Microzonation study around Lake Toba, North Sumatra, Indonesia based on horizontal – vertical spectral ratio

Abstract Lake Toba, North Sumatra, Indonesia, is one of the biggest calderas in the world. The area is also prone to earthquake damage because of the tectonic activity due to its location in the subduction zone of Indo-Australian and Eurasian Plates. In recent years, Lake Toba has become one of the most attractive tourist destinations in Indonesia. Hence, the Indonesian government has been considering infrastructure development in this region. The region vulnerability cause by soil amplification due to the earthquake activity can be quantitatively investigated. We carried out a microzonation analysis around Lake Toba using Horizontal-Vertical (H/V) Spectral Ratio method. A total of about 30 short-period seismometers around Lake Toba were used for the analysis. We estimated several parameters that may define some soil characteristics, such as H/V peak amplitudes, peak frequencies, average shear wave velocity of 30 m depth (Vs30), and vulnerability index (Kg). Spatial distribution of peak frequencies and Vs30 suggest that the region may fall into sites SC and SD showing characteristics of dense to stiff soil. We also observed areas with a high vulnerability index (Kg > 20), which may indicate the high possibility of soil vulnerability due to strong earthquakes. The findings may provide preliminary results or sources for future geophysical, geological, and geotechnical analysis for disaster risk reduction in this high-risk region.