The Baza Fault: a major active extensional fault in the central Betic Cordillera (south Spain)

In the Guadix-Baza Basin (Betic Cordillera) lies the Baza Fault, a structure that will be described for the first time in this paper. Eight gravity profiles and a seismic reflection profile, coupled with surface studies, indicate the existence of a NE-dipping normal fault with a variable strike with N-S and NW-SE segments. This 37-km long fault divides the basin into two sectors: Guadix to the West and Baza to the East. Since the Late Miocene, the activity of this fault has created a half-graben in its hanging wall. The seismic reflection profile shows that the fill of this 2,000–3,000 m thick asymmetric basin is syntectonic. The fault has associated seismicity, the most important of which is the 1531 Baza earthquake. Since the Late Tortonian to the present, i.e. over approximately the last 8 million years, extension rates obtained vary between 0.12 and 0.33 mm/year for the Baza Fault, being one of the major active normal faults to accommodate the current ENE–WSW extension produced in the central Betic Cordillera. The existence of this fault and other normal faults in the central Betic Cordillera enhanced the extension in the upper crust from the Late Miocene to the present in this regional compressive setting.     

活动断裂带附近地下水中的氢同位素变化与地震关系研究

本文主要研究活动断裂带附近地下水中的氢同位素变化与地震的相关性。2009年1月初,我们在汶川地震的发震断裂带——龙门山断裂带南缘的雅安进行考察,并开始采集地热深井的地下水样品,到5月初结束。在采样期间,龙门山断裂带发生4~5级余震多次。对所采地下水样品氢同位素δD值的分析结果表明,发震断裂带附近地下水中的氢同位素异常与该断裂带上4.0级以上地震有一定相关性。从氢同位素δD值随时间变化曲线可看出,(1)震前氢同位素δD值由背景值逐渐降低,震后升高,然后再逐渐回归背景值;(2)地震的级别越大,氢同位素δD值变化越显著;(3)从氢同位素δD值开始系统地降低到地震发生,这个过程大约可持续一周左右时间,属于临震预报的时间范畴。从本研究结果可得出地下水中氢同位素δD值异常能够反映断裂带的地震活动性,可作为5.0级以上地震临震预测的一种方法。     

郯庐断裂带安徽段活断层特征与成因

详细的野外地质调查表明, 郯庐断裂带安徽段活断层广泛存在, 自北向南分别沿着嘉山盆地的东、西边界、合肥盆地东界与大别造山带东界分布。走向上自北向南由北北东向转为北东向, 倾向上与早期盆缘正断层一致。这些活断层以逆右行平移活动为主, 显示了逆冲分量随着断层倾角变小而增大的现象。依据一系列活断层擦痕实测数据的反演, 指示它们活动时的应力状态为北东东—南西西向挤压, 与现代应力状态一致。从多种现象综合分析, 本文认为郯庐断裂带安徽段活断层的最新活动时代应为中更新世, 从而该段没有强震记录或小震群集现象。本次工作表明, 区内活断层主体上是早期盆缘正断层直接复活成因。由于其第四纪期间有限的累计垂直位移分量, 并没有改变白垩—古近纪盆地发育阶段的地貌格局。     

依兰-伊通断裂新构造活动规律分析

作为郯庐断裂带北段主干的依兰-伊通断裂, 其新构造活动性与活动规律仍然存在不同的认识.本次工作通过详细的野外调查, 发现该断裂内活断层广泛存在, 由东、西两支北东走向的主干活断层构成, 沿着古近纪地堑边界断层发育.这些活断层主要呈破碎型结构, 多为逆右行平移活动.通过对这些活断层一系列实测擦痕反演应力场, 显示它们多是在东西向挤压中活动的, 而现今应力场转变为北东东-南西西向区域性挤压.依据本次野外观察与¹⁴ C定年, 并结合前人定年结果与近代地震分布, 表明依兰-伊通西支活断层的最新活动时代为全新世与晚更新世相间, 而东支活断层的最新活动时代主要为早-中更新世.依兰-伊通断裂内活断层显示了明显的差异性活动, 表现为西支的活动强度明显大于东支, 西支的最新活动时代皆晚于东支, 沿走向上活动性强、弱相间与最新活动时代不断变化, 以及近代地震活动不均一分布.它们沿走向上的分段性、差异性活动主要是因为被一系列北西向断层切断所致.     

