Study of mechanical behavior and strain localization of methane hydrate bearing sediments with different saturations by a new DEM model

This paper presents a numerical investigation into mechanical behavior and strain localization in methane hydrate (MH) bearing sediments using the distinct element method (DEM). Based on the results of a series of laboratory tests on the bonded granules idealized by two glued aluminum rods and the available experimental data of methane hydrate samples, a pressure and temperature dependent bond contact model was proposed and implemented into a two-dimensional (2D) DEM code. This 2D DEM code was then used to numerically carry out a series of biaxial compression tests on the MH samples with different methane hydrate saturations, whose results were then compared with the experimental data obtained by Masui et al. [9]. In addition, stress, strain, void ratio and velocity fields, the distributions of bond breakage and averaged pure rotation rate (APR) as well as the evolution of strain localization were examined to investigate the relationships between micromechanical variables and macromechanical responses in the DEM MH samples. The numerical results show that: (1) the shear strength increases as methane hydrate saturation S_MH increases, which is in good agreement with the experimental observation; (2) the strain localization in all the DEM MH samples develops with onset of inhomogeneity of void ratio, velocity, strain, APR, and distortion of stress fields and contact force chains; and (3) the methane hydrate saturation affects the type of strain localization, with one shear band developed in the case of 40.9% and 67.8% methane saturation samples, and two shear bands formed for 50.1% methane saturation sample.     

Strain partitioning between the Suruga Trough and the Zenisu Thrust: Neogene to present tectonic evolution in the onland and offshore Tokai district, Japan

The Tokai district in central Japan is located close to the convergent boundary between the Philippine Sea and Eurasian plates, and has experienced not only repeated large interplate earthquakes but also intense aseismic movement. In this paper, the spatial and temporal tectonic evolution of the Tokai district, particularly around the Omaezaki area, is discussed to assess whether the district has been and will be active or inactive. According to a geological survey, the horizontal crustal shortening strain can imply the hypothetical tectonic model that the area has been getting less active and the strain rate since the Neogene can be calculated as 12% and 2×10⁻⁶%/year, respectively. The present interseismic horizontal crustal strain and strain rate around the Omaezaki area are approximately 4×10⁻⁷% and 4×10⁻⁹%/year. By comparing these rates, the decrease since Neogene can imply the hypothetical tectonic model that the area has been getting less active influenced by the strain partitioning between the Suruga Trough and the Zenisu Thrust.     

Kinematics and geochronology of Early Cretaceous thrusting in the northwestern paleozoic of Graz (Eastern Alps)

Abstract

The multiply deformed Upper Austro-Alpine nappe pile of the Graz area is built up of low-grade metamorphosed Paleozoic rocks which are discordantly overlain by sediments of Santonian (Late Cretaceous) age (“Gosau” formation). Slices of Permo-Mesozoic rocks are absent. Analyses of structures, microfabrics, strain and shear directions were used to decipher the kinematic history; geochronological investigations to date the age of thrusting. K/Ar and Rb/Sr ages of synkinematically grown mica suggest an eo-Alpine (Early Cretaceous) age for the major deformation D1. D1 is characterized by non-coaxial rock flow which caused SW- to W directed nappe imbrication. Incremental strain measurements indicate the progressive superposition of D2 over Dl. In the higher nappe (Rannach Nappe) nappe imbrication continued during D2 changing the direction of nappe transport from SW to NW. Enhanced flattening strain in the deeper nappe (Schöckel Nappe) led to recumbent folds in all scales during D2. This study emphasized two interpretations : (1) The Alpine deformation in the Upper Austro-Alpine nappe pile of the Paleozoic of Graz started in the Earliest Cretaceous (about 125 Ma.). (2) The emplacement of nappes followed a curved translation path in the studied area.     

