Ground Motion and Liquefaction Study at Opak River Estuary Bantul

Abstract

In Mei 2006, an earthquake occurred, and liquefaction was induced in Bantul Regency in the form of lateral spreading and sand boiling. Based on Indonesia's liquefaction susceptibility zone map 2019, the Bantul coast was identified as a high liquefaction hazard zone. There is a high risk of liquefaction in that location, and if liquefaction occurs, the effective stresses of the soil will be lost. This study aimed to determine ground motion-induced liquefaction potential in Opak river estuary in Bantul. The peak ground acceleration can be predicted by probabilistic seismic hazard analysis (PSHA) based on Indonesian spectra design application, deterministic seismic hazard analysis (DSHA) based on the mechanism of a recent earthquake in Bantul, and non-linear earthquake site response analysis (NERA) based on the 2004 Parkfield California earthquake, the 1986 Chalfant Valley California earthquake, and the 1987 Superstition Hill California earthquake. The value of acceleration from site-specific responses with DSHA and NERA is 0.558 g. Then, the maximum acceleration, seismic magnitude, N-SPT bore log soil, and other soil parameters are analyzed with a semi-empirical simplified method to estimate the safety factor of liquefaction potential using Settle3. The liquefaction safety factor was calculated by dividing the cyclic resistance ratio by the cyclic stress ratio; liquefaction occurs if the safety factor is less than 1. The result indicates that the Opak river estuary has a liquefaction potential beneath the surface from -1.5 to -16.5 m depth with various thick each boreholes. It implies that the bearing capacity design of the foundation system in this area and the liquefaction effect that will emerge when an earthquake occurs must be appropriately considered.