Seismicity and stress evolution in heterogeneous faults with various degrees of roughness

We investigate the effects of the spatial distribution of stress drop ratios (SDRs) on the fault of an earthquake sequence as well as stress evolution. To achieve this, we test four fault models, each with its own SDRs which differ with regard to wavelength components (roughness) but which share first-order similarity and the same mean value. We assume rupture processes are quasi-static and fluid-controlled. The evidence clearly shows that the locations of large earthquakes are more sensitive to distribution than to the values of the SDRs. For this reason, it is expected that reducing fault roughness should increase the number of large earthquakes but decrease the number of small ones. It is also clearly apparent that a clustered distribution of large earthquakes should be more frequently observed on heterogeneous faults, especially in areas with small gradient values of the SDRs. These phenomena are consistent with our finding that long-wavelength components of stress usually develop in areas with low gradients of the SDRs. A fault with a smoother distribution of SDRs should, therefore, exhibit a greater likelihood to generate large earthquakes. By contrast, in that a rough fault covers a smaller area on which long-wavelengths can develop, it should result in fewer large earthquakes but a much more highly clustered distribution.     

全球海啸地震震源机制解及海啸成因探讨

海啸作为五大海洋自然灾害之一,严重威胁着人类生命财产安全。近些年来,国内外学者对地震海啸进行了大量研究,主要针对海啸的生成、传播、爬高和淹没的数值模拟,以及古海啸沉积物进行研究,但是对于海啸地震震源机制的研究还比较欠缺,尤其是缺乏对震级小于6.5的海啸地震的研究。针对我国的地震海啸研究现状,强调震级小于6.5地震引发海啸的问题不容忽视。本文归纳整理了全球766次地震海啸,利用三角图分类基本法则对海啸地震震源机制解进行分类,并对其中341个发生在1976年后的海啸地震进行震源机制解分析,对其中633次海啸浪高进行统计学方法分析研究。本文认为逆冲型、正断型、走滑型和奇异型机制地震均能引发海啸,逆冲型地震引发的海啸占比最大,震级小于6.5级地震引发的海啸的浪高也有高达10 m的情况,也能产生巨大破坏性。逆冲型、正断型、奇异型地震可直接引起海底地形垂向变化,进而引发海啸,而走滑型地震引发海啸则可能有两种原因,一种是走滑型地震并非纯走滑型而是带有正断或逆冲分量从而引发海啸,另外一种是走滑型地震引发海底滑坡导致海底地形变化进而产生海啸。从海啸地震震源深度分析,能产生海啸的地震震源深度97%以上都是浅源地震,主要集中在30 km深度以内,但是也有中深源地震海啸。本文综合海啸地震的震源特点、我国地理位置以及以往海啸发生的情况,认为未来我国沿海地区威胁性的地震海啸主要集中在马尼拉海沟和台湾海峡区域,在今后海啸预警方面需要格外重视这些区域,通过建立完善海啸预警系统来减少损失。     

Quantitative Tectonic Markers of Large Coastal Earthquakes in Fujian, Guangdong, Taiwan and Hainan——with a Discussion of Qualitative Indicators

This paper expounds the quantitative tectonic indicators and some qualitative indicators of large earthquakes in the coast areas of Fujian, Guangdong, Taiwan and Hainan. The main quantitative indicators include uplift amplitude of the Moho, Quaternary and Late Holocene coasts. The paper also gives a brief account of the research method on quantitative indicators of surface uplifted zones. Taiwan is a famous neotectonic zone and an area of large earthquakes in the world. There is only one large-earthquake area in each of Fujian, Guangdong and Hainan Provinces. Along the coast large earthquake areas there are certainly many remains of crustal activity. Among these remains, coast activity, taking the sea level as the accurate marker horizon, can determine not only the amplitude of coastal elevation and subsidence in a certain period, but also the cycle and rate of positive or negative movements.     

