ANALYSIS OF SHEAR WAVES VELOCITY (VS) USING HILBERT-HUANG TRANSFORM ON HORIZONTAL TO VERTICAL SPECTRAL RATIO (HVSR) METHOD IN MAMUJU, WEST SULAWESI

• Main Author: Feriansyah, Sofyan
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/81784
• Institution: Institut Teknologi Bandung

Mamuju, which is the capital city of West Sulawesi, has the potential to have high losses due to earthquakes due to geological conditions dominated by alluvial deposits which are soft soil types and has a relatively dense population of 293,326 people and is likely to continue to grow given its location as the provincial capital. The earthquakes that occurred on 15 January 2021 and 8 June 2022 which caused significant damage can be evidence of the influence of geological conditions on earthquake damage. The results of a survey conducted by the Meteorology Climatology and Geophysics Agency (BMKG) show that the impact of the two earthquakes included 1016 fatalities and 7923 buildings damaged. Apart from the two earthquakes that occurred in 2021 and 2022, there have been several other earthquakes that have caused significant damage in the last three decades. The 7.0 magnitude earthquake that occurred on 08 January 1984 caused casualties and many buildings collapsed, both residential houses and government agency buildings. On 18 August 2012 Mamuju was also affected by an earthquake with a magnitude of 6.2. The earthquake resulted in loss of life and building infrastructure in Mamuju. To reduce the potential damage induced by future earthquakes, it is necessary to analyse seismic vulnerability and seismicity risk through shear wave velocity profiles at the location. In this study, the Horizontal to Vertical Spectral Ratio (HVSR) method was applied to analyse microtremor data in Mamuju. Microtremor data were obtained from direct measurements after the 15 January 2021 and 8 June 2022 earthquakes occurred. The microtremor data obtained can be contaminated with local noise because the survey location is a densely populated area and is carried out during the day. Hilbert Huang Transform (HHT) is used in this study to eliminate local noise. This method has been shown to be more effective in eliminating local noise than using the Fast Fourier Transform (FFT) in the Horizontal to Vertical Spectral Ratio (HVSR) method. The resulting HVSR curves were inversed to obtain shear wave velocity (Vs) profiles. The inversion process in this study is based on the Neighbourhood Algorithm method so that the shear wave velocity profile (Vs) at each point is known. The shear wave profile (Vs) values obtained were used to determine the dynamic response of the soil in Mamuju and the results were compared with damage data from the 2021 and 2022 earthquakes. The results of the study generally showed agreement with the survey data after the 2021 and 2022 Mamuju earthquakes where heavy damage occurred in the red zone and light to moderate damage occurred in the yellow to green zones. Classification of soil types in Mamuju based on Vs30 values shows that Mamuju is composed of medium soil types (SD) and hard soil types (SC) with medium soil types (SD) dominating the study area. The depth of bedrock in Mamuju shows that the western region of Mamuju has the deepest bedrock of 330 m (thick sediment) and the eastern region has the deepest bedrock of 200m. We also made subsurface vertical crosssections of Mamuju with North - South and West - East trajectories. The vertical cross section was used to interpret the subsurface profile in Mamuju, the result is that Mamuju is composed of three rock formations, namely alluvial deposits in the uppermost layer, the Mamuju formation which underlies the alluvial deposits and the Adang Volcanic Rock formation which is the hardest rock formation in the study area. This result is based on velocity contrast matched with the geology of the study area.