Faults Exist Below Homes, Schools

KOMPAS/RONY ARIYANTO NUGROHO

The Lembang fault, which marks the border between Lembang district, West Bandung regency, and Bandung city, West Java, will have a disastrous impact on the region if there is a land shift.

JAKARTA, KOMPAS -- Millions of people in Java have no idea that they are living above active fault lines. Other than the Lembang fault in West Java, other faults under major cities across Java are not well-mapped. In Semarang, Central Java, at least one fault is known to exist under residential areas.

“We began a detailed study on the fault line in Semarang several years ago and it is still ongoing. We cannot yet release the detailed map but the general location of the fault is in around Mt. Kebo,” Gadjah Mada University seismologist Gayatri Indah Marliyani said on Wednesday (9/1/2019).

According to Gayatri, it is difficult to go further with the research as the fault area is covered by homes. “Much of the area is covered by buildings and other infrastructure. This is a dilemma for researchers like us. On the one hand, we have ongoing research. On the other hand, we are certain that this area is dangerous for houses,” she said.

Gayatri explained that various research in scientific journals had proven the existence of active faults in Java, including in Central Java. Apart from Endra Gunawan’s latest study (2019), a 2016 study by A. Koulali cites the Kendeng fault on northern Central Java that extends to Surabaya before going under the Madura Strait. Meanwhile, the Baribis and Lembang faults, as well the Jakarta fault more recently, have been found in West Java.

“The Kendeng fault is active in Central Java. In my opinion, the fault is divided into several blocks or smaller segments. However, Koulali predicted that earthquakes along the fault will be massive,” Gayatri said. Other than the Kendeng fault, other, smaller faults include the Semarang fault, the Ambarawa fault and the Lasem fault. The repetition cycle for these faults are not know for certain.

“The Semarang fault is on the western end of the Kendeng fault. If the Kendeng fault extends from west to east, the Semarang fault extends in a northwest-southeast direction,” Gayatri said. However, she said, further studies, including paleoseismic tests, are required to find more details on the faults, including their repetition cycle and maximum strength of earthquake.

Only Lembang fault is well-mapped in Java, thanks to the work of Indonesian Institute of Sciences (LIPI) geotechnology center research Mudrik R Daryono and his team. Mudrik’s paper was recently published in an international journal. Mudrik said that, based on LIDAR-based maps with a 90-centimeter resolution, it could be ascertained that the Lembang fault went below residential areas, schools, community health centers (Puskesmas) and military installations. It will affect not only the surrounding areas but also places as far as Bandung.

Research conducted by Ayu Retnowati, Irwan Meilano, Akhmad Riqqi and Rahma Hanifa from the Bandung Institute of Technology (ITB) found in 2017 that economic loss due to earthquakes along the Lembang fault could reach Rp 51 trillion (US$3.62 billion). This value covers only residential damage.

Based on the team’s calculation, 2.5 million homes may be affected by earthquakes along the fault line, which extends 29 kilometers from the west to east in northern Bandung. Around 1 million homes may be lightly damaged, 500,000 will be totally damaged and the remaining 1 million will be moderately damaged. “Currently, we are updating our calculations with the latest data in collaboration with the National Development Planning Agency [Bappenas] and the National Disaster Mitigation Agency [BNPB],” Rahma said.

Earthquake history

The fault line is visibly marked by rivers, the presence of river sediment on hilltops and weak points that trigger landslides, as well as a spread of recorded earthquakes. “Semarang has quite a lot of weak points and they are in line with the active fault line,” Gayatri explained.

Meteorology, Climatology and Geophysics Agency (BMKG) earthquake information and tsunami early warning division head Daryono said earthquakes were recorded in Semarang in 1856, 1958, 1959 and 1966.

Other earthquakes in Central Java include ones that took place in Ambarawa in 1865, in Pati in 1890, in Lasem in 1847 and in Kudus in 1877. A destuctive earthquake also occurred in Wonosobo in 1924. Most recently, an earthquake was recorded near Salatiga on Jan. 15, 2017. Despite the strength being only 2.6-magnitude, it marks how active the earthquake zone really is.

Gayatri said that, apart from building damage along the active fault line, the danger in tectonic earthquake lies in its amplification. Buildings on active fault lines should be surrounded with an empty area in a radius of at least 15 meters and the construction must apply earthquake resistance standards.

Learning from Palu, liquefaction is a threat for Indonesian cities constructed on sand sediment layers. “Semarang and Jakarta have similar characteristics. The soil comprises a thick layer of soft sand, which is prone to liquefaction during earthquakes,” she said. (AIK)