Present-day Crustal Strain Field In The Taiwan Active Collision Zone And Its Geodynamic Mechanisms:Insight From FEM Simulations

The Taiwan Island is the product of convergence and collision between the Eurasian plate and the Philippine Sea plate, where the geological structure is complex, seismicity is high and deformation is strong. As ongoing arc-continent collision around Taiwan is rare in the world, Taiwan has been described as an ideal natural laboratory for plate collision studies. Taiwan has established its GPS Network since 1980s, which has cumulated mass data since then and make it possible to study the crustal deformation in and around it.In order to quantitatively study the characteristics of crustal deformation in Taiwan and to understand their geodynamic mechanisms, we calculated the strain rate field in and around Taiwan by using finite element method (FEM), utilizing GPS data from 1995 to 2005 as boundary constraints in simulation. The heterogeneity of the medium, slippage of the faults and some other factors are taken into consideration to quantitatively study the interseismic deformation in Taiwan and we try to find out the dominant factors that control the deformation. To acquire more ideal computed results, we have taken advantage of the optimal design function of the ANSYS(Design Opt). We take the fitted difference between GPS observations and their corresponding finite element calculated results as objective function, take the material parameters of each subregion and friction coefficients of the faults as design parameters to obtain the optimal finite element model.The computed results of the optimal model show that the largest calculated maximum principal strain of Taiwan locate on the Coastal Range and adjacent waters in the eastern Taiwan, the superiority trend of maximum principal compressive strain is NW-SE, which is agree with the orientation of arc-continent collision between the Eurasian plate and the Philippine Sea plate and converge obliquely with Taiwan Island. The computed results also show that contractions are predominant in the central part of Taiwan, while extensions exist in the northeastern and southern parts, respectively. The principal tensile strain rate is larger than the principal compressive strain rate in I-lan Plain, northeast Taiwan, indicating that the region is in the tensile stress environment which may closely related to the extension of the Okinawa trough. Similarly, tensile stress environment also appears in the southern Taiwan. In addition, the azimuths of calculated principal tensile strain are mainly consistent with the directions of the world stress map results and the P and T axes of focal mechanism solutions.The computed velocity fields in Taiwan show a fan shape. The displacement vectors in central and eastern in the direction of NW, the direction of velocity moving clockwise towards north and anticlockwise towards south, which reflect the characteristics of Taiwanese materials escape to northeast and southwest. Philippine Sea Plate squeeze Taiwan in the direction of NW, but the movement to northwest is blocked by the Kuanyin High(KH) and Peikang High(PH). This may result the escape phenomenon. In general, magnitudes of velocities gradually decrease northwestward from about 81.5 mm/yr at Lanyu and Lutao, SE offshore of Taiwan, to nearly no deformation in NW Taiwan.Besides, the simulation results show that most faults are locked(or with large friction coefficients) during interseismic deformation, while the calculated friction coefficient of the Longitudinal Valley Fault (LVF) is 0.5, and the slip rate is about 13.8-23.5 mm/yr. As part of the convergence is absorbed by LVE, deformation west of LVF decays rapidly westwards and northwestwards.Further tests of the model parameters show that the present-day framework of crustal strain field in Taiwan results from interactions by many factors such as the NW squeeze of Philippine Sea Plate, profile of collision plate, the resistance of KH and PH, back-arc extension of Okinawa trough, retreat towards south of Ryukyu arc together with faults ruptureBecause of the special geology structure in Taiwan region, the crustal strain field is complicated. Despite we have simulated the interseismic crustal deformation and velocity field by finite element model, the results is just first approximation. To obtain ideal results accord with actual geological structure, more factors should be taken into consideration and 3D FEM simulation is necessary.