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[SCG61-14] Why did the 1984 Western Nagano Prefecture earthquake (Mj6.8) occurred (ser.2)
Keywords:the 1984 Western Nagano Prefecture earthquake , stress, focal mechanism, aseismic slip
In the western Nagano Prefecture region, seismic activity became more active after the 1979 eruption of Mt. Ontake, and the 1984 Western Nagano Prefecture earthquake (Mj6.8) occurred in 1984. Based on the fact that the focal mechanism by the Japan Meteorological Agency has a normal fault component, the distribution of boulders ejected directly above the source fault suggests normal fault-like slip (Iio & Yoshioka, 1992), and the spatial distribution of stress estimated from NEDO borehole cores (Yamamoto et al., 1990), where the closer to the source fault the smaller the minimum compressive stress and the more tensile the stress field, a hypothesis was proposed that aseismic slip occurred on the S-wave reflection plane (Inamori et al., 1992) directly below the source region to the north of the source fault, generating stress concentration on the source fault (Iio, 1994).
On the other hand, the joint seismic observations conducted in 1986 reported that, while most of the aftershock area were characterized by reverse fault-type focal mechanisms, there was a concentration of strike-slip fault-type focal mechanisms near the source fault (Yamazaki et al., 1992). Although it was difficult to interpret these results, Yukutake et al.(2010) used data from a high-precision seismic observation network with 10 kHz sampling that was installed mainly in the eastern part of the aftershock area from 1995, and showed that most of the aftershocks with a strike-slip focal mechanisms were off-fault (occurring outside the fault plane) earthquakes, and that these aftershocks can be explained by the hypothesis that the stress concentration of the strike-slip type near the source fault was caused by aseismic slip of the lower extension of the source fault. Iio et al. (2017) also support this interpretation.
Recently, a more detailed stress field with a 500 m grid interval was estimated using a high-resolution stress inversion analysis that included Manten seismic observation data (Iio et al., 2025, in review). As a result, it was found that the width of the area of the stress field of the strike-slip fault type is narrower than 1 km in the direction perpendicular to the fault. When this is explained by stress concentration due to slip in the lower extension, it was found that it is not easy to create a narrow area of strike-slip stress field only in the vicinity of the fault, because the shallower the area is, the further it is from the lower extension, and the width of the area with high stress concentration increases.
This indicates that the proposed model based on the slip on the lower extension needs to be re-examined. The issues to be considered are whether the strike-slip stress field is not an apparent phenomenon by a lot of focal mechanisms of strike-slip type, and whether a mechanism that generates stress concentration in a narrow area can be proposed.
The following is our current hypothesis. Some of the focal mechanisms with high-angle nodal planes near the source fault show normal fault components, and it are difficult to explain these with a reverse fault-type stress field where one of the principal stresses is perpendicular to the ground surface, so it is thought that a strike-slip type stress field is likely. In order to cause lateral stress concentration in a narrow area, the area of the fault where slip occurs needs to be narrowed, and one possibility is to interpret the coseismic slip distribution as back-slip.
On the other hand, the joint seismic observations conducted in 1986 reported that, while most of the aftershock area were characterized by reverse fault-type focal mechanisms, there was a concentration of strike-slip fault-type focal mechanisms near the source fault (Yamazaki et al., 1992). Although it was difficult to interpret these results, Yukutake et al.(2010) used data from a high-precision seismic observation network with 10 kHz sampling that was installed mainly in the eastern part of the aftershock area from 1995, and showed that most of the aftershocks with a strike-slip focal mechanisms were off-fault (occurring outside the fault plane) earthquakes, and that these aftershocks can be explained by the hypothesis that the stress concentration of the strike-slip type near the source fault was caused by aseismic slip of the lower extension of the source fault. Iio et al. (2017) also support this interpretation.
Recently, a more detailed stress field with a 500 m grid interval was estimated using a high-resolution stress inversion analysis that included Manten seismic observation data (Iio et al., 2025, in review). As a result, it was found that the width of the area of the stress field of the strike-slip fault type is narrower than 1 km in the direction perpendicular to the fault. When this is explained by stress concentration due to slip in the lower extension, it was found that it is not easy to create a narrow area of strike-slip stress field only in the vicinity of the fault, because the shallower the area is, the further it is from the lower extension, and the width of the area with high stress concentration increases.
This indicates that the proposed model based on the slip on the lower extension needs to be re-examined. The issues to be considered are whether the strike-slip stress field is not an apparent phenomenon by a lot of focal mechanisms of strike-slip type, and whether a mechanism that generates stress concentration in a narrow area can be proposed.
The following is our current hypothesis. Some of the focal mechanisms with high-angle nodal planes near the source fault show normal fault components, and it are difficult to explain these with a reverse fault-type stress field where one of the principal stresses is perpendicular to the ground surface, so it is thought that a strike-slip type stress field is likely. In order to cause lateral stress concentration in a narrow area, the area of the fault where slip occurs needs to be narrowed, and one possibility is to interpret the coseismic slip distribution as back-slip.