Stress relaxation arrested the mainshock rupture of the 2016 Central Tottori earthquake

地震の破壊はなぜ止まるのか? --2016年鳥取県中部地震の断層サイズを決めたもの--. 京都大学プレスリリース. 2021-08-12.After a large earthquake, many small earthquakes, called aftershocks, ensue. Additional large earthquakes typically do not occur, despite the fact that the large static stress near the edges of the fault is expected to trigger further large earthquakes at these locations. Here we analyse ~10, 000 highly accurate focal mechanism solutions of aftershocks of the 2016 Mw 6.2 Central Tottori earthquake in Japan. We determine the location of the horizontal edges of the mainshock fault relative to the aftershock hypocentres, with an accuracy of approximately 200 m. We find that aftershocks rarely occur near the horizontal edges and extensions of the fault. We propose that the mainshock rupture was arrested within areas characterised by substantial stress relaxation prior to the main earthquake. This stress relaxation along fault edges could explain why mainshocks are rarely followed by further large earthquakes

液状化による埋設構造物の浮上に関する遠心模型実験: 浮上量の低減に関する対策

大地震時に液状化による埋設構造物の浮き上がりが生じた事例は数多く報告されている．2004年新潟県中越地震では，長岡市，小千谷市などで1, 400箇所以上のマンホールの浮き上がりが発生し，緊急車両の通行が阻害されるなど，市民生活に大きな影響を与えた．マンホールの浮上防止対策についても，埋戻し土の締固め，固化改良，砕石による埋戻し，間隙水圧をマンホール内に逃がす方法(小西ら，2008)などが考案されている．しかし，既存のマンホールに対する浮上防止対策については，有効かつ経済的な方法がいまだ模索されている．本研究では，遠心模型実験を用い，既存および新設のマンホールに対して間隙水圧をマンホール内に逃がす方法の有効性を評価した．その結果，浮き上り量は対策なしよりマンホール長さの12%まで低減された．Many types of damage of buried structures occur due to liquefaction during an earthquake, such as flotation, settlement, bending and buckling of buried pipes. Among those, this paper focuses only on uplift of sewerage manholes. Uplift behavior of buried structures with and without a measure for uplift is investigated in model tests which are dynamically tested in a centrifuge modeling. In this study, effectiveness of the new measure for uplift which is to dissipate the excess pore water pressure is investigated to mitigate uplift displacement of the manhole during earthquakes (Konishi et al. 2008). The measure consisted of two configurations which are a filtering net and pipe. The tests showed that the mechanism of the uplift behavior and the effects of the measure for the uplift, but the uplift amount may be still too large (8% of the length of the manhole) when it is applied in practice.大地震時に液状化による埋設構造物の浮き上がりが生じた事例は数多く報告されている．2004年新潟県中越地震では，長岡市，小千谷市などで1,400箇所以上のマンホールの浮き上がりが発生し，緊急車両の通行が阻害されるなど，市民生活に大きな影響を与えた．マンホールの浮上防止対策についても，埋戻し土の締固め，固化改良，砕石による埋戻し，間隙水圧をマンホール内に逃がす方法(小西ら，2008)などが考案されている．しかし，既存のマンホールに対する浮上防止対策については，有効かつ経済的な方法がいまだ模索されている．本研究では，遠心模型実験を用い，既存および新設のマンホールに対して間隙水圧をマンホール内に逃がす方法の有効性を評価した．その結果，浮き上り量は対策なしよりマンホール長さの12%まで低減された．Many types of damage of buried structures occur due to liquefaction during an earthquake, such as flotation, settlement, bending and buckling of buried pipes. Among those, this paper focuses only on uplift of sewerage manholes. Uplift behavior of buried structures with and without a measure for uplift is investigated in model tests which are dynamically tested in a centrifuge modeling. In this study, effectiveness of the new measure for uplift which is to dissipate the excess pore water pressure is investigated to mitigate uplift displacement of the manhole during earthquakes (Konishi et al. 2008). The measure consisted of two configurations which are a filtering net and pipe. The tests showed that the mechanism of the uplift behavior and the effects of the measure for the uplift, but the uplift amount may be still too large (8% of the length of the manhole) when it is applied in practice

The relationship between S-wave reflectors and deep low-frequency earthquakes in the northern Kinki district, southwestern Japan

Abstract We conducted high-resolution reflection analysis of data from 168 seismic stations with an average spacing of about 5 km, in northern Kinki district, southwestern Japan. Reflection analysis has previously been conducted in this region, assuming a homogeneous horizontal structure, resulting in an inclined planar zone of high relative reflection strengths (S-wave reflector). However, if the reflector is actually inclined, the location of the S-wave reflector differs from that of an assumed homogeneous horizontal structure. Hence, this study conducted high-resolution reflection analysis to determine the accurate location of the S-wave reflector. We confirm the previously reported S-wave reflector (reflector W). Furthermore, we detected the accurate location of the S-wave reflector and obtained more detailed results that revealed a second S-wave reflection structure (reflector E) to the east of reflector W, in an area that has not be imaged by previous studies. The northern edges of reflector E and reflector W are located near different hypocentral areas of deep low-frequency earthquakes (DLFs). Reflector W exists along the Kyoto Nishiyama fault zone, and its position appears to change along the fault zone as it deepens. Similarly, reflector E exists along the Hanaore and Biwako Seigan fault zones and its position appears to change along these fault zones. The reflector W and reflector E are imaged as separate S-wave reflectors in deeper regions, but they coalesce in shallower regions. According to previous studies, crustal fluid by dehydration from the Philippine Sea plate exists near these epicenters and we infer that this crustal fluid causes DLFs and forms S-wave reflectors