Identification of Subsurface Structure Causing Liquefaction in Palu City Based on Gravity Anomaly

Abstract Liquefaction in Balaroa and Petobo, Palu City occurred following the September 28 2023 earthquake which was triggered by movement of the Palu-Koro fault. It is necessary to study the existence of local geological factors which may be the cause of liquefaction in these areas. One of the contributing factors could be the existence of small structures as a follow-up to the main structure of the Palu-Koro fault. Using gravity methods at both regional scales for the Palu area and detailed scales in the Balaroa and Petobo areas to identify the presence of subsurface structures. In June 2023, gravity measurements were carried out after the Palu-Donggala earthquake in the Palu City and its surroundings. The measurement results of the gravity anomaly map in the Palu City are in the range of gravity anomaly between -4 – 66 mGals. The Balaroa region is in the gravity anomaly between 19.5 – 34.5 mGals, while Petobo is in the range -2 – 10 mGals. The gravity anomaly map shows the appearance of a structure which is characterized by the contrast of the anomalies which form a lineament which is believed to be a structure. The results of the analysis of gravity anomalies confirm the existence of small structures trending west-east and north-south in the Balaroa and Petobo liquefaction areas which may be one of the causes of liquefaction.