Aftershocks in the South of Yogyakarta 44 Times, Hundreds of Houses Damaged Lightly

The M 6 earthquake that hit parts of DI Yogyakarta and Central Java, Friday (30/6/2023) night, was a warning of the active tectonic zone in southern Java.

The following article was translated using both Microsoft Azure Open AI and Google Translation AI.

KOMPAS/HARIS FIRDAUS

Residents are outside their homes in Surobyan Hamlet, Argomulyo Village, Sedayu District, Bantul Regency, Special Region of Yogyakarta, after an earthquake occurred on Friday (30/6/2023) night.

JAKARTA, KOMPAS — A total of 44 aftershocks have occurred in the southern Indian Ocean of Yogyakarta following a magnitude 6 earthquake that struck on Friday (30/6/2023) night. The earthquake caused hundreds of buildings to be damaged, one fatality, and six people injured.

The Head of the Earthquake and Tsunami Center of the Meteorology, Climatology, and Geophysics Agency (BMKG), Daryono, stated that the number of aftershocks until Saturday (1/7/2023) at 08.00 WIB reached 44 times. "The largest magnitude of the aftershock is M 4.2," he said.

Daryono suggests that the public should avoid buildings that are cracked or damaged due to earthquakes. "Check and ensure that your residential building is strong enough to withstand earthquakes, or that there is no damage due to earthquake vibrations that could endanger the stability of the building before you return home," he said.

According to Daryono, the hypocenter of the M 6 earthquake that occurred south of Yogyakarta on Friday night was below the contact area of ​​the broken plate. This location is part of the slab or arm of the Indo-Australian plate. This causes the ground to feel strong. "This earthquake is very similar to the southern earthquake in East Java on April 10, 2021 which caused damage to 16 regencies/cities in East Java," he said.

Also read: Dozens of Buildings in DIY Reported Damage due to the M 6 Earthquake

BMKG

Aftershocks south of Yogyakarta. Source: BMKG

The impact of the earthquake

Acting Head of the Data, Information, and Communication Center for Disaster Management of the National Disaster Management Agency (BNPB) Abdul Muhari stated that the earthquake had impacted Yogyakarta, Central Java and East Java. Generally, the damage to buildings was mild in scale.

"Based on the latest information, 1 resident has passed away and another has suffered minor injuries in Bantul Regency. Meanwhile, in Tegal Regency, Central Java Province, 1 resident has suffered minor injuries," he said on Friday night. "

The latest data from the Regional Disaster Management Agency (BPBD) Control Center in Yogyakarta until 11:30 pm recorded 1 person died and 5 injured in Bantul Regency, while 3 were injured in Gunungkidul Regency and Sleman Regency.

Currently, the number of damaged buildings in Yogyakarta City has reached 137, consisting of 106 houses, where 102 are mildly damaged and 4 are moderately damaged. The number of damaged office facilities is 12, consisting of 11 mildly damaged units and 1 moderately damaged unit. Meanwhile, there are 5 lightly damaged worship facilities, 3 business facilities, 2 educational facilities, and 2 health facilities.

The Indian Ocean south of Java, including the south of Yogyakarta, has been known to have the potential for a major earthquake, which could trigger a tsunami.

Muhari stated that the impact of the earthquake in Central Java Province includes Kebumen District, where 8 units of houses were slightly damaged and 2 units were moderately damaged. Magelang and Tegal Districts had one unit of damaged houses each, while Purbalingga District recorded 4 units of damaged houses.

The Regional Disaster Management Agency (BPBD) of Kebumen Regency in Central Java reported that 11 families were affected and one family among them had to evacuate. Meanwhile, in Purbalingga Regency, four families were affected.

Meanwhile, in East Java Province, the impact was identified in Pacitan Regency with a total of 28 damaged houses.

Southern Java Risk

The Indian Ocean south of Java, including south of Yogyakarta, is known to have the potential for a large earthquake, which can trigger a tsunami. A research report by S Widiantoro from the Global Geophysics Research Group, Bandung Institute of Technology (ITB), and a team in the journal Nature in 2020 has identified a seismic gap between the south coast of Java and the Java trough. Seismic gaps are active seismic zones that store energy and have the potential for major earthquakes in the future.

This elongated seismic gap can rupture separately or simultaneously during an earthquake. If only the segment in the southern part of West Java ruptures, the resulting earthquake could have a magnitude of 8.9, while the segment in Central Java-East Java could trigger an earthquake with a magnitude of 8.8. If both segments rupture simultaneously, the earthquake could have a magnitude of 9.1.

Next, they modeled the potential height of the tsunami if the earthquake occurred with three scenarios, namely only the West Java segment is released, the Central Java and East Java segments are released, and the West and East Java segments are released simultaneously.

Also read: The M 6 Earthquake in South Bantul Triggered Subduction Activity

The worst scenario is if the Java to East Java segment collapses at the same time, it could generate a tsunami with a maximum height of 20.2 meters near small islands off the south Banten. The average height of the tsunami along the southern coast of Java is 4.5 meters. The height of the tsunami can be even higher if the earthquake triggers underwater landslides, as happened in the M 7.5 earthquake in Palu in 2018.