Submarine paleoseismology offshore eastern Taiwan: New insights from turbidite records

It is now admitted that large earthquakes are one of the driving mechanisms for turbidity current generation. When the turbidite sequences as well as the trigger mechanism are well defined among the sedimentary archives, it is possible to use the turbidites records as a proxy for paleoseismicity investigation. The Taiwan area, where the Philippine Sea Plate collides with the Eurasian plate at a convergence rate of 80mm/yr, is one of the most seismically active areas in the world and has been consequently struck repeatedly by destructive earthquakes. To better constrain recurrence intervals of large past earthquakes, we have conducted two cruises in September 2012 and August 2013 during which piston cores (up to 4m long) and box-cores were retrieved in tectonically controlled basin. We aimed at deciphering turbidite units and hemipelagic sequences. We have paid attention to the source of the turbidites such as river discharges or slope sediments destabilized by earthquakes. From these cores we analyzed each event using several proxies: physical measurements such as P-wave velocity, gamma density and high-resolution magnetic susceptibility, as well as sedimentological analysis and vertical grain-size variation. Moreover fine details of vertical elemental distribution acquired by X-ray fluorescence (XRF) scanning allow us to accurately identify and characterize turbidite events. Our preliminary results suggest evidences of at least 25 beds that differ from pelagic sediments with at least 18 turbidite units possibly linked to slope instabilities in the Ryukyu fore-arc basin area. In the Luzon volcanic arc area, 44 layers have been identified among which 12 are interpreted as turbidite units, corresponding to possible slope instabilities. In contrast, in the Southern Longitudinal Trough, which is directly fed rivers on land, about 44 turbidites have been identified likely triggered by typhoons or/and floods. Ongoing 14C dating performed on planktonic foraminifera using an accelerator mass spectrometry (AMS), and 210Pb dating (last ~100yr-150yr), will provide the chronology of seismic events and the sedimentation rate of the Late Holocene. The 210Pb results show a sedimentation rate around 0.34 cm/yr near the surface. We thus expect to record approximately 100 to 1000 years of seismic events off east Taiwan.