川东南长宁地区地震活动及b值演化特征

利用全国统一目录和流动台站目录,研究了四川盆地东南部长宁地区的地震活动特征和b值的空间分布特征.研究结果显示,长宁地区的地震活动在时间上呈现明显的分段特征,地震活动在2015年后明显增强;在空间上,长宁地区的地震活动主要集中在以28.3°N为界限的南、北2个地区,对于这2个区域的b值演化,计算结果显示出不同的分段特征....     

Offset dependent amplitudes: Fault curvature effects and dip move-out corrections

Equations have been presented previously which predict that reflector curvature can affect significantly seismic reflection amplitudes at both zero and nonzero source-receiver offsets. Here the fact that faults are generally concave-upward is used to examine the curvature effect for compaction-driven faults which the sediments have both exponential and logarithmic porosity decreases with increasing depth.The curvature effect is generally larger over upwardly concave reflectors where amplitudes can either increase with offset (exposed focus) or decrease with offset (buried focus). The magnitude of the effect depends on the ratio between the depth to the structure and the radius of curvature of the structure. A phase change of 90° also occurs at a critical offset in the case of an exposed focus concave-upward reflector, with decreasing amplitude at offsets larger than the critical value. Dip move-out (DMO) dominantly removes the amplitude variation with offset due to curvature. These results suggest that when looking for amplitude variations with offset in a fault prospect, DMO should be applied as a preprocessing step. Compaction-driven faults have an exposed focus and, for an exponential variation of porosity with depth, there is a maximum curvature effect at a depth roughly the same as the scaling depth for the porosity. Logarithmic porosity with depth variations suggest a continued increase in the curvature effect of faults with increasing depth, to a maximum amplitude increase of about 35% relative to a plane interface.     

Strain segmentation and lateral membrane deformation rate of the subducted Ryukyu slab

Abstract Focal mechanisms of intermediate-depth earthquakes within the subducted Ryukyu slab indicate a systematic pattern of down-dip extension in the northern part under Kyushu. These mechanisms switch rapidly around the Tokara channel to down-dip compression in the southern part toward Taiwan. This intriguing pattern of strain segmentation, together with the differences between the slab geometry of the northern and southern parts, has raised the question of whether stress segmentation, as defined possibly by a tear fault, might exist in the slab. However the Ryukyu trench has a concave oceanward shape in the northern segment while it is convex in the southern oceanward part toward Taiwan. The inflection zone is located around the Tokara channel. The concept of Gaussian curvature of a curved surface suggests that the along-arc variation of the geometric configuration of a subducted slab is related to the shape of the trench. This is in order to accommodate the lateral membrane deformation of the slab as the oceanic lithosphere subducts from a spherical shell to the geometry delineated by the Wadati-Benioff zone. The membrane deformation regime of the subducted Ryukyu slab and its relation with the trench geometry was examined by assuming that the subduction be modelled by the flow field of a thin viscous sheet. A projection operator was utilized to compute the membrane strain-rate tensor of an arbitrary non-Euclidean surface. Numerical experiments indicated that the northern segment of the slab was dominated by lateral compression and down-dip extension and the southern part by lateral extension and down-dip compression. This transition is sharply located near the Tokara channel. These patterns were compatible with what had been observed from studies of focal mechanisms, suggesting that the strain segmentation might be controlled, at least in part, by the lateral membrane deformation within the slab due to the trench shape in this subduction zone. The slab geometry was predicted by minimizing the integrated total dissipation power; this revealed distinct features that were consistent with observations. This implies that the slab geometry may also be affected by the membrane deformation in a systematic fashion.     

Analysis of Error Caused by Replacing Spherical Shell with an Elastic Plate Model in Studying Bending Deformation of the Lithosphere

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PLAN FORM AND PROFILE FORM OF HILLSLOPES

For 27 hillslope profiles located within four first-order drainage basins on the dip slope of the South Downs in East Sussex, contour curvature (P), measured in degrees per 100 m, is highly variable within any one hillslope. Hillslopes may be described as wholly concave in plan where P is less than ?40, wholly convex in plan where P is greater than +40 and convexo-concave in plan where P lies between ?40 and +40. One measure of P taken at the profile sampling line is found to give a better estimate of P than measuring P at the steepest point on the hillslope. P is found to be significantly negatively correlated with slope in 38 per cent of cases, correlated with distance from the divide in 55 per cent of cases (positively where P <0 and negatively where P <0) and positively correlated with profile curvature in 70 per cent of cases. Implications of these correlations for hillslope process-response models are discussed.     