Deep structure of the Longling–Ruili fault underneath Ruili basin near the eastern Himalayan syntaxis: insights from magnetotelluric imaging

Magnetotelluric (MT) geophysical profiling has been applied to the determination of the deep structure of the Longling–Ruili fault (LRF), part of a convergent strike-slip fault system, underneath thick Caenozoic cover in Ruili basin in southwestern Yunnan, China. The recorded MT data have been inverted using a two-dimensional (2-D) nonlinear conjugate gradients scheme with a variety of smooth starting models, and the resulting models show common subsurface conductivity structures that are deemed geological significant. The models show the presence of a conductive (5–60 Ω m) cover sequence that is thickest (1–1.5 km) in the centre of the basin and rapidly pinches out towards the margins. A half-graben structure is interpreted for the Ruili basin. This is underlain by about 7–10 km thick upper crustal layer of high resistivity (>200–4000 Ω m) that is dissected by steep faults, which we interpret to flatten at depth and root into an underlying mid-crustal conductive layer at about 10 km depth. The mid-crustal layer does not appear to have been severely affected by faulting; we interpret it as a zone of partial melt or intracrustal detachment. The MT models suggest SE directed thrusting of basement rocks in the area. The Longling–Ruili fault is interpreted as a NW-dipping feature bounding one of the identified upper crustal fragments underneath Ruili city. We suggest that MT imaging is a potent tool for deep subsurface mapping in this terrain.     

Multimode migration of scattered and converted waves for the structure of the Hikurangi slab interface, New Zealand

Reflectivity imaging of local earthquake seismograms has revealed the structure of the Hikurangi subduction interface at the location of two strong earthquakes that occurred in 1990. The earthquakes originated within the continental plate of the North Island of New Zealand and below in the subducting Pacific slab. We used seismograms from 500 well-located events in two earthquake sequences recorded by a small temporary seismograph deployment to directly image the structure and multiphase reflectivity of the plate interface. Synthetic tests of the imaging method show the effects of the poor 3-d geometric coverage afforded by the seismometer array. Kirchhoff summation image sections computed from synthetics show accurate depth imaging of backscattering interfaces. Phase-converting interfaces imaged with forward-scattered waves are smeared by poor ray coverage to 5-km depth inaccuracy and are only imaged over a small range of their horizontal extent. From the data, we computed image sections for P–P, P–S, S–P and S–S scattering. We mitigated imaging artifacts due to poor ray coverage with an obliquity factor, an antialiasing criterion and enhancement by resampling statistics. Imaging used a sharply layered velocity model. We tested for the effects of imaging with first-arriving headwaves by imaging through smoothly varying velocity models. For our ray geometry, early-arrival headwaves contribute little to the images.The plate interface appears as a 3–5-km thick P–P and possibly S–S backscatterer with 5° NW dip, offset 5 km down-to-the-NW above a normal fault in the slab. When illuminated from below, a wedge of the interface on the downdip side of the slab fault forms a very prominent P–P forward scatterer. The edges of the wedge forward-scatter some S–P and S–S energy, but an order of magnitude less than the P–P forward scattering. The imbalances between forward scattering of P and S energy suggest a wedge of subducted sediment retaining significant porosity but with rigidity close to that of surrounding rocks.     

An experimental study of soil memory and preconsolidation adjacent to an active tectonic structure: the Meers fault, Oklahoma, USA

Special consolidation tests were run on undisturbed samples to study the ability of Quaternary soils adjacent to the Meers fault in southwestern Oklahoma to record and remember the maximum effective (preconsolidation) stresses they experienced during the faulting process. The results show that the soils record >60% of the applied total stresses as preconsolidation stresses in 2 s of loading time, indicating that these stresses could have been recorded during an earthquake faulting event. To record all of the applied total stresses as preconsolidation stresses (100% recording or memory), the loading needs to last at least 4–5 min.     

