a better understanding of the Taiwan orogeny and subduction-collision system. The experiment consists of three profiles, one along each of the central and southern cross-island highways and another on the south-link highway of the island. For the first two lines, 35 three-component portable seismographs were deployed along each of the 116 and 135 km-long profiles onshore, with airgun shots being fired at distances eastward form the east coast of up to 133 km and 170 km, respectively. On the third line, 6 stations were deployed along the 20 km-long profile. For this line, the R/V Ewing provided shots from the Philippine Sea westward to the eastern coast of the Hengchun Peninsula and then from the western coast of the Hengchun Peninsula westward away from Taiwan. The total length of the third line was about 310 km. The average station intervals were about 4 km. The preliminary results along the three profiles show a very thick crust beneath Taiwan. In addition, the velocity structures show that lateral variation of velocity in the upper crust is generally larger than that in the lower crust and uppermost mantle. Large thickness variations in the crust were also obtained. The crustal thickness beneath Taiwan diminishes gradually toward the west, but dramatically toward the east. The topographic high is not well associated with the crustal thickness. The thickest crust is to the east of the central mountain range, which may indicate that Taiwan has not yet reached its isostatic equilibrium. In this paper, the preliminary results along the southern cross-island profile are given as an example. Detailed results of the three profiles will be given elsewhere