Field evidence of the greatest disaster in Balinese history: The 1815 Geger Bali multi-hazard event in Buleleng

On November 22, 1815, a M7.3 offshore earthquake and heavy rainfall triggered a major landslide on Bali's Buyan-Bratan caldera, initiating a cascading sequence of natural processes. This event, previously unstudied in scientific literature, is analyzed here using historical records, geomorphological observations, sediment analysis, radiocarbon dating, as well as textural, mineralogical, and geochemical analyses. The ~5.5 km horseshoe-shaped crown scarp on the caldera flank confirms that a translational landslide, triggered by a rock-slope failure along the rim, displaced approximately 64 × 10⁶ m³ of material over an area of 2.38 km². The landslide evolved as a debris avalanche involving basaltic-andesitic rock and silty-clay soils, depositing megaclasts within 10 km of the source. Midstream, it progressively transformed into a cohesive debris flow with multi-metric boulders, driven by progressive water saturation and reduced internal friction. Hydration of the mass facilitated fluidization and the transition from avalanche to cohesive debris flow. Channel bed erosion, lateral bank failures, secondary landslides, and probably rain-triggered lahars contributed to the bulking of the debris flow, increasing both its volume and thickness. After traveling 17 km, the flow entered the sea, likely triggering a local tsunami. However, no tsunami deposits associated with the 1815 landslide have been identified in coastal records, possibly due to the tsunami's limited size and/or post-depositional sediment alteration. This geomorphological study enhances the geohistorical understanding of the 1815 Gejer Bali disaster and underscores its relevance for current risk awareness and collective memory in the landslide-prone areas of Buleleng.