Free-Air Gravity Map of Taiwan and Its Applications

An island-wide gravity in Taiwan was conducted by the Institute of Earth Sciences, Academia Sinica, between 1980 and 1987. The 603 stations at which the gravity values were determined included 308 points in the 500 m or higher mountain range where few readings were available previously. The average spacing of the stations in the present survey is about 7 km apart. A new Free-air gravity anomaly map has been constructed based on these values. The map is dominated by a NNE-SSW gravity high trend with a maximum value of 300 mgal, that follows closely the Central Range, a folded and faulted mountain belt with many peaks 3000 m or higher. The magnitude of the Free-air anomaly in the Taiwan area is quite large compared to that elsewhere in the world. The good correlation between the Free-air anomaly and elevation suggests that the Taiwan area is not in isostatic equilibrium. An average surface rock density of 2.57 g cm-3 is estimated from the Free-air gravity data by using the least-squares method. This value can be used for both terrain and Bouguer corrections. The undulation of the geoid and the deflections of the vertical in the Taiwan area are also calculated by using the Free-air anomaly data. The geoid undulation is not rugged over the Taiwan area. The maximum difference is about 5 m. And the deflection of the vertical seems mainly to be affected by both land and submarine topographies