In search of extreme past events in marine sediments around Taiwan
The EAGER oceanographic cruise aboard the N / O Marion Dufresne from June 5 to 27, 2018, has sampled the sedimentary records of the seabed off the island of Taiwan. Its purpose was to trace the extreme earth events (earthquakes, typhoons, and volcanic eruptions) that occurred during the Quaternary period (over the last hundred thousand years), in order to constrain their cyclicities and variabilities, and thus better to deal with the associated hazards.
It was initiated as part of a Franco-Taiwanese partnership supported by the LIA CNRS D3E (From Deep Earth to Extreme Events), and led by the University of Western Brittany, Nice Sophia Antipolis University, and the National Central University of Taiwan. Forty-nine scientists from eleven different institutions * were on board. Among them, eleven Master students were able to participate in the campaign as part of their curriculum, through the organization of a Floating University.
Taiwan, offers an exceptional setting for the study of extreme telluric events. Indeed, its unique geodynamic context presents a double subduction: that of Ryukyus in the East and that of Manila in the Southwest, leading to seismicity and uplift rates among the highest in the world. In addition, the island is in the axis of the “Typhoon Alley”, subjecting it to an average of 4 typhoons per year. Historically, the region has experienced exceptional events such as the magnitude 7.6 earthquake of Chi-Chi in 1999, or the Meiwa tsunami in 1771, which caused nearly 12,000 casualties, most likely caused by magnitude earthquake ~ 7.5 to 8 off Ryukyu. Finally, Typhoon Morakot in 2009 was one of the most destructive to reach the island, because of the floods and the landslides that it generated.
By definition, these exceptional events are rare, and the instrumental and historical archives too limited. It is therefore essential to establish time series of several thousand years (or more) in order to constrain the recurrence and amplitude of these events, better understand the factors that control them in the long term, and in the end train predictive models of these natural hazards and to improve the prevention of associated risks.
Reading the geological archives is therefore necessary to establish long series. Because of the exceptional erosion occurring on the island, the geological traces on land of these events are limited or incomplete. The marine domain can offer a very good preservation of the history of events: These catastrophic phenomena generate submarine landslides and turbidity currents whose deposits (turbidites) contrast with the slow hemipelagic sedimentation: they are easily identifiable and datable by marine sedimentary cores. The detailed analysis of the sedimentary sequences and their contents can make it possible to go back to source zones, and to propose factors triggering among earthquakes of great magnitude, the leaching by a wave of tsunami, or exceptional floods caused by typhoons.
Thirty-two cores, up to 46 meters long, and fourteen short interface cores were collected during the EAGER campaign for a total of over 600m of deposits. Almost all of the cores at summer have been treated on board (measurements of the physical parameters, opening, description) and makes it possible to propose the following preliminary results:
In the West of the island, the analysis of the superficial deposits shows a stratigraphy a priori compatible with the Pintung earthquake in 2006 of magnitude 7, followed by the typhoon Morakot in 2009. These results are very promising for the study of the deposits older.
To the east, above the subsurface Ryukyus subduction plane, turbidite deposits show a very distinct origin, characterized by reef and continental content reworkings, and suggesting emplacement during tsunamis. . These deposits would thus represent the first marine record of tsunamis documented on land. Future dating will identify these events, and most likely extend the time series on land limited to ~ 2700 years. We also found on several sites for the first time in this region a level of volcanic ash Pleistocene age up to 40cm thick. To date, the volcanic caldera at the origin of this mega-eruption remains enigmatic, and the geochemical analyzes and dating in progress will reveal this mystery.