Japan is a volcanic country and there are many active volcanoes. Various physicalphenomena associated with activity of magma or hot water are observed in and aroundvolcanoes. Among these phenomena, volcanic earthquakes have unique featurescompared with the usual tectonic earthquakes. The volcanic earthquakes and tremorare very broadband and the way they occur differs greatly for every volcano.Moreover, even at the same volcano, the characteristics of seismic signals andtheir occurrence pattern often differ from event to event. It is thought that thediversity of such volcanic earthquakes is a manifestation of the complicatedphysical processes inside volcanoes.
In order to understand the physical process inside the active volcanoes, we analyzethe broadband seismic signals, and we compare the results with the various typesof other observations, such as ground deformation, gravity, and volcanic deposits.The waveforms observed in active volcanoes are strongly influenced and increase theircomplexity by the complicated topography of the volcanic edifices and the highlyheterogeneous structure in the propagation path between the source and seismic stations.To extract the information on the source of the volcanic earthquakes from the complexseismic waveforms, we use a waveform analysis technique that can remove the influencesdue to the complicated topography and the heterogeneous structure.
The origin of the volcanic earthquakes and tremor is presumed to be the oscillationof the volcanic conduit which extend deep into the volcanoes from the summit crater,or the oscillation of groups of fluid-filled cracks with a finite extent. Sinceno solid model of the seismic source like the fault model in the usual seismologyhas not yet been established in the volcano seismology, the analyses of the seismicsignals observed in volcanoes are often conducted by assuming a point source.We will develop an analysis technique for the seismic source with a finite extent,and will aim at establishment of a general source model widely applicable toanalyses of volcanic earthquakes.
For the observational aspects, we install broadband seismometers around activevolcanoes, especially in the vicinity of summit craters, and observe earthquakesaccompanying volcanic activity. Moreover, in order to know the structure ofvolcanoes, we perform observations and analyses, which use both the artificial andnatural earthquakes.
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