[1] To characterize the present-day vertical displacement field in the active Taiwan orogenic belt, 1843 precise leveling and 199 continuous GPS measurements from 2000 to 2008 are collected and analyzed in this study. Vertical velocities derived from the leveling data are placed in a reference frame of the Chinese continental margin using continuous GPS observations at nearby sites. The leveling and GPS vertical velocities generally reveal a dome-shaped pattern with uplift of ∼0.2-18.5 mm/yr in the interior of the mountain range and subsidence on the flanks of the mountains and coastal plains. Modern uplift rates in the active fold and thrust belt are generally consistent with geologic uplift rates. However, present-day uplift rates in the Central Range are faster than the million-years-averaged exhumation rates. The modern subsidence rates are generally consistent with geologic rates, except for the rates in western coastal areas due to groundwater pumping. Present-day subsidence in the southern Central Range and northern Coastal Range is, however, inconsistent with long-term uplift, which may reflect interseismic elastic strain accumulation across faults. Present-day subsidence in northern Taiwan occurs in a region of postcollisional orogenic collapse. We model the present-day and geologic vertical velocities and published GPS horizontal velocity data across southern Taiwan using a 2-D lithospheric model. The model suggests a combined slip rate of 40 mm/yr on the frontal thrusts and 45 mm/yr on the Longitudinal Valley fault. The model requires an additional source of crustal thickening under the Central Range to match the observed present-day uplift rates.

[1] An airborne gravity survey was conducted over Taiwan using a LaCoste and Romberg (LCR) System II air-sea gravimeter with gravity and global positioning system (GPS) data sampled at 1 Hz. The aircraft trajectories were determined using a GPS network kinematic adjustment relative to eight GPS tracking stations. Long-wavelength errors in position are reduced when doing numerical differentiations for velocity and acceleration. A procedure for computing resolvable wavelength of error-free airborne gravimetry is derived. The accuracy requirements of position, velocity, and accelerations for a 1-mgal accuracy in gravity anomaly are derived. GPS will fulfill these requirements except for vertical acceleration. An iterative Gaussian filter is used to reduce errors in vertical acceleration. A compromising filter width for noise reduction and gravity detail is 150 s. The airborne gravity anomalies are compared with surface values, and large differences are found over high mountains where the gravity field is rough and surface data density is low. The root mean square (RMS) crossover differences before and after a bias-only adjustment are 4.92 and 2.88 mgal, the latter corresponding to a 2-mgal standard error in gravity anomaly. Repeatability analyses at two survey lines suggest that GPS is the dominating factor affecting the repeatability. Fourier transform and least-squares collocation are used for downward continuation, and the latter produces a better result. Two geoid models are computed, one using airborne and surface gravity data and the other using surface data only, and the former yields a better agreement with the GPS-derived geoidal heights. Bouguer anomalies derived from airborne gravity by a rigorous numerical integration reveal important tectonic features.

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