Inversion of breakout data from inclined boreholes for stress state of the upper crust in Jizhong depression

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Stress constraints and hydrofracturing stress data for the continental crust

Faulting and seismicity in the upper continental crust require considerable differential stresses. Application of experimentally developed friction, fracture and flow laws shows that high differential stresses can only exist in the uppermost crust. Direct hydraulic fracturing measurements in deep boreholes seem to support this rock mechanics conclusion. The experimental data base presently consists of approximately 500 hydrofrac tests conducted in about 100 boreholes at about 30 different geographical locations. To illustrate the variation of measured stresses with depth, the data are expressed as dimensionless horizontal stresses in the formS _H,h/S_v=(/z)+, whereS _v=gz Extrapolation of the experimental data to greater depth shows that the minor horizontal stress approaches the valueS _h/S_v=0.5 which limits friction on wet faults, and that the major horizontal stress approaches a value close toS _H/S_v=1 at rather shallow depth (5 to 10 km.). This limits faulting and seismicity in most of the upper crust to either strike-slip or normal faults. The lower boundary for seismicity is mainly dependent on tectonic strain accumulation and rock creep at the environmental conditions at depth.     

Study on in-situ stress measurement around coastal marginal land in Fujian

Introduction The southeast coastal margin of Chinese mainland is important to understand the mutual movement, matter exchange and energy transfer between ocean and land lithospheres, as well as their effects. The coastal area in Fujian Province is advanced in economy development but it has frequent earthquakes, or even strong earthquakes. Therefore, the current study on in-situ stress state measurement is of great significance for studying continental dynamics, marginal sea dy- namics, seismo…     

Crustal stress field in Yunnan: implication for crust-mantle coupling

We applied the g CAP algorithm to determine239 focal mechanism solutions 3:0 M We 6:0T with records of dense Chin Array stations deployed in Yunnan,and then inverted 686 focal mechanisms(including 447 previous results) for the regional crustal stress field with a damped linear inversion. The results indicate dominantly strike-slip environment in Yunnan as both the maximum(r1) and minimum(r3) principal stress axes are sub-horizontal. We further calculated the horizontal stress orientations(i.e., maximum and minimum horizontal compressive stress axes: S H and S h, respectively) accordingly and found an abrupt change near *26°N. To the north, S H aligns NW-SE to nearly E-W while S h aligns nearly N-S. In contrast, to the south, both S H and S h rotate laterally and show dominantly fan-shaped patterns. The minimum horizontal stress(i.e., maximum strain axis) S h rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in west Yunnan, and further toNE-SW in the South China block in the east. The crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions. Therefore, the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.     

Borehole Breakouts in Berea Sandstone Reveal a New Fracture Mechanism

— Vertical drilling experiments in high-porosity (22% and 25%) Berea sandstone subjected to critical true triaxial far-field stresses, in which σ _H (maximum horizontal stress) >σ _v (vertical stress) >σ _h (least horizontal stress), revealed a new and non-dilatant failure mechanism that results in thin and very long tabular borehole breakouts that have the appearance of fractures, and which counterintuitively develop orthogonally to σ _H . These breakouts are fundamentally different from those induced in crystalline rocks, as well as limestones and medium-porosity Berea sandstone. Breakouts in these rocks are typically dog-eared in shape, a result of dilatant multi-cracking tangential to the hole and subparallel to the maximum far-field horizontal stress σ _H , followed by progressive buckling and shearing of detached rock flakes created by the cracks. In the high-porosity sandstone a narrow layer of grains compacted normal to σ _H is observed just ahead of the breakout tip. This layer is nearly identical to “compaction bands” observed in the field. It is suggested that when a critical tangential stress concentration is reached along the σ _h spring line at the borehole wall, grain bonding breaks down and a compaction band is formed normal to σ _H . Debonded loose grains are expelled into the borehole, assisted by the circulating drilling fluid. As the breakout tip advances, the stress concentration ahead of it persists or may even increase, extending the compaction band, which in turn leads to breakout lengthening.     

