巴基斯坦北Potwar形变区地震的震源机制研究

巴基斯坦北Potwar形变区是西北喜马拉雅褶皱逆冲带前陆区的一部分，绘制了该区的地震活动性图. 与相邻地区比较，该区地震并不活跃，没有显示出与地表地质构造相关的清晰地震活动图象. 做出了4次地震的震源机制解. 结果表明，有3次地震是左旋走滑断层活动，另一次地震是逆断层活动. 地震震源机制解的P轴方向为NW-SE和NE-SW. 现今的构造形变很可能也包括基底的形变.      

巴楚-伽师Ms6.8地震地磁异常特点分析

通过分析、比较1996--2003年喀什地震台站记录到的新疆伽师及附近地区发生的多次6级地震的地磁前兆异常特征，发现在这几组地震活动过程中喀什地震台的地磁表现出不同的异常特征，说明与地震破裂特征有更深层次的相关性，且地磁响应比异常和总强度差值异常在一定程度上反映了台站周围小范围的震源区介质电性结构的变化。巴楚-伽师6．8级地震前地磁异常特征并不十分突出，有别于1996—1998年伽师强震群震前地磁异常量较丰富的特点。     

A study of microearthquake seismicity and focal mechanisms within the Sea of Marmara (NW Turkey) using ocean bottom seismometers (OBSs)

We have carried out seismological observations within the Sea of Marmara (NW Turkey) in order to investigate the seismicity induced after Gölcük–İzmit (Kocaeli) earthquake (M_w 7.4) of August 17, 1999, using ocean bottom seismometers (OBSs). High-resolution hypocenters and focal mechanisms of microearthquakes have been investigated during this Marmara Sea OBS project involving deployment of 10 OBSs within the Çınarcık (eastern Marmara Sea) and Central-Tekirdağ (western Marmara Sea) basins during April–July 2000. Little was known about microearthquake activity and their source mechanisms in the Marmara Sea. We have detected numerous microearthquakes within the main basins of the Sea of Marmara along the imaged strands of the North Anatolian Fault (NAF). We obtained more than 350 well-constrained hypocenters and nine composite focal mechanisms during 70 days of observation. Microseismicity mainly occurred along the Main Marmara Fault (MMF) in the Marmara Sea. There are a few events along the Southern Shelf. Seismic activity along the Main Marmara Fault is quite high, and focal depth distribution was shallower than 20 km along the western part of this fault, and shallower than 15 km along its eastern part. From high-resolution relative relocation studies of some of the microearthquake clusters, we suggest that the western Main Marmara Fault is subvertical and the eastern Main Marmara Fault dips to south at 45°. Composite focal mechanisms show a strike-slip regime on the western Main Marmara Fault and complex faulting (strike-slip and normal faulting) on the eastern Main Marmara Fault.     

Spatial damage distribution of August 16,2003,Inner Mongolia,China,M_S=5.9 earth-quake and analysis

Introduction The ground motion is generally estimated by attenuation relation in seismic hazard assesment.The attenuation relation is usually the function of earthquake magnitude and distance.Actually the focal mechanism and other source parameters may also have significant impacts on the ground motion,especially in the near-source region.Recent post-earthquake investigations show that the damages have close relation with the closest fault-plane distance.On August16,2003,a MS=5.9earthquake o…     

A study on the focal parameters of Shidian M =5.9 earthquake sequences in 2001

On the basis of about 300 earthquake wave forms observed in the Shidian M _S=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5–5.3, the seismic moments are 10¹⁰–10¹⁶ N·m, the corner frequencies are 0.2–0.8 Hz, radii of the focal rupture are 200–2 500 m and the stress drops are 0.1×10⁵–20×10⁵Pa. Through the statistical analyses of variation of corner frequency f _c and stress drop Δσ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops Δσ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of average corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses. Foundation item: Scientific and Technological Key Project of Yunnan Province (2001NG46)     

Current states of stress in the northern Andes as indicated by focal mechanisms of earthquakes

Systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern Andes reveals relatively homogeneous patterns of crustal stress in three main regions. The first region, presently under the influence of the Caribbean plate, includes the northern segment of the Eastern Cordillera of Colombia and the western flank of the Central Cordillera (north of 4°N). It is characterized by WNW–ESE compression of dominantly reverse type that deflects to NW–SE in the Merida Andes of Venezuela, where it becomes mainly strike–slip in type. A major bend of the Eastern thrust front of the Eastern Cordillera, near its junction with the Merida Andes, coincides with a local deflection of the stress regime (SW–NE compression), suggesting local accommodation of the thrust belt to a rigid indenter in this area. The second region includes the SW Pacific coast of Colombia and Ecuador, currently under the influence of the Nazca plate. In this area, approximately E–W compression is mainly reverse in type. It deflects to WSW–ENE in the northern Andes south of 4°N, where it is accommodated by right-lateral displacement of the Romeral fault complex and the Eastern front of the northern Andes. The third, and most complex, region is the area of the triple junction between the South American, Nazca and Caribbean plates. It reveals two major stress regimes, both mainly strike–slip in type. The first regime involves SW–NE compression related to the interaction between the Nazca and Caribbean plates and the Panama micro-plate, typically accommodated in an E–W left-lateral shear zone. The second regime involves NW–SE compression, mainly related to the interaction between the Caribbean plate and the North Andes block which induces left-lateral displacement on the Uramita and Romeral faults north of 4°N.Deep seismicity (about 150–170 km) concentrates in the Bucaramanga nest and Cauca Valley areas. The inversion reveals a rather homogeneous attitude of the minimum stress axis, which dips towards the E. This extension is consistent with the present plunge of the Nazca and Caribbean slabs, suggesting that a broken slab may be torn under gravitational stresses in the Bucaramanga nest. This model is compatible with current blocking of the subduction in the western northern Andes, inhibiting the eastward displacement of slabs, which are forced to break and sink in to the asthenosphere under their own weight.     

