Bawean Earthquake Series Still Shaking, Mitigation is Needed

It is not yet possible to predict when the Bawean earthquake will stop, so strengthening mitigation is needed in the rehabilitation process.

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Residents were in tents while health workers from the Sangkapura Community Health Center examined their health at a shelter for earthquake victims in Suwari Village, Sangkapura District on Bawean Island, Gresik, on Tuesday (26/3/2024).

A series of earthquakes in the Java Sea, precisely north of Tuban Regency, which were triggered by the reactivation of old faults, are still continuing to shake East Java and its surroundings. The phenomenon known as the Bawean earthquake cannot be predicted when it will stop, so it is necessary to strengthen mitigation in the rehabilitation and reconstruction process for disaster management.

The Meteorology, Climatology and Geophysics Agency recorded 608 earthquakes occurring between March 22, 2024, and April 26, 2024, at 06.00 am WIB. Of this figure, the largest earthquake had a magnitude of 6.5 and the smallest was 2.1.

The series of continuous earthquakes that have occurred up until now are still difficult to predict when they will stop. This phenomenon has prompted the East Java Provincial Disaster Management Agency to be cautious in handling disasters, especially during the rehabilitation and reconstruction stages in infrastructure.

BPBD Jatim data shows that the maximum Bawean earthquake shock with a 6 MMI scale caused damage to buildings. For example, in Sangkapura District, Gresik, there are 17 affected villages with a total of 1,943 damaged houses, as well as 84 places of worship, and 31 school buildings. There are also six affected offices, one healthcare facility, and eight pesantren facilities.

Also read: Bawean Island Residents Still Traumatized by Earthquake

In Tambak District, there are 13 affected villages, 2,711 households, 97 places of worship, and 57 schools. Additionally, 12 office buildings and eight traditional markets were impacted. The damage to these structures varied from minor to moderate, severe, and even collapsed or demolished.

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Health workers at the Sangkapura Community Health Center are examining the health of infants in the refugee camp for earthquake victims in Dekatagung Village, Sangkapura District on Bawean Island, Gresik, on Tuesday (26/3/2024).

To address the impacts of the Bawean earthquake, the central, provincial, and district governments are collaborating to rehabilitate and reconstruct damaged buildings, particularly the homes of residents and public facilities, including schools, places of worship, healthcare facilities, and markets.

However, the rehabilitation and reconstruction phase of the disaster management is full of challenges due to the numerous damaged buildings. As rehabilitation efforts begin, aftershocks continue to shake the area. Therefore, apart from implementing the principle of caution, they are collaborating with competent experts from various universities.

"A hospital in Bawean, for example, is unsure about the condition of its building and has requested a building safety survey following the continuous aftershocks of the earthquake," said the Head of the East Java Regional Disaster Management Agency, Gatot Soebroto, during a webinar on the Bawean earthquake in East Java, held by the Sepuluh Nopember Institute of Technology (ITS) Surabaya on Saturday (27/4/2024).

The webinar organized by Geophysical Engineering also presented the Head of the BMKG Earthquake and Tsunami Center Daryono, Utama PVMBG Earth researcher Supartoyo, and BRIN researcher Susilo Hadi as speakers. The webinar participants came from students, the general public and academics.

Also read: Dozens of Schools Damaged, Educational Activities on Bawean Island Almost Paralyzed

Chief Earth Investigator of PVMBG, Supartoyo, stated that Bawean Island is a vulnerable area to earthquakes due to its proximity to the earthquake source, which is an old fault around the island experiencing reactivation. In addition to building damage, the series of Bawean earthquakes with a maximum shake of VI MMI scale triggered liquefaction disasters, ground cracks, and ground movements on Bawean Island.

AMBROSIUS HARTO MANUMOYOSO

A screenshot on the official BMKG application shows a tectonic earthquake in the Java Sea which shook again on Wednesday (3/4/2024).

"The location of the building damage is located in undulating to hilly terrain that is composed of soft, loose, and uncompacted Quaternary deposits. The occurrence of building damage in Bawean is caused by various factors," said Supartoyo.

One of the causes, as stated by Supartoyo, is the close proximity to earthquake sources, poor building quality, and geological conditions. In addition, it is located in a liquefaction and soil cracking zone, as well as being affected by topography.

