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High groundwater pressure led to deadly mudslides after Indonesian quake: researchers

Destroyed paddies and deep cracks in the ground are visible in the Sigi Regency village of Pombewe, thought to be the origin of the mud flow caused by liquefaction after a large earthquake, in Central Sulawesi, Indonesia, on Oct. 11, 2018. (Mainichi/Hiroshi Koizumi)

TOKYO -- Mudflows triggered by liquefaction that followed an earthquake on Sulawesi Island, Indonesia, are likely to have been caused by high pressure groundwater, a Japanese research group lead by University of Tokyo associate professor Takashi Kiyota has found.

More people died as a result of mudflows caused by liquefaction than from tsunami surges generated by the Sept. 28 temblor, which killed at least 2,000 people and left at least 1,300 others missing. Most of those missing are believed to remain buried.

Liquefaction occurs when the pressure of water contained in the ground increases as a result of seismic shaking. This causes sand particles to lose their stiffness and the land becomes like liquid. The liquefaction following the Indonesian quake was extensive -- covering an area as large as 5 square kilometers. The surface of the ground at one location moved 1 kilometer horizontally.

The mudflows occurred on gentle slopes whose inclination was less than 1 degree, in three areas of the island including the city of Palu. In a bid to understand the mechanism of the disaster, associate professor Kiyota, who specializes in geotechnical engineering, and other researchers conducted two surveys in the disaster-hit area in mid-October and earlier this month.

Researchers said they believe that artesian aquifers -- geological strata where groundwater pressure is high -- exist in the disaster-hit areas. If the ground in these areas is dug, water gushes out. A local resident said, "Even before the quake, water sprayed out if a rod was driven into the ground."

Normally, liquefaction settles after sand and water gush out of the ground and the water pressure decreases. However, it is believed that the liquefaction was prolonged on Sulawesi Island because water pressure remained high near the surface of the ground for a long time, causing massive mudflows.

The surface soil around the areas where liquefaction occurred was viscous, blocking groundwater that flowed from high altitudes from breaching the ground, functioning akin to a "lid," the researchers said. This apparently heightened the groundwater pressure, but only a small amount of water came out of the ground following the disaster, they noted.

There are numerous artesian aquifers across Japan but there are no records that such a large-scale mudflow has ever occurred in the country. Kiyota, however, advised caution. "Mudflows could occur in Japan if all the necessary conditions are met. To prevent damage from such a disaster, ground surveys will be increasingly important."

(Japanese original by Tomohiro Ikeda, Science & Environment News Department)

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