Do seismic slip deficits indicate an underestimated earthquake potential along the Vienna Basin Transfer Fault System?

Seismic slip rates of about 0.2 mm yr^?1 calculated from cumulative seismic moments of earthquakes along the Vienna Basin Transfer Fault (VBTF) between the Alps and the Carpathians are very low compared to geologically and geodetically determined slip rates of 1–2 mm yr^?1, proving a significant seismic slip deficit. Additional seismic slip calculations for arbitrarily selected fault sectors reveal large differences along strike ranging from c. 0.02 to 0.5 mm slip yr^?1. As the earthquake frequency distribution suggests seismically coupled deformation, these variations might indicate locked fault segments. Results suggest that (1) the seismic cycle of the VBTF exceeds the length of available seismological observation, and (2) larger earthquakes than those recorded may occur along the fault. Thus, current local seismic hazard estimates, which are solely based on this historical database, probably underestimate the earthquake potential of the fault system.     

Backstripping dip-slip fault histories: apparent slip rates for the Miocene of the Vienna Basin

Backstripped basement subsidence histories from both the hanging wall and the footwall blocks adjacent to synsedimentary normal faults can be used to reconstruct the sense of fault motion through time and to quantify the vertical component of fault slip. Consequently, apparent dip-slip rates of faults can be calculated for each stratigraphic interval and times of increased fault activity can be distinguished. An application of this method to well data along a transect through the central part of the Miocene Vienna Basin indicates that two distinct phases of faulting occurred during the Karpatian, with rates as high as 3000 m Myr⁻¹. Changes in the sense of movements during the early Karpatian and the earliest Badenian indicate a major rearrangement in the fault patterns. During the early Sarmatian another short pulse of dip-slip is recorded along the investigated faults.     

Late Quaternary slip rates and slip partitioning on the Alpine Fault,New Zealand

Published geological data on Late Quaternary offsets on the Alpine Fault, New Zealand, have been assembled into a common format and analysed with respect to uncertainties. Uncertainties arise mainly from measurement of offset features, relating apparent offsets to actual fault slip, and dating the offset features. Despite the considerable uncertainties, the data form a coherent set consistent with a relatively constant rate of strike-slip of 27±5 mm/year between Milford Sound and Hokitika. This rate represents 70–75% of the fault-parallel interplate motion. North of the confluence with the Hope Fault, the rate drops substantially. Dip-slip rates, on the other hand, show considerable variation along strike, rising to a maximum of more than 10 mm/year in the central section and decreasing to zero at the southern end. Partitioning of c. 25% of the interplate slip on to structures east of the Alpine Fault occurs in the central section, consistent with predictions from critical wedge models. The partitioning of all the fault-normal component of displacement on to other structures in the south may be related, in part, to a doubling in width of the deforming wedge to the east. Most probably, however, the fault-normal displacement is mainly accommodated by underthrusting of the Australian plate offshore, due to a change in the nature of the crust from continental to oceanic.     

Seismic hazard map of Coimbatore using subsurface fault rupture

This study presents the future seismic hazard map of Coimbatore city, India, by considering rupture phenomenon. Seismotectonic map for Coimbatore has been generated using past earthquakes and seismic sources within 300 km radius around the city. The region experienced a largest earthquake of moment magnitude 6.3 in 1900. Available earthquakes are divided into two categories: one includes events having moment magnitude of 5.0 and above, i.e., damaging earthquakes in the region and the other includes the remaining, i.e., minor earthquakes. Subsurface rupture character of the region has been established by considering the damaging earthquakes and total length of seismic source. Magnitudes of each source are estimated by assuming the subsurface rupture length in terms of percentage of total length of sources and matched with reported earthquake. Estimated magnitudes match well with the reported earthquakes for a RLD of 5.2% of the total length of source. Zone of influence circles is also marked in the seismotectonic map by considering subsurface rupture length of fault associated with these earthquakes. As earthquakes relive strain energy that builds up on faults, it is assumed that all the earthquakes close to damaging earthquake have released the entire strain energy and it would take some time for the rebuilding of strain energy to cause a similar earthquake in the same location/fault. Area free from influence circles has potential for future earthquake, if there is seismogenic source and minor earthquake in the last 20 years. Based on this rupture phenomenon, eight probable locations have been identified and these locations might have the potential for the future earthquakes. Characteristic earthquake moment magnitude (M _w ) of 6.4 is estimated for the seismic study area considering seismic sources close to probable zones and 15% increased regional rupture character. The city is divided into several grid points at spacing of 0.01° and the peak ground acceleration (PGA) due to each probable earthquake is calculated at every grid point in city by using the regional attenuation model. The maximum of all these eight PGAs is taken for each grid point and the final PGA map is arrived. This map is compared to the PGA map developed based on the conventional deterministic seismic hazard analysis (DSHA) approach. The probable future rupture earthquakes gave less PGA than that of DSHA approach. The occurrence of any earthquake may be expected in near future in these eight zones, as these eight places have been experiencing minor earthquakes and are located in well-defined seismogenic sources.     

