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[SSS08-P13] About hypocentral distribution of foreshocks of M3-4 small earthquakes in the western Nagano Prefecture region
Keywords:10 kHz waveform, relative hypocenter determination, foreshock
Before a large earthquake (mainshock) occurs, small earthquakes (foreshocks) sometimes occur in the vicinity. It is believed that a mainshock doesn’t begin abruptly but requires a nucleation process, and the spatiotemporal proximity of foreshocks and a mainshock suggests the existence of a nucleation process. The nucleation process may be explained by stress change caused by the occurrence of earthquakes or some phenomena other than earthquakes, such as aseismic slip or fluid migration, but the question of which explanation is more accurate representation of the real foreshock generation process remains unresolved. In this study, we performed redetermination of hypocenters of foreshocks and aftershocks of M3-4 earthquakes in the western Nagano Prefecture region, central Japan, and from spatiotemporal evolution of foreshocks, considered foreshock generation process. We defined mainshocks and their foreshocks spatiotemporally based on magnitudes, origin times, and distances between hypocenters, which are determined by an ordinary hypocenter determination method.
In the western Nagano region, a dense seismic observation network has been deployed since June 1995, and the seismic stations are located mainly in the eastern part of the aftershock area of the 1984 Western Nagano Prefecture Earthquake. Among the stations, those recording data at a sampling frequency of 10 kHz were installed at a maximum of 57 locations. Many earthquakes have shallow focal depths in this area and the surroundings of the stations are quiet and have low noise levels, so a lot of micro-seismicity data has been obtained. In order to perform high-precision relative hypocenter determination of the initial rupture point of each earthquake, using the 10 kHz waveform data obtained from the dense seismic observation, we calculated the P-wave arrival time differences of two earthquakes at each station by the cross-correlation of windows with a width of 0.01s that include only initial P-waves. With the differences in P-wave arrival time obtained in this way, we performed relative hypocenter determination referring to Ito (1985).
In the spatial relationship between redetermined foreshocks and aftershocks, the areas where the aftershocks were determined include the hypocenters of mainshocks and most foreshocks, and there is a tendency that there are few aftershocks determined in the areas where the foreshocks are concentrated, suggesting the possibility of mainshock rupture initiating within the slip area of foreshocks. In addition, for earthquakes selected for each M0.1 in the range of M3.0-4.3, from the area where aftershocks may have been occurred by stress concentration associated with each of them, we estimated the upper limit of the range in which the next earthquake can occur due to the static stress change caused by the occurrence of one earthquake. As a result, foreshock activity that may not be explained only by static stress change caused by the occurrence of earthquakes was found, suggesting the existence of factors in the occurrence of foreshocks other than stress transfer.
In the western Nagano region, a dense seismic observation network has been deployed since June 1995, and the seismic stations are located mainly in the eastern part of the aftershock area of the 1984 Western Nagano Prefecture Earthquake. Among the stations, those recording data at a sampling frequency of 10 kHz were installed at a maximum of 57 locations. Many earthquakes have shallow focal depths in this area and the surroundings of the stations are quiet and have low noise levels, so a lot of micro-seismicity data has been obtained. In order to perform high-precision relative hypocenter determination of the initial rupture point of each earthquake, using the 10 kHz waveform data obtained from the dense seismic observation, we calculated the P-wave arrival time differences of two earthquakes at each station by the cross-correlation of windows with a width of 0.01s that include only initial P-waves. With the differences in P-wave arrival time obtained in this way, we performed relative hypocenter determination referring to Ito (1985).
In the spatial relationship between redetermined foreshocks and aftershocks, the areas where the aftershocks were determined include the hypocenters of mainshocks and most foreshocks, and there is a tendency that there are few aftershocks determined in the areas where the foreshocks are concentrated, suggesting the possibility of mainshock rupture initiating within the slip area of foreshocks. In addition, for earthquakes selected for each M0.1 in the range of M3.0-4.3, from the area where aftershocks may have been occurred by stress concentration associated with each of them, we estimated the upper limit of the range in which the next earthquake can occur due to the static stress change caused by the occurrence of one earthquake. As a result, foreshock activity that may not be explained only by static stress change caused by the occurrence of earthquakes was found, suggesting the existence of factors in the occurrence of foreshocks other than stress transfer.