Fault Structures and Lithological Boundaries Study on Weh Island Using Horizontal Gravity Analysis

The horizontal gradient method is a widely used qualitative approach for enhancing the interpretation of gravity anomaly data. This method is particularly effective in delineating lithological boundaries between rock units with contrasting densities, especially where contacts are horizontally aligned. Horizontal gradient calculations are performed by partially differentiating gravity data with respect to its horizontal position. This method was applied to gravity data of Weh Island, the westernmost part of Indonesia, to delineate fault zones and lithological boundaries in the region. The results of the study show that this method can effectively identify rock formations on Weh Island, such as the boundary between coral reef limestone and pyroclastic flow units, as well as tuff sandstone areas and surrounding rocks, including volcanic kulam units and volcanic pumice rocks. Applying the horizontal gradient method to gravity data on Weh Island produced maps that clearly outline rock formations. Combining these with geological maps shows a northwest–southeast trend aligned with the main strike of the Seulimuem Fault, which crosses the island. The results from analyzing horizontal derivatives offer key insights for studies of lithology, faulting, and subsurface layers in complex geological regions like Weh Island.