Illite diagenesis in the Vienna Basin,Austria

Summary The fine clay fractions less than 0.2 micron isolated from Tertiary shales of the Vienna Basin, Austria, were characterized mineralogically and chemically (including c.e.c.). Over the depth (885–2802 m) and temperature (45–120 °C) intervals represented mixed layer smectite-illites change from 74/26 to 20/80, the deeper samples showing IM ordering. Inhibition of illitization at about 2700 m is attributed to unusually high Mg content of pore solutions at this depth.Utilizing a chemical-statistical approach, the illite component of each smectite-illite mixed-layer phase has been characterized as to magnitude of interlayer charge, and octahedral and tetrahedral substitutions. The illite compositions fall neatly within the normal illite field defined by Köster.During the initial stage of illitization, smectite to illite transformation proceeds toward phengitic compositions. As illitization continues (greater depth and T), the illite component progressively changes composition with loss of Mg toward that of ideal 50/50 allevardite.

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Illit-Diagenese im Wiener Becken

Zusammenfassung Feintonproben (kleiner 0,2 Mikron) aus neogenen pelitischen Sedimenten aus dem Wiener Becken wurden mineralogisch und chemisch (einschließlich Ionenaustauschkapazität charakterisiert. Die im Teufenbereich zwischen 885–2802 m und Temperature zwischen 45–120 °C angetroffenen Smektit-Illit Mischphasen haben eine Zusammensetuzung von 74/26 bis 20/80, wobei im untersten Teufenbereich IM-Entwicklung vorliegt. Eine verzögerte Illitisierung bei ca. 2700 m wird lokal ungewöhnlich hohen Mg-Konzentrationen der Porenwässer zugeschrieben.Durch Anwendung eines chemisch-statistischen Näherungsverfahrens konnte die illitische Komponente der Smektit-Illit Mischphasen hinsichtlich Zwischenschichtladung sowie oktaedrischen und tetraedrischen Substitutionen charakterisiert werden. Die Zusammensetzungen der Illite liegen innerhald des Illitfeldes nach Köster.Die Umformung Smektit zu Illit zeigt außerdem zu Beginn des Illitisierungsprozesses phengitische Zusammensetzungen der Phasen. Mit zunehmender Tiefe und Temperatur ist weiterhin eine Neubildung von Allevardit zu beobachten.

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地震危险性、地震危害性和地震易损性

做好城市防震减灾工作 ,关键是对未来可能遭遇地震灾害定量化预测。本文论述地震灾害定量化的三要素 :地震危险性 ,地震危害性和地震易损性 ,它们概念完全不同 ,且很容易混淆 ,但又存在因与果的关系。本文还简要介绍厦门市地震科技工作者开展的“闽南地区综合防震减灾示范工程”。     