Cretaceous deformation history of the middle Tan-Lu fault zone in Shandong Province, eastern China

Based on field analysis of fault-slip data from different rock units of the Cretaceous basins along the middle part of the Tan-Lu fault zone (Shandong Province, eastern China), we document polyphase tectonic stress fields and address the changes in sense of motion of the Tan-Lu fault zone during the Cretaceous. The Cretaceous deformation history of the Tan-Lu fault zone can be divided into four main stages. The first stage, during the earliest Cretaceous, was dominated by N-S extension responsible for the formation of the Jiaolai basin. We interpret this extension to be related to dextral strike-slip pull-apart opening guided by the Tan-Lu fault zone. The second stage, during the middle Early Cretaceous, was overwhelmingly rift-dominated and characterized by widespread silicic to intermediate volcanism, normal faulting and basin subsidence. It was at this stage that the Tan-Lu-parallel Yi-Shu Rift was initiated by E-W to WNW-ESE extension. The tectonic regime then changed during the late Early Cretaceous to NW-SE-oriented transpression, causing inversion of the Early Cretaceous rift basin and sinistral slip along the Tan-Lu fault zone. During the Late Cretaceous, dextral activation of the Tan-Lu fault zone resulted in pull-apart opening of the Zhucheng basin, which was subsequently deformed by NE-SW compression. This deformation chronology of the Tan-Lu fault zone and the associated Cretaceous basins allow us to constrain the regional kinematic models as related to subduction along the eastern margin of Asia, or related to collision in the Tibet region.     

Seismic velocity structure of a large mafic intrusion in the crust of central Denmark from project ESTRID

The origin of regional sedimentary basins is being investigated by the ESTRID project (Explosion Seismic Transects around a Rift In Denmark). This project investigates the mechanisms of the formation of wide, regional basins and their interrelation to previous rifting processes in the Danish–Norwegian Basin in the North Sea region. In May 2004 a 143 km long refraction seismic profile was acquired along the strike direction of a suspected major mafic intrusion in the crust in central Denmark. The data confirms the presence of a body with high seismic velocity (> 6.5 km/s) extending from a depth of  10–12 km depth into the lower crust. There is a remarkable Moho relief between 27 and 34 km depth along this new along-strike profile as based on ray-tracing modelling of PmP reflections. The lack of PmP reflections at a zone of very high velocity in the lowest crust (7.3–7.5 km/s) suggests a possible location of a feeder channel to the batholith. The presence of volcanic rocks of Carboniferous–Permian age above the intrusion (mafic batholith) suggests a similar age of the intrusion. An older obliquely crossing profile and two new fan profiles deployed perpendicular to the main ESTRID profile, show that the batholith is about 30–40 km wide. The existence of this large mafic batholith supports the hypothesis that the origin of the Danish–Norwegian Basin is related to cooling and contraction after intrusion of large amounts of mafic melts into the crust during the late Carboniferous and early Permian. The data and interpretations from project ESTRID will form the basis for subsidence modelling. Tentatively, we interpret the formation of the Danish–Norwegian Basin as a thermal subsidence basin, which developed after widespread rifting of the region.     

轮古东走滑断裂破碎带结构及与油气关系

利用最新处理完成的轮古东300 km2叠前深度偏移地震资料,多手段识别出轮古东气田发育3期4组断裂。断裂控制了裂缝走向与裂缝发育密度,裂缝主要为高角度(45°~75°)构造窄裂缝,沿裂缝存在溶蚀,走向主要为NESW。纵向上,一间房组裂缝发育密度最大(14条/100 m),其次为鹰山组(6条/100 m)和良里塔格组(4条/100 m);平面上,裂缝主要分布在主干断裂周边1 km范围内,随着距断裂距离增大,裂缝发育强度(裂缝线密度)呈指数降低。在此基础上,综合考虑主干断裂及伴生裂缝发育特征,将轮古东断裂破碎带平面上划分为"羽状破碎带、转换破碎带、斜列破碎带、复合破碎带"4种结构,羽状破碎带分布面积最广,是油气最富集的区域,是目前高效井的集中分布区,围绕羽状破碎带的钻探为走滑断裂控储控藏研究和寻找新的油气富集区域提供了新思路。     

