World scientists expressed surprise at the enormity of the tsunami strikes in Palu, Central Sulawesi (Central Sulawesi). They assessed that the earthquake that shook Donggala, Central Sulawesi was not expected to trigger a devastating wave like what happened in Palu on Friday (28/9).
"We estimate that the earthquake might trigger a tsunami, but not as big as that," said Jason Patton who is a geophysicist who works for the consulting company Temblor and teaches at the Humboldt State University in California, United States (US).
"When events like this occur, we usually find things that we have never observed before," Patton added as reported by the New York Times on Monday (1/10/2018).
An earthquake measuring 7.4 magnitude which shook Donggala on Friday (28/9) afternoon was known to be centered at a point 80 kilometers north of Palu. Shortly after that - about 30 minutes later - waves of sea water up to 5 meters high hit Palu, destroyed buildings, swept vehicles and killed hundreds of people.
It is known that devastating tsunamis are often caused by megathrust earthquakes when large Earth faults adjust by moving vertically along the Earth fault. In this event usually a large amount of sea water will be pushed, triggering waves that can move at high speed and trigger destruction at locations that are thousands of kilometers from the epicenter.
The devastating tsunami in the Indian Ocean in 2004, which triggered a tsunami up to 30 meters high and killed nearly a quarter of a million people from Indonesia to South Africa, triggered by a 9.1 magnitude megathrust earthquake in Sumatra.
Contrary to 2004, a broken fracture on Friday (28/9) was then called a strike-slip fault. Earth fault movements are mostly horizontal and such movements do not usually trigger tsunamis. But in certain situations, said Dr Patton, a tsunami could occur.
Strike-slip faults can trigger a number of vertical movements that can move large amounts of sea water. Or the rupture zone of the Earth, in this case which is estimated to reach 112 kilometers in length, passes through the area where the seabed moves up or down, so when the fault moves when the earthquake hits, it moves a large amount of sea water in the future.
Another possibility is that tsunamis are formed indirectly. Violent shocks during an earthquake may have caused an underwater landslide that would move seawater and trigger large waves. This kind of event is not unusual, because it happened when the earthquake 9.2 Magnitude shook Alaska in 1964 ago.
Dr Patton said, a combination of various factors might contribute to a tsunami. Studies of the ocean floor are crucial in an effort to understand the occurrence of a tsunami. "We will not know what caused it until the event is over," he said.
The tsunami was also caused by the location of Palu city which was at the end of the shallow bay. The coastline and the basic shape of the bay can focus ocean wave energy and direct it to the bay, with height increasing as it approaches the shore. Such an effect had happened before in Crescent City, California, USA which had been hit by more than 30 tsunamis.
Tsunami experts say, the large number of dead victims reflects the lack of sophisticated systems for tsunami detection and warning in Indonesia.
Currently Indonesia is known to only use seismographs, GPS equipment (global positioning system) and tide gauge (measuring instruments for changes in sea level) to detect tsunamis. Professor at the University of Pittsburgh, Louise Comfort, said the equipment had very limited effectiveness. Comfort is currently involved in a project to bring a tsunami sensor in Indonesia.
The US itself has 39 sophisticated sensor networks at the bottom of the ocean that can detect even the slightest change in pressure that indicates the appearance of a tsunami. The data from the sensor will be delivered via satellite and analyzed, then a warning will be released if needed.
Dr Comfort said Indonesia had 22 similar sensor networks but most were no longer used because they were not treated or stolen by irresponsible parties. The project being worked on by Dr. Comfort will bring a new system to Indonesia, which will later use underwater communications to avoid using buoys at sea level that can be stolen or hit by ships.
Dr Comfort said that he was discussing this project with three Indonesian government agencies. The plan to install the prototype system in Sumatra was delayed this month. "They can't find a way to work