Role of African–Eurasian plate setting in the felt areas of intermediate‐depth earthquakes: an investigation using crowdsourced data

Greek intermediate‐depth earthquakes, occurring in the subducted plate of the Hellenic Arc, are felt at greater distances than expected, reaching Italy in some cases. We study in detail macroseismic intensity data from intermediate‐depth Italian and Greek earthquakes collected from Internet users who felt the shaking in Italy. The huge amount of data allowed us to outline the felt/not‐felt limit and to find a correspondence between attenuation areas and the presence of asthenospheric material at shallow depths. We show that plate boundaries, known to produce the majority of earthquakes, are, in some specific cases, the boundaries of areas in which earthquakes are felt. The Ionian subducted lithosphere propagates seismic waves with low attenuation over large distances, whereas high‐attenuation zones in Italy, linked to asthenospheric upwelling, limit the propagation, as evidenced also by PGA values. We identify a typical pattern that can be used to recognize intermediate‐depth earthquakes, and to properly locate historical events.     

Relationship between Crustal 3D Density Structure and the Earthquakes in the Longmenshan Range and Adjacent Areas

This paper presents the 3D density structure of crust in the Longmenshan range and adjacent areas,with constraints from seismic and density data.The density structure of crust shows that the immense boundary plane of density distribution in relation to the Longmeshan fault belt is extended downward to～80 km deep.This density boundary plane dips towards the northwest and crosses the Moho.With the proximity to the Longmenshan fault belt,it has a larger magnitude of undulation in the upper and middle crust ...     

Earthquake risk and slope stability in Jerusalem

Jerusalem is located 25 km from the active Dead Sea fault, which is a part of the Dead Sea Rift System Despite its proximity to the fault, the city has escaped past seismic events relatively undamaged. In contrast to the rest of the city, the Mount of Olives did suffer damage as a result of landslides, as evidenced by a large landslide scarp found in the western slope The unstable slopes in Jerusalem are located on soft Senonian chalk. In the past, these areas were left undeveloped and as a result, damage from earthquakes was relatively slight However, during the past 15 years, with the expansion of Jerusalem, construction has been taking place on unstable slopes as well This could result in heavy damage during future earthquakes A map showing the areas of highest risk is presented. It is recommended that the unstable slopes be reserved as green areas.     

Seismic Activity in the Himalayan Orogenic Belt and Its Related Geohazards

Himalayan orogenic belt is the highest and largest continental collision and subduction zone on the Earth. The Himalayan orogenic belt has produced frequent large earthquakes and caused several geohazards due to landslides and housing collapse, having an impact on the safety of life and property along a length of over 2500 km. Here we took three earthquake clusters as examples, which occurred at Nepal Himalaya, eastern Himalayan syntaxis and western Himalayan syntaxis, respectively. Here we calculated the earthquake locations and fault plane solutions based on the waveform data recorded by seismic stations deployed in source areas by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. We found that at the Nepal Himalayan, the Main Himalayan Thrust is the major tectonic structure for large earthquakes to occur. At the eastern Himalayan syntaxis, most earthquakes are of the reverse or strike-slip faulting. The major tectonic feature is the combination of the NE-dipping thrust with the southeastern escape of the Tibetan plateau. At the western Himalayan syntaxis, intermediate-depth earthquakes are active. These observations reveal the geometry of the deep subduction of the continental plate with steep dipping angle.     

Relationship between Crustal 3D Density Structure and the Earthquakes in the Longmenshan Range and Adjacent Areas

Abstract: This paper presents the 3D density structure of crust in the Longmenshan range and adjacent areas, with constraints from seismic and density data. The density structure of crust shows that the immense boundary plane of density distribution in relation to the Longmeshan fault belt is extended downward to ~80 km deep. This density boundary plane dips towards the northwest and crosses the Moho. With the proximity to the Longmenshan fault belt, it has a larger magnitude of undulation in the upper and middle crust levels. Density changes abruptly across Longmeshan fault belt. Seismic data show that most of the earthquakes in the Longmenshan area after the 2008 Ms8.0 Wenchuan Earthquake occurred within the upper to middle crust. These earthquakes are clearly distributed in the uplifted region of the basement. A few of them occurs in the transitional zone between the uplifted and subsided areas. But most of the earthquakes distributes in transitional zone from subsided to uplifted areas in the upper and middle crust where relatively large density changes occurr The 3D density structure of crust in the Longmenshan and adjacent areas can thus help us to understand the pattern of overthrusting from the standpoint of deep crust and where the earthquakes occurred.     