New Damage-locating Method for Bridges Subjected to a Moving Load

A new damage-locating method for bridges subjected to a moving load is presented, and a new ‘moving load damage-locating indicator＇ （MLDI） is introduced. A vehicle is modeled as a moving load, the bridge is simplified as an Euler-Bernoulli beam, and the damage is simulated by a reduction of stiffness properties of the elements. The curvature and MLDI values at each node of the baseline model （undamaged） and the damage model are computed respectively. Then the damage or damages can be located from a sudden change of the MLDI value. The feasibility and effectiveness of the proposed method are validated by numerical simulation. The results indicate that the method is effective, being able to not only locate a single damage accurately, but also locate multiple damages in simply-supported bridges, including multiple damages in continuous bridges. The results also indicate that the MLDI can accurately locate damages under 5% measurement noise.     

小尺度断裂沿层曲率敏感性研究

沿层曲率作为地震属性的一种,不仅有数学几何意义更有着重要的地质意义。和相干体相比,沿层曲率在油气勘探中常被用于刻画地质构造形态及断裂精细解释,特别是在小尺度断裂发育的地区更为有效。而在实际工作中,有时利用沿层曲率所描述的断裂效果不太明显。为进一步明确对于刻画断裂结果的影响因素及其适用条件,以鄂尔多斯盆地Q区Tj9为研究对象,基于Geoeast软件对该区小尺度断裂系统从选取属性、方位角、频带三个方面展开分析。结果表明：（1）曲率比相干体更适用于刻画较小尺度的断层;（2）平面上从平行断裂的方位观测,断裂识别不清晰;从垂直断裂的方位观测,易于识别;（3）在反射层同相轴较强时,中高频段对断裂识别更清楚;在反射层同相轴较弱时,中频段对断裂识别更清楚。因此可以根据不同属性、不同方位角、不同频带对断裂的敏感程度,用不同的数据体解释小尺度断裂,提高对断层刻画的精度,为复杂油气勘探寻找有利区奠定基础。     

采空塌陷区地表建筑物变形特征分析及破坏机理研究

采空区建筑物的变形特性研究,一直是采空塌陷灾害的一个重要方向。本文以北京市房山区史家营乡大村涧村采空塌陷为背景,在充分分析采空区上方建筑物砖墙上裂缝形态及分布规律的基础上,从建筑物受力方面探讨了裂缝产生的原因,并采用数值模拟方法对采矿影响下建筑物的变形破坏机理进行浅析,研究结果不仅可为建筑物加固和维修提供依据,还可为采空塌陷区治理提供参考。     

MORPHOLOGICAL CHARACTERISTICS OF FREE BENDS

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基于多项式拟合均匀荷载面曲率曲线的结构损伤识别方法

为仅利用结构损伤状态的柔度矩阵对结构进行损伤程度识别,先对损伤状态的均匀荷载面曲率曲线进行最小二乘法拟合。根据曲率曲线差判断结构的损伤位置,对损伤位置的点进行剔除后,再利用未损伤位置上的点进行局部最小二乘法拟合,代替损伤前的均匀荷载面曲率曲线,用于结构的损伤定位与定量。通过一简支梁数值算例,先以理论的二次多项式进行拟合,考虑单损伤和多损伤的情况,进行损伤识别分析,再分析多项式次数、测点数目以及不同噪声水平对损伤定量精度的影响。结果表明:在一定范围内,次数越高拟合误差越小,但差别不明显,采用理论的二次多项式拟合即可满足结构损伤识别要求,无噪声的情况下,测点数目减半不影响损伤识别的精度,该方法具有一定的抗噪声能力。