The zone under threat from hot mud bubbling from the ground in the Porong area of Sidoarjo, East Java, is expanding further, as geologists warn land subsidence will continue at a massive rate there.
Geologist Andang Bachtiar, who chairs the Indonesian Geologists Association, said land subsidence of up to 12 meters, found at one of the affected houses in Siring Barat in mid-July, should be seen as a natural sign of a collapsed zone.
“The collapsed zone extends beyond the mudflow gush point, and could bury hundreds of homes and roads in dozens of meters deep,” he told The Jakarta Post on Thursday.
“We must be alert to this.”
Andang added the 12-meter sink hole was believed to be an initial sign of a collapsed zone, such as occurred in Porong during the Pleistosen-Holosen era between 1 million and 11 million years ago.
“According to seismic data belonging to PT Lapindo Brantas Inc., during that era, a cauldron 6 kilometers in diameter and between 100 and 300 meters deep was formed around the first former Porong well, or around 7 kilometers northeast of the Banjar Panji I well,” he said.
Andang said the current collapsed zone could threaten areas in West Porong district in a 6 kilometer radius from the main gush point, including West Jatirejo, Gedang, Juwetkenongo subdistricts, and Wunut, Pamotan and Kesambi villages, or the relocation areas of the Porong infrastructure, which the government claimed would remain safe for the next 45 years.
“Areas to the north and east could be affected, including Glagaharum village in Porong district, and Kalidawir, Gempolsari, Kalitengah, Kalisampurno and Ketapang villages in Tanggulangin district,” Andang said.
Amien Widodo, a mud expert from the November 10 Institute of Technology in Surabaya (ITS), said Andang’s theory might be valid because geologists had no data on the condition inside the mud pocket.
“That data is with Lapindo.”
Sidoarjo Mudflow Mitigation agency (BPLS) deputy operations manager Soffian Hadi said the collapse in Porong would probably not occur for decades to come.
“The latest analysis on the Sidoarjo mudflow shows the radius of geological deformation is only around 1.5 kilometers,” he said Thursday.
“We’ll certainly not keep monitoring the impact of the mudflow.”
However, he said the deformation had affected the Porong highway and Buntung toll road north of the gush point, with land subsidence there of 1.5 meters.
“We can’t prevent the deformation happening, but we can maintain the Porong highway by raising it, and demolish the Buntung toll road due to the dangers it faces,” he said.
BPLS head Sunarso said it would be difficult for the government to change the infrastructure relocation area it had set in response to warnings sounded by geologists.
He said his office had asked residents in Siring Barat, included in the area prone to land subsidence, to leave the village, especially an ice factory there. “Based on detection with a ground-penetrating radar, a 15-meter sink hole has formed within the factory compound,” he said.
“A lot of cracks can also be noticed on the walls of the factory.”
Sunarso warned the factory’s ammonia tanks could explode.
[Indra Harsaputra]
(c) The Jakarta Post