COULOMB STRESS CHANGE ON ACTIVE FAULTS IN SICHUAN-YUNNAN REGION AND ITS IMPLICATIONS FOR SEISMIC HAZARD

Coulomb stress change on active faults is critical for seismic hazard analysis and has been widely used at home and abroad. The Sichuan-Yunnan region is one of the most tectonically and seismically active regions in Mainland China, considering some highly-populated cities and the historical earthquake records in this region, stress evolution and seismic hazard on these active faults capture much attention. From the physical principal, the occurrence of earthquakes will not only cause stress drop and strain energy release on the seismogenic faults, but also transfer stress to the surrounding faults, hence alter the shear and normal stress on the surrounding faults that may delay, hasten or even trigger subsequent earthquakes. Previously, most studies focus on the coseismic Coulomb stress change according to the elastic dislocation model. However, the gradually plentiful observation data attest to the importance of postseismic viscoelastic relaxation effect during the analysis of seismic interactions, stress evolution along faults and the cumulative effect on the longer time scale of the surrounding fault zone. In this paper, in order to assess the seismic hazard in Sichuan-Yunnan region, based on the elastic dislocation theory and the stratified viscoelastic model, we employ the PSGRN/PSCMP program to calculate the cumulative Coulomb stress change on the main boundary faults and in inner blocks in this region, by combining the influence of coseismic dislocations of the M≥7.0 historical strong earthquakes since the Yongsheng M7.8 earthquake in 1515 in Sichuan-Yunnan region and M≥8.0 events in the neighboring area, and the postseismic viscoelastic relaxation effect of the lower crust and upper mantle. The results show that the Coulomb stress change increases significantly in the south section of the Xianshuihe Fault, the Anninghe Fault, the northern section of the Xiaojiang Fault, the southern section of the Longmen Shan Fault, the intersection of the Chuxiong-Jianshui Fault and the Xiaojiang Fault, and the Shawan section of the Litang Fault, in which the cumulative Coulomb stress change exceeds 0.1MPa. The assuming different friction coefficient has little effect on the stress change, as for the strike-slip dominated faults, the shear stress change is much larger than the normal stress change, and the shear stress change is the main factor controlling the Coulomb stress change on the fault plane. Meanwhile, we compare the Coulomb stress change in the 10km and 15km depths, and find that for most faults, the results are slightly different. Additionally, based on the existing focal mechanism solutions, we add the focal mechanism solutions of the 5 675 small-medium earthquakes(2.5≤M≤4.9)in Sichuan-Yunnan region from January 2009 to July 2019, and invert the directions of the three principal stresses and the stress shape factor in 0.1°×0.1° grid points; by combining the grid search method, we compare the inverted stress tensors with that from the actual seismic data, and further obtain the optimal stress tensors. Then, we project the stress tensors on the two inverted nodal planes separately, and select the maximum Coulomb stress change to represent the stress change at the node. The results show that the cumulative Coulomb stress change increase in the triple-junction of Sichuan-Yunnan-Tibet region is also significant, and the stress change exceeds 0.1MPa. Comprehensive analysis of the Coulomb stress change, seismic gaps and seismicity parameters suggest that more attention should be paid to the Anninghe Fault, the northern section of the Xiaojiang Fault, the south section of the Xianshuihe Fault, the southern section of the Longmen Shan Fault and the triple-junction of the Sichuan-Yunnan-Tibet region. These results provide a basis for future seismic hazard analysis in the Sichuan-Yunnan region.     

NEAR-FAULT DISPLACEMENT AND DEFORMATION OBTAINED FROM ONE-KILOMETER-LONG FAULT-CROSSING BASELINE MEASUREMENTS-A PRELIMINARY EXPERIMENT AT 2 SITES ON THE EASTERN BOUNDARY OF THE SICHUAN-YUNNAN BLOCK

