Pingtung coastal plain, located at the active convergent boundary between Philippine Sea Plate and Eurasian Plate, is one of the most active areas regarding tectonic deformation in Taiwan. Groundwater over-pumping for aquaculture along the coast area of Pingtung plain induced a serious land subsidence problem for decades. How much land subsidence contributed by tectonic activity and groundwater pumping is a crucial issue for tectonic study and groundwater management in this area. This study collected the data in different fields and proposed a conceptual model to calculate the quantities of land subsidence caused by natural (tectonic) and human (pumping) factors. The data from the Global Positioning System (GPS) are used to illustrate the total subsidence concerning vertical displacement. A system called the multi-level compaction monitoring well (MCMW) is able to measure the vertical compaction in different depths from the earth surface to the depth of 200 m. Two GPS stations, named CLON and FALI, close to two MCMWs, named Jiadong and Fangliao, are adopted for analysis The data during 2007 and 2016 taken from MCMWs and groundwater observation wells indicate that the compaction in the shallow depth should be mainly caused by groundwater over-pumping due to their high correlation coefficients (from 0.58–0.95). The difference of the vertical deformation between GPS and MCMW indicates that there is deformation beyond the depth within 200 m. From the data and literature, the further vertical deformation should be due to tectonic activity associated with tectonic escape and extrusion of the Taiwan orogen with average vertical deformation from −3.0 to −4.4 mm. Therefore, the quantities of land subsidence contributed by local groundwater over-pumping and regional tectonic activities are successfully separated. The method and concept proposed in this study can be used in land subsidence quantification due to both tectonic activity and groundwater over-pumping.

Previous studies showed that there is a discrepancy between total subsidence (measured on the ground surface) and shallow compression (measured within several hundred meters beneath the ground surface) in Taiwan. This difference is referred to as deep displacement in this study. The variations of deep displacement are opposite to those of ground surface displacement and groundwater level within the depth of several hundred meters. The mechanism is unknown and requires further investigation. This study adopts two kinds of geodetic observation data and separates land subsidence into shallow compression and deep displacement to investigate the mechanism of deep displacement. A tectonically active coastal area in Taiwan is selected as the study area. The assessment results show that the associated variations are likely due to cyclic hydraulic loading and unloading. The variations of deep displacement are opposite to those of ground surface displacement. This study proposes that these variations are due to hydraulic expansion and contraction. The mechanism is demonstrated using a hydromechanical model. The results of a cross-correlation analysis show that hydraulic expansion and contraction occur at certain depths. The study results provide important information on the mechanism of deep displacement that can be used in tectonophysical and land subsidence investigations. Graphical Abstract

Taiwan is one of the most seismically active countries in the world, and the Taipei basin is particularly susceptible to earthquake hazards due to its high population density and local geology. Site effects, caused by soft sediment deposits overlying hard rock, amplify seismic waves and intensify seismic ground motion, posing significant challenges for seismic hazard assessments in the Taipei basin. To represent site effects, the average shear-wave velocity of the upper 30 m (VS30) is commonly used. However, direct VS measurements are relatively scarce compared to the abundance of engineering boreholes, which often include soil parameters such as void ratio and unit weight. This study proposes new estimation functions that correlate these parameters with VS, enabling the inference of VS at borehole locations without direct measurements. This study also builds upon previous research by incorporating a larger and more comprehensive database, made possible by the availability of additional data and the extrapolation of VS profiles from shallow boreholes (less than 30 m in depth). In addition, we employed Kriging with varying local means to generate a VS30 distribution map across the Taipei basin. The results indicate that the VS30 distribution in the Taipei basin ranges from ∼150–600 m/s, with some areas in the northern part of the basin identified as having lower VS30-values (<180 m/s), which may result in amplified ground motion. Compared with previous research, our study significantly enhances the spatial coverage and resolution of VS30 mapping in the Taipei basin, providing valuable insights for assessing seismic hazard.