Abstract
In this study, the rupture process of the main shock of the Kumamoto earthquake, particularly the generation of strong ground motions in the frequency range relevant to structural damage, was investigated based on the inversion of strong ground motions. Strong-motion records in the near-source region were mainly utilized because the authors were interested in the generation mechanism of damaging ground motions in the near-source region. Empirical Green’s functions (EGFs) were applied to avoid uncertainty in the subsurface structure model. Four cases of inversions with different combinations of small events were used to investigate the dependence of the inversion results on the selection of the small events. It was found that the dependence of the final slip distribution and peak slip velocity distribution on the selection of the EGF events is small. The results clearly indicate that a region of significantly large slip and slip velocity existed approximately 15 km northeast of the hypocenter. However, no “asperity” was observed between the hypocenter and Mashiki. Thus, it is not appropriate to conclude that the large-amplitude pulse-like ground motion in Mashiki was generated by the forward-directivity effect associated with the rupture of an asperity. As far as the source effect is concerned, the ground motion in Mashiki cannot be interpreted as the worst case scenario. On the other hand, the rupture of the “asperity” 15 km northeast of the hypocenter should have caused significantly large ground motions in regions close to the asperity. The significant damage of highway bridges in the region can potentially be attributed to the rupture of the asperity. The result of this study was compared with an inversion result obtained from numerical Green’s functions for a layered half-space. The two results share the main features in spite of the difference of the Green’s functions and stations used. Therefore, it can be concluded that these two source models capture the main features of the rupture process of the earthquake.Graphical abstractIn this study, the rupture process of the main shock of the Kumamoto earthquake, particularly the generation of strong ground motions in the frequency range relevant to structural damage, was investigated based on the inversion of strong ground motions. Strong-motion records in the near-source region were mainly utilized because the authors were interested in the generation mechanism of damaging ground motions in the near-source region. Empirical Green’s functions (EGFs) were applied to avoid uncertainty in the subsurface structure model. Four cases of inversions with different combinations of small events were used to investigate the dependence of the inversion results on the selection of the small events. It was found that the dependence of the final slip distribution and peak slip velocity distribution on the selection of the EGF events is small. The results clearly indicate that a region of significantly large slip and slip velocity existed approximately 15 km northeast of the hypocenter. However, no “asperity” was observed between the hypocenter and Mashiki. Thus, it is not appropriate to conclude that the large-amplitude pulse-like ground motion in Mashiki was generated by the forward-directivity effect associated with the rupture of an asperity. As far as the source effect is concerned, the ground motion in Mashiki cannot be interpreted as the worst case scenario. On the other hand, the rupture of the “asperity” 15 km northeast of the hypocenter should have caused significantly large ground motions in regions close to the asperity. The significant damage of highway bridges in the region can potentially be attributed to the rupture of the asperity. The result of this study was compared with an inversion result obtained from numerical Green’s functions for a layered half-space. The two results share the main features in spite of the difference of the Green’s functions and stations used. Therefore, it can be concluded that these two source models capture the main features of the rupture process of the earthquake.
Highlights
A series of damaging earthquakes hit the Kumamoto and Oita prefectures, Kyushu, Japan, including the Mw6.1 foreshock (April 14, 21:26 JST) and the Mw7.1 main shock (April 16, 1:25 JST)
Nozu and Nagasaka Earth, Planets and Space (2017) 69:22 (Kinoshita 1998; Aoi et al 2004), KiK-net (Aoi et al 2000, 2004), and the Japan Meteorological Agency (JMA) network (Nishimae 2004), and by a temporary seismic array that was deployed after the Mw6.1 foreshock in Mashiki, Kumamoto Prefecture, located in the near-source region (Hata et al 2016)
The importance of their observation is that they successfully recorded strong ground motions during the main shock in a heavily damaged zone in which the ratio of totally collapsed wooden houses exceeded 50% (Kikuchi and Tanaka 2016), the damage could have partly been due to the Mw6.1 foreshock
Summary
A series of damaging earthquakes hit the Kumamoto and Oita prefectures, Kyushu, Japan, including the Mw6.1 foreshock (April 14, 21:26 JST) and the Mw7.1 main shock (April 16, 1:25 JST). (Kinoshita 1998; Aoi et al 2004), KiK-net (Aoi et al 2000, 2004), and the JMA network (Nishimae 2004), and by a temporary seismic array that was deployed after the Mw6.1 foreshock in Mashiki (see Fig. 1), Kumamoto Prefecture, located in the near-source region (Hata et al 2016) The importance of their observation is that they successfully recorded strong ground motions during the main shock in a heavily damaged zone in which the ratio of totally collapsed wooden houses exceeded 50% (Kikuchi and Tanaka 2016), the damage could have partly been due to the Mw6.1 foreshock. It is important to understand what kind of rupture process contributed to the generation of such strong ground motions
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