the phenomenon of Liquidation and Sinking of Houses in Petobo

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Hundreds of houses in Petobo Village, Palu City, Central Sulawesi (Central Sulawesi) were still buried in black mud after the earthquake measuring 7.4 on the Richter Scale shook the area on Friday (28/9). Black mud comes from river dikes located in the eastern part of Petobo Village on Jalan H.M. Suharto.

The embankment collapsed when the earthquake shook the area and immediately the mud hit people's homes in the Ranjule section of Petobo Village at around 18.07 West Indonesia Time. At that time, it coincided with the Maghrib prayer time. Many of the main Muslim communities are in the mosque.

Some other residents are at home. They cannot do much primarily to save themselves. Until yesterday, the search for victims had not been carried out.

Petobo village is one of the most affected locations of the earthquake, in addition to the Balaroa National Housing area. Thousands of victims are estimated to still be buried in the ground with buildings in the two locations.

"We have not identified it at the Balaroa National Park and Petobo Village because the location is very severe," said Palu City Regional Disaster Management Agency Head Fresly Tampubolon in Palu, Monday (1/10).

The two regions, namely Balaroa and Petobo, are the most devastating centers of damage because houses and public facilities at that point are buried in the ground like they were swallowed up by the earth. According to a number of witnesses, a few seconds after the 7.4 SR earthquake shook Palu, the village area was seen with a fairly high burst of water, then suddenly the land surface dropped so that it dragged all the objects above it.

In fact, some buildings like mosques shifted about 50 meters away from their original position. "My wife and children cannot be saved. I expect them to be trapped in the house and then rolled up in the ground," said Husnan, one of the victims' families.

When the incident happened, Husnan was in the office, while his wife and children were at home. The same conditions also occur in the neighborhood of Kawatuna. However, the location was accompanied by water so it was not possible to be touched by the disaster management team.

Palu Deputy Mayor Sigit Purnomo Said said that the two worst disaster points had not been touched because of broken access. Sigit said, the disaster management team prioritized disaster locations that could be reached quickly.

The Hammer-Kuro Fault Moves

On Monday (1/10), the Indonesian Institute of Sciences (LIPI) released a brief analysis of the earthquake and tsunami that occurred in Palu, Central Sulawesi. What happened in Petobo Village was the phenomenon of liquefaction or soil liquefaction.

LIPI seismic expert Danny Hilman Natawidaja revealed that there were details of natural phenomena that made the Palu earthquake and tsunami deserve attention. "There was a tsunami that actually happened in the mechanism of the horizontal fault structure movement as well as land liquefaction," he said in a written statement received by, Monday (1/10).

The Head of the LIPI Geotechnology Research Center Eko Yulianto said that the location of Palu City was above the Palu-Koro fault. The Palu-Koro fault is a fault that divides Sulawesi into two parts West and East. On Friday last week, the fault was active.

"This fault has an active movement and is a concern for geological researchers," said Eko.

Researcher in marine geophysics from the Oceanographic Research Center, Nugroho Dwi Hananto, believes that it is possible that a fault in the Palu-Koro landscape which has a vertical deformation component on the seabed triggers a tsunami. He explained, the Palu to Donggala Bay area also has a closed canal-like shape with a steep seabed shape.

As a result, when there is a sea water mass coming, the waves are higher and the speed is faster. He also noted the possibility of underwater landslides caused by underwater cliffs collapsing due to the earthquake.

"The earthquake and tsunami in Palu became an important lesson in the need for more complete geo-scientific data to be able to assess the potential for earthquakes whose sources come from under the sea," he said.

Liquidation Phenomenon

Meanwhile, liquefaction or liquefaction of land that occurred when the earthquake shook the city of Palu, Central Sulawesi, on Friday (28/9) was a new phenomenon for the people of Indonesia. In various videos spread on social media, liquefaction is characterized by the movement of buildings on the ground as if dragged down by mud, as happened in Petobo Village.

The Head of the Information and Public Relations Center, National Disaster Management Agency (BNPB), Sutopo Purwo Nugroho said that the liquefaction incident in Palu City was not the first time this had happened. In the earthquake that occurred in Lombok, liquefaction also occurred. However, the scale is smaller.

"Not all places where the earthquake occurred, liquefaction also occurred. In Lombok, it happened, but it was small. But if we look at Palu, the liquefaction happened so much," he said at a press conference at Graha BNPB, Monday (1/10).

He explained, the occurrence of liquefaction was caused by earthquake shocks. The condition of the geological material on the ground also affects. When shocks occur, the soil becomes liquid due to high water material.

In large volumes of water, he said, the land became loose. As a result, buildings above land, housing and trees, walked slowly until they finally collapsed and were buried by mud.

In other words, liquefaction is the process of removing mud from the soil layer due to earthquake shaking and causing the soil layer that was initially compact, mixed with water to become mud. The reduced strength of the soil resulted in the building being destroyed.

According to Sutopo, liquefaction is not arbitrarily found in the path of the earthquake that will occur. This means that not all regions are earthquake prone, liquefaction is also prone to occur.

The phenomenon of severe liquefaction that befell Petobo Housing and Balaroa Housing in Palu City must be a learning for all parties. In the earthquake incident in the city of Palu, liquefaction occurred at the Balaroa National Park which sank around 1,747 housing units. Meanwhile, in the Housing Patobo there are about 744 units of drowning houses.

Sutopo said, BNPB had not received information related to the number of victims and damage that occurred in the two Housing Authority. However, he believes the high amount of damage and liquefaction swallowed many victims.
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