Three-dimensional P-and S-wave velocities（vP,vS）,Poisson＇s ratio（σ）,crack-density（ε） and bulk-sound velocity（vφ） structures along the slab upper boundary beneath the fore-arc regions were determined using ...Three-dimensional P-and S-wave velocities（vP,vS）,Poisson＇s ratio（σ）,crack-density（ε） and bulk-sound velocity（vφ） structures along the slab upper boundary beneath the fore-arc regions were determined using a large number of high-quality P-wave and S-wave arrival times from both onshore and offshore earthquakes in Japan.The velocity and Poisson＇s ratio images provide a compelling evidence for a highly hydrated and serpentinized fore-arc mantle and fluid-bearing anomalous low velocity and high Poisson＇s ratio associated with slab dehydration under the fore-arc areas.Most great thrust earthquakes（M〉7.5） occurred at or close to the high-velocity areas along the slab interface under the fore-arc areas,suggesting strong interplate coupling（asperities） with slab subduction.On the other hand,prominent low-velocity anomalies were revealed along the slab upper boundary,which may reflect weak coupled or decoupled patches（aseismicity） of the plates due to serpentinization of the fore-arc mantle wedge.The crack-density and bulk-sound velocity images,calculated from the corresponding velocity models,indicate that the interplate coupling in northeastern Japan is different from that under central and southwestern Japan owing to differences between the tectonic backgrounds of the subduction system,such as the geological age,thermal regime and dipping angle of the oceanic plates.A comparison between fluid-related anomalies of Japan,Cascadia,Chile,and Costa Rica subduction zones suggests that seismic mantle may be common in fore-arc settings and these reflect similar 3-D seismic structures relatively to fluid liberating processes.We consider that the fluid-bearing anomalies along the interface of the subducting slab,attributing to processes such as slab dehydration and serpentinization of the fore-arc mantle,are mainly contributed to the interplate coupling and the repeated generation of the great thrust earthquakes under the fore-arc regions in Japan.展开更多
Master event location method was described in detail in this paper. Some problems in the application of master event method have been analyzed and some improvements of the method have been made. As compared the locat...Master event location method was described in detail in this paper. Some problems in the application of master event method have been analyzed and some improvements of the method have been made. As compared the location results of MS>=3.0 earthquakes of Jiashi swarm obtained by using this method with that by the traditional absolute method, the result obtained by using the master event method shows more reasonable and more consistent with that from the focal mechanism solutions. After relocation, we can see, the epicenters of M>=5.0 earthquakes show an echelon-type alignment along NNW-SSE direction, and all earthquakes concentrate nearly in a volume region about 30 km (N-S) × 15 km (E-W) × 15 km (U-D). Earthquake focal depths are mainly in the range of 15-28 km.展开更多
基金supported by the research grants of the National Natural Science Foundation of China(Nos.40872148 and 40974024)the New Century Excellent Talents in University,Ministry of Education(NCET2009)
文摘Three-dimensional P-and S-wave velocities（vP,vS）,Poisson＇s ratio（σ）,crack-density（ε） and bulk-sound velocity（vφ） structures along the slab upper boundary beneath the fore-arc regions were determined using a large number of high-quality P-wave and S-wave arrival times from both onshore and offshore earthquakes in Japan.The velocity and Poisson＇s ratio images provide a compelling evidence for a highly hydrated and serpentinized fore-arc mantle and fluid-bearing anomalous low velocity and high Poisson＇s ratio associated with slab dehydration under the fore-arc areas.Most great thrust earthquakes（M〉7.5） occurred at or close to the high-velocity areas along the slab interface under the fore-arc areas,suggesting strong interplate coupling（asperities） with slab subduction.On the other hand,prominent low-velocity anomalies were revealed along the slab upper boundary,which may reflect weak coupled or decoupled patches（aseismicity） of the plates due to serpentinization of the fore-arc mantle wedge.The crack-density and bulk-sound velocity images,calculated from the corresponding velocity models,indicate that the interplate coupling in northeastern Japan is different from that under central and southwestern Japan owing to differences between the tectonic backgrounds of the subduction system,such as the geological age,thermal regime and dipping angle of the oceanic plates.A comparison between fluid-related anomalies of Japan,Cascadia,Chile,and Costa Rica subduction zones suggests that seismic mantle may be common in fore-arc settings and these reflect similar 3-D seismic structures relatively to fluid liberating processes.We consider that the fluid-bearing anomalies along the interface of the subducting slab,attributing to processes such as slab dehydration and serpentinization of the fore-arc mantle,are mainly contributed to the interplate coupling and the repeated generation of the great thrust earthquakes under the fore-arc regions in Japan.
文摘Master event location method was described in detail in this paper. Some problems in the application of master event method have been analyzed and some improvements of the method have been made. As compared the location results of MS>=3.0 earthquakes of Jiashi swarm obtained by using this method with that by the traditional absolute method, the result obtained by using the master event method shows more reasonable and more consistent with that from the focal mechanism solutions. After relocation, we can see, the epicenters of M>=5.0 earthquakes show an echelon-type alignment along NNW-SSE direction, and all earthquakes concentrate nearly in a volume region about 30 km (N-S) × 15 km (E-W) × 15 km (U-D). Earthquake focal depths are mainly in the range of 15-28 km.
