1 1Graduate School of Mathematics and Applied Sciences, Universitas Syiah Kuala, Banda Aceh, Indonesia

2 Meteorological, Climatological, and Geophysical Agency, BMKG, Banda Aceh, Aceh, Indonesia

3 Tsunami and Disaster Mitigation Research Center, Universitas Syiah Kuala, Gampong Pie, Indonesia

4 Earth Observatory of Singapore, Nanyang Technological of Singapore, Singapore

5 Department of Science and Technology, Universitas Islam Negeri Ar-Raniry, Kopelma Darussalam, Banda Aceh, Indonesia

6 Meteorological, Climatological, and Geophysical Agency, BMKG, Medan, Indonesia

7 Graduate School of Mathematics and Applied Sciences, Universitas Syiah Kuala, Banda Aceh Indonesia

8 Asian School of the Environment, Nanyang Technological University of Singapore


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.

Graphical Abstract

Environmental vulnerability characteristics in an active swarm region


  • The microtremor results find five clusters in Toba and its surroundings that describe a recent subsurface condition in the swarm zone;
  • Samosir Island as the center of swarm location is categorized as the prone area with highest vulnerability level. The highest value is relevant with the earthquake intensity felt by the local population in Samosir Island;
  • The microtremor results desrcibe a potential amplification that can be influenced by a swarm earthquake. It will be important to plan the mitigation program for swarm earthquakes that may occur at shallow depths, based on subsurface soil properties.


Main Subjects


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