Seismic event stratigraphy of tectonic Lake Towuti, Sulawesi, Indonesia: a 15 kyrs record of seismo-turbidites

Located at the triple junction of the Pacific, Eurasian and Sunda plates, the island of Sulawesi in Indonesia is one of the most tectonically active places on Earth. This is highlighted by the recurrence of devastating earthquakes such as the 2018 Mw 7.5 earthquake that destroyed the city of Palu and caused several thousand deaths in central Sulawesi. The majority of large magnitude earthquakes on Sulawesi are related to stress release along major strike-slip faults such as the Palu-Koro fault and its southern extensions the Matano and Lawanopo faults. To date, information on the frequency and magnitude of major events on these faults is limited to instrumental records, whereas information from historical sources and natural archives is completely lacking. Considering the increase in population density and its extension into distant areas, it is important to better quantify the seismic hazard. Therefore, a systemic catalogue of past earthquakes is essential for the understanding of tectonic dynamics of the area.

            Lake Towuti, situated in Eastern Sulawesi, is a key site to study the paleoseismology on the island. The lake lies close to the Matano strike-slip fault and is hence an ideal archive for past earthquakes that have occurred in the surrounding area. Moreover, its morphology allows a temporally continuous sedimentary succession. The large and deep central basins of the lake preserve the deposits linked to seismic activity. We combine high-resolution Chirp seismic data with sedimentary analyses of sediment piston cores to assess the recurrence of major earthquakes (Mw > 6) in the area, which are expressed by earthquake-triggered Mass Wasting Deposits (MWD). Five major seismic-stratigraphic units are identified in the upper 200 milliseconds TWT and show different depositional mechanisms. MWD’s and associated seismoturbidites can be easily distinguished in seismic data and are well preserved in the cored sedimentary successions in the topmost Unit 1.1. Chronologically Unit 1.1 covers the last 15 kyrs and enables the establishment of an event chronostratigraphy for Lake Towuti’s recent past. The most recent MWD likely corresponds to an AD 1924 Mw 6.5 earthquake, which was recorded to the south of Lake Towuti. In addition, 11 seismoturbidites have been observed in the Late Glacial to Holocene (~15 kyrs) sediment succession. These results tentatively suggest an average recurrence of major events every 1300 to 1400 years.