Paleo-earthquake records of the Hengchun offshore structure, southern Taiwan, revealed by uplifted coral colonies

In the southernmost part of Taiwan, the Western Hengchun Tableland is a prominent topographic feature, with several steps of late Pleistocene marine terraces forming the top of the eastward tilting tableland, and uplifted Holocene coral reefs along the coast. An active Hengchun offshore structure, located offshore to the west, was proposed to be the primary formation mechanism of the tableland. However, the presence of this structure is under debate, and some argued that the tableland was formed by aseismic deformations of mud diapirism, because no record of paleo-earthquakes related to the structure was identified. In this study, we utilized fossil coral colonies as paleo-sea-level indicators to identify land-level uplifts and paleo-earthquakes probably produced by this structure over the past 2.5 thousand years. We identified uplifted Porites coral colonies at five sites along the coast and found that they can be separated into six groups based on their elevations and ages. Since the corals in the same age group have similar elevations at one site, and corals found at different sites along the coast can all be correlated into the six groups, these coral colonies likely recorded co-seismic uplifts of six paleo-earthquake events of the proposed Hengchun offshore structure. Co-seismic deformation of this structure is also likely responsible for the long-term topographic development of the Western Hengchun Tableland. Our results suggest that mud diapirism is not the primary structural mechanism in the area, and the Hengchun offshore structure poses significant regional seismic hazard potentials in the future. •6 uplifted coral groups are present along the Western Hengchun Tableland coast.•These corals represent episodic co-seismic uplift of the Hengchun offshore structure.•The Hengchun offshore structure poses great future earthquake hazards.