Preface

The relationship between the behavior of crustal fluid and the occurrence of large inland earthquakes has long been discussed (e.g. Sibson 1992). After the 1995 Kobe earthquake (M 7.2) in Japan, studies of seismic tomography in the fore-arc region show the low seismic velocity and high Poisson’s ratio anomaly in and below the hypocenter (Zhao et al. 1996). The abnormal region beneath the hypocenter is considered that the fluid migrated from the downgoing slab is stored there. Along the volcanic front, the relationship between the distinct S waves reflector and the cutoff depth for shallow seismicity are detected beneath NikkoShirane volcano in Northeast Japan (Matsumoto & Hasegawa 1996). Hasegawa et al. (2000) pointed out that the structural heterogeneities are to be related to the nucleation process of large inland earthquakes. The important role of fluids in the earthquake generating processes as trigger or initiation became a highly critical problem and has been focused. The study of physical properties of crustal rocks, including fluid phases, will greatly improve our interpretation of seismic observations and our understanding about crustal structure, distribution of fluids, and crustal dynamics. This thematic section is an outcome of the symposium on ‘Fluid-rock interaction in the bottom of the inland seismogenic zone: crustal fluid behavior deduced from “paleo” bright-layers’ organized by M. Terabayashi, K. Okamoto and H. Yamamoto at the 114th Annual Meeting of the Geological Society of Japan in 2007, where eight oral presentations related to this topic were given during a half-day session. In response to call for submission of papers for a special issue, four articles were contributed from the presentations given at the 114th Annual Meeting of the Geological Society of Japan, and the guest editors adopted one article not presented there. In the first paper of this thematic section, by Zhao et al. (2010), recent high-resolution tomographic studies of large crustal earthquakes on the Japanese islands during 1995–2008 are reviewed. In the source areas of some large earthquakes, tomographic imaging has detected structural heterogeneities, which may be related to the nucleation process of the large earthquakes. Large crustal earthquakes in the fore-arc region such as the 1995 Kobe earthquake may be triggered by fluids that are released from the dehydration of subducting slab, and directly ascend to the crust and active fault zone (Zhao et al. 1996). In contrast, along the volcanic front and in the back-arc regions, the seismogenic layer in the upper crust is thinned and its mechanical strength is weakened because of the ascending hot magma and fluids which are produced by a combination of slab dehydration and corner flow in the mantle wedge. Large crustal earthquakes are apt to take place at the edge portion of the thinned seismogenic layer, which exhibits low velocity, high Poisson’s ratio and high electric-conductivity. The second paper, by Terabayashi et al. (2010), reports the geological evidence of brittle-ductile transition in the island arc crust. They propose that competent layers composed of silicified rocks in the Cretaceous Ryoke low-pressure/temperature metamorphic belt in the Iwakuni-Yanai area, Southwest Japan, may be an exhumed equivalent of a seismically-reflective layer in the middle crust. Silicified rocks crop out as layers or long lenticular bodies several to fifteen meters in thickness, and they are restricted to the area of greenschist facies conditions within a structural thickness about one kilometer. The mode of occurrences of quartz veins indicates that silicified rocks were competent relative to underlying biotite schist. The combination of a contact with high competence contrast adjacent to low-permeability silicified rocks may be a good reflector of seismic waves. The laser Raman spectra for the vapor phase within water-rich inclusions indicate that they contain CO2 alone or CO2 and CH4, in varying molar proportions. The homogenization-temperature of fluid inclusions from vein quartz in silicified rocks and matrix quartz in silicified rocks exhibits peak within the range of 300–320°C. If the described silicified rocks are distributed in the middle crust to a considerable Island Arc (2010) 19, 1–3