Study of crustal structure and geological implications of southwestern margin of Northeast India

It is noticed that few geophysical studies have been carried out to decipher the crustal structure of southwestern part of the Northeast India comprising of Tripura fold belt and Bengal basin as compared to the Shillong plateau and the Brahmaputra basin. This region has a long history of seismicity that is still continuing. We have determined first-order crustal features in terms of Moho depths (H) and average V_P/V_S ratios (κ) using H-κ stacking technique. The inversion of receiver functions data yields near surface thick sedimentary layer in the Bengal basin, which is nearly absent in the Shillong plateau and Tripura fold belt. Our result suggests that the crust is thicker (38–45 km) in the Tripura fold belt region with higher shear-wave velocity in the lower crust than the Shillong plateau. The distribution of V_P/V_S ratio indicates heterogeneity throughout the whole region. While low to medium value of Poisson’s ratio (1.69–1.75) indicates the presence of felsic crust in the Shillong plateau of the extended Indian Archean crust. The medium to high values of V_P/V_S ratio (> 1.780) in the Bengal basin and the Tripura fold belt region represent mafic crust during the formation of the Bengal delta and the Tripura fold belt creation in the Precambrian to the Permian age. The depth of the sediments in the Bengal basin is up to 8 km on its eastern margin, which get shallower toward its northeastern and southeastern margins.     

Northeast-southwest compressive stress in Alberta evidence from oil wells

The introduction of the four-arm dipmeter well-logging tool has permitted hole asymmetry to be recognised and hole ellipticity to be measured and oriented. Many wells in Alberta, western Canada, have been shown to exhibit depth ranges over which they are non-circular as a result of caving of their walls. These break-outs occur so as to elongate the holes in a northwest-southeast direction. This direction of elongation is consistent between the break-outs in a given well, and between wells distributed over an area of more than 3 × 10⁵ km². Asymmetric hole elongation occurs in siltstones, sandstones, limestones, dolomites and one shale formation, and through the stratigraphic column from Devonian to Cretaceous. It is unrelated to dip of the strata. The hypothesis is advanced that the break-outs are caused by concentration of stress at the walls of the wells, in a stress field with large, unequal horizontal principal stresses, the larger oriented NE-SW. It is shown that a normal stress field (with the largest principal stress, σ₁, vertical) is unlikely to produce large enough horizontal stresses to produce the break-outs. Consequently our hypothesis requires σ₁ to be horizontal and oriented NE-SW. The vertical principal stress could be either σ₂ (strike-slip stress field) or σ₃ (thrust stress field). The orientation of σ₁ at right angles to the Rocky Mountains fold axes suggests the possibility that the stress field responsible for the thrust faulting in the mountains is still present. A strike-slip stress field is also possible. The most prominent system of surface joints outcropping in Alberta, shown by Babcock to have sets oriented NE-SW and NW-SE, could have been formed in the proposed stress field. Direct measurements of the stress tensor at two or three points, if they verified this interpretation, would combine with the oil well break-outs to demonstrate a remarkably uniform stress field over a substantial part of the North American plate. The stresses in the Alberta crust here proposed may be related to tractions now acting on the edges and underside of the North American plate, including those producing postglacial uplift of Hudson's Bay; or may be residual stresses from past tectonic events such as the Laramide compression of the Rocky Mountains.     

Normal and shear stresses acting on arbitrarily oriented faults, earthquake energy, crustal GPE change, and the coefficient of friction

As usual, earthquake energy is defined as the total energy released from an earthquake, which is partitioned into radiated energy, friction energy, and rupture energy regardless of crustal gravitational potential energy (GPE) change. We analyze the energy and stress parameters in earthquake energy budget. For arbitrarily oriented faults, we deduce the formulas for calculating the normal and shear stresses acting on the fault under principal stresses. We show that shear stress is composed of horizontal and vertical shear stresses. Then, we provide the expressions for computing crustal GPE change and the coefficient of friction. The GPE change should be considered, except strike-slip faulting, when investigating earthquakes. Also, for various faulting types, we show that the ratio of differential stresses is related to the fault orientation and the relative magnitudes of stresses. Finally, “12 May, 2008, Wenchuan, Sichuan, China, M_W 7.9 Earthquake” is cited to analyze and calculate various energy/stress parameters and the coefficient of friction. Our result of GPE change coincides with the post-event field observations.     

Crustal thickness,VP/VS ratio,and shear wave velocity structures beneath Myanmar and their tectonic implications

The crustal structure in Myanmar can provide valuable information for the eastern margin of the ongoing Indo-Eurasian collision system. We successively performed H–k stacking of the receiver function and joint inversion of the receiver function and surface wave dispersion to invert the crustal thickness (H), shear wave velocity (V_S), and the V_P/V_S ratio (k) beneath nine permanent seismic stations in Myanmar. H was found to increase from 26 ?km in the south and east of the study area to 51 ?km in the north and west, and the V_P/V_S ratio was complex and high. Striking differences in the crust were observed for different tectonic areas. In the Indo-Burma Range, the thick crust (H ?～ ?51 ?km) and lower velocities may be related to the accretionary wedge from the Indian Plate. In the Central Myanmar Basin, the thin crust (H ?= ?26.9–35.5 ?km) and complex V_P/V_S ratio and V_S suggest extensional tectonics. In the Eastern Shan Plateau, the relatively thick crust and normal V_P/V_S ratio are consistent with its location along the western edge of the rigid Sunda Block.     

