Fig. 2
Daily frequency of volcanic earthquakes at Meakan-dake volcano. The top graph shows long-term seismicity since 1973 recorded by Sapporo Meteorological Observatory, Japan Meteorological Agency. Five recent eruptions are included in this period. The middle graph expresses medium-term seismicity since 2005, which includes the eruptions in 2006 and 2008. The bottom graph shows short-term seismicity during 4 months from September to December 2008. Gray triangles at the top of each graph express eruptive activities. No evident increase in seismicity was observed prior to the eruption in 1998. However, intense seismic activities with no subsequent eruption have been observed