An Interpretation of the 1914 Eruption of Sakurajima Volcano

The 1914 eruption of Sakurajima volcano, Japan was one of the largest volcanic eruptions in the history of this country. Hitherto, numerous papers have appeared discussing this eruption from various standpoints. Nevertheless, there still remain many data not fully analyzed from viewpoints of increased volcanological knowledge. In the present paper, its eruption processes are interpreted utilizing the observational data with relation to subsurface structure. Benchmark (BM) 2469 near the Kagoshima harbor proved to have upheaved approximately 12cm relative to 1891 elevation before the 1914 eruption based on precise leveling surveys and the temporary sea level observations at the Kagoshima harbor. This was a part of the precursory upheaval around the volcano. BM. 2474 about 10km northwest of the center of the volcano subsided approximately 80cm by the eruption relative to the 1891 elevation, and 5cm more during subsequent 7 months. Such subsidence was common to the other benchmarks around the volcano. This means that the subsidence was rheological: we may assume that a part of the subsidence was elastic and the rest was viscoelastic. Such movements can not be attributed to lava effusions that continued for only 1 month. Considering the sequences of the volcanic activity in connection with the subsurface structure of the volcano, one can surmise the magma movements: The lava flows began to effuse 36 hours after the first outburst of a column of white smoke from the summit crater. The first outburst may have been caused by contact of magma with the aquifer at a depth of 2.5km below sea level based on analyses of the gravity anomalies there. If we adopt that the magma reached the first craterlet at 0.5km above sea level after 36 hours, we may calculate magma ascent velocity of 2km/day. Prior to the 1914 eruption, the first precursory shock of volcanic origin was observed 4 days prior to lava effusion. Thus, the starting point may be about 8km below the sea level. This is the depth of a short lived pressure source which is interpreted as the cause of surface subsidence.