Earthquakes Along the Passive Margin of Greenland: Evidence for Postglacial Rebound Control

—?An intriguing observation in Greenland is a clear spatial correlation between seismicity and deglaciated areas along passive continental margins, a piece of evidence for earthquake triggering due to postglacial rebound. Another piece of evidence for induced seismicity due to deglaciation derives from earthquake source mechanisms. Sparse, low magnitude seismicity has made it difficult to determine focal mechanisms from Greenland earthquakes. On the basis of two normal faulting events along deglaciated margins and from the spatial distribution of epicenters, earlier investigators suggested that the earthquakes of Greenland are due to postglacial rebound. This interpretation is tested here by using more recent data. Broadband waveforms of teleseismic P waves from the August 10, 1993 (m _b = 5.4) and October 14, 1998 (m _b = 5.1) earthquakes have been inverted for moment tensors and source parameters. Both mechanisms indicate normal faulting with small strike-slip components: the 1993 event, strike = 348.9°, dip = 41.0°, rake =?56.3°, focal depth = 11?km, seismic moment = 1.03?×?10²⁴ dyne-cm, and M _w = 5.3; the 1998 event, strike = 61.6°, dip = 58.0°, rake =?95.5°, focal depth = 5?km, seismic moment = 5.72?×?10²³ dyne-cm, and M _w = 5.1. These and the two prior events support the theory that the shallow part of the lithosphere beneath the deglaciated margins is under horizontal extension. The observed stress field can be explained as flexural stresses due to removal of ice loads and surface loads by glacial erosion. These local extensional stresses are further enhanced by the spreading stress of continental crust and reactivate preexisting faults. Earthquake characteristics observed from Greenland suggest that the dominant seismogenic stresses are from postglacial rebound and spreading of the continental lithosphere.     

True Triaxial Stresses and the Brittle Fracture of Rock

This paper reviews the efforts made in the last 100 years to characterize the effect of the intermediate principal stress σ ₂ on brittle fracture of rocks, and on their strength criteria. The most common theories of failure in geomechanics, such as those of Coulomb, and Mohr, disregard σ ₂ and are typically based on triaxial testing of cylindrical rock samples subjected to equal minimum and intermediate principal stresses (σ ₃=σ ₂). However, as early as 1915 Böker conducted conventional triaxial extension tests (σ ₁=σ ₂) on the same Carrara marble tested earlier in conventional triaxial compression by von Kármán that showed a different strength behavior. Efforts to incorporate the effect of σ ₂ on rock strength continued in the second half of the last century through the work of Nadai, Drucker and Prager, Murrell, Handin, Wiebols and Cook, and others. In 1971 Mogi designed a high-capacity true triaxial testing machine, and was the first to obtain complete true triaxial strength criteria for several rocks based on experimental data. Following his pioneering work, several other laboratories developed equipment and conducted true triaxial tests revealing the extent of σ ₂ effect on rock strength (e.g., Takahashi and Koide, Michelis, Smart, Wawersik). Testing equipment emulating Mogi's but considerably more compact was developed at the University of Wisconsin and used for true triaxial testing of some very strong crystalline rocks. Test results revealed three distinct compressive failure mechanisms, depending on loading mode and rock type: shear faulting resulting from extensile microcrack localization, multiple splitting along the σ ₁ axis, and nondilatant shear failure. The true triaxial strength criterion for the KTB amphibolite derived from such tests was used in conjunction with logged breakout dimensions to estimate the maximum horizontal in situ stress in the KTB ultra deep scientific hole.     