Liquefaction occurred in Raba and Tanjunganyar hamlets in Lebak village, as well as Nyiur hamlet in Teluk Jatidawang village. Soil cracks also occurred in Teluk Jatidawang village. Meanwhile, a landslides disaster occurred in Celok-Celok hamlet in Kelubang Gubuk village.

In response to the situation in Bawean, Supartoyo urged residents to stay calm, be vigilant, follow the instructions of local BPBD officers, and not be influenced by unreliable information. To avoid the risk of damage due to earthquakes, buildings and public facilities should use earthquake-resistant construction and be equipped with evacuation routes.

According to him, buildings damaged by liquefaction and soil cracks can still be constructed with earthquake-resistant structures. Meanwhile, buildings located on the edge of slopes need to have their construction strengthened.

KOMPAS/BAHANA PATRIA GUPTA

The Al-Muhajirin Mosque which was damaged by the earthquake in Gunungteguh Village, Sangkapura District, Bawean Island, Gresik Regency, Sunday (24/3/2024).

PVMBG recommends that the East Java Provincial Government and Gresik Regency Government revise the spatial plan of Bawean Island based on data from the Geological Agency, such as the KRBG map (earthquake-prone disaster zone), land movement vulnerability zone map, and data from field inspections by the Emergency Disaster Response Team for Earthquakes. To strengthen early mitigation efforts, PVMBG recommends that local governments include geological disaster material such as volcanoes, earthquakes, tsunamis, and land movements in the education curriculum on Bawean Island.

The Bawean earthquake was centered in a low seismicity activity zone, so it is understandable that the community perceives it as an unusual earthquake because it occurred in an area where shallow earthquakes rarely happen.

The Head of the BMKG Earthquake and Tsunami Center, Daryono, said that the series of Bawean earthquakes started with an earthquake that occurred on March 22, 2024 with the epicenter in the Java Sea. The magnitude 6 earthquake was centered at a depth of 10 kilometers. The earthquake event began with an opening earthquake and was followed by a series of aftershocks.

"The Bawean earthquake was centered in a zone of low seismicity so it is natural for people to consider it an unusual earthquake because it occurred in an area where shallow earthquakes are rare," said Daryono.

So far, said Daryono, the Java Sea region has usually been the epicenter of deep hypocenter earthquakes (deep focus) due to slab deformation of the Indo-Australian Plate which was subducted under the Eurasian Plate, precisely under the Java Sea at a depth of around 500-600 km.

The Bawean earthquake measuring magnitudes 5.9 and 6.5 on March 22 2024 was a type of shallow crust triggered by active fault activity with a strike-slip mechanism in the Java Sea. The Bawean earthquake was centered in the old fault zone because the Java Sea area north of East Java is geologically and tectonically located in the old fault zone of the Meratus pattern.

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Prospective passengers board the Bahari Express Fast Ship at Bawean Harbor, Bawean Island, Gresik Regency, Wednesday (27/3/3034).

"If we look at the location of the Bawean earthquake center, the epicenter is located precisely on the Muria Fault (sea) line according to a journal published by Peter Lunt (2019)," said Daryono.

According to Daryono, there are many aftershocks due to the characteristics of shallow crust earthquakes in Bawean, occurring on the earth's surface rock formations that are heterogeneous in nature and therefore brittle. Aftershocks commonly occur after a strong earthquake, but there is no need to fear them.

"The number of aftershocks can actually provide information on earthquake decay so that we can estimate when the aftershocks will end," said Daryono.

Geology expert from ITS, Amin Widodo, emphasized that the Bawean earthquake provides many valuable lessons for the community. One of them is to remind the community and the government to always be alert and strengthen mitigation efforts when living in earthquake-prone areas.

Furthermore, the reactivation of old faults should serve as a momentum for the government to investigate and map the characteristics of all faults in the archipelago, both old and new.

Also read: Residents of Pawean Island are again panicked

At the same time, there is a need to remind the public and policymakers that it is not earthquakes that cause disasters for humans, but the damage to buildings caused by earthquakes. Therefore, there is a need to reevaluate building constructions to use earthquake-resistant constructions in order to reduce disaster risks.