Tsunami on 28 September 2018 IN Palu Bay AN Eyewitnesses Account at Wani, Tanjung Karang-Donggala and Tide Gauge Measurement at Pantoloan Port, Central Sulawesi, Indonesia

The strike-slip earthquake with magnitude M7.4 on 28 September 2018 hit the Central Sulawesi Province of Indonesia at 18:02:53 local time. This strike-slip earthquake caused by the movement of Palu-Koro Transform fault was surprisingly generated a devastated tsunami that hit most of the coastal area inside the Palu Bay and killed more than hundred of people. This event also arose a polemic as the warning system had been lifted about half-an-hour after the quake based on small tsunami signal recorded at Mamuju Port tide gauge 240 km southwest of Palu Bay at The Makassar Straits. While shortly after that, the devastated of tsunami attack along the coast of Palu had been reported and viral in the social media. About three days later, the tide gauge record inside the Palu Bay at Pantoloan Port had been recovered and revealed the recorded of tsunami timeseries with wave height 4.0 m. The main negative leading tsunami wave arrive 5 minutes after the quake preceded by small positive leading waves which arrived 1.5 minutes earlier with height of 0.4m. The second waves arrived 4 minutes later. The time series plot of the tide at Pantoloan Port before and after earthquake indicate that this site did not experience of subsidence or uplitft.

The tsunami evidences at Tanjung Karang, Wani and Pantoloan Port provide a remarkable information that constraint of tsunami source region, mechanism and their impact. The arrival time of tsunami at Wani is nearly similar with the arrival of first positive leading waves at Pantoloan tide record with the incoming wave is from southeast which is from inside the Bay. The eyewitnesses account and field observation at Tanjung karang confirming that the major tsunami is generated inside the Bay. Question then arise on whether these major tsunami is generated by the strike-slip event or other sources? and why the main tsunami wave recorded at Pantoloan Port Tide Station - the negative leading wave of 4.0m - was arrived 1.5 minutes after the biggest wave impacted at Wani considering these two locations is only 2.4 km apart? Is this preceded positive leading tsunami wave (0.4m) at Pantoloan Port is similar to the tsunami waves that arrived and attacked Wani since their arrival time nearly similar? Are these then indicate that these tsunamis are generated from different source?