花岗岩变形破坏的阶段性模型--应力速度及预存微裂纹密度对断层形成过程的影响

依据对具有较高微裂纹密度的筑波花岗岩和极低微裂纹密度的花岗斑岩 2种极端的岩石标本、在等应力速率的快速加载 (约 6MPa/min)和蠕变加载 (轴向应力保持在约 95 %破坏强度 ) 2个极端的加载条件下的实验结果 ,讨论了预存微裂纹密度与加载速度对多晶质结晶岩变形破坏过程的影响。实验中利用高速多通道声发射波形数字记录系统 (每秒可记录多达 5 0 0 0个声发射事件的 32通道波形 ) ,获得了岩石标本破坏前微破裂活动的详细时空分布数据。声发射的发生率、震级 -频度关系中的b值及震源的空间分布揭示出岩石变形破坏过程中微破裂活动具有 3个典型阶段 :初期阶段、第 2阶段和成核阶段。尤其是在成核阶段 ,还观测到发生率和b值有大幅度前兆性起伏。一般而言 ,在相同加载条件下预存微破裂密度越高 ,或对同一岩石加载速度越慢 ,对应的断层成核过程就越长 ,因此最终破坏的可预报性也越高。为了进一步探讨微破裂活动的阶段性特征及其物理本质 ,还利用亚临界破裂扩展模型对声发射数据进行了理论分析     

Assessing flood disaster impacts in agriculture under climate change in the river basins of Southeast Asia

Natural Hazards - This study focused on flood damage assessment for future floods under the impact of climate change. Four river basins of Southeast Asia were selected for the study. They included...     

Presence of high nitrate to phosphate ratio subsurface water in the Tsushima Strait during summer

To identify water with an excess nitrate concentration to phosphate ratio and its potential source, the nutrient concentrations in the Tsushima Strait (TSS) were investigated over ten cruises in August and September 2007–2014, excluding 2010. On the basis of the Redfield ratio, water with an excess nitrate concentration of >1 μM (positive ExNOx water) was identified below the surface mixed layer during four cruises in 2011–2013. Positive ExNOx water was present mainly in less-saline (<34) waters with a density of 22–25 σ _θ , and 25–75 m depth. However, in August 2012, positive ExNOx was detected in dense (25–25.5 σ _θ ) and deep (50–110 m depth) waters near the salinity maximum, although the salinity during this period was significantly lower than that in other years. The horizontal length of positive ExNOx water was >100 km across the TSS during two cruises in August 2012 and September 2013, respectively. According to multi-regression analysis conducted on the silicate concentration, temperature, and salinity, the silicate concentration was increased in the less-saline subsurface water. The required amount of original freshwater was 10^8–9 m³ day^?1 based on the excess nitrate concentration. This evidence indicates that the positive ExNOx water originated from large river waters such as the Changjiang. Thus, discharged water from the rivers of the East Asia is contributing to the increased N:P ratio in the Tsushima Warm Current, southern Japan Sea.     

Fifteen years progress of the TRITON array in the Western Pacific and Eastern Indian Oceans

The Triangle Trans‐Ocean Buoy Network (TRITON) project by the Japan Agency for Marine-Earth Science and Technology began with deployment in the western tropical Pacific Ocean in 1998 and has shifted to steady, long-term observations since 1999. After on-site inter-comparison with the Autonomous Temperature Line Acquisition System mooring system of the Tropical Atmosphere and Ocean (TAO) array by the National Oceanic and Atmospheric Administration, the TRITON array became the international TAO/TRITON array in 2000 as a key component of the Global Ocean and Climate Observing Systems. The TAO/TRITON array took over from the TAO array, which was developed during the Tropical Ocean and Global Atmosphere program (1985–1994), and replaced the western part of TAO with new additional real-time measurements of salinity and ocean currents. In 2001, two TRITON moorings were deployed in the eastern Indian Ocean for capturing the eastern pole of the Indian Ocean Dipole. From this initiative, the Indian Ocean Observing System (IndOOS) was designed, and the Indian Ocean mooring array (Research Moored Array for Africa–Asian–Australian Monsoon Analysis and Prediction) was developed as a key component of IndOOS. In this paper, 15 years of progress in the TRITON project in the western Pacific and eastern Indian Oceans is reviewed with regards to scientific outcomes, technological development, and collaborations with international and domestic partners. Future directions for sustainable observation in the Pacific and Indian Oceans are also discussed.     