新加坡地应力测量

2008年和2009年, 通过和新加坡宜康地质技术有限责任公司合作, 在新加坡18个浅钻孔中进行了水压致裂地应力测量. 通过测量获得了新加坡地壳浅部的现今应力状态, 为新加坡拟建地下管缆隧道设计施工提供了地应力依据, 也为世界应力环境数据库提供了参考. 测试结果表明, 在38.00~56.95 m深度域内最大水平主应力σ_h值为3.45~9.64 MPa, 最小水平主应力σ_h为2.49~6.28 MPa. 水平主应力和用上覆岩层重量计算的垂直应力σv之间的关系为σ_h>σ_h>σv,最大、最小水平主应力平均值分别是垂直应力平均值的5.13倍和3.23倍, 表明新加坡应力场以水平应力为主导. 初步分析认为, 新加坡陆域浅部构造应力场的量值大致呈南、北两端偏低, 中间高的态势, 在北端最大与最小水平主应力值都小于它们各自的平均值, 而南端则与平均值相当. 最大水平主应力优势方向为NE—NEE向, 该方向与前人在其邻区获得的构造应力场方向和由震源机制解反映的主压应力方向较为一致. 新加坡现今构造应力场状态与印度板块、澳大利亚板块、菲律宾板块与欧亚板块的汇聚作用有着密切的联系. 根据库仑摩擦滑动准则, 断层面摩擦系数取0.6~1.0, 分析了研究区内断层产生摩擦滑动的可能性.     

On gradings for one — dimensional seismologic predictions

This paper gives a complete commentary on some popular gradings which based on the 2×2 contingency table, for one—dimensional seismologic prediction. It contains:

1. In view of the statistical correlation, the Wallen gradingV=√QS is the most equitable one amomg all gradings.
2. Each one of the three gradings: Zhenchao Gu gradingS, Obuhov gradingQ and Shaoxie Xu gradingR, has its own inadequacy although they are so easy for use.
3. A set of gradings can be derived fromV. The harmonic mean ofS andQ, H(S,Q) and the Hedike gradingS _H are the members of this set. The order of some popular gradings are given in Theorem 5. 3.
4. In view of the measure of the overlapping part a gradingW is introduced. Another set of gradings may be derived from it. The index of efficiencyh is the severest one among this set.

     

Stress Patterns in Northern Iraq and Surrounding Regions from Formal Stress Inversion of Earthquake Focal Mechanism Solutions

The collision zone between the Arabian and Eurasian plates is one of the most seismically active regions. Northern Iraq represents the northeastern part of the Arabian plate that has a suture zone with the Turkish and Iranian plates called the Bitlis–Zagros suture zone. The orientations of the principal stress axes can be estimated by the formal stress inversion of focal mechanism solutions. The waveform moment tensor inversion method was used to derive a focal mechanism solution of 65 earthquakes with magnitudes range from 3.5 to 5.66 in the study area. From focal mechanism solutions, the direction of slip and the orientations of the moment stress axes (P, N, and T) on the causative fault surface during an earthquake were determined. The dataset of the moment stress axes have been used to infer the regional principal stress axes (σ ₁, σ ₂, and σ ₃) by the formal stress inversion method. Two inversion methods, which are the new right dihedron and the rotational optimization methods, were used. The results show that six stress regime categories exist in the study area. However, the most common tectonic regimes are the strike-slip faulting (43.94 %), unspecified oblique faulting (27.27 %), and thrust faulting (13.64 %) regimes. In most cases, the strike-slip movement on the fault surfaces consists of left-lateral (sinistral) movement. The normal faulting is located in one small area and is due to a local tensional stress regime that develops in areas of strike-slip displacements as pull-apart basins. The directions of the horizontal stress axes show that the compressional stress regime at the Bitlis–Zagros suture zone has two directions. One is perpendicular to the suture zone near the Iraq–Iran border and the second is parallel in places as well as perpendicular in others to the suture zone near the Iraq–Turkey border. In addition, the principal stress axes in the Sinjar area near the Iraq–Syria border have a E–W direction. These results are compatible with the tectonic setting of the Arabian–Eurasian continental collision zone and the anticlockwise rotation of the Arabian plate that is evidently responsible for the strike-slip displacements on fault surfaces.     

Numerical simulation of the detachment dynamics in North China Basin

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Crustal Heterogeneity and Seismotectonics of the National Capital Region, Delhi, India

To better understand the relationship between crustal heterogeneity and seismotectonics in the National Capital Region (NCR), Delhi, India, we carried out local P- and S-velocity tomography beneath the NCR. First arrival times of the first P- and S-wave from 275 crustal earthquakes recorded by the Seismic Telemetry Network in and around Delhi of the India Meteorological Department, India, are inverted to obtain crustal P-, S-velocity and V _P /V _S variations in the region. Our tomographic images of the upper crust reflect well the surface geological and tectonic features. The Delhi fold belt is identified as low V _P, high V _S and low V _P/V _S. The Sohna hot spring region is appearing as low V _P, low V _S and high V _P /V _S correlating with the possible presence of fluid-filled rocks. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the low V _P /V _S region.     

