Earthquake swarm activity in Changbaishan Tianchi volcano

In summers of 2002 and 2003,temporary seismic stations recorded a large number of small earthquakes and a series of swarms near Changbaishan Tianchi volcano. Earthquake location result shows that these earthquakes mainly occurred near the Tianchi caldera. Almost all swarms were concentrated at southwest of the caldera, and no swarms occurred in northeast area where earthquakes densely concentrated. In other seasons, the permanent station CBS is the only seismic station to monitor volcanic earthquakes in Tianchi volcano. Waveform correlation analysis of seismic data recorded at CBS in different time shows that earthquake swarms occurred in other seasons also concentrated in the southwest Tianchi Lake. High-precision relative location shows that hypocenters of earthquake swarm distributed along northwest with a dip angle of 80° to southwest. During the July 13, 2003 earthquake swarm the hypocenters migrated from deep to shallow and deeper events had upward first motion directions at almost all stations. That means they had obvious expansion component in source mechanism. Taking into account of significant surface deformation in Changbaishan Tianchi volcano since 2002, geochemical anomalies and harmonic-spectral earthquakes, we believe that the earthquake swarm activity was caused by magmatic and hydrothermal activity and pressure increase of magma near the depth of 5 km.