PEEWAN

The island of Taiwan is located in a unique geodynamical context characterized by intense seismicity, with over 23 earthquakes of magnitude Mw>7 in the last century, onshore or offshore. Additionally, Taiwan's climate is mainly influenced by the East Asian monsoon, which plays a dominant role in local rainfall. Directly located within "Typhoon Alley", Taiwan is impacted by three to four typhoons each year. These events generate heavy and potentially devastating rainfalls and landslides. For instance, 2,700 mm of rainfall accumulated in 24 hours during typhoon Morakot in 2008 (compared with ~600mm during Storm Alex in 2020 in Roya and Tinée, one of the biggest historical events in France).

By definition, extreme events are rare, and instrumental and historical records are too limited. It is therefore essential to establish time series of several thousands of years (or more), first, to constrain the recurrence and intensity of such events; second, to better understand the factors that control them in the long term; and third, ultimately draw up predictive models of these natural hazards and improve the prevention of the associated risks.

Deciphering the geological record is thus necessary to establish long time series. Due to the high level of erosion on the island, the geological traces of these events on land are limited or incomplete. In contrast, in the marine realm, where deposition prevails, traces of such events are very well preserved. Catastrophic events generate submarine landslides and turbidity currents whose deposits (turbidites) contrast with the slow background sedimentation; they are identifiable and datable by marine sedimentary cores. Detailed analysis of the sedimentary sequences and their contents can be used to trace the source areas, and to discriminate between triggering factors such as large earthquakes, tsunami wave washout, or exceptional floods caused by typhoons.