Survey Reveals Mystery of Palu Tsunami

KOMPAS/BAHANA PATRIA GUPTA

Gempa Residents at Layana Indah Village, Mantikulore district, Palu city, start building houses from the remains of their homes, which were completely destroyed by the tsunami on Sunday (07/10/2018). Rather than being bored in evacuation centers, they want to re-organize their lives for a better future after falling victims of the earthquake and tsunami.

JAKARTA, KOMPAS – The earthquake with a magnitude of 7.4 and a sliding fault mechanism, which hit Donggala on Sept. 28 , 2018, should normally not trigger a large tsunami.

A survey conducted by a joint team of Indonesian and Japanese tsunami experts found that an undersea landslide was the potential source of the tsunami. Underwater mapping should be carried out to prove the hypothesis.

The results of the field survey, which was jointly conducted by tsunami experts including Fumihiko Imamura from Tohoku University, Taro Arikawa from Chuo University and Abdul Muhari from the Maritime Affairs and Fisheries Ministry, were unveiled by the Office of the Coordinating Maritime Affairs Minister in Jakarta on Sunday.

Imamura said the inundation distance of the tsunami was only 200-300 meters from the shore, while the height of the tsunami on the shore (inundation depth) was 2-5 meters. "This was smaller than the initial estimate," he said.

The short inundation distance of the tsunami on the land indicates that the tsunami had a short wave, in contrast to the wavelength of the tsunami in Aceh in 2004, or the tsunami in Japan in 2011, which reached tens of kilometers and reached almost 10 kilometers inland. Tsunami wavelengths are usually determined by the short width of the sliding area on the seabed triggering the tsunami.

The results of the survey on the height of the tsunami were slightly different from the initial findings made by the Meteorology, Climatology and Geophysics Agency (BMKG) of Indonesia, which said the height of the tsunami on the coast of Palu city reached around 6-7 meters. In fact, in certain areas, it reached 11.31 meters.

Taro Arikawa said the initially presumed height of the tsunami was possibility splash height. From the findings of his team, the water splashes caused by the tsunami could more than double its height. This showed that the momentum and strength of the tsunami were greater than usual.

The pattern of the tsunami meant that its height of just half a meter was able to sweep people away. "If the tsunami height was over four meters, at least 80 percent in the affected area would have been swept away," he said.

The survey also confirmed that the tsunami had arrived on the coast less than 10 minutes after the earthquake. "People would not have had enough time to evacuate even if the tsunami warning [siren] was on," said Arikawa.

As it was reported, the only tsunami siren in Palu City was not functioning. In addition, text messages containing tsunami warnings that should have been sent to residents in affected areas were not working.

Abdul Muhari said the initial survey conducted by the team had begun to uncover the mystery of the tsunami that hit Palu. Typically, a ground-based earthquake triggered by a sliding fault would not trigger a big tsunami.

"In the field, we have found tsunami debris in high trees, but the surrounding buildings were not damaged. This proves that the debris was brought by a water splash, not by a tsunami, because if it was a tsunami, the buildings would have been destroyed," said Muhari.

From the data, the temporary conclusion was that the tsunami in Palu was triggered by an underwater avalanche that occurred after the earthquake. "We need an underwater survey to prove this, but in the meantime our suspicions are heading to that conclusion," he said.

Compared to the previous tsunami that had hit the region, according to Muhari, the Palu tsunami was relatively small. For example, in 1927, a 6.3-magnitude earthquake triggered a tsunami of up to 15 meters in Palu bay. In 1968, a 7.4-magnitude earthquake triggered a tsunami in Palu bay up to 10 meters.

"The tsunamis in the past could also have been triggered by underwater landslides. As it is known, underwater landslides can trigger very high tsunamis even though the radius is focused only on relatively narrow areas, such as the tsunami in Ambon in 1674, which reached more than 100 meters,” he said.

With tsunamis occurring in Palu bay every 30 years, and with the potential for higher tsunamis in the future, the Japanese survey team recommended that the coastal city of Palu should no longer have people living there. The tsunami-affected areas are recommended to be used as open spaces, not as residential areas.

Underwater survey

A team of Indonesian scientists with a more complete composition and better equipment departed to carry out an underwater survey using the Baruna Jaya ship operated by the Agency for the Assessment and Application of Technology (BPPT).

A BPPT engineer, Udrekh, who led the team, said the team, consisting of a number of research institutions and universities in Indonesia, would be divided between land and sea. The survey also involved the Navy\'s Hydrographic and Oceanographic Center.

"For the sea, we will conduct bathymetric surveys (underwater mapping) to find out the changes after the earthquake. If there is evidence of underwater landslides and their magnitude, the impact is expected to be known," he said.

In addition, the team will study the tsunami inundation on land, as well as the affected area of liquefaction. "The ship carrying the survey team will also bring aid to victims. The funds for survey and donations for victims are contributed by a number of parties," Udrekh said.

The results of the survey are expected to help determine recommendations for the rebuilding of Palu city and other cities in Indonesia that are located in areas vulnerable to earthquake and tsunamis. (AIK)