Landslide Susceptibility Assessment in Trenggalek, East Java, Indonesia: A Geological Overview

The number of landslide occurrences in Trenggalek tends to increase every year, and it represents a significant constraint on development, causing high levels of economic loss and substantial numbers of fatalities. A complete understanding of geological conditions is essential needed for spatial planning and to reduce the risk of lost lives and infrastructure damage. Surface mapping at regional scale comprised of lithology, geomorphology, geological structures, and land-use conditions was conducted in an area of 1261 km² to develop a detailed landslide susceptibility map. The surface mapping showed that landslides commonly occurred on steep slopes of the fault-controlled hills that having inclination ranging from 16° to 28°, in the areas covered by residual soils of Mandalika Breccia unit, with high geological structure density in the settlement areas. The high susceptibility zone covered 46,75% of the research area and was characterized by having slope inclination ranging from 8° to 45°, geological units of Mandalika Breccia and Arjosari Sandstone, and land uses of settlement, dry fields, and plantation. Undisturbed soil samples collected from the high susceptibility area exemplify the clay–silt material, and XRD analysis showed the dominant clay mineral composition of halloysite and smectite. Major ion components from precipitation water, seepage, and surface water samples were used to determine hydraulic connectivity in high susceptibility zone. Both seepage and surface water indicate sodium bicarbonate-type water similar to the chemical composition of the precipitation water. The landslide mechanism in the research area is mainly controlled by lithological conditions and triggered by high rainfall.