3D seismic analysis of the structure and evolution of a salt-influenced normal fault zone: A test of competing fault growth models

In this paper we determine the structure and evolution of a normal fault system by applying qualitative and quantitative fault analysis techniques to a 3D seismic reflection dataset from the Suez Rift, Egypt. Our analysis indicates that the October Fault Zone is composed of two fault systems that are locally decoupled across a salt-bearing interval of Late Miocene (Messinian) age. The sub-salt system offsets pre-rift crystalline basement, and was active during the Late Oligocene-early Middle Miocene. It is composed of four, planar, NW–SE-striking segments that are hard- linked by N–S-striking segments, and up to 2 km of displacement occurs at top basement, suggesting that this fault system nucleated at or, more likely, below this structural level. The supra-salt system was active during the Pliocene-Holocene, and is composed of four, NW–SE-striking, listric fault segments, which are soft-linked by unbreached relay zones. Segments in the supra-salt fault system nucleated within Pliocene strata and have maximum throws of up to 482 m. Locally, the segments of the supra-salt fault system breach the Messinian salt to hard-link downwards with the underlying, sub-salt fault system, thus forming the upper part of a fault zone composed of: (i) a single, amalgamated fault system below the salt and (ii) a fault system composed of multiple soft-linked segments above the salt. Analysis of throw-distance (T-x) and throw-depth (T-z) plots for the supra-salt fault system, isopach maps of the associated growth strata and backstripping of intervening relay zones indicates that these faults rapidly established their lengths during the early stages of their slip history. The fault tips were then effectively ‘pinned’ and the faults accumulated displacement via predominantly downward propagation. We interpret that the October Fault Zone had the following evolutionary trend; (i) growth of the sub-salt fault system during the Oligocene-to-early Middle Miocene; (ii) cessation of activity on the sub-salt fault system during the Middle Miocene-to-?Early Pliocene; (iii) stretching of the sub- and supra-salt intervals during Pliocene regional extension, which resulted in mild reactivation of the sub-salt fault system and nucleation of the segmented supra-salt fault system, which at this time was geometrically decoupled from the sub-salt fault system; and (iv) Pliocene-to-Holocene growth of the supra-salt fault system by downwards vertical tip line propagation, which resulted in downward breaching of the salt and dip-linkage with the sub-salt fault system. The structure of the October Fault Zone and the rapid establishment of supra-salt fault lengths are compatible with the predictions of the coherent fault model, although we note that individual segments in the supra-salt array grew in accordance with the isolated fault model. Our study thereby indicates that both coherent and isolated fault models may be applicable to the growth of kilometre-scale, basin-bounding faults. Furthermore, we highlight the role that fault reactivation and dip-linkage in mechanically layered sequences can play in controlling the three-dimensional geometry of normal faults.     

Fracture-zone conditions on a recently active fault: insights from mineralogical and geochemical analyses of the Hirabayashi NIED drill core on the Nojima fault, southwest Japan, which ruptured in the 1995 Kobe earthquake

