Great interplate earthquakes have repeatedly occurred in pairs along the Nankai trough. Ruptures started to the east of the Kii Peninsula and propagated to the west within a few years, and, especially for the last three events, the delay within pairs of earthquakes gradually increased as the magnitude and recurrence interval of events decreased. Recent numerical simulations of the earthquake cycles, based on a plane fault model with depth-dependent frictional properties and anomalous structure beneath the Kii Peninsula, have qualitatively reproduced the characteristics of earthquakes along the Nankai trough. We used a similar numerical simulation to examine the effect of the 3D geometry of the Philippine Sea (PHS) plate on the earthquake cycles. To the east of the Kii Peninsula, where the PHS plate is relatively flat, the stress increase rate is similar to that of the plane fault model. In contrast, the stress increase rate west of the Kii Peninsula is considerably lower; in this region there is marked flexure of the PHS plate. Furthermore, east-west gaps in the stress increase rates of the 3D plate model greatly increase the delay time (by∼60 years) between simulated Tonankai and Nankai earthquakes. However, we tested our 3D plate model with minor adjustments of the frictional parameters beneath the Kii Peninsula (reducing characteristic slip distance by only a few percent) and showed that the earthquake cycles thus simulated were nearly the same as those of the plane fault model. Therefore, frictional properties at the boundary between Tonankai and Nankai earthquakes have greater influence on earthquake occurrence patterns than stress changes related to 3D plate geometry.

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Using the empirical attenuation relationship between the maximum acceleration and source distance, we evaluated the amplitude level of the strong ground motion which might be possible at stations of the DONET (Dense Ocean-floor Network System for Earthquakes and Tsunamis), deployed in the source area of the Tonankai earthquake. We firstly studied on how often the maximum acceleration data exceeds the criteria, such as 1g, 2g, 3g, and 4g, using the maximum acceleration data from recent fourteen off-shore earthquakes of magnitude around 7 or more and 2008 Iwate-Miyagi Inland earthquake (M_w6.9). We modified the maximum accelerations by compensating the attenuation effect, so as to derive the distributions of the corrected acceleration values which might be observed at an epicentral area. Next, we evaluated the exceeding probabilities of such criteria at the DONET stations against the 1944 Tonankai earthquake (M_w8.1). After selecting five reference events who involved the maximum acceleration data larger than 3g after such correction, we obtained the distribution of the ratio between the data and the prediction and evaluated the exceeding probabilities for above criteria. Through the study, it is clarified that the upper limit of the maximum measurable acceleration of the DONET should be 4g, to avoid saturation by the extremely strong ground shaking from nearby source rupturing as much as possible.

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We have isolated thirteen different bacterial strains as poly ε-caprolactone (PCL)-degrading bacteria from the Kurile and Japan Trenches at a depth of 5,000-7,000 m (deeper ocean bottoms). The isolates belong to the Shewanella, Moritella, Psychrobacter and Pseudomonas genera. This is the first record of PCL degrading bacteria isolated from deep-sea environments at depth of over 5,000 m. Six strains of the isolates, numbered CT01 in genus Shewanella, CT12, JT01 and JT04 in genus Moritella, JT05 in genus Psychrobacter, and JT08 in genus Pseudomonas were selected for investigation of their cell shapes, degrading abilities for several aliphatic polyesters, and growth profiles. The cell shapes of the strains, except JT05, were rod-shaped, non-spore-forming and motile by means of a single or multi polar flagella. The cell shapes of JT05 were coccal with no visible flagella. From the results of degradation tests on six different alihphatic polyesters, all strains could degrade only PCL. Strains CT01, CT12, JT01 and JT04 are psychrophilic and pressure tolerant bacteria and three strains except JT04 showed typical piezophilic growth profiles. Therefore, it is possible that these strains might play a role in degrading aliphatic polyesters under deep-sea conditions, ie., low-temperatures and high hydrostatic pressures.

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Reef-building corals are susceptible to environmental stresses such as high water temperature and strong light intensity. Growth-ring formation and isotopic compositions of coral skeletons are known to be affected by various environmental factors. To understand the effect of light on skeletogenesis of corals, we conducted a long-term indoor culture experiment of Porites under different light conditions and analyzed the correlation between the carbon isotope fractionation and the photosynthetic parameters of the symbiotic alga. Here, we report the detailed protocols about over 500-day coral culture and biological and geochemical analyses.

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