A M7.5 earthquake occurred on September 28, 2018, at 6:02 pm (7:02 pm Japan time) on the Indonesian island of Sulawesi. A significant amount of damage was caused by the earthquake, subsequent tsunami, liquefaction, and landslides. On October 4–6, Prof. Fumihiko Imamura (Tsunami Engineering) of IRIDeS conducted the first field survey to investigate the damage caused by the tsunami in and around the major city of Palu in collaboration with other researchers and government officials in Indonesia. After returning to Japan, he reported his observations to IRIDeS on October 11.
Based on the water level differences in the tide-level records observed by the Indonesian government at that time, a tsunami with a wave height of 4 m occurred exactly during high tide and arrived six minutes after the occurrence of the earthquake. The tsunami was large and arrived at Palu Bay, which was not close to the epicenter of the earthquake, within a short period of time. Prof. Imamura observed that the water level in Palu Bay was several tens of cm different from that recorded in other areas, such as Mamuju in the coastal area of Sulawesi Island, and that the wave characteristics were considerably different. He noted that the tide-level record obtained from Palu Bay was particularly valuable for its ability to reveal the actual nature of the tsunami that struck Palu.
Prof. Imamura also reported that buildings were damaged in Palu from the coastline to approximately 200 to 300 m inland. The first floors of the buildings located near the coastline were destroyed, and the lack of traces of muddy water indicated that the water current was fast. Further, ground subsidence was confirmed. Based on eye-witness reports, the tsunami may have reached a wave height of 10.4 m because of splashing. The large tsunami in Palu Bay was likely caused by an underwater landslide caused by shaking instead of the earthquake itself. Prof. Imamura supported this observation using topographical maps to
explain the scale and direction of the landslide.