喜马拉雅造山带地震活动及其相关地质灾害

喜马拉雅造山带是地球上海拔最高、规模最大的陆陆板块俯冲碰撞带在这条长达2 500 km的板块边界上,近年来多次发生破坏性地震,造成大规模的滑坡、房屋倒塌等次生灾害,给人民生命和财产安全造成严重的威胁。分别选取尼泊尔喜马拉雅、喜马拉雅东构造结和喜马拉雅西构造结地区近期发生的3个地震震群作为研究实例,基于中国科学院青藏高原研究所在研究区架设的区域流动地震台站记录的波形资料,对地震的震源位置和震源机制解进行计算。结果表明,在尼泊尔喜马拉雅地区,主喜马拉雅逆冲断裂是大地震的主要发震构造;东构造结地区的地震以逆冲和走滑型为主,表明印度板块向北东方向的逆冲推覆和青藏高原向东南逃逸的侧向挤出是该地区的主要构造背景;西构造结地区中深源地震多发,揭示了高角度大陆深俯冲的几何形态。     

Crustal heterogeneity and seismotectonics of the region around Beijing, China

A detailed three-dimensional (3-D) P-wave velocity model of the crust and uppermost mantle under the Chinese capital (Beijing) region is determined with a spatial resolution of 25 km in the horizontal direction and 4–17 km in depth. We used 48,750 precise P-wave arrival times from 2973 events of local crustal earthquakes, controlled seismic explosions and quarry blasts. These events were recorded by a new digital seismic network consisting of 101 seismic stations equipped with high-sensitivity seismometers. The data are analyzed by using a 3-D seismic tomography method. Our tomographic model provides new insights into the geological structure and tectonics of the region, such as the lithological variations and large fault zones across the major geological terranes like the North China Basin, the Taihangshan and the Yanshan mountainous areas. The velocity images of the upper crust reflect well the surface geological and topographic features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocities, respectively. The Taihangshan and Yanshan mountainous regions are generally imaged as broad high-velocity zones, while the Quaternary intermountain basins show up as small low-velocity anomalies. Velocity changes are visible across some of the large fault zones. Large crustal earthquakes, such as the 1976 Tangshan earthquake (M=7.8) and the 1679 Sanhe earthquake (M=8.0), generally occurred in high-velocity areas in the upper to middle crust. In the lower crust to the uppermost mantle under the source zones of the large earthquakes, however, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids. The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes.     

Geophysical properties and seismotectonics of the Tohoku forearc region

We investigated the detailed three-dimensional (3-D) isotropic and anisotropic structures of the crust and upper mantle under the NE Japan forearc region using a large number of P and S wave arrival-time data from onshore and offshore earthquakes. The suboceanic earthquakes used in this study are well relocated using the sP depth phases. We also determined the 3-D distribution of Poisson’s ratio, crack density and saturation rate using the 3-D P and S wave velocity model obtained by this study. The relatively complex anisotropic structures in the megathrust zone may reflect the complex geological structures, lithological variations and fluids in the accretional prism under the forearc region. The tomographic images reflect strong lateral heterogeneities in the megathrust zone under the Tohoku forearc. Areas with low velocity, high Poisson’s ratio, high crack density and high saturation rate may be due to entrapment of fluid-filled, unsolidated sediments on the plate interface close to the Japan Trench. Most of the large megathrust earthquakes since 1900 (M ⩾ 6.0) and the large 2011 Tohoku-oki earthquakes (M 6.0–9.0) are located in areas with high velocity, high Poisson’s ratio, low crack density and high saturation rate, which may represent strongly-coupled asperities in the megathrust zone resulting from the subducted oceanic ridges and/or seamounts. In contrast, the areas with high Poisson’s ratio may indicate that the fluids have infiltrated into the strongly coupled patches. We think that the great Tohoku-oki earthquakes were caused by not only the stress concentration but also the in situ structural heterogeneities in the megathrust zone.     

Active fault mapping: An initiative towards seismic hazard assessment in India

Identification and characterization of active faults and deciphering their seismic potential are of vital importance in seismic hazard assessment of any region. Seismic vulnerability of India is well known as more than 60 % of its area lies in high hazard zones due to the presence of major active faults in its plate boundaries and continental interiors, which produced large earthquakes in the past and have potential to generate major earthquakes in future. The safety of critical establishments, like Power plants, Refinaries and other lifeline structures is a major concern in these areas and calls for a better characterization of these faults to help mitigate the impact of future earthquakes. The paper provides a brief overview of the work carried out in India on active fault research, its limitations and immediate priorities.     