从地震信息看阿尔金断裂带构造和塔里木盆地花彩弧断裂体系

阿尔金断裂带由多条断裂组成,主要有阿尔金断裂、且末断裂、三危山断裂。其中阿尔金断裂为主断裂,它呈左旋走滑兼具逆冲性质,中生代—古近纪为左旋走滑,新近纪由东南向西北逆冲推覆。且末断裂和三危山断裂均具左旋走滑性质。且末断裂受统一的阿尔金断裂带左旋应力场控制,但又叠加了塔里木台盆区向南挤压的应力场,从而具有双重属性。塔里木盆地的断裂总体上组成古生界塔北花彩弧断裂束和塔南花彩弧断裂束,展布成全盆地的菱形断裂系统,且末断裂构成其东南边界。在该菱形断裂系统的北弧顶和菱形内的中央轴部为背冲式的构造断裂带,显示挤压特征;在花彩弧两翼转弯处展布正花状构造样式,显示走滑特征。阿尔金断裂带及其两侧,主要在柴达木、塔里木两大盆地发现了大油气田,两者都是由断层控制油气的垂向运移与分布。柴达木盆地具有双重断—坳的特点,但油气田只分布在中—新生界构造层内;塔里木盆地,南北翘板式的构造运动是其形成复式油气区的最重要的地质构造条件。     

万安盆地构造沉降与沉积速率及其油气资源前景

万安盆地是南海重要的新生代含油气盆地之一,是较为典型的新生代走滑拉张盆地。根据对横穿盆地的地震反射剖面的平衡剖面模拟和构造沉降速率、沉积速率的计算,认为万安盆地曾经历了3次快速的构造沉降。2.6Ma以来,沉积速率最大。万安盆地的沉降与沉积速率和盆地构造演化过程密切相关,各构造演化阶段分别控制了盆地中油气的生成、运聚和保存等成藏条件与过程。     

Spatial Characteristics and Controlling Factors of the Strike‐slip Fault Zones in the Northern Slope of Tazhong Uplift,Tarim Basin: Insight from 3D Seismic Data

The detailed characteristics of the Paleozoic strike-slip fault zones developed in the northern slope of Tazhong uplift are closely related to hydrocarbon explorations. In this study, five major strike-slip fault zones that cut through the Cambrian-Middle Devonian units are identified, by using 3D seismic data. Each of the strike-slip fault zones is characterized by two styles of deformation, namely deeper strike-slip faults and shallower en-echelon faults. By counting the reverse separation of the horizon along the deeper faults, activity intensity on the deeper strike-slip faults in the south is stronger than that on the northern ones. The angle between the strike of the shallower en-echelon normal faults and the principal displacement zone(PDZ) below them is likely to have a tendency to decrease slightly from the south to the north, which may indicate that activity intensity on the shallower southern en-echelon faults is stronger than that on the northern ones. Comparing the reverse separation along the deeper faults and the fault throw of the shallower faults, activity intensity of the Fault zone S1 is similar across different layers, while the activity intensity of the southern faults is larger than that of the northern ones. It is obvious that both the activity intensity of the same layer in different fault zones and different layers in the same fault zone have a macro characteristic in that the southern faults show stronger activity intensity than the northern ones. The Late Ordovician décollement layer developed in the Tazhong area and the peripheral tectonic events of the Tarim Basin have been considered two main factors in the differential deformation characteristics of the strike-slip fault zones in the northern slope of Tazhong uplift. They controlled the differences in the multi-level and multi-stage deformations of the strike-slip faults, respectively. In particular, peripheral tectonic events of the Tarim Basin were the dynamic source of the formatting and evolution of the strike-slip fault zones, and good candidates to accommodate the differential activity intensity of these faults.     

