Seismicity and magma supply rate of the 1998 failed eruption at Iwate volcano, Japan

Iwate volcano, Japan,showed significant volcanic activity including earthquake swarms and volcano inflation from the beginning of 1998.A large earthquake of magnitude 6.1 hit the south-west of the volcano on September 3.Although a 1 km²no magmatic eruptions occurred.We reconcile the spatio-temporal distributions of volcanic pressure sources determined by previously reported studies in which GPS,strain and tilt data from dense geodetic station networks are analyzed (Miura et al. Earth Planet Space 52:1003-1008,2000; Sato and Hamaguchi J Volcanol Geotherm Res 155:244-262,2006).We calculate the magma supply rates from their results and compare them with the occurrence rates of volcanic earthquakes. The results show that the magma supply rates are almost constant or even decrease with time while the earthquake occurrence rate increases with time.This contrast in their temporal changes is interpreted to result from stress accumulation in the volcanic edifice caused by constant magma supply without effusion of magma to the surface. We further show that data showing slight acceleration in strain can be best explained by magma ascent at a constant velocity, and that there is no evidence for increased magma buoyancy resulting from gas bubble growth. This consideration supports the interpretation that the magma stayed at 2 km depth and horizontally migrated.These findings relating magma supply rate and seismicity to magma ascent process are clues to understanding why no magmatic eruption occurred at Iwate volcano in 1998.