Structural evolution beneath Sakurajima Volcano, Japan, revealed through rounds of controlled seismic experiments

• Repeating seismic experiments reveal variation in seismic reflectivity beneath Sakurajima Volcano, Japan.
• Reflectivity variation locates northeastern Sakurajima at the depth of 6.2 km.
• Seismic reflectivity decreased by 30% with deflation after 2009.
• Change in seismic velocity in the intermediate layer at the reflector can explain the reflectivity variation.
• The velocity change can be caused by temperature change through the progress of activity.

Structural evolution beneath an active volcano is detected as the variation of seismic reflectivity through controlled seismic experiments, which is interpreted as being associated with discharging magma. The target of the present study is Sakurajima Volcano, which is one of the most active volcanoes in Japan. Six rounds of seismic experiments with controlled sources have been conducted annually at the volcano. Two seismic reflection profiles are obtained from the datasets for each successful round of experiments. The experiments reveal clear annual variation in seismic reflectivity at a depth of 6.2 km in the northeastern part of Sakurajima. The reflectivity is maximum in December 2009 upon the first intrusion of magma and decreases gradually until December 2013, which coincides with the inflation and deflation cycle of Sakurajima Volcano. Reflectivity variation occurred in the embedded clear reflector at depth. An evolving sandwiched structure in the intermediate layer is used as the reflector model. Lower-velocity magma embedded in the intermediate layer and its succeeding velocity increment explain the variation range of reflectivity. This is interpreted as a temperature decrease associated with discharging magma at depth. The present study describes a new approach for instantaneously sensing magma properties and for monitoring active volcanoes.