Basin Structure and Numerical Simulation for the Mechanisms of Seismic Disasters

In the present study seismic wave propagation in heterogeneous media is numerically simulated by using the pseudospectral method with the staggered grid RFFT differentiation in order to clarify the cause for the complicated distribution characteristics of strong ground motion in regions with basin structure. The results show that the maximum amplitudes of simulated ground acceleration waveforms are closely related to the basin structure. Interference of seismic waves in the basin strongly affects the distribution of maximum seismic waveforms, which may result in peak disasters during earthquakes. Peak disasters might be away from basin boundaries or earthquake faults. Seismic energy transmitted into the basin from the bedrock can hardly penetrate the bottom of the basin and then travel back into the bedrock region. The seismic energy is absorbed by basin media, and transferred into the kinematical energy of seismic waves with great amplitude in the basin. Seismic waves between basins may result in serious damage to buildings over the basin. This is significant for aseismatic research. Geological surveys in and around urban areas would benefit aseismatic research and mitigation of seismic disasters of a city. Such geological surveys should involve seismic velocity structure in the media above the bedrock besides such subjects as active faults and geological structure.

Development characteristics of the fault system and its control on basin structure, Bodong Sag, East China

The Bodong Sag,located in the Bohai Sea,offshore China,is one of the most petroliferous basins in China.Based on three dimensional seismic reflection data and time slice data,we analyze the fault system of the Bodong area in detail,establish the fault structure pattern of different types and summarize the distribution of the fault system.It is concluded that the development characteristics of the Cenozoic fault system are in accordance with the dextral stress field of the Tanlu Fault,which displayed a brush structure with NNE strike-slip faults as its principal faults,NE-trending extensional faults as secondary faults and EW-trending faults as minor faults.Faults can be divided into （1） strike-slip type,（2） extensional type,（3） strike-slip extensional type and （4） extensional strike-slip type.The spatial structures of different faults have obvious differences because of the fault properties and activity intensity.The fault system at different stages shows tremendous differences because of the transition of the Tanlu Fault from sinistral strike-slip to dextral strike-slip,the transition between extension and strike-slip,and the transition from mantle upwelling to thermal subsidence.According to the controlling effect of faults on basin structure,the Cenozoic basin experienced four evolutionary stages,（a） transition stage from sinistral strike-slip to dextral strike-slip,（b） strike-slip extensional faulted stage,（c） extensional strike-slip faulted stage and （d） strike-slip depression stage.The identification of temporal and spatial differences of faults could be used as a significant guideline for oil and gas exploration in the Bodong area.

Structures of the Bohai Petroliferous Area,Bohai Bay Basin

This paper, for the first time, deals with a more systematic study of the structures in the Bohai petroliferous area that covers nearly one third of the Bohai Bay basin. The study mainly involves the effects of preexisting basement faults on the basin formation, the characteristics of basin geometry and kinetics, the modelling of the tectonic-thermal history, the polycyclicity and heterogeneity in the structural evolution and the natural seismic tomographic images of the crust and upper mantle. The authors analyze the features of the dynamic evolution of the basin in the paper and point out that the basin in the Bohai petroliferous area is an extensional pull-apart basin.

The Three-layer Structure and Systems of Petroleum Migration and Accumulation of Continental Basins

The concept of the three-layer structure of continental basins is presented based on the characteristics of layered structure of basins. The reservoir could be classified into accumulation system assemblage, accumulation system, accumulation assemblage and reservoir. This paper discusses the characteristics of hydrocarbon accumulation system assemblages of the Zhanhua Depression, which include four kinds of genetic patterns： （1） buried-hill hydrocarbon accumulation system assemblage; （2） self-sourced accumulation system assemblage from the upper interval of member 4 to member 1 of Shahejie formation; （3） transition accumulation system assemblage from member 1 of the Shahejie formation to Dongying formation and （4） externally sourccd accumulation system assemblage in the late Tertiary. The hydrocarbon-source transport network layer consisted of faults and unconformities, which connected with the reservoir layer.