阿尔金南缘断裂东段晚第四纪活动特征

阿尔金断裂的几何学和运动学特征对研究青藏高原构造演化和陆内地震机理非常重要。为定量科学研究较为薄弱的阿尔金南缘断裂东段的运动学参数,我们采用航卫片的解译、野外调查、地形测绘和年代学样品测试等多种方法对该段进行了初步研究。结果显示该段断裂主要以左旋走滑运动为主,且沿线存在较多的小位移,其位移空间分布的3个明显峰值分别为4.5m、8m和13m。根据河流阶地的左旋位移和相应的地貌面沉积年龄,得到晚第四纪以来阿尔金断裂东段的走滑速率约为2.7±0.9mm/a。基于所获得的定量参数,认为该段断裂向东以应变分解的形式将其应变量传递到相邻的逆冲断裂上。     

渤海湾区域应变应力场演化特征

根据渤海湾区域1999~2013年多期GPS站点速率资料,以营潍断裂带为界分为东西两个地块计算运动参数,求解主应变率和断裂带的应变率。基于应变率计算结果研究了区域应变-应力场。结果表明,1999~2011年渤海湾区域地壳整体受北东东-南西西向压应力和北北西-南南东向张应力作用,2011~2013年区域地壳压应力主方向为近南北向,张应力主方向呈近东西向,致使这一变化的主要原因是2011年日本3.11大地震。渤海湾区域不同地段的应力也存在差异,局部性变化明显。营潍断裂带不同时段的正应变率和剪应变率亦不同,1999~2011年为右旋剪应变-剪应力作用,2011~2013年为左旋剪应变-剪应力作用。     

断层活动协调比在地震预测中的应用

为了有效捕获潜在震源区前兆变化信息与断层应变积累状态, 本文应用断层活动协调比概念, 对川滇地区和首都圈地区断层活动进行了分析。 结果表明: ① 丽江7.0级地震前震源区经历了至少有10多年的断层闭锁阶段, 不同观测场地协调比变化的时空差异性特征可能反映了震源区以及外围的异常演化特征; ② 断层活动协调比参数可帮助判识断层活动的性质, 当基线、 水准原始观测曲线出现显著的变化, 而协调比处于正常水平时, 断层活动即为无应变积累的自由蠕滑; ③ 断层活动协调比在大部分地震前都出现一定的离散性变化, 对分析强震前的断层活动性质和应变积累状态以及地震中长期预测具有一定的参考价值。     

渤海盆地的现今扩张运动

关于渤海盆地的扩张,前人已有很多研究成果.但是,对渤海盆地现今扩张的主方向,扩张的速率和笵围迄今还不清楚,本文试图探讨之.根据中国地壳运动观测网络GPS网1998~2004年的观测结果,建立了渤海盆地周围地区相对于欧亚板块的速度场.将该区划分为8个块体,建立了各块体的速度模型,得到该区的水平形变速度场和应变场.分析形变速度场和应变场的空间变化,发现渤海及周围地区呈现明显的NW-SE方向的双向扩张运动,扩张速率为2.5±1.8 mm/a.NW-SE与NE-SW方向扩张范围分别为310 km与 410 km.渤海盆地扩张的范围与华北地壳最薄的区域基本一致,这表明上地壳的扩张变形与上地幔的隆升密切相关,岩石圈减薄和岩浆上涌使上地壳发生拉张变形,使渤海地区发育成一个拉张盆地.     

汶川MS8.0地震孕育发生的机制与动力学问题

2008年5月12日四川省汶川县发生了MS8.0强烈地震.发震断层是龙门山断裂带的映秀—北川断裂.分析震前的GPS速度场发现,从巴颜喀拉块体西部到龙门山断裂带沿大约N103°E方向的缩短速率为13.0 mm/a,龙门山断裂带的右旋走滑速率1.1 mm/a,断裂带处于闭锁状态.四川盆地沿大约N103°E方向有少量的压缩变形,而沿SW方向有少量的拉张变形.同震位移场显示,这次地震可能是巴颜喀拉块体SE向逆冲与四川盆地NW向俯冲同时发生的.应变场分析发现,震前震中区的主压与主张应变率分别为-30.840×10^-9/a与13.956×10^-9/a,主压应变轴N105.4°E与震源机制解得到的主压应力轴的方向N103°E一致.由本文提出的应力-应变机制得到的断层滑动方向和走向与地表破裂调查和震源机制解得到的结果一致.印度、太平洋和菲律宾海板块与欧洲板块的相互作用是龙门山断裂带积累弹性应变能和孕育汶川地震的长期作用力.苏门达腊大地震使青藏高原和华南块体的相互作用加强,促进了汶川地震的发生.     

