Preliminary Crustal Structures Across Central Taiwan from Modeling of the Onshore-Offshore Wide-Angle Seismic Data

Wide-angle reflection and refraction data are used to illustrate the crustal structures in the central Taiwan area, across the Taiwan Orogen and in the westernmost part of the Ryukyu subduction system. The preliminary structures were derived by modeling the data set collected using onshore seismic recording instruments along the central cross-island highway in 1995. Seismic signals were generated by the powerful airgun arrays of the R/V Ewing in the seas east of Taiwan. For the shallow structures, results from the forward modeling of the Pg phases show that strong lateral variations of P-wave velocities in the upper crust were obtained. Velocities at the uppermost crust have a wide range from 3.0 km/sec to 5.2 km/sec, and increase from 5.8 km/sec to 6.8 km/sec at the bottom of the upper crust. The thickness of the upper crust increases from 22 km beneath the Western Central Range to 25 km beneath the Hsincheng Ridge, and it then dramatically decrease to only 10 km beneath the Hoping and Nanao basins. For the deep crust structures, similar pattern of thickness variations, from 12 km to 20 km, was obtained. Velocities at the lower crust range from 6.4 - 6.7 km/sec at its top to 7.2 - 7.3 km/sec at its bottom. In summary, the crustal thickness beneath the island could be over 40 km, but only about 25 km beneath the westernmost part of the Ryukyu forearc region. The thickest crust is not right beneath the highest mountains on the island but with an offset of about 40 km toward the east. The result indicates that Taiwan has still not yet reached to its isostatic equilibrium yet.