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Upper-plate structures that splay out from the megathrusts are common features along major convergent plate boundaries. However, their earthquake and tsunami hazard potentials have not yet received significant attention. In this study, we identified at least one earthquake event that may have been produced by an upper-plate splay fault offshore western Myanmar, based on U–Th ages of uplifted coral microatolls. This event is likely an earthquake that was documented historically in C.E. 1848, with an estimated magnitude between 6.8 and 7.2 based on regional structural characteristics. Such magnitude is consistent with the observed co-seismic uplift amount of ∼0.5 m. Although these events are smaller in magnitude than events produced by megathrusts, they may produce higher earthquake and tsunami hazards for local coastal communities due to their proximity. Our results also indicate that earthquake events with co-seismic uplift along the coast may not necessarily produce a flight of marine terraces. Therefore, using only records of uplifted marine terraces as megathrust earthquake proxies may overlook the importance of upper-plate splay fault ruptures, and underestimate the overall earthquake frequency for future seismic and tsunami hazards along major subduction zones of the world.

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.