Micro-Zonation of hot spring manifestation site at Umbul Niti, South Lampung based on microtremor

Abstract

In 2021, an intriguing event unfolded at Umbul Niti village when a resident’s well drilling endeavor unexpectedly yielded to a hot spring, thus transforming the site into a small local tourist attraction that captivates visitors and enhances local economies. However, it is crucial to prioritize the safety of visitors and infrastructure, considering the geological implications of hot springs are related to unstable weak zones that would be severely damaged when an earthquake strikes. Weak zones often exhibit heightened susceptibility to seismic events. Using Horizontal to Vertical Spectral Ratio (HVSR) analysis of micro-tremor data that was recorded for 40 minutes at 25 locations, the Amplification (A 0) and Natural Frequency (f 0) were obtained; then the Seismic Vulnerability Index (Kg ) were calculated. The study area has a dominant frequency of around 4.74 - 9.7 Hz indicating the dominance of hard Tertiary Type I soil from Lampung Formation (QTI). Kg value ranges from 0.08 - 3.57 interpreted as mostly low hazard level. However, in the NW - SE direction the value indicates a high Kg value which means a relatively high hazard level, passing through the hot spring location. Thus, the area at the NW-SE zone might require more specific treatment.