In order to analyze the seismicity in recent years in Aso volcano, hypocenters relocation was performed using the first arrival times of P wave from 13 stations belonging to Aso Volcanological Observatory from 1977 to 2001. This relocation was made using HypoDD software developed by Waldhauser F. and Ellsworth W. in 2000 from USGS. For making the relocation two different input data were used with the purpose of having a comparison data for a more accurate judgment of results. Prior the application case, the basic description of Volcano Seismology is presented as well as the geological background of Aso volcanic complex in Kyushu Island, Japan.
The results obtained from the relocation were plotted using Seis-PC Ver.1.24 (Developed by Ishikawa and Nakamura in 2002). With this software a database was created and the seismicity was analyzed for different periods of time taking as a reference points three eruptive events, 1979, 1989-91, and 1994 as well as a calm period after 1994 until 2001. The analysis made can be divided into two parts, firstly a correlation between the hypocenters location in space and time and the extrusive activity is discussed, finding that in this period of time (1977-2001) the seismicity in the three eruptive events differ in occurrence and location between them, therefore no clear judgment about volcanic eruption forecast can be made only from analyzing this type of seismicity, proving that earthquakes beneath the volcanic edifice only represent the stress distribution but another geophysical methods are needed in order to improve accurately enough the understanding of magmatic processes. The second analysis has to do with the crack-like conduit proposed by Yamamoto in 1999, which is widely accepted among the Aso volcano researchers. This crack-like conduit which according to Yamamoto is the source of long period volcanic tremor in Aso, presents a geometry of an inclined plane which apparently provokes a triangular shape pattern in the occurrence of volcano-tectonic earthquakes. An explanation of this phenomenon is proposed based on stress symmetry distribution.