Attenuation of coda waves at the Changbaishan Tianchi volcanic area in Northeast China

摘要

21 earthquakes recorded by a temporary seismic network in the Changbaishan Tianchi volcanic area in Northeast China operated during the summer of 2002 and 2003 were analyzed to estimate the S coda attenuation. The attenuation quality factor Q(c) was estimated using the single scattering attenuation model of Sato (1977) in the frequency band from 4 to 24 Hz. All the events studied in this paper occurred at depths from 2 to 6 km with ML of 1.4-2.8. The epicentral distances are less than 25 km. For all events which occurred near the Tianchi Lake (caldera), the Q(c) patterns obtained at the stations near the lake are similar, and the Q(c) values are relatively small. At the stations located about 15 km east of the Tianchi Lake, however, the average Q(c) is significantly higher. For an event which occurred 25km from the lake to the west, Q(c) patterns derived at the stations near the lake are quite similar to the above mentioned Q(c) for stations located in the east. Further study shows that Q(c) value in the north and central areas of the volcano is relatively lower than that in the surrounding area. Compared to other volcanic areas in the world, the average Q(c) of the Changbaishan Tianchi volcanic area is obviously lower. The deep seismic sounding and teleseismic receiver function studies indicated more than one lower velocity layer in the crust. The MT studies suggested the presence of high conductive bodies beneath the area. We interpret the strong attenuation of coda waves near the Changbaishan Tianchi volcano as being possibly related to high temperature medium caused by shallow magma chambers.