| Summary: | P(論文) In order to monitor the long-term seismicity of Mount Erebus, Antarctica, seismic observations have been made since 1980 using radio-telemetered and local networks. The observations in the 1981-1982 field season enabled us to determine to hypocenters of 162 local events. The hypocenters can be grouped into two sets, group I with focal depths ranging from the elevation of the summit to sea level, and group II with depths ranging from sea level to 15km. The epicenters of group I earthquakes are concentrated 1 to 2.5km north-northeast of the summit crater, and have a nearly vertical focal depth distribution. They are accompanied by infrasonic signals of two distinct waveform types, which allowed the earthquakes to be classified into two sub-sets, called arbitrarily α-and β-type events. There are about twice as many α-type events than β-Type events. The epicenters of the deeper α-type events are more tightly clustered than those of the shallower β-type events. A comparison of the arrival times of the infrasonic signals with those of the seismic signals at the summit station indicates that infrasonic signals associated with α-type events may be shock waves traveling at a sound speed of around Mach 2. The shock waves may be generated at the surface of the lava lake that occupies an inner summit crater. The different character of α-and β-type infrasonic signals is probably the result of different partitioning of acoustic and seismic energies for the shallower β-type events and the somewhat deeper α-type events. The number of group Ii earthquakes is smaller by one order of magnitude compared to group I earthquakes. The hypocenters are scattered more widely and form an elongated pattern that is slightly inclined to a northeast. Group II earthquakes are not accompanied by infrasonic signals but are characterized by higher-frequency seismic signals at all stations. departmental bulletin paper
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