Bulletin of the Geological Society of Malaysia, Volume 73, May 2022, pp. 139 – 149

Geophysics and geomorphic observation for near-surface structures mapping of Seulimeum Fault on Lamtamot area, Northern Sumatra

Muhammad Yanis1, Gozian Islami1, Nazli Ismail1,2,*

1 Department of Geophysical Engineering, Faculty of Engineering, Universitas Syiah Kuala, Darussalam-Banda Aceh 23111, Indonesia
2 Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Syiah Kuala, Darussalam-Banda Aceh 23111, Indonesia
* Corresponding author email address: nazli.ismail@unsyiah.ac.id

Abstract: In the northern part of Sumatra Island, Indonesia, the Great Sumatran Fault, which can cause an earthquake, was divided into two segments: the Aceh and Seulimeum. An effort to reduce the risk is mapping the fault area, especially in the region that does not clearly show the sign on the surface, e.g., in the Lamtamot area, Indonesia. Electrical resistivity is widely used to study shallow structures, but the method requires more time when applied in a large area. This research explores the potential of an extremely low frequency (very low frequency-electromagnetic; VLF-EM) method to investigate the shallow fault of the Seulimeum segment. Here, the VLF-EM is compared with other geophysical methods such as resistivity and magnetic methods. For comprehensive results, the geomorphic observation that was conducted covered outcrops of the fault and trenching sites in the geophysical study for validating the model. A similar pattern of the VLF-EM and electrical resistivity data has been shown in a two-dimensional profile using data processing. The fault structure can be mapped at a distance of 20–24 m from the profile measurement, which is demonstrated by the low current density associated with the conductive zone from the VLF-EM, and low resistive anomaly in electrical resistivity. The fault can also be confirmed via magnetic intensity, which significantly increases at the same distance (20–25 m) of the VLF-EM and electrical resistivity. The geomorphic observation shows outcrops of fault activity, such as fault scarp, fractures, and faults, in the same direction as the Seulimeum segment, while scrap extends in the northwest direction up to ~20
m around the geophysical surveys. As revealed by the results, the VLF-EM method combined with other geophysical surveys and geomorphic observation can be used as a technique to image the fault that shows the shallow structure of the Seulimeum fault at 20–32 m along the profile.

Keywords: Near-surface geophysics, Seulimeum segment, VLF-EM, resistivity, fault structure

DOI: https://doi.org/10.7186/bgsm73202211

0126-6187; 2637-109X / Published by the Geological Society of Malaysia.
© 2022 by the Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution (CC-BY) License 4.0.