Why is the Southern Java Region Prone to Landslides?

As of Tuesday (18/11/2025) morning, there are still 34 residents reported missing in the landslide disaster in Banjarnegara and Cilacap, Central Java, who are still being searched for. They are divided into seven victims in Cibeunying Village, Majenang District, Cilacap Regency, and 27 others in Pandanarum Village/ District, Banjarnegara Regency.

In Cilacap, the combined search and rescue (SAR) team successfully evacuated 16 deceased victims within four days of the search process. The frequently rainy weather posed challenges for the search team in locating and evacuating the victims. In addition to the relatively large landslide terrain, the length of the landslide area in Cilacap extends approximately 900 meters with a height difference of 100 meters.

The Head of the Data, Information, and Disaster Communication Center of the National Disaster Mitigation Agency (BNPB), Abdul Muhari, in a written statement on Tuesday morning, stated that based on the analysis results from the Geological Agency, there is potential for subsequent land movement around the landslide-affected area in Majenang.

The area falls within the medium landslide movement forecast zone. Based on the morphological data, the location consists of hills with gentle to steep slopes. Geologically, the soil in this area is characterized by thick weathered soil, brown, loose, and saturated with water, with a thickness of more than 10 meters.

While giving a presentation at the "Disaster Briefing on Cilacap Landslide Handling Efforts", Monday (11/17/2025) afternoon, Muhari said that Central Java (Jateng) is one of the provinces that most frequently experiences landslides and flash floods after West Java.

If viewed from its topography, both West Java, Central Java, and East Java have an average area prone to landslides located in the central to southern regions. In West Java, there are Garut, Tasikmalaya, and Ciamis. In Central Java, there are Cilacap, Banjarnegara, and Kebumen, while in East Java, there are Malang, Batu, and Trenggalek.

In Central Java itself, the most prominent disasters in the last 10 years occurred in 2017 and 2020. The areas most frequently affected are Banjarnegara and Cilacap, with a significant number of victims. The National Disaster Management Agency (BNPB) recorded that the number of victims suffering and displaced during the period of 2015-2024 was 13,351 individuals and 9,641 individuals, respectively.

"If you look at the characteristics, the soil in West Java and the central to southern parts of Central Java is loose. It has high porosity. Even if there are cracks, even if there are no cracks, high-intensity rain can cause landslides. There's a saturation point, which will cause the top layer of soil to slide," Muhari explained.

When contacted separately, Professor of Geophysical Disaster Science and Natural Resource Exploration at Brawijaya University Malang, Adi Susilo, stated that the central to southern part of Java Island is formed from limestone resulting from uplift from the sea. Limestone itself is a sediment from the sea that has subsequently been buried by layers of soil.

This layer of soil was once anchored by a multitude of plants. The roots of these plants penetrated deep into the ground and served as a natural barrier. However, recently, many of these plant roots have begun to disappear due to human activities. Meanwhile, on the other hand, the southern region of Java is generally characterized by hills with a fairly steep slope.

"Perhaps landslides also occurred in the past. However, at that time, settlements were not as numerous, making them less known to the public and not recorded as easily as they are today. With the sloping terrain, coupled with heavy rainfall and water saturation due to inability to seep into the ground, there is a potential for landslides to occur," he said.

The soil layer on the southern side of Java differs from the northern side, which is predominantly formed by fluvial deposits or sediments from rivers. As is well known, many rivers flow into the northern coast of Java. Meanwhile, on the other hand, the northern region tends to be flat with an elevation of around 100 meters, lower than the southern side, which can reach 500 meters above sea level (masl).

"Actually, both are dangerous, but the problems are different. On the northern side, the threats are tidal flooding and land subsidence due to excessive groundwater exploitation, as seen in Semarang and Jakarta, while on the southern side, it is landslides," said Adi Susilo, who also serves as the Chairman of the Earth Science and Disaster Mitigation Study Center at Brawijaya University.

To eliminate the impact, according to him, the plan for the construction of houses and buildings in areas located below the hill should be reconsidered or reviewed. In the past, such matters were never an issue because buildings below the hill were not as massive as they are today. Meanwhile, on the other hand, rainwater can no longer directly seep into the ground.

During the Disaster Briefing, Muhari also explained how to significantly improve the existing landscape and environment, including vegetation. However, the results of reforestation will not be visible in a short period of 3-5 years, but rather over a longer period of 15-20 years.

Meanwhile, as quoted from the vsi.esdm.go.id page, the Geological Agency responded to the landslide in Cilacap, stating that the morphology of the affected area is a moderately undulating structural hill with a steep, gentle slope. The affected area is at an altitude of 22-115 meters above sea level. Its shape resembles a truncated hill with a height difference of 50-90 meters.

Based on the Regional Geological Map Sheet 1308-5 Majenang (Kastowo and Suwarna, Geological Research and Development Center, 1998), the disaster-affected location is situated at the contact boundary between the Tapak Formation (Tpt) and the Kumbang Formation (Tmpk). The Tapak Formation is located above the Kumbang Formation. This condition is influenced by geological structures in the form of folds and faults.

The weathered soil appears very thick, brown in color, loose, friable, and saturated with water, with a thickness of more than 10 meters. The regional geological structure is influenced by tectonic processes such as folding, thrust faults, and strike-slip faults.

Therefore, referring to the landslide movement forecast map of Central Java from the Center for Volcanology and Geological Hazard Mitigation (PVMBG) of the Geological Agency in November 2025, the disaster area in Majenang falls within the forecast zone for medium landslides or ground movements. The most vulnerable zones are particularly in areas bordering river valleys, cliffs, road embankments, or slopes that are experiencing disturbances.

The disaster location in Pandanarum is situated on a moderately undulating structural hill morphology with gentle to steep slope inclinations. The height difference of the slopes ranges from 70 to 100 meters, with an elevation of the location between 450 and 1,048 meters above sea level. The morphology indicates a cut slope shape with several indications of old land movement.

Based on the Geological Map of the Banjarnegara and Pekalongan sheets (Condon et al. 1996), the location is situated at the contact between the Kumbang Formation (Tmpk) and the Halang Formation (Tmph). The Kumbang Formation consists of andesite-basalt lava, breccia, tuff, pumice breccia, sandy tuff, and intercalated marl. Meanwhile, the Halang Formation comprises tuffaceous sandstone, conglomerate, marl, and claystone that are plastic and easily saturated with water.

The geological structure around the location consists of a left-lateral fault oriented north-south to the west, and a thrust fault oriented northwest-southeast to the north. This weak structural zone is suspected to influence the stability of the slope. Rocks from the Halang Formation are estimated to be the sliding plane of this landslide.

Based on the landslide prediction map for Central Java from the PVMBG Geological Agency in November 2025, the disaster area in Pandanarum, Banjarnegara, is included in the medium landslide prediction zone. This means that the region may experience landslide events that are adjacent to river valleys, cliffs, road cut slopes, and on slopes that are experiencing disturbances.

Old landslides can become active again triggered by heavy rainfall and/or vibrations. Generally, the slope range is more than 9 degrees, depending on local geological conditions and slopes formed by deposited materials.

The controlling factors and triggers of this landslide incident are suspected to include steep slopes with significant height differences; volcanic material resting on top of the clay-silt rock of the Halang Formation, which is a slip surface for the frequent landslides occurring in Banjarnegara; and weathered soil that is loose, prone to collapse, and saturated with water.

In addition, high and prolonged rainfall that increases pore pressure, the possibility of seepage or subsurface flow, and slope cutting for settlement or access roads.