Delineation of the geometry of nodes in the Alps–Dinarides hinge zone and recognition of seismogenic nodes (M ≥ 6)

We introduce a new method based on the analysis of the topographical alignments and used to delineate the structural boundaries of the nodes, which permits the definition of relatively narrow earthquake-prone areas by the pattern recognition approach. The structurally bounded nodes capable of earthquakes with M  ≥ 6.0, identified with pattern recognition, in fact, cover a significantly smaller area of the study region as compared with that defined by Gorshkov et al. (2004) who used conventional circles. The proposed method thus improves the precision in the location of potential large earthquakes.     

青藏高原周边地区河流分形特征与地貌、构造活动耦合关系

青藏高原周边地区的地貌特征与形成演化机制一直是科学界研究的热点。选择青藏高原周边典型地区河流分形特征、地貌特征及构造活动性进行研究,发现喜马拉雅断裂带、龙门山断裂带和阿尔金断裂带控制的区域构造活动性强烈,历史地震记录频繁,大震较多,河流形态与地貌演化特征也非常相似,河流纵剖面变化很快,长波长下凹型,河流坡降比大,地形起伏度大,河流形态变化简单,河流分维值低;青藏高原东北缘构造活动性不强烈,历史地震记录偏低,大震极少,河流纵剖面变化缓慢,近似长波长微振幅上凸型,河流坡降比小,地形起伏度较小,河流形态错综复杂、分维值高;青藏高原东南缘,构造活动性较强烈,历史地震记录频繁,大震较多,但由于该区域平均多年侵蚀速率比较低,同时河流下切深度大,河流纵剖面变化缓慢,也是近似长波长微振幅上凸型,河流坡降比小,河网发育较成熟,河网分维值较高。通过对比发现,降水量的变化对该区域侵蚀速率的影响远小于构造活动性的作用,在分析河网形态特征时可以不考虑降水量空间变化的影响。     

青藏高原东南缘活动断层相互作用、应力触发与差别响应

岩石圈、地壳由众多板块、地块及层圈组合而成,是开放性复杂巨系统,活断层在地壳中犹如神经网络也是复杂的开放体系。因此活断层之间存在复杂的相互作用,例如地震断层破裂产生应力扰动,可能触发其他断层破裂,不仅在近处,也会在远处发生。不同的活断层构造产状、活动方式及应变阶段不同,对同一次触发响应不同。触发与差别响应二者的叠加效应在地震活动性上有重要表现。表现之一是区域大震后,余震区外地震活动显著增强处发生继发性大震的概率最大,也即率先出现"远余震、诱发前震、响应震"的地方地震危险性增大,对预测未来地震位置有效。对本区1950—2013年地震统计表明,预测成功率W=1-漏报率-虚报率=80%。同时,对当前地震危险区作了预测。     

青藏高原东南缘活动断层相互作用、应力触发与差别响应

岩石圈、地壳由众多板块、地块及层圈组合而成,是开放性复杂巨系统,活断层在地壳中犹如神经网络也是复杂的开放体系。因此活断层之间存在复杂的相互作用,例如地震断层破裂产生应力扰动,可能触发其他断层破裂,不仅在近处,也会在远处发生。不同的活断层构造产状、活动方式及应变阶段不同,对同一次触发响应不同。触发与差别响应二者的叠加效应在地震活动性上有重要表现。表现之一是区域大震后,余震区外地震活动显著增强处发生继发性大震的概率最大,也即率先出现“远余震、诱发前震、响应震”的地方地震危险性增大,对预测未来地震位置有效。对本区1950—2013年地震统计表明,预测成功率W=1－漏报率－虚报率=80%。同时,对当前地震危险区作了预测。     

Seismicity changes associated with reservoir loading

Changes in seismic activity have been related to the filling of large reservoirs in over thirty cases. These changes range from variations in the level of micro-earthquake activity detectable only with instruments of high sensitivity to destructive earthquakes with magnitudes greater than 6. On the other hand, the filling of many other large reservoirs has not been accompanied by increased seismicity.