Geological structure and dynamic mechanism of the Termit rift basin in West African rift system

Based on seismic and drilling data in the study area,the geological structure and kinematic process of the Termit rift basin were studied using seismic profile interpretation and balanced restoration to find out the dynamic mechanism of the basin.(1)The geological structure of the Termit Basin is represented as a narrow rift basin,with development of series of structural styles in extensional,extensional strike-slip and compressional stress setting.On plane,it is narrow in the north and wide in the south,and transitions from graben to half-graben from north to south;it features a graben controlled by the boundary faults in the north and a fault-overlapped half-graben in the south.(2)Before the Cretaceous,a series of hidden faults developed in the West African rift system,which laid the foundation for the development location and distribution direction of the Termit Basin;during the Cretaceous to Paleogene periods,the basin experienced two phases of rifting in Early Cretaceous and Paleogene,which controlled the initial structure and current structural shape of the basin respectively;during the Neogene to Quaternary,the basin was subjected to weak transformation.(3)In the Precambrian,the Pan-African movement gave rise to a narrow and long weak zone within the African plate,which provided the pre-existing structural conditions for the formation of the Termit Basin.In the Early Cretaceous,affected by the South Atlantic rifting,the Pan African weak zone was reactivated,resulting in the first stage of rifting and the basic structural framework of the Termit Basin.In the Paleogene,affected by the subduction and convergence of the Neo-Tethys Ocean,the African-Arabian plate extended in near E-W trending,and the Termit Basin experienced the second stage of rifting.The oblique extension in this period caused intense structural differentiation,and the current structural pattern of alternate uplifts and depressions took shape gradually.

New Recognition about Faulted Structures in Tarim Basin

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Structural Variability and Rifting Process of the Segmented Cenozoic Pearl River Mouth Basin,Northern Continental Margin of the South China Sea

The Pearl River Mouth Basin(PRMB)is an important area for studying the evolution of continental marginal basins in the northern South China Sea(SCS),but the structural variability and spatiotemporal rifting process remains poorly understood.This study investigates the differential structural features of the eastern,middle and western PRMB,as well as the extensional deformation laws in operation during the rifting stage,according to an integrated analysis of geometric characteristics and kinematic parameters,i.e.,horizontal displacement and stretching factors of basin and crust.The PRMB underwent at least three phases of intense extension,which varied in time and space.(1)During the middle Eocene,most sags in the PRMB were intensely stretched and high-angle planar to listric boundary faults controlled the wedge-shaped stratigraphic geometry.(2)During the late Eocene-to-early Oligocene,the stratigraphic geometry of the sags was slightly wedge-shaped and continuously controlled by boundary faults,however,the extensional strength decreased relatively in the Northern depression zone,but increased in the Southern depression zone.(3)During the late Oligocene,the extension was extremely weak in the northeast PRMB,but relatively strong in the southwest PRMB,leading to tabular stratigraphic geometry in the northeast PRMB,but localized slightly wedge-shaped stratigraphic geometry in the southwest.The southwest PRMB still underwent relatively strong extension during the early Miocene.The southwest PRMB that was induced by a small-scale localized mantle convection system constantly rifted during the late Oligocene,controlled by the weak lithosphere,westward(southwestward)diachronous opening and southward jump of the ocean ridge.The applied quantitative parameters and spatiotemporal rifting process may be used as a reference with which to study the segmented continental margin rifts.

Superimposed versus residual basin:The North Yellow Sea Basin

The North Yellow Sea Basin is a Mesozoic and Cenozoic basin. Based on basin-margin facies, sedimentary thinning, size and shape of the basin and vitrinite reflectance, North Yellow Sea Basin is not a residual basin. Analysis of the development of the basin＇s three structural layers, self-contained petroleum systems, boundary fault activity, migration of the Mesozoic--Cenozoic sedimentation centers, different basin structures formed during different periods, and superposition of a two-stage extended basin and one-stage depression basin, the North Yellow Sea Basin is recognized as a superimposed basin.