Preferred orientation of the trace fossil Entradichnus meniscus in eolian dune strata (Djadokhta Formation) at Tugrikiin Shiree,southern Mongolia and its paleoecological implications

Eolian sand dune deposits of the Upper Cretaceous Djadokhta Formation at Tugrikiin Shiree, southern Mongolia, yield not only dinosaur skeletal remains but also numerous trace fossils produced by invertebrates. This paper describes the trace fossil Entradichnus meniscus, a long unlined and unbranched trail that is filled with meniscate laminae and occurs characteristically in positive epirelief. The trail is straight to gently meandering, parallel to the foreset laminae of the eolian dunes, and exhibits a significant preferred orientation parallel to the depositional dip of the cross‐stratification laminae. In addition, almost all the crescentic internal laminae of the trail show concave down‐dips. These features indicate that the trails were produced beneath the slipface of eolian dunes by the downward burrowing of the trace‐makers. This occurrence mode of E. meniscus of the Mongolian Cretaceous is very similar to that described from the Jurassic eolian dune deposits in North America. Hence, the downward burrowing of the E. meniscus animal might be a common feature in arid eolian dune deposits at least during the Jurassic and Cretaceous, and possibly reflecting a behavioral response to the morphology of large sand dunes under an arid climate.     

Millennium‐scale recurrent uplift inferred from beach deposits bordering the eastern Nankai Trough,Omaezaki area,central Japan

A flight of Holocene marine terraces on the southwestern coast of Cape Omaezaki of central Japan provides evidence of recurrent millennium‐scale uplift events. We reconstructed the uplift history of these terraces by using facies analysis of drill core and geoslicer samples, environmental analysis of trace fossils, and ¹⁴C age determinations. Coastal uplift can be identified by the displacement of beach deposits such as foreshore deposits, which represent the intertidal swash zone of a wave‐dominated sandy coast. Three levels of former beach deposits facing the Nankai Trough were identified near the coast in the Omaezaki area. The highest of these, dated at about 3020–2880 BC, records a maximum of 2.2–2.7 m of emergence. The middle beach surface, of minimum age 370–190 BC, shows 1.6–2.8 m of emergence. The lowest beach surface, which is older than 1300–1370 AD, records 0.4–1.6 m of emergence. Our analysis of vertical crustal deformation data during the Holocene in this region suggests that rapid and strong uplift was restricted to the southwestern coast of the Omaezaki area and was probably caused by high‐angle thrusting on subsidiary faults branching from the underlying plate boundary megathrust.     

Influence of interaction between small asperities on various types of slow earthquakes in a 3-D simulation for a subduction plate boundary

Recently, the occurrence of slow earthquakes such as low-frequency earthquakes and very low-frequency earthquakes have been recognized at depths of about 30 km in southwest Japan and Cascadia. These slow earthquakes occur sometimes in isolation and sometimes break into chain-reaction, producing tremor that migrates at a speed of about 5–15 km/day and suggesting a strong interaction among nearby small asperities. In this study, we formulate a 3-D subduction plate boundary model with two types of small asperities chained along the trench at the depth of 30 km. Our simulation succeeds in representing various types of slow earthquakes including low-frequency earthquakes and rapid slip velocity in the same asperity, and indicates that interaction between asperities may cause the very low-frequency earthquakes. Our simulation also shows chain reaction along trench with propagation speed that can be made consistent with observations by adjusting model parameters, which suggests that the interactions also explain the observed migration of slow earthquakes.     

Hydrogen isotopic ratios of plant wax n-alkanes in a peat bog deposited in northeast China during the last 16 kyr

Based on paleoclimatic reconstructions using various proxies, the Holocene Climate Optimum (10.5–6 ka) has been characterized as a warmer and wetter period in most of East Asia. The summer monsoons associated with the East Asian Monsoon evidently intensified and extended further inland from the Pacific Ocean, a source region of moisture. A notable exception to this general pattern exists in northeast China, where less wet conditions are recorded. We determined molecular compositions of individual plant wax hydrocarbons and their hydrogen isotope compositions (δD values) in a radiocarbon-dated peat core recovered from the Hani marsh in Jilin Province (China) and confirmed that the temperature-dependent effective precipitation in northeast China decreased during the Holocene Climate Optimum. A combination of P_aq, an indicator of the relative contribution of aquatic to terrestrial plants, and the difference in δD between low (C₂₃, C₂₅ and C₂₇) and high molecular weight (C₃₁) n-alkanes in the Hani peat bog indicates a dramatic change in vegetation from the deglaciation to the Holocene. No significant differences were observed between the δD values of low and high molecular weight n-alkanes with relatively high δD values and low P_aq during the early Holocene, indicating that all n-alkanes were produced by evapotranspiration-sensitive terrestrial plants during that time. However, lower δD values of mid-chain n-alkanes (C₂₃, C₂₅ and C₂₇) relative to the long chain n-alkane (C₃₁), together with higher P_aq values during the deglaciation (14–11 ka), suggest an increase in the contribution of aquatic plants and a higher water level during the period. The study demonstrates that northeast China was under a markedly wetter climate condition during the late deglaciation. For the 16 kyr record in the Hani peat sequence, we infer that moisture delivery by the East Asian Monsoon was relatively invariable in northeast China, but increased evaporation during the warmer Holocene Climate Optimum reduced the effective precipitation, defined by the balance between precipitation and evaporation.