A number of factors may contribute to the generation or absence of post-impounding seismicity. Increased vertical stress due to the load of the reservoir and decreased effective stress due to increased pore pressure can modify the stress regime in the reservoir region. Whether or not these stress changes are sufficient to generate earthquake activity will depend on a complex interaction of the induced stress with the state of pre-existing stress near the reservoir, and on the geologic and hydrologic conditions at the site. The combined effect of increased vertical load and increased pore pressure will have the greatest tendency to increase activity in regions where the maximum compressive stress is vertical (normal faulting). In regions where the minimum compressive stress is vertical (thrust faulting) increased stress due to a vertical load should have a minimum effect. For all of the larger reservoir-induced earthquakes the stress system determined from fault plane solutions is in agreement with the pre-existing stress field in the region of the reservoir. These earthquakes are all of strike-slip or normal type, there being no reported cases of large induced earthquakes with thrusting mechanisms.

The potential for major changes in seismicity may be highest in regions of moderate strain accumulation (low to moderate natural seismicity). In areas of high strain accumulation and high levels of natural seismicity, the stress changes induced by the reservoir will be small compared to natural variations. In aseismic areas, with low strain accumulation, the reservoir-induced stresses may be insufficient to raise the stress level to a state of failure.     

中国大陆及其邻区强震活动与活动地块关系研究

从活动地块假说出发 ,在活动地块研究的基础上 ,探讨了中国大陆及邻区活动地块与强震活动的关系。研究指出 ,主要构造变形和强烈地震大都发生在活动地块边界。在占总面积 17%的活动地块边界上 ,集中了全部的 8级以上巨大地震和 86 %的 7级以上大地震 ,其释放能量占全部总能量的 95 %以上 ,表明中国大陆及其邻区活动地块边界带控制了绝大部分的强地震。从活动地块的整体来看 ,强震活动不仅显示出显著的韵律性特征 ,而且其高、低起伏基本上与中国大陆地区一致 ,只是强震活跃时段有时稍长于中国大陆。各轮回强震活动都有各自活动的主体地区 ,反映了不同活跃期内地块的不同活动方式。文中还从现今地壳运动角度 ,讨论了活动地块运动速率与强地震活动水平之间的可能联系。     

中国活动构造研究的进展与展望

在简要介绍中国活动构造基本特征的基础上,分析了21世纪以来中国大陆7级以上大地震活动出现的2个新特点：一是大地震主要沿青藏地块区的巴颜喀拉活动地块边界断裂带丛集发生;二是在活动盆地内部或盆山交界地带发生了多次逆断裂–褶皱型地震。面对新的地震活动特点以及工程建设和社会公众对防灾减灾的新需求,分析了活动构造研究中的一些关键环节或研究薄弱领域,探讨了进一步加强相关研究的发展方向和技术途径。一是要注重活动构造研究自身的理论提升与应用实践,包括活动断裂几何学的精细研究,注重断裂活动习性与破裂分段性相结合的运动学研究,切实加强古地震探槽和断错微地貌的互补性研究,更为重要的是应切实加强活动构造研究与构造地貌、地球物理勘探、地壳形变、地震学等相关学科的交叉融合与应用实践。二是强化区域活动构造研究与大震丛集迁移规律和动力学机制的认识,提升对活动块体及其边界断裂带强震构造活动的整体认识。认真梳理和分析我国各活动地块区发生丛集地震各自的孕震构造环境和发震机制,综合历史地震和古地震详细研究结果,识别可能存在地震丛集活动的活动构造带或地震构造区,为巴颜喀拉活动块体大地震主体活动区结束之后可能的迁移地区将面对的大震灾害风险做好科技支撑。三是关注逆断裂–褶皱型地震构造研究等薄弱环节。除采用传统的活动构造研究技术之外,应结合构造地貌学开展地表地貌变形特征研究,以及开展深部地震构造探测和地震学研究等,以获取其深部结构构造特征,综合分析其深浅构造关系、构建变形模式和讨论发震机制等。重点关注上述关键技术环节和未来发展趋势,为活动构造深入研究和防灾减灾提供参考依据。

     

极震区的地震动与潜在震源区内重大工程安全

潜在震源区是未来可能发生破坏性地震的震源所在地区 ,区内的地震属近场或直下型 ,其地震破坏和地震动特征相应于已发生地震的极震区。近期国内外诸多强烈地震的实际资料和相关研究成果表明 ,直下型地震不仅地震峰值加速度大 ,且竖向和水平峰值加速度比值也有别于远场地震的统计关系。文中从极震区岩土体破坏、地震动特点及地震地质灾害等方面对潜在震源区内的重大工程问题进行了探讨