A coastal geodetic GNSS station for tectonic and sea-level variation study in the South China Sea

Abstract. South China Sea (SCS) separates the land from north to south by nearly two-thousand km in distance. In this framework, land-based observation is limited, thus, little is known about the tectonics in this widely gapped area. Here, we present a coastal geodetic GNSS station that is recently deployed in Nansha southern SCS. The station measures mm precision of land displacements that show ~4–5 mm range of variation in horizontal components and ~1 cm range of changes in vertical nearly a year. These displacements are characterized by a flat trend, with occasionally disturbed by subcentimeter changes. The GNSS-IR retrieved sea-level variations over both short- and long-term time span are comparable with tide gauge recordings, offering a complement equipment to detect amplitude variations and trend adjustments in both relative and absolute sea levels. Although varying with time, the average rate is commensurate with that of the global mean sea-level changes. With such capacity, we filter the sea-level retrievals to be equally sampled as a conventual tide gauge using a Kalman filter, and conduct experimental tests to investigate whether it can capture sea-level disturbances from extreme events i.e., tsunamis. Our results show that as long as the disturbances are twice larger than the standard deviation of the filtered time series, then such disturbances are successfully detected. This criterion could be largely reduced if the GNSS site was built particularly for GNSS-IR. In any case, the GNSS-IR detectability is particularly helpful to capture sea-level disturbances triggered by quiet submarine landslides as they often don’t send sensible signals as earthquakes do. Our station adds an extra connecting pod to fill in the existing few stations and pushes one-step forward to link the separated land, and affords another example to study regional tectonics and sea-level variations simultaneously.