Numerical simulation of pancake-shaped lava dome overflow from the summit crater: the 2018 Shinmoedake eruption, Japan

In 2018, Shinmoedake volcano, Japan, produced a rapidly emplaced, pancake-shaped andesitic lava dome within approximately three days. The lava filled the summit crater and slightly overflowed from the crater rim. Because collapse of lava domes can generate hazardous pyroclastic flows, understanding the behavior of lava overflow is essential for hazard assessment. To investigate the controls on overflow behavior, we conduct numerical simulations of lava extrusion within the Shinmoedake crater, using a depth-averaged Bingham-fluid model. The comprehensive simulations show that the overflow direction is primarily controlled by lava viscosity. When the viscosity is lower than 10^9 Pa s, lava overflows from the western side. In contrast, when the viscosity exceeds 10^9 Pa s, lava overflows from the eastern side. This difference in overflow direction is explained by a geometric effect: at higher viscosity, reduced lateral spreading leads to thicker lava accumulation, allowing the flow to overcome the higher eastern crater wall. By comparing the numerical results with SAR observations capturing the detailed evolution of dome morphology, we further constrain the lava viscosity to values lower than 10^9 Pa s and estimate the corresponding yield strength during the 2018 eruption. Using these rheological parameters, we discuss implications for predicting the extent of future lava emplacement and associated pyroclastic-flow hazard areas at Shinmoedake volcano.