Evidence for a Tang-Song Dynasty great earthquake along the Longmen Shan Thrust Belt prior to the 2008 <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> 7.9 Wenchuan earthquake,China

The magnitude (M _w) 7.9 Wenchuan earthquake occurred on 12 May 2008 in the Longmen Shan region of China, the transition zone between the Tibetan Plateau and the Sichuan Basin, resulting in widespread damage throughout central and western China. The steep, high-relief eastern margin of the Tibetan Plateau has undergone rapid Cenozoic uplift and denudation accompanied by folding and thrusting, yet no large thrust earthquakes are known prior to the 2008 M _w 7.9 Wenchuan earthquake. Field and excavation investigations reveal that a great historical earthquake occurred in the Sichuan region that ruptured a >200-km-long thrust fault within the Longmen Shan Thrust Belt, China, which also triggered the 2008 M _w 7.9 Wenchuan earthquake. The average co-seismic slip amount produced by this historical earthquake is estimated to be 2–3 m, comparable with that caused by the 2008 Wenchuan earthquake. Paleoseismic and archaeological evidence and radiocarbon dating results show that the penultimate great earthquake occurred in the Sichuan region during the late Tang-Song Dynasty, between AD 800 and 1000, suggesting a recurrence interval of ~1,000–1,200 years for Wenchuan-magnitude (M = ~8) earthquakes in the late Holocene within the Longmen Shan Thrust Belt. This finding is in contrast with previous estimates of 2,000–10,000 years for the recurrence interval of large earthquakes within the Longmen Shan Thrust Belt, as obtained from long-term slip rates based on the Global Positioning System and geological data, thereby necessitating substantial modifications to existing seismic-hazard models for the densely populated region at the eastern marginal zone of the Tibetan Plateau.     

风向对半固定沙垄表面风速影响——以古尔班通古特沙漠为例

利用DETI可移动测风系统对古尔班通古特沙漠半固定沙垄表面风速进行实地观测,获取了主要风季不同风向条件下沙垄表面7个典型部位距离地表20 cm、40 cm、60 cm、100 cm和200 cm 5个高度的风速系列数据,系统研究风向对半固定沙垄表面坡面风速及风速廓线的影响。结果表明,迎风坡的气流加速和背风坡的风速降低现象在实际观测中得到证实,但受风向的影响甚大。垄顶风速放大率随入射角的增大呈指数关系递增,而背风坡风速占垄顶风速的比率则随入射角的增大呈线性关系递减。大角度入射气流速度变化主要受控于沙垄形态,小角度入射气流速度变化主要受控于地表植被状况。无论气流以何种角度入射,距沙垄表面20 cm高度气流的加速均较其他高度缓和。沙垄底部和坡中下部的风速廓线变化趋势基本一致,且呈较好的对数拟合关系;两坡中上部和垄顶部的对数拟合方程相关系数偏小;受回旋涡流的影响,风速廓线在背风坡上部有明显偏折,初步断定涡流中心在距地表40 cm左右的高度。     

Effects of excitation frequency on detection accuracy of orthogonal wavelet decomposition for structural health monitoring

Accurate estimation of stiffness loss is a challenging problem in structural health monitoring. In this study orthogonal wavelet decomposition is used for identifying the stiffness loss in a single degree of freedom spring-mass-damper system. The effects of excitation frequency on accuracy of damage detection is investigated. Results show that pseudo-alias effects caused by the orthogonal wavelet decomposition (OWD), affect damage detectability. It is demonstrated that theproposed approach is sunable for damage detection when the excitation frequency is relatively low. This study shows how a priori knowledge about the signal and ability to control the sampling frequency can enhance damage detectability.     

Study on the crust-mantle structure in the central and southern parts of Shanxi

Another comparative interpretation was conducted with respect to the data from 5 DSS profiles in the central and southern parts of Shanxi, leading to the conclusion that in Linxian, Linfen and Xingtai earthquake regions, through which the five profiles pass, there exist anomalous crust-mantle structure and abyssal crustal faults extending to Moho, all being regarded as the deep indications for earthquake occurrence.     

渤海南部海区风浪推算与分析

根据文氏建立的风与浪理论关系及其由风推算风浪的计算方法,利用马山子气象站３３ａ的大风资料,推算出黄河口近海水域波浪要求,并与中科院尜所实测波浪数值和青岛海洋大学推算的多一遇波浪数值进行了比较,其结果是十分相近的。     

