Shallow seismic imaging of sub-seafloor structures off the subsiding area of Linbian estuary in Pingtung, SW Taiwan: implications for recent tectonic activities and focused fluid migrations

Recently, subsidence at the coast of Pingtung alluvial plain (SW Taiwan) considerably accelerated, mainly caused by excessive groundwater exploitation from shallow aquifers. To better understand the subsidence pattern and groundwater flow, investigating the structural setting of the Pingtung Basin is essential. The present investigation has revealed that the shallow-marine region off the most rapidly subsiding area of Linbian estuary and Dapeng Bay is characterised by extensive fault and fracture networks, along with a buried syncline, as evidenced by the seismic records. Nearshore, seismic records reveal tectonic fault blocks situated only a few meters beneath the modern seafloor, within the upper 200 m of the seafloor sediments corresponding to the upper interval of the syncline’s infill. This syncline has the same width as the Pingtung basin on land and likely represents its marine extension. Seaward of the most rapidly subsiding area, namely the Linbian estuary, a depression developed on the modern seafloor by both sagging above the syncline centre towards the western flank and extensional faults, which are indicative of recent sinking (= reactivation) of the syncline underneath. The faults, the submarine depression and the coastal subsidence are primarily manifest above the western flank and the centre of the syncline, possibly due to asymmetric reactivation prograding from the syncline's centre towards its western flank. The western flank is intersected by the Liuchu Hsu mud-diapir ridge, which started to rise further and thus, likely triggered the formation of a series of extensional faults above the syncline, followed by minor fault inversions. Previous studies have described freshwater leakage from land aquifers to the seafloor near the subsiding area and at the locations of faults. In fact, these aquifers extend to the adjacent seafloor. Furthermore, faults and fractures in the sub-seafloor deposits in vicinity to the subsiding land areas likely act as conduits in two ways: (1) saline water can infiltrate into the aquifers or (2) freshwater flows out of them. Therefore, these conduits facilitate flow of water from land towards the sea and vice versa. Consequently, the human-induced groundwater overdraft at the Pingtung coast represents a primary factor, which causes seawater to intrude inland whereas tectonic subsidence of the Pingtung Basin is of subordinate importance. The extensive fault and fracture networks, however, have the potential to amplify seawater intrusion inland or seaward freshwater leakage by providing pathways, as highlighted by this study results.