On 17th of June 2015, rockfall occurred on the South Coast at Gunung Kidul Regency taking a few casualties. The preliminary investigation results concluded that many cliffs on the South Coast are dangerous, and there is a potential that similar events might happen in the future. Therefore, geotechnical investigation in order to plan mitigation action is necessary. Field investigations, laboratory work, and numerical modeling were employed in this study, focusing on Sadranan beach where the incident happened. Rock slope stability analyses by means two-dimensional finite element in both static and seismic conditions were conducted. The investigation results show that both biological decay and wave erosion influence the rock slope instability. Moreover, the infilling material in rock joint has considerable effect on the stability of the slope. However, the results from numerical simulation suggested that potential failure was originating from the intact rock body instead of the plane of discontinuities. In the seismic condition, the slopes are more vulnerable and further mitigation action should be considered in the future.
Landslides are one of the most disastrous natural hazards that frequently occur in Indonesia. In 2017, Balai Sabo developed an Indonesia Landslide Early Warning System (ILEWS) by utilizing a single rainfall threshold for an entire nation, leading to inaccuracy in landslide predictions. The study aimed to improve the accuracy of the system by updating the rainfall threshold. We analyzed 420 landslide events in Java with the 1-day and 3-day effective antecedent rainfall for each landslide event. Rainfall data were obtained from the Global Precipitation Measurement (GPM), which is also used in the ILEWS. We propose four methods to derive the thresholds: the first is the existing threshold applied in the Balai Sabo ILEWS, the second and third use the average and minimum values of rainfall that trigger landslides, respectively, and the fourth uses the minimum value of rainfall that induces major landslides. We used receiver operating characteristic (ROC) analysis to evaluate the predictability of the rainfall thresholds. The fourth method showed the best results compared with the others, and this method provided a good prediction of landslide events with a low error value. The chosen threshold was then applied in the Balai Sabo-ILEWS.
The Malalayang Beach area is a strategic tourist destination close to Manado city. Arrangement of the area is carried out by building several buildings that can support this area as a tourist destination. Liquefaction is a phenomenon when the soil loses its strength of contact between particles. This is due to earthquake shocks that trigger an increase of water pressure in areas with loose sand characteristics (not dense). The settlement of soil due to liquefaction is a vertical deformation of the soil in the soil layer caused by soil compaction due to earthquakes. The study purpose is to determine how much reduction could occur at the research site as an early stage of the early stages of risk management. With the N-SPT data, the Yoshimine method and the computational method Rocscience Settle 3D can be used to analyze the settlement. Yoshimine method indicates settlement with a very low classification in BH-MLY-01 and high in BH-MLY-05. In RS Settle 3D, the location of BH-MLY-01 has a very low classification, and the location of BH-MLY-05 is dominated by low classification. The maximum settlement resulted from the Rocscience Settle 3D at the BH-MLY-05 location by 11,588 cm.
Terjadi longsor di area tebing batu (Rocky Wall) jalan akses fasilitas produksi pipa penyalur uap dan brine Pembangkit Listrik Tenaga Panas bumi (PLTP) milik PT. Pertamina Gothermal Energy (PT.PGE) di Kabupaten Muara Enim, Provinsi Sumatera Selatan, yang menyebabkan terjadinya penurunan produksi (derating) di PLTP akibat rusaknya fasilitas produksi dari sumur produksi klaster 1 ke PLTP. Tujuan dari penelitian ini untuk mengidentifikasi struktur lapisan bawah permukaan longsoran di area rocky wall dan untuk memprediksi kedalaman bidang gelincir (slip surface) berdasarkan nilai resistivitas lapisan tanah dengan metode geofisika ERT, sehingga metode geoteknik yang dipilih untuk penanganan longsor adalah tepat. Penelitian ini dilakukan dengan metode geofisika Electrical Resistivity Tomography (ERT) konfigurasi Wenner, Schlumberger dan Dipole-dipole dengan Ares II (Automatic Resistivity System II) sebagai peralatan utama ERT yang digunakan dalam pengambilan data. Luas daerah penelitian adalah 150 m × 200 m dengan jumlah pengukuran sebanyak 4 lintasan. Dari hasil pengolahan data pengukuran resistivitas menggunakan perangkat lunak Res2dinv dan Zondres2D, hasilnya menunjukkan bahwa sebaran batuan di daerah longsor rocky wall didominasi oleh lapisan breksi andesit lapuk dan lanau berpasir berlempung. Berdasarkan back analysis dan analisis stabilitas lereng sebelum kondisi longsor terjadi dan setelah longsor terjadi menggunakan Slope/W, didapatkan lapisan yang berpotensi menjadi bidang gelincir berkesesuaian pada kedalaman 10,43 m di bawah permukaan lereng.