FIRST REPORT OF BERO ZECO ACTIVE FAULT IN GÊRZÊ, NORTHERN TIBET

In the interior of the Tibetan Plateau, the active tectonics are primarily marked by conjugate strike slip faults and north-trending rifts, which represent the E-W extension since late Cenozoic of the plateau. The conjugate faults are mainly composed of NE-trending left-lateral strike-slip faults in Qiangtang terrane and NW-trending right-lateral strike-slip faults in Lhasa terrane. While, the rifts mainly strike N, NNW and NNE within southern Tibet. However, it is still a debate on the deformational style and specific adjustment mechanism of E-W extension. One of key reasons causing this debate is the lack of detailed investigation of these active faults, especially within the northwestern plateau. Recently, we found a 20km long, NNW-trending active fault at Bero Zeco in northwestern Tibet. This fault is presented as fault sag ponds, channel offsets and fault scarps. Displacement of channels and geomorphic features suggested that the Bero Zeco Fault(BZF)is a dextral strike-slip fault with a small amount of normal slip component, which may result from the E-W extensional deformation in the interior of Tibet. BZF strikes N330°~340°W, as shown on the satellite image. The main Quaternary strata in the studied area are two stages alluvial fans around the Bero Zeco. From the satellite images, the old alluvial fans were cut by the lake shoreline leaving many of lake terraces. And the young fans cut across the lake terraces and the old fans. By contrasting to the "Paleo-Qiangtang Huge Lake" since late Quaternary, these old alluvial fans could be late Pleistocene with age ranging from 40ka to 50ka. And the young fans could be Holocene. The sag ponds along the BZF are distributed in the late Pleistocene alluvial fans. Also, the BZF displaced the late Pleistocene fans without traces within Holocene fans, suggesting that the BZF is a late Pleistocene active fault. The fault scarps are gentler with the slope angle of around 10° and the vertical offset is about 2m by field measurement. Reconstruction of the offset of channels suggested that the accumulated dextral offset could be about 44m on the late Pleistocene alluvial fans. Therefore, we infer that the dextral slip-rate could be around 1mm/a showing a low-rate deformation characteristic. The angle between the strike of BZF and principal compressive stress axis(σ₁)is around 30°, which is significantly different to the other faults within the conjugate strike-slip fault zones that is 60°~75°. Now, the deformation mechanisms on these conjugate faults are mainly proposed in the studies of obtuse angle between the faults and σ₁, which is likely not applicable for the BZF. We infer that the BZF could be the northward prolongation of the north-trending rifts based on the geometry. This difference suggests that the conjugate strike-slip faults may be formed by two different groups:one is obtuse angle, which is related to block extrusion or shear zones in Lhasa and Qiangtang terranes possibly; the other is acute angle, which may represent the characteristics of new-born fractures. And more studies are needed on their deformation mechanisms.     

考虑地震动持时效应的输电塔-线体系响应研究

研究了地震动持时效应对输电塔-线体系动力响应的影响。基于能量持时的定义方法,选取了一组包括20条长持时和20条短持时的地震动作为输入。地震动调幅至0.2g和1.2g来分别研究地震动持时效应对结构弹性和塑性响应的影响,并选取输电塔的塔顶峰值加速度、位移、主材应力以及节间位移角作为评价指标。结果表明:地震动持时效应对所选取的评价指标具有不同的影响规律,在输电塔-线体系动力响应研究中持时效应不容忽视。可以为输电塔的抗震设计提供有益的参考。     

多维地震激励下考虑构件滞回性能的输电塔倒塌分析

本文基于角钢新型非线性材料本构模型对多维地震激励下输电塔倒塌破坏进行模拟研究。基于ABAQUS中RIKS算法得到不同长细比角钢的屈曲承载力,并通过承载力试验验证有限元分析结果。基于角钢滞回性能试验结果,采用Fortran语言编制角钢新型非线性材料本构模型,并与试验结果对比。建立输电塔三维有限元模型,采用动力显式分析方法,对多维地震激励下输电塔倒塌进行模拟,总结输电塔的极限承载力、杆件屈曲情况和倒塌位置。研究结果可为多维地震激励下输电塔抗倒塌设计提供参考。     

Morphology and Dynamics of Alluvial Fan and Its Research Prospects

Alluvial fans can preserve historical records of sediment transport to middle and lower river systems or piedmont basins, which are considered to be sensitive recorders of climate change and tectonic activity. In this paper, the morphological characteristics, control factors and future development trend of alluvial fan are summarized and described. The main understanding is as follows: \mathrm{①} According to the gravity flow and traction flow process, fan can be divided into debris flow alluvial fan and fluvial fan. The former is formed under the action of debris gravity flow deposits, which is related to the occasional flood and burst flow in a short time. The latter is braided tributaries depositions which are gradually shallower and spread radially in the direction of fan toe under the traction water transport. \mathrm{②} The erodibility of underlying bedrock can affect the scale of downstream alluvial fan, which depends on the sediment production and store factors in the catchment. The easily eroded bedrock may produce more sediment, making the alluvial fan area larger. In the contrast, the erodibility of rocks in the source area can also affect the slope and hydrological characteristics of the valley so that more sediment is deposited in the upstream basin and the alluvial fan formed in the downstream is smaller. \mathrm{③} Tectonic activity is the pre-condition for the development of alluvial fans, which provides a space for alluvial fans depositions. Faulting in the piedmont can change the position and morphology of the ancient alluvial fan, and also cause deformation or distortion of the thick sedimentary sequence to record the regional tectonic activity. The quaternary alluvial fan sequence corresponds well to the climate change during the glacial-interglacial period. However, the influence of the flood events caused by extreme meteorological events on alluvial fan deposition should be focused on. \mathrm{④} The application of a series of new techniques and methods will help to carry out deep research on alluvial fan in the future, such as high-resolution observation technique, physical simulation experiment, and precise dating.