Mechanism of vertical displacement beneath shallow compression zone in coastal area.

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. [ABSTRACT FROM AUTHOR]