The 2000 Western Tottori Earthquake (Mw 6.6, Mjma 7.3) in Japan occurred in an area where no active faults are known to exist. We examined the distribution, occurrence, paleostress field, and K–Ar ages of minor faults in the aftershock region of this earthquake to understand the tectonics and deformation process in a blind active fault area. Five stress stages were identified: NW–SE compressional strike-slip and NE–SW extensional normal faulting stress field in Stage 1; clockwise rotation of ~ 40° between 18 and 16 Ma in Stage 2; N–S compressional strike-slip and N–S extensional normal faulting stress field in Stage 3; stress state with σHmin (σ3) oriented NE–SW in Stage 4; and E–W compressional strike-slip faulting stress field in Stage 5. The results from the relationship between stress fields and K–Ar ages of fault indicate that most minor faults in the epicentral area of the 2000 Western Tottori Earthquake were formed along pre-existing joints during rifting and opening of the Japan Sea during the late Oligocene. Some minor faults show signs of reactivation under the present stress field. However, a large active fault with a clear topographic expression has not yet been developed due to the small displacement of minor faults. These findings indicate that detailed structural analysis of minor faults and reconstruction of stress history are important for seismic hazard assessment in areas where fault activities are initial stages.

Drill sites in the southern Bay of Bengal at 3 degrees N 91 degrees E (International Ocean Discovery Program Expedition 362) have sampled for the first time a complete section of the Nicobar Fan and below to the oceanic crust. This generally overlooked part of the Bengal-Nicobar Fan System may provide new insights into uplift and denudation rates of the Himalayas and Tibetan Plateau. The Nicobar Fan comprises sediment gravity-flow deposits, mostly turbidites, that alternate with hemipelagite drapes and pelagite intervals of varying thicknesses. The decimetre-thick to metre-thick oldest pre-fan sediments (limestones/chalks) dated at 69 Ma are overlain by volcanic material and slowly accumulated pelagites (0.5 g cm(-2) kyr(-1)). At Expedition 362 Site U1480, terrigenous input began in the early Miocene at ca 22.5 Ma as muds, overlain by very thin-bedded and thin-bedded muddy turbidites at ca 19.5 Ma. From 9.5 Ma, sand content and sediment supply sharply increase (from 1-5 to 10-50 g cm(-2) kyr(-1)). Despite the abundant normal faulting in the Nicobar Fan compared with the Bengal Fan, it offers a better-preserved and more homogeneous sedimentary record with fewer unconformities. The persistent connection between the two fans ceased at 0.28 Ma when the Nicobar Fan became inactive. The Nicobar Fan is a major sink for Himalaya-derived material. This study presents integrated results of International Ocean Discovery Program Expedition 362 with older Deep Sea Drilling Project/Ocean Drilling Program/International Ocean Discovery Program sites that show that the Bengal-Nicobar Fan System experienced successive large-scale avulsion processes that switched sediment supply between the Bengal Fan (middle Miocene and late Pleistocene) and the Nicobar Fan (late Miocene to early Pleistocene). A quantitative analysis of the submarine channels of the Nicobar Fan is also presented, including their stratigraphic frequency, showing that channel size/area and abundance peaked at ca 2 to 3 Ma, but with a distinct low at 3 to 7 Ma: the intervening stratigraphic unit was a time of reduced sediment accumulation rates.

Understanding variations of slip distance along major thrust systems at convergent margins is an important issue for evaluation of near-trench slip and the potential generation of large tsunamis. We derived quantitative estimates of slip along ancient subduction fault systems by using the maturity of carbonaceous material (CM) of discrete slip zones as a proxy for temperature. We first obtained the Raman spectra of CM in ultracataclasite and pseudotachylyte layers in discrete slip zones at depths below the seafloor of 1–4 km and 2.5–5.5 km, respectively. By comparing the area-under-the-peak ratios of graphitic and disordered bands in those Raman spectra with spectra of experimentally heated CM from surrounding rocks, we determined that the ultracataclasite and pseudotachylyte layers had been heated to temperatures of up to 700 and 1300 °C, respectively. Numerical simulation of the thermal history of CM extracted from rocks near the two slip zones, taking into consideration these temperature constraints, indicated that slip distances in the ultracataclasite and pseudotachylyte layers were more than 3 and 7 m, respectively. Thus, potential distance of coseismic slip along the subduction-zone fault system could have regional variations even at shallow depth (≤ 5.5 km). The slip distances we determined probably represent minimum slips for subduction-zone thrusts and thus provide an important contribution to earthquake preparedness plans in coastal areas facing the Nankai and Sagami Troughs.