Diagenesis of Tertiary marlstones in the Vienna Basin

Summary To investigate the diagenetic changes of Tertiary pelitic sediments in the Vienna Basin a suite of drill core samples from the OeMV-bore-hole Aderklaa 78 has been studied using mineralogical and chemical methods.The sediments generally correspond to shales and cover a section from 780–2812 m depth. Their mineralogical composition is complex and variable and mainly due to the initial variation in detrital constituents at the time of sedimentation and the local sedimentary environments. Quartz, feldspars (sodic-plagioclase, K-feldspar), carbonates (calcite, dolomite, siderite), phyllosilicates (mica, chlorite, kaolinite, illite-smectite mixed-layers), and pyrite to a minor extent are the essential components of the shales.The analytical data of bulk samples and the less than 0.2 micron fractions point out that mechanical compaction of the sediments was accom inied by isochemical changes within the shale sequence. The diagenetic overprint of the mineral constituents is revealed by a decrease with depth of K-feldspar and an increase of quartz respectively, but especially by the gradual illitization of smectite via mixed-layer intermediates. Burial diagenesis in this Vienna Basin profile therefore can be expressed by the mineral reaction: K-feldspar+smectiteillite+quartz.A local reversal in all these trends in the basal Tertiary section can be explained by unusually high magnesium contents of the pore waters which inhibited illitization by promoting the stability of smectite relative to illite.

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Diagenetische Veränderungen an neogenen tonigen Sedimenten des Wiener Beckens

Zusammenfassung Zur Feststellung diagenetischer Veränderungen an den neogenen pelitischen Sedimenten des Wiener Beckens wurde eine Serie von Tonmergeln der ÖMV-Tiefbohrung Aderklaa 78 dem Teufenbereich zwischen 780–2812 m entnommen und mineralogisch sowie chemisch untersucht.Der Mineralbestand der Proben ist primär vom Einzugsgebiet und dem Ablagerungsmilieu der Sedimente geprägt. Quarz, Feldspäte (Albit-reicher Plagioklas, Alkalifeldspat), Karbonate (Calcit, Dolomit, Siderit) und Schichtsilikate (Glimmer, Chlorit, Kaolinit, Illit-Smektit Mischphasen) sowie untergeordneter Pyrit sind die wesentlichen Gemengteile der Ablagerungen.Die Untersuchungsergebnisse der Gesamtproben und der Fraktionen kleiner 0,2 Mikron zeigen, daß die mechanische Kompaktion der Sedimente von isochemischen Umwandlungsprozessen begleitet wird. Die diagenetische Überprägung des Mineralbestandes äußert sich mit zunehmender Tiefe in der Abnahme des Alkalifeldspates und der Zunahme des Quarzes, vor allem aber in der fortschreitenden Illitisierung des Smektits, die über Mixed-Layer Zwischenbildungen verschiedenen Ordnungsgrades abläuft.Die Mineralreaktion Alkalifeldspat+SmektitIllit+Quarz entspricht der versenkungsdiagenetischen Entwicklung dieses Bereiches.Eine lokale Trendumkehr der mineralogisch-chemischen Veränderungen im Nahbereich zur mesozoischen Basis des Neogens ist mit einer extremen Zusammensetzung der Porenwässer zu erklären, deren hoher Mg-Gehalt eine kontinuierliche Entwicklung des Illitisierungsprozesses verhindert.

     

Structure of the Transkei Basin and Natal Valley, Southwest Indian Ocean, from seismic reflection and potential field data

Marine geophysical data from the southern Natal Valley and northern Transkei Basin, offshore southeast Africa, were used to study the structure of the crust and sedimentary cover in the area. The data includes seismic reflection, gravity and magnetics and provides information on the acoustic basement geometry (where available), features of the sedimentary cover and the basin's development. Previously mapped Mesozoic magnetic anomalies over a part of the basin are now recognized over wider areas of the basin. The ability to extend the correlation to the southeast within the Natal Valley further confirms an oceanic origin for this region and provides an opportunity to amplify the existing plate boundary reconstructions.The stratigraphic structure of the southern Natal Valley and the northern Transkei Basin reflects processes of the ocean crust formation and subsequent evolution. The highly variable relief of the acoustic basement may relate to the crust formation in the immediate vicinity of the continental transform margin. Renewed submarine seismicity and neotectonic activity in the area is probably related to the diffuse boundary between the Nubia and Somalia plates.2.5-D crustal models show that a 1.7–3.2-km-thick sediment sequence overlies a 6.3±1.2-km-thick normal oceanic crust in the deep southern Natal Valley and Transkei Basin. The oceanic crust in the study area is heterogeneous, made up of blocks of laterally varying remanent magnetization (0.5–3.5 A/m) and density (2850–2900 kg/m³). Strong modifications of accretionary processes near ridge/fracture zone intersections may be a reason of such heterogeneity.     

