Assessment of Seismic Vulnerability and Shear Wave Velocity Estimation Using the Horizontal-to-Vertical Spectral Ratio (HVSR) Method in Umbul Niti Geothermal Manifestations, Jatimulyo Village, South Lampung

Abstract The discovery of geothermal manifestations in 2021 in Umbul Niti, Jatimulyo Village, South Lampung Regency, reveals significant potential for geotourism development, which can help reduce poverty. However, this potential is accompanied by an unstable zone due to a fault, increasing earthquake risks. Additionally, population growth and the presence of the Sumatera Institute of Technology (ITERA) pose development challenges related to population density and potential losses. Therefore, investigating soil characteristics based on natural frequency ( f 0 ), amplification ( A 0 ), and shear wave velocity at depths of 0-30 meters ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>V</mml:mi> <mml:mi>s</mml:mi> <mml:mrow> <mml:mn>30</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) is crucial for development planning and earthquake risk evaluation through seismic vulnerability values ( K g ). The Horizontal to Vertical Spectral Ratio (HVSR) method was used, revealing f 0 values from 3.876 Hz to 23.695 Hz, indicating sediment thickness variations from thick to very thin, dominated by very thin layers with very low amplification. The f 0 results, correlating with geothermal areas, show the same anomaly contrast as previous studies with 3D gravity model results and residual magnetic anomalies. Additionally, the Particle Swarm Optimization (PSO) method was used to obtain <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>V</mml:mi> <mml:mi>s</mml:mi> <mml:mrow> <mml:mn>30</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> variations, ranging from hard to soft soil, with the lowest <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>V</mml:mi> <mml:mi>s</mml:mi> <mml:mrow> <mml:mn>30</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> values near the manifestation. The seismic vulnerability index was generally low, indicating a low earthquake disaster risk in the study area. The cross-section results identified three faults in cross-section A-A’ and two faults in cross-section B-B’, corresponding with the forward modeling in the magnetic method. This research is expected to contribute positively to geotourism development, particularly in planning and disaster risk mitigation. These findings highlight the need for careful planning and risk mitigation to leverage geotourism potential while managing seismic risks. Although the seismic vulnerability index is generally low, some medium risk zones were detected, emphasizing the importance of integrating geotourism development with comprehensive risk management strategies to ensure sustainable geotourism development and community resilience.