In 1997, two strong earthquakes occurred on March 26 (M j 6.6) and May 13 (M j 6.4) in the northwestern part of Kagoshima Prefecture, Japan (termed the 1997 Northwestern Kagoshima Earthquakes).However no seismogenic faults associated with these earthquakes had previously been recognized at the surface.

Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records.

A holistic view of the Bengal–Nicobar Fan system requires sampling the full sedimentary section of the Nicobar Fan, which was achieved for the first time by International Ocean Discovery Program (IODP) Expedition 362 west of North Sumatra. We identified a distinct rise in sediment accumulation rate (SAR) beginning ∼9.5 Ma and reaching 250–350 m/Myr in the 9.5–2 Ma interval, which equal or far exceed rates on the Bengal Fan at similar latitudes. This marked rise in SAR and a constant Himalayan-derived provenance necessitates a major restructuring of sediment routing in the Bengal–Nicobar submarine fan. This coincides with the inversion of the Eastern Himalayan Shillong Plateau and encroachment of the west-propagating Indo–Burmese wedge, which reduced continental accommodation space and increased sediment supply directly to the fan. Our results challenge a commonly held view that changes in sediment flux seen in the Bengal–Nicobar submarine fan were caused by discrete tectonic or climatic events acting on the Himalayan–Tibetan Plateau. Instead, an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation.

Coseismic shear stress and slip distance affect subduction-related earthquake processes. They need to be understood to evaluate the earthquake's mechanism and the tsunami generation potential near trenches. The amount of frictional heat generated depends on the shear stress and slip distance, which are therefore able to be derived from the temperature recorded in the fault. Here we developed a new temperature proxy for carbonaceous materials by performing spectroscopic, thermogravimetric, and organic elemental analyses in conjunction with heating experiments. We found marked anomalies in the infrared and Raman spectra and atomic compositions of carbonaceous materials retrieved from the slip zone of an ancient megasplay fault in the Cretaceous Shimanto accretionary complex, Japan: the infrared spectra show extinction of aliphatic C-H bonding and very weak aromatic C=C bonding, and the Raman spectra show a slightly elevated ratio of disordered band intensity to graphitic band intensity and relatively low H/C and O/C ratios. These correlate well with the spectral and elemental features of host-rock carbonaceous materials after heating to 600 degrees C. Thus, we conclude that the slip zone experienced a temperature of 600 degrees C during a past earthquake event, indicating coseismic slip of 2-9 m, which could have generated a large tsunami if the ruptures propagated to the seafloor.

2015年 1月 27日受付. 2015年 2月 13日受理. Corresponding author: A. Yamaguchi, asuka@aori.u-tokyo.ac.jp 1 東京大学大気海洋研究所,Atmosphere and Ocean Research Institute, the University of Tokyo, Kashiwa 277-8564, Japan 2 広島大学大学院理学研究科地球惑星システム学専 攻,Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashihiroshima 739-8526, Japan 3 海洋研究開発機構高知コア研究所,Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, Nankoku 7838502, Japan 4 高知大学海洋コア総合研究センター,Center for Advanced Marine Core Research, Kochi University, Nankoku 783-8502, Japan 5 島根大学総合理工学部地球資源環境学科,Department of Geoscience, Shimane University, Matsue 690-8504, Japan

Crystallographic orientations of minerals were determined by electron backscatter diffraction (EBSD) under a scanning electron microscope. In this study, we introduce a technique for vibratory polishing of rock samples using colloidal silica (CS), for EBSD analysis of quartz in granite and calcite in limestone. Evaluation of the surface state consisted of a band contrast (BC) measure to represent the surface state, a hit rate to represent the success rate of indexing, and a mean angular deviation (MAD) to represent the accuracy of indexing. The factor that most affected the BC and the hit rate was the total amount of displacement of the sample. However, the BC and hit rate also showed a dependency on the crystallographic orientation. When eight Kikuchi bands were detected, the value of the MAD was < 1°, and the MAD was independent of the BC. The BC measure showed that the optimal CS polishing times for quartz and calcite were 3 hours and 1 hour, respectively. When preparing a sample with for the first time, CS polishing for 3 hours and detection of the eight Kikuchi bands provide the most effective method for determining crystallographic orientations.