An 1800-m-deep borehole into the Nojima fault zone was drilled at Nojima-Hirabayashi, Japan, after the 1995 Hyogo-ken Nanbu (Kobe) earthquake. Three possible fracture zones were detected at depths of about 1140, 1300, and 1800 m. To assess these fracture zones in this recently active fault, we analyzed the distributions of fault rocks, minerals, and chemical elements in these zones. The central fault plane in the shallowest fracture zone was identified by foliated blue-gray gouge at a depth of 1140 m. The degree of fracturing was evidently greater in the hanging wall than in the footwall. Minerals detected in this zone were quartz, orthoclase, plagioclase, and biotite, as in the parent rock (granodiorite), and also kaolinite, smectite, laumontite, stilbite, calcite, ankerite, and siderite, which are related to hydrothermal alteration. Biotite was absent in both the hanging wall and footwall across the central fault plane, but it was absent over a greater distance from the central fault plane in the hanging wall than in the footwall. Major element compositions across this zone suggested that hydrothermal alteration minerals such as kaolinite and smectite occurred across the central fault plane for a greater distance in the hanging wall than in the footwall. Similarly, H₂O+ and CO₂ had higher concentrations in the hanging wall than in the footwall. This asymmetrical distribution pattern is probably due to the greater degree of wall–rock fracturing and associated alteration in the hanging wall. We attributed the characteristics of this zone to fault activity and fluid–rock interactions. We analyzed the other fracture zones along this fault in the same way. In the fracture zone at about 1300 m depth, we detected the same kinds of hydrothermal alteration minerals as in the shallower zone, but they were in fewer samples. We detected relatively little H₂O+ and CO₂, and little evidence for movement of the major chemical elements, indicating little past fluid–rock interaction. In the fracture zone at about 1800 m depth, H₂O+ and CO₂ were very enriched throughout the interval, as in the fracture zone at about 1140 m depth. However, smectite was absent and chlorite was present, indicating the occurrence of chloritization, which requires a temperature of more than 200 °C. Only smectite can form under the present conditions in these fracture zones. The chloritization probably occurred in the past when the fracture zone was deeper than it is now. These observations suggest that among the three fracture zones, that at about 1140 m depth was the most activated at the time of the 1995 Hyogo-ken Nanbu (Kobe) earthquake.     

The identification of an active fault by a multidisciplinary study at the archaeological site of Sagalassos (SW Turkey)

The archaeological site of Sagalassos (SW Turkey) is located in a region characterized by the absence of any significant recent seismic activity, contrary to adjacent regions. However, the assessment of earthquake-related damage at the site suggests that the earthquakes that have been demonstrated to have struck this Pisidian city in ca. AD 500 and in the middle or second half of the 7th century AD are characterized by an MSK intensity of at least VIII and occurred on a fault very close to the city. Different investigation techniques (archaeoseismology, remote sensing and geomorphology, surface geology and structural data, 2D resistivity imaging and palaeoseismological trenching) have been applied at the archaeological site and its direct surroundings in search for the causative fault of these earthquakes. This multidisciplinary approach shows that each of the different approaches independently provides only partial, non-conclusive information with respect to the fault identification. Integration is imperative to give a conclusive answer in the search for the causative fault. This study has, indeed, revealed the existence of a to date unknown active normal fault system passing underneath ancient Sagalassos, i.e. the Sagalassos fault. A historical coseismic surface rupture event on this fault could be identified. This event possibly corresponds to the devastating Sagalassos earthquakes of ca. AD 500 and the middle or second half of the 7th century AD. Finally, this study demonstrates that in the particular geodynamic setting of SW Turkey archaeological sites with extensive earthquake-related damage form an important tool in any attempt to asses the seismic hazard.     

小子域滤波联合总水平导数在重力资料处理中的应用——以内蒙古银额盆地EQ区块断裂构造识别为例

为了进一步提高小子域滤波法识别地质体边界的有效性和实用性,以银额盆地EQ区块1∶5万高精度重力资料为基础,分别利用小子域滤波、总水平导数、改进后的小子域滤波联合总水平导数等方法对重力资料进行处理,探讨各种方法的处理效果,并采用联合方法处理结果作为断裂识别的重要依据,推断出EQ区块的断裂构造格架。与地震剖面和电法剖面的解释结果进行对比,证实了推断的断裂,证明该联合方法的有效性。理论模型试验与实际应用结果表明,改进后的小子域滤波与总水平导数计算的联合处理方法将场源体边界锐化,提高了横向分辨能力,突出了地质体边界特征,能更清晰地表现研究区的断裂构造格架。