The current and conventional fault-crossing short baseline measurement has a relatively high precision, but its measurement arrays usually fail to or cannot completely span major active fault zones due to the short length of the baselines, which are only tens to 100 meters. GNSS measurement has relatively low resolution on near-fault deformation and hence is not suitable for monitoring those faults with low motion and deformation rates, due to sparse stations and relatively low accuracy of the GNSS observation. We recently built up two experimental sites on the eastern boundary of the active Sichuan-Yunnan block, one crossing the Daqing section of the Zemuhe Fault and the other crossing the Longshu section of the Zhaotong Fault, aiming to test the measurement of near-fault motion and deformation by using fault-crossing arrays of one-kilometer-long baselines. In this paper, from a three-year-long data set we firstly introduce the selection of the sites and the methods of the measurement. We then calculate and analyze the near-field displacement and strain of the two sites by using three hypothetical models, the rigid body, elastic and composed models, proposed by previous researchers. In the rigid body model, we assume that an observed fault is located between two rigid blocks and the observed variances in baseline lengths result from the relative motion of the blocks. In the elastic model, we assume that a fault deforms uniformly within the fault zone over which a baseline array spans, and in the array baselines in different directions may play roles as strainmeters whose observations allow us to calculate three components of near-fault horizontal strain. In the composed model, we assume that both displacement and strain are accumulated within the fault zone that a baseline array spans, and both contribute to the observed variances in baseline lengths. Our results show that, from the rigid body model, variations in horizontal fault-parallel displacement component of the Zemuhe Fault at the Daqing site fluctuate within 3mm without obvious tendencies. The displacement variation in the fault-normal component keeps dropping in 2015 and 2016 with a cumulative decrease of 6mm, reflecting transverse horizontal compression, and it turns to rise slightly(suggesting extension)in 2017. From the elastic model, the variation in horizontal fault-normal strain component of the fault at Daqing shows mainly compression, with an annual variation close to 10^-5, and variations in the other two strain components are at the order of 10^-6. For the Longshu Fault, the rigid-body displacement of the fault varies totally within a few millimeters, but shows a dextral strike-slip tendency that is consistent with the fault motion known from geological investigation, and the observed dextral-slip rate is about 0.7mm/a on average. The fault-parallel strain component of the Longshu Fault is compressional within 2×10^-6, and the fault-normal strain component is mainly extensional. Restricted by the assumption of rigid-body model, we have to ignore homolateral deformation on either side of an observed fault and attribute such deformation to the fault displacement, resulting in an upper limit estimate of the fault displacement. The elastic model emphasizes more the deformation on an observed fault zone and may give us information about localizations of near-fault strain. The results of the two sites from the composed model suggest that it needs caution when using this model due to that big uncertainty would be introduced in solving relevant equations. Level surveying has also been carried out at the meantime at the two sites. The leveling series of the Daqing site fluctuates within 4mm and shows no tendency, meaning little vertical component of fault motion has been observed at this site; while, from the rigid-body model, the fault-normal motion shows transverse-horizontal compression of up to 6mm, indicating that the motion of the Zemuhe Fault at Daqing is dominantly horizontal. The leveling series of the Longshu site shows a variation with amplitude comparable with that observed from the baseline series here, suggesting a minor component of thrust faulting; while the baseline series of the same site do not present tendencies of fault-normal displacement. Since the steep-dip faults at the two sites are dominantly strike-slip in geological time scale, we ignore probable vertical movement temporarily. In addition, lengths of homolateral baselines on either side of the faults change somewhat over time, and this makes us consider the existence of minor faults on either side of the main faults. These probable minor faults may not reach to the surface and have not been identified through geological mapping; they might result in the observed variances in lengths of homolateral baselines, fortunately such variations are small relative to those in fault-crossing baselines. In summary, the fault-crossing measurement using arrays with one-kilometer-long baselines provides us information about near-fault movement and strain, and has a slightly higher resolution relative to current GNSS observation at similar time and space scales, and therefore this geodetic technology will be used until GNSS networks with dense near-fault stations are available in the future.     