On September 30, 2009, extensive landslides occurred in Tandikat, Padang Pariaman Regency, West Sumatra Province, Indonesia, burying hundreds of people, and flattening some villages after a Mw 7.6 earthquake hit West Sumatra coast. The landslides occurred during rainfall, and originated on mountains mantled with loose pumice. The probability of concurrent earthquake and rainfall event in this area requires landslide hazard evaluation by considering the effect of the unfortunate combination of rainfalls and earthquakes. To evaluate the landslide hazard level, the term of specific volume ratio (RSV) was used as the ratio of displaced volume to the maximum volume per unit width that could collapse from the slope. Combined analysis of groundwater model and slope stability was utilized to determine the landslide hazard level. Stochastic analysis using Monte-Carlo method was implemented to deal with uncertainties in determining slope stability. The stochastic analysis of the Tandikat landslide confirms that smaller earthquakes could possibly trigger catastrophic landslides during rainfall. Smaller peak ground acceleration of approximately 0.15 g could result in a more than 60% chance of Rsv >0.75, while the analysis of dry condition yields a 30% chance of catastrophic level of landslide hazard. This suggests that rainfall condition increases the probability of catastrophic landslide. The stochastic analysis of the Tandikat landslide confirms that smaller earthquakes could possibly trigger catastrophic landslides during rainfall. The results also suggest that peak ground acceleration of approximately 0.3 g is considered as the critical magnitude of ground acceleration that could result in a nearly 100% probability of catastrophic level of landslide hazard in the area.
A large earthquake struck Padang Province, West Sumatra, Indonesia, at 17:16 on September 30, 2009. The earthquake had a moment magnitude of Mw 7.6, and triggered landslides in Tandikat, Padang Pariaman Regency. The landslides occurred during rainfall, and originated on mountains mantled with loose pumice, taking many lives. The unfortunate combination of intensive rainfall and strong earthquake probably decreased slope stability. This study seeks to examine the initiation mechanism of earthquake- induced landslides during rainfall, and to develop a new approach to predict pore pressure increase by assuming reciprocal relationships between strain, stiffness, and pore pressure. In order to assess slope stability, the concept of stiffness degradation was used to predict pore pressure increase due to earthquake. This was achieved by developing empirical formulation based on cyclic triaxial test results. A new procedure based on the "rigid block on quasi plastic layer" assumption was developed to assess slope stability of earthquake-induced landslides during heavy rainfall. Results from cyclic triaxial test experiments showed that effective confining pressure and initial shear stress had considerable influence on increase in pore pressure. Slope stability analysis using actual earthquake acceleration suggest that landslide occurred due to pore pressure build up and the factor of safety decreased rapidly before earthquake acceleration reached its peak. The results emphasize the high risk of catastrophic earthquake-triggered landslides in tropical regions with high rainfall. It also suggest that landslide with similar mechanism of pore pressure increase are likely to occur on saturated sliding zones during smaller earthquakes.
The UNESCO Chair Workshop on Geoenvironmental Disaster Reduction was held on 28 April – 1 May 2019 in Palu City and Jakarta, Indonesia. Firstly, this article introduces the background and objectives of the workshop, reports the field trip around disaster area of 7.4 Mw 2018.9.28 Palu-Donggala earthquake, and then reports the ICGdR 2019 Council Meeting and UNESCO Chair Workshop in UNESCO Office, Jakarta. Finally, this article oulines the visit on Indonesian Meteorological, Climatology and Geophysical Agency (BMKG), Jakarta.
On 10 January 2023, a strong earthquake with a moment magnitude (Mw) of 7.5 occurred in Banda Sea, Maluku Province. This earthquake caused casualties around the epicentre due to the building collapse which was not an earthquake-resistant structure. A high level of seismic vulnerability and non-standard infrastructure often result in large losses when earthquakes occur. Therefore, mapping of earthquake-prone areas needs to be carried out to optimize the mitigation efforts based on Peak Ground Acceleration (PGA) and Spectrum Acceleration (SA) values. In this paper, mitigation efforts are carried out by mapping earthquake-prone areas using Probabilistic Seismic Hazard Analysis (PSHA) which considers the potential of each complex earthquake source in Maluku Province. Input data was obtained from several earthquake catalogues such as Advanced National Seismic System (ANSS), International Seismological Center (ISC), and United States Geological Survey (USGS) which were processed and analyzed using the Matlab, ZMap, and R-CRISIS programs and mapped using the ArcMap program. The results of PSHA show that Maluku Province is a region with varying levels of earthquake vulnerability. The research results show that the distribution hazard value on PGA in Maluku Province reaches 0.02g to 0.48g for a return period of 500 years and 0.04g to 0.79g for a return period of 2,500 years and on SA for periods of 0.20 seconds and 1.00 second reaches 0.04g to 0.89g and 0.01g to 0.22g for a return period of 500 years and 0.07g to 1.48g and 0.02g to 0.35g for a return period of 2,500 years, respectively. The highest level of vulnerability is in the northern region including Seram Island, Way Apu Island and Ambon Island and the lowest level of vulnerability is in the eastern region including Aru Island.
Bunaken Island is a quaternary deposit of Holocene age. Dominating sand layers, shallow groundwater table, and near active fault locations make the area highly susceptible to liquefaction. This study aims to determine the minimum ground acceleration that can potentially trigger liquefaction in the area. In this study, earthquakes originating in the North Sulawesi Thrust were modelled with various magnitudes. PGA was calculated using the attenuation function from : Liu and Tsai (2005), Abrahamson et al. (2016), Atkinson and Boore (2003), and Zhao et al. (2006). Each earthquake parameter was analyzed for its liquefaction potential using the simplified procedure by Idriss and Boulanger (2008), and then the minimum earthquake parameter value that can cause liquefaction was determined. The analyses show that the study site has the potential for liquefaction if more than Mw 5.8 earthquake occurs with a PGA value of above 0.17g. The BKN-BH02 borehole is the most critical point of the four boreholes made at the study site.
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