Activity of Earthquakes with Similar Waveforms Around Ontake Volcano, Central Japan

Ontake volcano area is known for its very high seismicity. Swarm activity has been repeatedly observed since 1976. The volcano erupted in 1979 and a large earthquake with a magnitude of 6.8 occurred in 1984 in the southeastern part of the volcano. A dense seismic network has been constructed for monitoring in this area. Recent studies using leveling survey reveal uplift above the swarm area. Earthquakes with similar waveforms (similar earthquakes) have been observed mainly at geothermal areas and plate boundaries. It is possible to use them to detect temporal change of crustal structure due to large earthquakes or volcanic eruptions using repeating similar earthquakes. With a view to use them as a monitoring tool, we search for similar earthquakes in Ontake area and examine temporal change of observed seismic waveforms. We use ~3300 events which occurred in this area from January 1999 to August 2000. First we pick up event pairs of which hypocenters are manually determined within 4 km. Then we calculate cross-correlation coefficients at each station from the waveform pairs band-pass-filtered at 4 to 8 Hz. The calculation uses 15-second time window that contains both P- and S-waves. We define similar earthquake pairs as event pairs with cross-correlation coefficients larger than 0.9 at more than 8 stations. Such event pairs are distributed mainly in earthquake clusters located to the east and southeastern flank of the volcano. The similar earthquakes have magnitude ranging from 1 to 2 and focal depths of 1 to 7 km. Occurrence intervals of the most pairs are very short, between a few hours and a few days. A few pairs have intervals of several months. But repeating groups with nearly the same interval have not been found due probably to the short data period. We will further analyze using expanded data period and area.