新疆西克尔地区微震波谱的初步研究

为探索大地震发生前孕震区内微震辐射波谱的变化,在新疆西克尔地区,用选频测震仪对微震波谱进行了观测和研究。选频测震仪分为6个频道,各道中心频率分别为6.0周、8.1周、15.7周、30.0周、50.0周、75.0周。初步结果表明:在一个地震系列中,随着临近较强地震（M_L≥3.2）的发生,微震P波在10周-60周频率范围内的谱值增高,其置信水平一般在70％以上。较强地震发生以后,微震P波在相应频率范围内的谱值下降。 文中还简要地分析了选频地震记录的物理意义,并对较强地震前,微震辐射波谱变化的原因作了初步说明。     

全球过去千年典型暖期温度空间格局重建

利用过去两千年全球变化研究网络(PAGES 2k network)最新公布的501条代用记录, 重建了全球过去千年全年平均温度空间格局的演化特征, 对比分析了中世纪暖期及其最暖100年与20世纪现代暖期、中世纪暖期和小冰期最暖30年与20世纪最近30年的年平均温度空间模态异同。结果显示, 在世纪尺度上, 现代暖期与历史上中世纪暖期的温度异常空间格局大致相同, 变化幅度也在大部分区域相当, 但从年代际尺度上, 最近30年的升温比过去千年中世纪暖期和小冰期两个典型时期都明显。值得一提的是北大西洋中高纬度海温变化与上述特征并不相同, 在年代际和世纪尺度上小冰期和中世纪暖期海温均高于20世纪。可能原因是大西洋经圈翻转环流在中世纪暖期、 小冰期和20世纪现代暖期等3个特征时段对太阳辐射、火山活动和温室气体等外强迫的响应不同。     

新疆牙哈地区新近系沉积相及油气勘探意义

利用地震资料具有横向分辨率高的优点,将地震资料与录井资料和测井资料相结合,用地震相分析、地震振幅属性分析、相干分析和可视化等手段对牙哈地区新近系的河道展布进行了刻画,确定了河道、河漫滩、决口扇和曲流砂坝等沉积相类型的平面展布.在新近系共识别出7期大型河道,总面积为619 km2,可形成有利勘探面积75 km2,为牙哈地区新近系下一步勘探提供了依据.     

Numerical simulation of the Qulong Paleolandslide Dam event in the late pleistocene using the finite volume type shallow water model

Natural Hazards - The Qulong paleolandslide dam event lies in the Benzilan-Batang zone of the upper Jinsha River. The Jinsha River is one of the most extensive water resources in southwest China....     

Modeling of the Correlation Between Mineral Size and Shale Pore Structure at Meso- and Macroscales

Mathematical Geosciences - Inorganic pore structures are critical to understand the oil and gas transport and storage properties of unconventional reservoirs. However, it can be difficult to...     

油田开发中后期岩相单元的细分研究

油田开发中后期建立在原小层对比和沉积微相基础上的储层地质模型已不能适应调整挖潜和剩余油分布研究的要求,需要在小层划分的基础上,进一步细分到岩相单元。本文以大庆长垣南部葡北油田葡I组油层为例提出岩相单元的概念,通过现代沉积研究论证岩相单元细分的可行性,结合密井网测井曲线分析旋回性、隔夹层分布的稳定性及目前经济技术和采油工艺等,探讨细分单元的基本原则。最后提出岩相单元细分的基本方法。     

地学三维模型光照与动态显示

介绍了一种具有真实感的地学三维模型光照显示的新方法,给出了三角网构建和曲面样条函数插值方法,讨论了适应三维模型光照显示的可见性测试、光照模型等算法和方法,实现了模型动态显示和剖面绘制。     

Numerical modeling of cyclic mobility based on fuzzy-set concepts in plasticity theory

In this paper, the application of an efficient, transparent and accurate kinematic-cyclic constitutive model based on the fuzzy-set concepts and incremental plasticity theory is presented to show its capability in modeling cyclic mobility of saturated granular soil. The nature and kinematic mechanism of the membership functions in the fuzzy-set constitutive model are illustrated. The model’s capability of modeling soil dilatancy is investigated. Important features of volume change and pore water pressure build-up related to soil cyclic mobility are captured. The formulation of the proposed model is relatively simple and it can be readily implemented in finite element codes. The enhanced fuzzy-set model is capable of simulating ground motion problems particularly related to cyclic mobility, soil liquefaction, and spreading behavior.