2013年芦山地震序列震源机制与震源区构造变形特征分析

基于四川区域地震台网记录的波形资料,利用CAP波形反演方法,同时获取了2013年4月20日芦山M7.0级地震序列中88个M≥3.0级地震的震源机制解、震源矩心深度与矩震级,进而利用应变花（strain rosette）和面应变（areal strain）As值,分析了芦山地震序列震源机制和震源区构造运动与变形特征.获得的主要结果有：（1）芦山M7.0级主震破裂面参数为走向219°/倾角43°/滑动角101°,矩震级为M_W6.55,震源矩心深度15 km.芦山地震余震区沿龙门山断裂带走向长约37 km、垂直断裂带走向宽约16 km.主震两侧余震呈不对称分布,主震南西侧余震区长约27 km、北东侧长约10 km.余震分布在7~22 km深度区间,优势分布深度为9~14 km,序列平均深度约13 km,多数余震分布在主震上部.粗略估计的芦山地震震源体体积为37 km×16 km×16 km.（2）面应变As值统计显示,芦山地震序列以逆冲型地震占绝对优势,所占比例超过93%.序列主要受倾向NW、倾角约45°的近NE-SW向逆冲断层控制;部分余震发生在与上述主发震断层近乎垂直的倾向SE的反冲断层上;龙门山断裂带前山断裂可能参与了部分余震活动.P轴近水平且优势方位单一,呈NW-SE向,与龙门山断裂带南段所处区域构造应力场方向一致,反映芦山地震震源区主要受区域构造应力场控制,芦山地震是近NE-SW向断层在近水平的NW-SE向主压应力挤压作用下发生逆冲运动的结果.序列中6次非逆冲型地震均发生在主震震中附近,且主震震中附近P轴仰角变化明显,表明主震对其震中附近局部区域存在明显的应力扰动.（3）序列整体及不同震级段的应变花均呈NW向挤压白瓣形态,显示芦山地震震源区深部构造呈逆冲运动、NW向纯挤压变形.各震级段的应变花方位与形状一致,具有震级自相似性特征,揭示震源区深部构造运动和变形模式与震级无关.（4）不同深度的应变花形态以NW-NWW向挤压白瓣为优势,显示震源区构造无论是总体还是分段均以NW-NWW向挤压变形为特征.但应变花方位与形状随深度仍具有较明显的变化,可能反映了震源区构造变形在深度方向上存在分段差异.（5）芦山地震震源体尺度较小,且主震未发生在龙门山断裂带南段主干断裂上,南段长期积累的应变能未能得到充分释放,南段仍存在发生强震的危险.     

利用强震震源机制解资料反演中国大陆及其邻区的形变场模型

搜集了1900-2013年间发生在中国大陆及其邻区的震源机制解资料,详细整理了其中的70个7级及以上大震的震源参数、地表破裂带和地表位移资料。根据资料的完整程度将地震分成三类: A类存在地表破裂和地表位移观测资料; B类存在地表破裂资料,但缺少地表位移观测数据;C类缺少地表破裂带和地表位移观测资料。对B类和C类缺少地表位移观测数据的地震,利用三角形模型模拟其位移分布。再根据地表位移分布及地震破裂带与本文使用网格模型之间的位置关系将地震分段。最后,利用分段前、后的地震数据和改进的双三次样条方法反演研究区域的形变场模型。结果表明:①大震资料的分段处理改进了地震数据的反演结果,提高了反演模型的合理性和空间一致性;在喜马拉雅断裂带,形变场具有更好的连续性,其变形特征与地质等数据的反演结果基本吻合;塔里木盆地和阿尔金断裂的形变量减小,与该区域较低的地震活动性一致;戈壁一阿尔泰的变形从SE的挤压和NE的拉张调整到NE的挤压和NW的拉张;鄂尔多斯西缘的拉张分量明显减小。②113年的地震资料解释了印度板块向欧亚板块运动总速率的30~50%,存在20 mm/a左右的速度亏损,该亏损量可能包括断层蠕动、褶皱等非震形变,未监测到或者缺失的地震,及以弹性应变能形式存在通过潜在地震释放的应变