A new model for volcanic earthquake at Unzen Volcano: Melt rupture model

Small earthquakes occurred inside the flowing magma during the 1991-1995 activities of Unzen Volcano. This paradoxical phenomenon can be explained when we consider that even a flowing magma fails brittly under the condition that the product of viscosity and strain rate of magma exceeds a certain threshold value. The estimated strain rate of magma in the conduit leads us to the conclusion the brittle failure is expected when the viscosity is over 10^11.5 Pa s. Laboratory measurements of samples show that this viscosity range can be attained when the temperature is under 850°C in dry condition, and 800°C with 0.6 wt.% water. Considering the temperature from Fe-Ti oxide geothermometer (780-880°C) and the gas temperature (800°C), the brittle failure condition could be satisfied inside the conduit. The melt failure by shear flow could be the source of the earthquakes occurred at the summit area.