The genetic algorithm method is combined with the finite‐element method for the first time as an alternative method to invert gravity anomaly data for reconstructing the 3D density structure in the subsurface. The method provides a global search in the model space for all acceptable models. The computational efficiency is significantly improved by storing the coefficient matrix and using it in all forward calculations, then by dividing the region of interest into many subregions and applying parallel processing to the subregions. Central Taiwan, a geologically complex region, is used as an example to demonstrate the utility of the method. A crustal block 120 × 150 km2 in area and 34 km in thickness is represented by a finite‐element model of 76 500 cubic elements, each 2 × 2 × 2 km3 in size. An initial density model is reconstructed from the regional 3D tomographic seismic velocity using an empirical relation between velocity and density. The difference between the calculated and the observed gravity anomaly (i.e., the residual anomaly) shows an elongated minimum of large magnitude that extends along the axis of the Taiwan mountain belt. Among the interpretive models tested, the best model shows a crustal root extending to depths of 50 to 60 km beneath the axis of the Western Central and Eastern Central Ranges with a density contrast of 400 or 500 kg/m3 across the Moho. Both predictions appear to be supported by independent seismological and laboratory evidence.

The three-dimensional P-wave velocity structure of the obliquely convergent zone in the eastern Taiwan area has been determined by using traveltimes of seismic waves from 1826 local earthquakes and air-gun shots recorded by the Central Weather Bureau Seismographic Network, and 8334 earthquakes have been relocated for better understanding of the current tectonics. The possible location of the plate boundary between the Eurasian and Philippine Sea plates, characterized by a sharp gradient in the velocity structure, is found beneath the eastern flank of the Central Range to the north of 23.5ЊN, eastern Taiwan. To the south of 23.5ЊN, this boundary is generally along the Longitudinal Valley fault and its southern projection. The distribution of the relocated earthquakes also shows a spatial pattern closely related to the boundary and the state of plate collision. To the east of the boundary, a prominent high-velocity anomaly in the middle to lower crust is found beneath the Longitudinal Valley and the Coastal Range in eastern Taiwan; this anomaly could be interpreted as the oceanic crust of the Luzon forearc. To the west of the boundary, the Central Range has a relatively low velocity at the same depth. The velocity structure and relocated seismicity have led to the recognition of interaction between the materials on opposite sides of the boundary. The relatively high P-wave velocity of the Luzon forearc suggests that it can accumulate strain energy and then release it as brittle failure. However, the relatively low P-wave velocity of the Central Range implies that it responds to the convergence by silent or ductile deformation.

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Using travel time data from local earthquakes and air-gun shots re corded by the Central Weather Bureau Seismographic Network, the tran sition from a typical subduction to a collision suture in the southeastern Taiwan area is imaged in terms of a three-dimensional Vp structure. The southern prolongation of the Longitudinal Valley Fault (PL VF), which is characterized by a sharp contrast in velocity on either side, is the primary feature in the velocity structure. West of the PLVF, a high velocity volume exists from the surface to about 9-km in depth, which can be interpreted as being related to the Central Range. The Central Range structure seems to end near 22.2° N beneath the Hengchun Peninsula. East of the PLVF, a major high velocity anomaly in the middle-to lower-crust beneath the South ern Longitudinal Trough and Huatung Ridge is observed. According to the velocity structure and the estimated composition, the high velocity body could be the forearc oceanic crust, which might have been torn off and separated from the Philippine Sea plate after the Luzon arc was formed, and has been shortened during the collision of the Eurasian and Philippine Sea plates. The other conspicuous feature of the Vp model is a clearly lat eral velocity variation across the Taitung Canyon from the surface to about 25-km in depth, which might be associated with the segmentation of the Luzon arc. Using the three-dimensional Vp model, earthquake events that occurred from 1990 to 1997 were relocated. Most of the relocated hypo centers in the study area tend to lie on the locations where there is a greater gradient in the Vp model.

Seismic refraction data from onshore and offshore experiments in the eastern-northeastern Taiwan region \\'ere used to study the velocity struc ture by the two-dimensional ray-t . racing method. In the \1elocity model, a structural fault boundary located beneath the Longitudinal Valley "ras used to separate the northern Coastal Range (CR) on the eastern side from the eastern flank of the Central Range (EFCR) on the western side. The P 'vave 'relocities from the surface to the dept .h of 12-15 km varied from 3.9 to 5.8 km/s beneath the CR and from 4.8 to 6.1 km/s beneath the EFCR. Com paring the velocity structures along various latitudes, it \\'as found that the CR extends northward to 24.2 <> N. The velocity structures of the CR, the Hsinchen Ridge (HR) and the Yaeyama Ridge (YR) indicate that the HR and the YR both belong to t . he same type of tectonic unit as the CR. To the north of 24.2 <• N, the velocit)' structure of the Ilan Ridge (IR), located be t,veen the EFCR and the southwest . ern end of the Ryukyu arc, is similar to that . of the EFCR; hence, probably indicating it is the northeastern exten sion of the EFCR. This suggests that the EFCR bends eastward and be longs to the same tectonic unit as the southwestern Ryukyu arc. From a comparison of the velocity structures of the CR, EFCR and of other typical continental arcs, orogens and oceanic arcs in the literature, it can be con cluded that the northern CR b�longs to an oceanic arc and that the EFCR is a continental arc. Further more, from the analysis of the velocity struc tures beneath the CR and EFCR, it is believed that the upper crust of the CR is weaker in strength than the EFCR, which means that the arc-conti nent collision is not an appropriate model for the formation of Tai\\ran is land.