On the pore‐scale mechanisms leading to brittle and ductile deformation behavior of crystalline rocks

Deformation mechanisms at the pore scale are responsible for producing large strains in porous rocks. They include cataclastic flow, dislocation creep, dynamic recrystallization, diffusive mass transfer, and grain boundary sliding, among others. In this paper, we focus on two dominant pore‐scale mechanisms resulting from purely mechanical, isothermal loading: crystal plasticity and crofracturing. We examine the contributions of each mechanism to the overall behavior at a scale larger than the grains but smaller than the specimen, which is commonly referred to as the mesoscale. Crystal plasticity is assumed to occur as dislocations along the many crystallographic slip planes, whereas microfracturing entails slip and frictional sliding on microcracks. It is observed that under combined shear and tensile loading, microfracturing generates a softer response compared with crystal plasticity alone, which is attributed to slip weakening where the shear stress drops to a residual level determined by the frictional strength. For compressive loading, however, microfracturing produces a stiffer response than crystal plasticity because of the presence of frictional resistance on the slip surface. Behaviors under tensile, compressive, and shear loading invariably show that porosity plays a critical role in the initiation of the deformation mechanisms. Both crystal plasticity and microfracturing are observed to initiate at the peripheries of the pores, consistent with results of experimental studies. Copyright © 2015 John Wiley & Sons, Ltd.     

Laboratory Simulation of the Formation Process of Surface Geochemical Anomalies Applied to Hydrocarbon Exploration

The formation mechanisms and processes of geochemical anomalies used as proxies in surface geochemistry exploration (SGE) have not been well understood. Previous studies cannot realize 3D measurement of microseeping hydrocarbons from reservoirs to the surface, which made it difficult to understand the features and pathways of deep hydrocarbon microseepages. Understanding the processes of hydrocarbon microseepages will contribute to the acceptance and effectiveness of surface geochemistry. Based on a simplified geological model of hydrocarbon microseepages, including hydrocarbon reservoir, direct caprock, overlying strata and Quaternary sediments, this work established a 3D experimental system to simulate the mechanisms and processes of deep hydrocarbon microseepes extending to the surface. The dispersive halos of microseeping hydrocarbons in the subsurface were adequately described by using this 3D experimental system. Results indicate that different migration patterns of hydrocarbons above the point gas source within the simulated caprock and overlying strata can be reflected by the ratio of i-butane to n-butane (i-C4/n-C4), which follow diffusion and infiltration (buoyancy) mechanisms. This is not the case for vertical measurement lines far from the point gas source. A vertical gas flow in the form of a plume was found during hydrocarbon microseepage. For sampling methods, the high-density grid sampling is favorable for delineating prospecting targets. Hydrocarbon infiltration or buoyancy flow occurs in the zones of infiltration clusters, coupling with a diffusion mechanism at the top of the water table and forming surface geochemical anomalies. These results are significant in understanding hydrocarbon microseepage and interpreting SGE data.     

2017年九寨沟7.0级地震序列震源机制解和构造应力场特征

九寨沟余震序列的震源机制和构造应力场有助于认识本次地震的发震构造和孕震机理。本文基于四川区域地震台网的波形资料, 采用波形拟合(CAP)方法和P波初动+振幅比(HASH)方法反演得到2017年8月8日九寨沟7.0级地震序列中59次ML≥3.0地震的震源机制解, 并基于该结果采用阻尼线性逆推法(DRSSI), 计算研究区域的平均构造应力场, 给出该区域的应力场特征。结果显示, 利用CAP方法反演得到的本次主震的最佳双力偶机制解节面I: 走向248°/倾角86°/滑动角–169°, 节面II: 走向157°/倾角79°/滑动角–4°, 矩震级为Mw6.31, 矩心深度5 km, 属走滑型地震事件; 大部分余震的震源机制解错动类型与主震一致, 矩心深度集中在3~10 km; 应力场反演结果显示, 该区域周边的应力性质为走滑型, 最大主应力方向呈NWW–SEE向, 与该区域的应力场方向一致, 表明本次地震主要受区域应力的控制。结合该区域的地震地质构造等已有研究成果, 分析认为此次地震的发震断层为走向NW–SE、倾向SW的左旋走滑断裂——树正断裂, 巴颜喀拉块体向E-SE向的水平运动受到华南块体的强烈阻挡导致此次地震的发生, 汶川地震的发生对本次地震具有一定的促进作用。