Because a megasplay fault branching from the deep subduction boundary megathrust in the Nankai Trough is thought to be the source of large tsunamis associated with past Tonankai earthquakes, investigation of the heat signal due to frictional slip recorded in the fault is important for estimating the earthquake slip parameters. We performed X-ray diffraction and infrared spectroscopic analyses of a megasplay fault-rock sample and re-examined previously reported trace-element and isotope compositions, but observed no specific change related to high temperature (≥250°C). In addition, although a qualitative increase of the illite content in illite/smectite mixed-layer minerals within the slip-zone sample was previously reported, our kinetic evaluation of illitization, taking into consideration the coseismic temperature change due to frictional heating and heat conduction, revealed that the illitization reaction hardly progresses at temperatures under 250°C. Alternatively, we suggest that the illite content in mixed-layer minerals might increase progressively via a comminution–dissolution–recrystallization process during multiple past slips. Accurate assessment of the slip behavior of the megasplay fault could be efficiently obtained by drilling to penetrate the fault zone at a deeper depth of approximately 1.5km, where records of high temperatures would be detectable.

The correlation between the displacement and thickness of faults is an important parameter that enables an increased understanding of the processes involved in fault growth and earthquake generation. We determined this correlation by measuring the thicknesses of outcropping faults and using vitrinite reflectance (Ro) to determine paleotemperature estimates for the hangingwall and footwall units of 18 faults exposed in the Upper Cretaceous Shimanto accretionary complex of southeast Kochi prefecture, Japan. Fault displacement was estimated from the temperature difference across the faults, assuming a paleothermal gradient and the dip of the fault plane. The ratio of fault thickness to displacement ranges from ~10 to ~10, far lower than compared with previously identified thickness–displacement scaling relationships (10 to 10; e.g., Shipton et al., 2006). Permeability measurements within fault zones in the study area indicate that both hangingwall and footwall units have low permeability (less than 10 m at an effective pressure of 100 MPa), although fault zones have permeabilities that are four orders of magnitude higher than these units. This type of fault zone permeability structure means that similar fault zones in accretionary wedges could sustain high fluid pressures, promoting fault slips without significant generation of fault gouge and resulting in the formation of fault zones with lower ratios of thickness to displacement, as observed in the study area.

Thermal-hydrological-mechanical coupling processes suggest that fault permeability should undergo dynamic change as a result of seismic slip. In igneous rocks, a fault's slip surface may have much higher permeability than the surrounding rock matrix and therefore operate as a conduit for fluids. We conducted laboratory experiments to investigate changes in fracture permeability (or transmissivity) of a fault in granite due to shear slip and cyclic heating and cooling. Our experiments showed that high initial fracture transmissivity (>10(-18)m(3)) was associated with a high friction coefficient and that transmissivity decreased during slip. We propose that this reduction in transmissivity reflects the presence of gouge in fracture voids, increasing the area of contact in the fault plane and reducing the hydraulic aperture. In contrast, when initial fracture transmissivity was low (<10(-18)m(3)), we observed that friction was lower and transmissivity increased during slip. The high transmissivity and high friction may be explained by large areas of bare rock being in contact on the slip surface. Slip velocity had little influence on the evolution of permeability, probably because gouge produced at different slip velocities had similar grain size distributions, or because gouge leaked from the slip surface. Transmissivity decreased with increasing temperature in heating tests, probably due to thermal expansion increasing normal stress on the fracture. Frictional heating did not influence transmissivity during the shearing tests.

We measured fluid transport properties at an effective pressure of 40 MPa in core samples of sediments and fault rocks collected by the Integrated Ocean Drilling Program (IODP) NanTroSEIZE drilling project Expedition 316 from the megasplay fault system (site C0004) and the frontal thrust (site C0007) in the Nankai subduction zone. Permeability decreased with effective pressure as a power law function. Permeability values in the fault zones were 8 × 10 −18 m 2 at site C0004 and 9 × 10 −18 m 2 at site C0007. Stratigraphic variation in transport properties suggests that the megasplay fault zone may act as a barrier to fluid flow, but the frontal thrust fault zone might not. Depth variation in permeability at site C0007 is probably controlled by the mechanical compaction of sediment. Hydraulic diffusivity at shallow depths was approximately 1 × 10 −6 m 2 s −1 in both fault zones, which is small enough to lead to pore pressure generation that can cause dynamic fault weakening. However, absence of a very low permeable zone, which may have formed in the Japan Trench subduction zone, might prevent facilitation of huge shallow slips during Nankai subduction zone earthquakes. Porosity tests under dry conditions might have overestimated the porosity.