Raising Awareness of Living on Fault Lines

· 5 minutes read

A number of new studies have revealed that major cities in Indonesia lie on top of active fault lines and are prone to catastrophic disasters. However, with preparedness and mitigation, the risks can be managed.

Fault lines are cracks in the ground caused by seismic activity influenced by endogenous forces from within the earth. Earthquakes occur when rocks in the field of movement are exposed to pressure and break. Researcher Rahma Hanifa from the National Center for Earthquake Studies (Pusgen) said a fault line was generally considered to be active if there had been seismic activity marked by at least one earthquake in 10,000 years.

Shifts at the convergent boundaries of continental plates can trigger large earthquakes, like the one in Aceh in 2004. Fault lines can exist on open land or below large cities, such as Jakarta and Surabaya. Even if earthquakes triggered by land fault lines are not as strong as those in subduction zones, they can be destructive, as their epicenters are close to human dwellings and activities. Records show that earthquakes on land fault lines often lead to large casualty numbers.

According to modeling by Ngoc Nguyen, Phil R. Cummins and an Australian National University team in 2015, if an earthquake similar in strength to the one that hit Jakarta in 1699 was to happen today, the number of casualties may reach 100,000. This is based on the city’s population density and high risk of collapsing buildings.

Mitigation the risks

To reduce disaster risk, we need to increase our awareness that we are living on active fault lines and prone to disasters. We need to accept, not neglect, such risks, especially because history has shown that massive earthquakes have occurred repeatedly.

Disasters lead to high casualty figures because we neglect the risks of our dwellings. Therefore, it is important to realize National Disaster Mitigation Agency (BNPB) chief Doni Monardo’s plan to establish active fault signs on land. “Whether they like or not, the people have to understand and be aware that they are living in an earthquake-prone country,” he said.

We need to assess how far our homes are from fault lines, so that we can mitigate potential disasters. The closer our homes are to fault lines, the bigger an impact an earthquake could have on us.

Apart from the soil amplification factor, certain types of soil, such as deep alluvial sediments in Palu, Jakarta and Bandung, will lead to stronger earthquakes. Therefore, seismic microzonation is important.

The National Earthquake Map, compiled by Pusgen and published by the Public Works and Housing Ministry in 2017, maps out all known fault lines and their earthquake probability in the next 50 years. The map includes 295 active fault lines all over Indonesia, an increase of 214 from the 2010 earthquake map.

The increase was due to the discovery of new fault lines in previously unmapped zones, such as the Pidie Jaya fault in 2016, or based on new research.

An earthquake map is important to devise an Indonesian National Standard (SNI) on construction safety and security. Plans in line with SNI serve as reference in proposing building permits (IMB). However, once the IMB is issued, officials often neglect their duty to ensure that the actual building is in line with the plan. Many buildings collapse during earthquakes due to negligence in construction rather than in planning.

Apart from poor monitoring of buildings, old buildings are also more vulnerable to earthquakes. Structural audits on old buildings are not mandatory, and private buildings are often constructed without any monitoring.

Construction is merely one factor in earthquake mitigation. Of no less importance is the building’s location, which must not be on fault lines. Infrastructure damage on an active fault line could be seen clearly in the Palu earthquake in September last year. The damage extended on both sides of the surface cracks that marked the fault line.

Shockwave

Indonesian Institute of Sciences (LIPI) geotechnology research center head Eko Yulianto said building were destroyed along fault lines because of the initial shockwave during an earthquake and damage to the soil structure around the fault line. “The fault lines should have been free of buildings,” Eko said.

Pusgen head and Bandung Institute of Technology (ITB) professor of civil engineering Masyhur Irsyam said that, in many countries, fault lines were taken into consideration when determining zones to be kept free of buildings. In the US, areas within 150 meters from main fault lines and 60 m from branch faults have been declared building-free zones since 1977.

In Taiwan, fault lines with historical earthquakes of 7-magnitude or higher demand a 100-m building-free zone. For fault lines with historical earthquakes of less than 6-magnitude, the building-free zone is 30 m.

Rahma said there was no national-level regulation yet on building-free zones around fault lines in Indonesia, as most fault lines in the country had not yet been mapped out in detail. Only West Java has a regulation on building-free zones around fault lines.

West Java Gubernatorial Decree No. 2/2016 stipulates that all construction in disaster-prone areas, including in the Lembang Fault corridor, must be in line with disaster mitigation studies. The Lembang Fault has a 250-m building-free zone. “However, many homes, schools and military installations are found on the fault,” Eko said.

Gadjah Mada University seismologist Gayatri Indah Marliyani said that, apart from limited in-depth studies on fault lines, Indonesia also suffered from poor law

enforcement and a lack of awareness of disaster risks. Consequently, the country was highly vulnerable to earthquakes. “Many regional governments are unaware of the danger of fault lines. Studies on the danger of earthquakes are often seen as fear mongering and a hindrance to investment,” Gayatri said. (AHMAD ARIF)

Editor:

DISASTER MITIGATION earthquakes