Numerical simulation of the detachment dynamics in North China Basin

Refering to geological evidences and recent Lincheng-Julu deep seismic reflection profile in the west-central part of North China Basin, it is concluded preliminarily that a low angle detachment structure may exist in the central part of North China depression. Numerical method is used to simulate the influence of hot mantle intrusive bodies to Cenozoic basin tectonic movements. Numerical simulations show that,① The intrusion of hot mantle material has led to an extensional stress state in the upper crust in central North China depression. As time increasing, the extensional stress state changed slightly in the upper crust and was in keeping with the normal faulting tectonics in the upper crust in depression area. ② In Cenozoic era, under the effects of magmatic intrusion and the resistance of Taihang Mountain, the weak zone produced by the Mesozoic thrust faulting would become a detachment structure.③ With the elapse of time, the horizontal compressive stress gradually concentrated in the median crust, and the concentration of stress may generate strike-slip earthquakes in the median crust above the intrusive body.

Structural Features and Proto-Type Basin Reconstructions of the Bay of Bengal Basin:A Remnant Ocean Basin Model

Remnant ocean basin is a key to understand the plate suturing and subsequent uplift and erosion of orogen. The Bay of Bengal Basin （BOBB） provides a typical example to analyze the remnant ocean basin structures, evolution, and relationships between depositional filling and uplifting of the Himalayan Orogen. Thirty-nine seismic profiles as well as interval velocities of well BODC3 were used to compile isopach maps of the basin. Among the seismic data, 26 seismic profiles were applied to estab- lish 8 cross sections. The cross sections suggest the basin is asymmetric, bounded to the west by the eastern continental margin of India （ECMI） with graben-horst and to the east by the Sunda conver- gence margin dominated by trench-arc system. The BOBB is characterized by a prominent down flex- ure structures caused by huge amount of Bengal fan turbidite sediments accumulation. Our isopach maps and chronology data collected from adjacent regions reveal the initial development and fast southward growth of the Bengal fan were related to the early and major stage uplift and erosion of the Himalayan Orogen, respectively. The BOBB has experienced a critical transition from an ocean basin to a remnant ocean basin at Late Oligocene. Such basin structures and evolution features indicate the BOBB provides whole records of oblique convergence of the India and Asia plates, and the early and major stage evolution of the Himalayan Orogen.

Slumping in the Upper Jurassic Baisakhi Formation of the Jaisalmer Basin, western India: Sign of synsedimentary tectonics?

A spectacularly exposed slump is described from a 120-m-long road cut between the villages of Kanod and Deva in the northeastern Jaisalmer Basin of Rajasthan,India.The Upper Jurassic part of the sediments at the outcrop was formed in a near-shore setting and belongs to the Ludharwa Member of the Baisakhi Formation.The 3-m-thick unit shows a number of asymmetric folds and thrust faults leading to an imbrication of partly lithified sandstone beds.The deformation structures allow the reconstruction of a movement towards the northwest.This agrees well with the basin configuration that shows a deepening into this direction.Although the determination of a specific trigger mechanism is difficult for soft-sediment deformation structures,an earthquake caused by synsedimentary tectonics in the basin seems to be the most likely explanation.

Geological Factors for the Formation of Xi＇an Ground Fractures

Xi＇an ground fractures are the most typical ground fractures in China. Fourteen fractures have nearly divided the historical city into several distinct sections. These fractures are parallel and dis- tributed in NEE direction at the same interval, with all features exhibiting a down dropping southerly block which extends to connect with the underlying fault. The activities of fractures are primarily ex- pressed as normal faults. The faulted strata are well defined and dislocation displacement increases with depth. Thus, fractures have the characteristics of syn-sedimentary faults, which constitute the hanging wall of the Lintong-Chang＇an fault branch system. Crustal thinning caused by the uplifting of upper man- tle provides a power source for extension and stretching along the fracture surface of the upper crust, which results in a series of extensional faults and the suitable conditions for forming massive ground frac- tures. The movement of tectonic blocks influences the normal dip-slipping tension of Lintong-Chang＇an fault branches, and produces a series of secondary tectonic fractures adjacent to surface, which constitute the prototype of ground fractures. The recent regional tensile stress produced by modern mainland de- formation, also profoundly influences the current activity of Xi＇an ground fractures.