Environmental Management
A.V.H. Simanjuntak; U Muksin; A. Arifullah; K. Lythgoe; Y. Asnawi; M. Sinambela; S. Rizal; S. Wei
Abstract
BACKGROUND AND OBJECTIVES: For the first time, an earthquake swarm occurred from April to August 2021 in Lake Toba; Indonesia, the world’s largest caldera lake. Although the earthquakes were located in a volcanic environment, the swarm activities could also be related to tectonic activities on ...
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BACKGROUND AND OBJECTIVES: For the first time, an earthquake swarm occurred from April to August 2021 in Lake Toba; Indonesia, the world’s largest caldera lake. Although the earthquakes were located in a volcanic environment, the swarm activities could also be related to tectonic activities on the Sumatran fault. The swarm activities occurred at shallow depths and may influence the ground surface condition in which soil or rock below the subsurface can amplify the shaking. The research objective was to investigate the characteristics of the earthquake swarm in the Toba Caldera from the spectrum of the earthquake waveforms, site frequency, and horizontal-to-vertical ratio of sites.METHODS: The spectra of very closely located swarm and nonswarm earthquakes were analyzed to investigate the differences between both types of seismic events. The seismic spectral ratio of horizontal-over-vertical components was applied to calculate the spectrum in the active swarm region from all newly installed seismic sensors. The root mean square was applied to average the amplitude of the horizontal components. Then, the values of the horizontal-to-vertical ratios were obtained by comparing the average values of the horizontal and vertical components.FINDING: The microtremor study showed a more complete spectrum waveform from the low-to-high frequency of a non swarm earthquake, while the swarm earthquakes generated high-frequency seismograms. From the combination values of natural site frequencies and the horizontal-to-vertical ratios, the Toba environment can be classified into five clusters: I) Samosir–Hasinggaan, II) Samosir–Parapat, III) Silimapuluh, IV) Balige–Paropo, and V) Panjaitan. Samosir Island located in the middle of the Toba Caldera has the highest frequency and amplification, which are divided into two clusters.CONCLUSION: Cluster I, with high amplification corresponding to the earthquake intensity, was felt by people in northern Samosir. Cluster II is located in the southern part of Samosir Island. Cluster III features moderate values of amplification and seismic vulnerability and therefore needs attention before future infrastructure development. Cluster IV, located in the southern and northern regions with high amplification and vulnerability, is associated with the Quaternary eruption. Cluster V, situated in northeastern Toba, has the lowest amplification and vulnerability compared to other clusters. The microtremor results provide good correlation with the geology in the volcanic environment of the Toba region.
Environmental Management
U. Muksin; E. Riana; A. Rudiyanto; K. Bauer; A.V.H. Simanjuntak; M. Weber
Abstract
BACKGROUND AND OBJECTIVES: Soil or rock types in a region are often interpreted qualitatively by visually comparing various geophysical properties such as seismic wave velocity and vulnerability, as well as gravity data. Better insight and less human-dependent interpretation of soil types can be obtained ...
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BACKGROUND AND OBJECTIVES: Soil or rock types in a region are often interpreted qualitatively by visually comparing various geophysical properties such as seismic wave velocity and vulnerability, as well as gravity data. Better insight and less human-dependent interpretation of soil types can be obtained from a joint analysis of separated and independent geophysical parameters. This paper discusses the application of a neural network approach to derive rock properties and seismic vulnerability from horizontal-to-vertical seismic ratio and seismic wave velocity data recorded in Majalengka-West Java, Indonesia.METHODS: Seismic microtremors were recorded at 54 locations and additionally multichannel analyses of surface wave experiments were performed at 18 locations because the multichannel analyses of surface wave experiment needs more effort and space. From the two methods, the values of the average shear wave velocity for the upper 30 meters, peak amplitudes and the dominant frequency between the measurement points were obtained from the interpolation of those geophysical data. Neural network was then applied to adaptively cluster and map the geophysical parameters. Four learning model clusters were developed from the three input seismic parameters: shear wave velocity, peak amplitude, and dominant frequency.FINDINGS: Generally, the values of the horizontal to vertical spectral ratios in the west of the study area were low (less than 5) compared with those in the southeastern part. The dominant frequency values in the west were mostly low at around 0.1–3 Hertz, associated with thick sedimentary layer. The pattern of the shear wave velocity map correlates with that of the horizontal to vertical spectral ratio map as the amplification is related to the soil or rock rigidity represented by the shear wave velocity. The combination of the geophysical data showed new features which is not found on the geological map such as in the eastern part of the study area.CONCLUSION: The application of the neural network based clustering analysis to the geophysical data revealed four rock types which are difficult to observe visually. The four clusters classified based on the variation of the geophysical parameters show a good correlation to rock types obtained from previous geological surveys. The clustering classified safe and vulnerable regions although detailed investigation is still required for confirmation before further development. This study demonstrates that low-cost geophysical experiments combined with neural network-based clustering can provide additional information which is important for seismic hazard mitigation in densely populated areas.
Environmental Management
Y. Asnawi; A.V.H. Simanjuntak; U. Muksin; M. Okubo; S.I. Putri; S. Rizal; M. Syukri
Abstract
BACKGROUND AND OBJECTIVES: Soil or rock properties where buildings are situated play an important role in the ground shaking caused by an earthquake. The highly populated Banda Aceh city in the northernmost Sumatra is flanked by two active faults, the Seulimeum and the Aceh segment. Therefore, it is ...
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BACKGROUND AND OBJECTIVES: Soil or rock properties where buildings are situated play an important role in the ground shaking caused by an earthquake. The highly populated Banda Aceh city in the northernmost Sumatra is flanked by two active faults, the Seulimeum and the Aceh segment. Therefore, it is crucial to investigate the subsurface characteristics of the region to reduce the earthquake risk as there was no regional study has been conducted so far.METHODS: Characteristics of the soil or rock of the subsurface were derived from various seismic parameters. The seismic microtremors were recorded at 36 sites covering the highly populated city and the two active faults. The spatial autocorrelation method was used to obtain a dispersion curve based on the relationship between seismic frequencies and phase velocity from triangular geophones array to determine the shear wave velocity of the subsurface layer. The seismic amplification, dominant frequency and vulnerability value at each measurement point were measured using the horizontal-to-vertical spectral ratio method. The maps of velocity structure and HVSR parameters were generated from the interpolation of those seismic parameters.FINDING: Based on the variation of the four geophysical parameters: shear wave velocity, seismic amplification, dominant frequency, and seismic vulnerability, the study area can be clustered into five different groups: I) Banda Aceh, II) Jantho, III) Krueng Raya, IV) Lhoknga-Lhoong, and V) Seulawah, which classify the different types of rocks. The classification of soil properties from the combination of shear wave and horizontal-to-vertical spectral ratio data correlates with the geology of the study area.CONCLUSION: The Banda Aceh city, flanked by the two active faults, is characterized by low shear wave velocity and high amplification because the city stands on the sedimentary basin; thus, it requires a detailed investigation prior to constructing infrastructures. The other clusters are located on the relatively less vulnerable areas, indicated by moderate shear wave velocity and moderate to low seismic vulnerability indexes. The joint analysis shows that the combination of physical properties, including the shear wave velocity, seismic amplification, and dominant frequency, can be used to investigate lithology and seismic vulnerability into a specific cluster. The research results are essential for hazard mitigation and can be used for disaster risk management by the local government. A detailed investigation with denser measurement points needs to be conducted to comprehensively describe the types of rocks in Banda Aceh and its surrounding.