Southern Java Tectonic Zone Increasingly Active

The earthquake on Tuesday (23/1) is a warning to improve disaster readiness for earthquake hazards.

KOMPAS/DWI BAYU RADIUS

Atin (right) and his wife Mimin look at what remains of their kitchen on Tuesday (23/1) in Sawarna village, Bayah district, Lebak regency, Banten. Their home was destroyed by the earthquake that happened earlier in the day.

JAKARTA, KOMPAS — The earthquake of 6.1 magnitude that struck Jakarta and its surrounding areas at 1:34 p.m. on Tuesday (23/1) occurred in the Indian Ocean to the southwest of Sukabumi regency, West Java. The southern Java earthquake was part of the active tectonic zone of this area, and has given a warning that its impact can reach the capital city.

The earthquake’s epicenter was located at a depth of 61 kilometers under the seabed and about 67 kilometers southwest of Sukabumi. Based on its depth and impact, Daryono, head of the Earthquake Information and Early Warning at the Meteorology, Climatology and Geophysics Agency (BMKG), said that Tuesday\'s temblor was believed to have been an intraslab earthquake on the Eurasian Plate, caused by the strain of the Indo-Australian Plate’s movement.

"The characteristics of an intraslab earthquake is that it creates a powerful shock. An earthquake in this zone is similar to what happened in Tasikmalaya on Dec. 15, 2017," he said.

Even though its power was relatively small, the earthquake was felt in a number of cities, including Jakarta, South Tangerang and Bogor. The earthquake was also felt in Bandung, Lampung and Yogyakarta.

Increasing activities

Irwan Meilano, a geodesy expert at the Bandung Institute of Technology (ITB), said that seismic activities had been on the rise in the subduction zone off the southern coast of Java. This was detected especially after the 7.8 magnitude earthquake in Pangandaran that triggered a tsunami in 2006. A 7.3 magnitude earthquake later occurred to the south of Tasikmalaya in 2009 and a 6.9 magnitude earthquake also occurred in the area in December 2007.

"We do not know yet why there has been an increase in activities in this zone. However, our assumption is that this is not a release of energy that will reduce the risk of a major earthquake in the Megathrust Zone to the south of the Sunda Strait and southern Java," Irwan said.

An earthquake expert at the Indonesian Institute of Sciences (LIPI), Danny Hilman Natawidjaya, also expressed concern over the increasing activities of the tectonic zone to the south of Java. "Even though the earthquake occurred in different locations, from the aspect of disaster mitigation, what has to be most taken into account is the Megathrust Zone to the south of Java," he said.

The 2017 national earthquake map indicates that an earthquake of a potential 8.7 magnitude could occur to the south of West Java. However, according to the assessments of the ITB earthquake researcher, if the fault segment to the south of Java collapsed simultaneously, the resulting destructive power could reach M 9.2, equivalent to the 2004 Aceh earthquake.

Danny said even though more data had been discovered on the potential for a major earthquake to the south of Java, no predictions had been made on when and where such an earthquake might occur. Moreover, a large part of Indonesia’s seismic zones had not been mapped properly.

However, Danny said, Tuesday\'s earthquake had to be taken as a warning to improve readiness for earthquake hazards, including in Jakarta. "Jakarta, which is relatively far from the southern Java zone, was also strongly rocked. This was a result of its soft soil with deep bedrocks, which heightened the shock from the earthquake," he said.

Not ready

Yesterday’s earthquake caused panic in the capital, indicating that the city’s residents are not prepared for an earthquake. As soon as the earthquake hit, residents ran out of multi-story buildings. In Jakarta’s Sudirman Central Business District, for example, office workers ran out of the buildings and gathered in a number of locations on the main thoroughfare.

A sociology research professor at LIPI\'s Center for Community and Culture Research, Henny Warsilah, said that Jakartans had not been taught about earthquake mitigation.

In Depok, a woman fainted and a pregnant woman went into labor and was rushed to a hospital after they were jostled in a panicked crowd rushing to exit a building. A student at SMK Negeri 1 Tanggeung vocational school in Tanggeung district, Cianjur regency, West Java, jumped from the second floor of the school building in a panicked response to the strong shock from the earthquake, suffering injuries.

No casualties resulted from yesterday\'s earthquake. However, hundreds of houses and a number of buildings were damaged. The greatest damage occurred in Lebak regency, with 842 houses damaged across 16 districts.

"The earthquake caused the biggest damage in Malingping district, Pandeglang regency. At least 330 houses showed minor damage," said Madlias, head of the Emergency and Logistics Division of Lebak\'s Regional Disaster Mitigation Agency.

The widespread damage, according to ITB\'s civil engineering professor Masyhur Irsyam, pointed to the poor quality of the buildings. Aside from the damage to residential buildings, which are classified as non-engineering, the damage to public buildings, such schools, indicated poor implementation of earthquake-resistant building standards.

Masyhur said that yesterday\'s earthquake was a warning for both residents and the government to improve the quality of buildings so they were resistant to being damaged by earthquakes.

(AIK/UTI/RTS/INK/PIN/BAY/VIO/COK)