Overcoring measurements in the příbram mine area and their relation to the regional stress field

Summary A set of overcoring measurements in deep mines near Píbram is re-interpreted in order to obtain information on the regional component of in situ stress. This is achieved by averaging 17 complete stress tensors measured at the deepest level of the mine, at a depth of 1240 m. The computations indicate that (1) the mean principal stresses act roughly in the horizontal and vertical directions, and (2) the maximum principal stress is horizontal, acting NW - SE, which was also confirmed by the data from all 42 sites of the mine area. The NW - SE orientation of S_H is consistent with the stress pattern in Western Europe and also with borehole breakouts observed within the Bohemian Massif, but the quality of the stress indicator obtained is poor — the standard deviation of the S_H direction is 26°. High variance of the directional data is discussed with respect to stress redistribution due to the mine workings and the contact separating Proterozoic sediments from the Variscan pluton.     

Three‐dimensional VS profiling using microtremors in Kushiro,Japan

A practical method is presented for determining three‐dimensional S‐wave velocity (V_S) profile from microtremor measurements. Frequency–wave number (f–k) spectral analyses of microtremor array records are combined, for this purpose, with microtremor horizontal‐to‐vertical (H/V) spectral ratio techniques. To demonstrate the effectiveness of the proposed method, microtremor measurements using arrays of sensors were conducted at six sites in the city of Kushiro, Japan. The spectral analyses of the array records yield dispersion characteristics of Rayleigh waves and H/V spectra of surface waves, and joint inversion of these data results in V_S profiles down to bedrock at the sites. Conventional microtremor measurements were performed at 230 stations within Kushiro city, resulting in the H/V spectra within the city. Three‐dimensional V_S structure is then estimated from inversion of the H/V spectra with the V_S values determined from the microtremor array data. This reveals three‐dimensional V_S profile of Kushiro city, together with an unknown hidden valley that crosses the central part of the city. The estimated V_S profile is consistent with available velocity logs and results of subsequent borings, indicating the effectiveness of the proposed method. Copyright © 2008 John Wiley & Sons, Ltd.     

Stress indications in the Bohemian Massif: Reinterpretation of borehole televiewer data

Summary This paper presents the first results of stress determinations in the Bohemian Massif utilizing televiewer data from deeper boreholes. The televiewer survey was originally carried out by Geological Survey Ostrava for the purpose of studying fractures and bedding in coal basins to depths of 2 km. A reinterpretation of the amplitude logs reveal a large number of stress-induced borehole elongations (breakouts) which occur in a direction perpendicular to the maximum horizontal principal stress (S_H). The breakout analysis in the Upper Silesian, Meno and Kladno Basins indicates a dominant NW-SE direction of S_H which is consistent with the pattern of stress orientation in Western Europe. The NE-SW trend observed in the Plze Basin is less reliable. A model for stress magnitude determinations is formulated as a linear programming problem. If the a priori information on rock strength properties is limited, it reduces to explicit formulas for stress regime estimates. The model is applied to data from the Upper Silesian Basin for which a compressional stress regime is derived.     

Estimation of sedimentary layer shear wave velocity using micro-tremor H/V ratio measurements for Bangalore city

For assessing earthquake hazard of metro cities, knowledge of soil amplification, thickness and properties of sedimentary layer are essential. In order to map the soil thickness using microtremor survey method, in Bangalore city, it is required to calibrate the relation between fundamental resonance frequency of the soil layer and its thickness for the region. For this purpose microtremor survey was carried out at 34 locations in the city where borehole log was available. The resonance frequency of the soil is evaluated from the microtremor recordings using the H/V ratio technique. A nonlinear regression relation between the thickness of sedimentary layer h (m), from the borehole logs, and the resonance frequency f_r (Hz), was derived as h=(58.3±8.8)f_r^{−(0.95)±0.1}. Using the model of shear wave velocity increasing with depth at these locations, the derived average shear wave velocity and the corresponding soil thickness were used, to get an empirical relation between V_S (m/s) and depth z(m), as V_s=(174±28)(1+z)^0.16±0.07. This relation also compares reasonably with the fit obtained between simulated V_S and depth from borehole logs for Bangalore city. The calibrated relations can be used at locations in Bangalore city where borehole logs are not available, for finding the thicknesses and shear wave velocities of the local soil layers at the survey locations.