Petrography and provenance of subsurface Neogene sandstones of Bengal Basin, Bangladesh

This study deals with petrography and provenance of the Neogene reservoir sandstones encountered in the Kailas Tila, Titas, Bakhrabad and Shahbazpur Gas Fields of Bengal Basin. Framework grains are sand-sized to silt-sized particles of mainly detrital origin. The most common detrital grains are quartz, feldspars, and rock fragments. Mica occurred as minor and non-opaque heavy minerals found as minor accessories. Among the main detrital framework grains, quartz constitutes 51–60%, feldspar 3–15%, lithic fragments 8–22%. Sandstones encountered in the studied wells have been classified as sublithic arenite, feldspathic arenite and lithic arenite in order of abundance. Different triangular plots reveal that the Neogene sandstones of the studied wells exhibit a quartzolithic composition, low feldspar, very low volcanic grains and abundant sedimentary and low grade metamorphic lithic clasts indicating that the sands were derived from quartzose recycled orogen province, such as a fold thrust province or a collision suture zone. This study suggests that either the eastern Himalayas or Indo-Burman Ranges might act as the source of the sandstones of the studied wells of the Bengal Basin.     

鄂尔多斯盆地南部黄土塬区三维地震激发关键技术研究

鄂尔多斯盆地南部黄土塬区是中国特有的地形地貌,巨厚黄土层弱弹性介质地震激发能量下传问题是地震数据采集的难题。中石化华北分公司经过多年的攻关与试验,对影响黄土塬地震资料品质的主要因素进行了深入分析,形成了一套黄土塬区激发关键技术——选取合理的激发点位、尽可能减少干燥疏松黄土对地震有效信息的吸收与衰减、选择合理的激发药型和激发参数增加子波的能量及拓展子波频带宽度、利用组合效应压制部分侧面干扰等。鄂尔多斯盆地南部黄土塬区三维地震采集关键技术的突破,结束了黄土塬区地震数据采集只能进行沿沟、谷进行二维弯线勘探的历史,使黄土塬区三维地震勘探成为可能。     

Experimental frictional heating of coal gouge at seismic slip rates: Evidence for devolatilization and thermal pressurization of gouge fluids

High velocity (1 m/s) friction experiments on bituminous coal gouge display several earthquake-related phenomena, including devolatilization by frictional heating, gas pressurization, and slip weakening. Stage I is characterized by sample shortening and reduction in the coefficient of friction (μ) from  1 to 0.6. Stage II is characterized by high frequency ( 5 Hz) oscillations in stress and strain records and by gas emissions. Stage III is marked by rapid weakening (μ  0.1 to 0.35) and sample shortening, together with continued gas emissions. Stage IV produces stable stress records and continued weakness (μ  0.2), but without gas emission. Stage I shortening is due to compaction of the gouge and the weakening is attributed to mechanical or thermal effects. Stage II behavior is interpreted as due to coal gasification and fluctuations in fluid pressure, resulting in high frequency stick-slip type behavior. Dramatic reduction in shear stress in stage III is attributed to gas pressurization by pore collapse and corresponds to a frictional instability, analogous to nucleation of an earthquake. Microstructural observations indicate the deformation was brittle during stages I and II but ductile during stages III and IV. Time dependent finite element frictional heat models indicate the center of the samples became hot ( 900 °C) during stage II, whereas the edge of samples remained relatively cold (< 300 °C). Vitrinite reflectance of coal samples shows an increase in reflectance from  0.5 to  0.8% over the displacement interval 20–40 m (20–40 s), indicating that the reflectance responds to frictional heating on a short time scale. The energy expended per unit area in these low stress, large displacement experiments is similar to that of higher stress ( 50 MPa), short displacement ( 1 m) earthquakes ( 10⁷ J/m²).     

Petrophysical evaluation of subsurface reservoir sandstones of Bengal Basin,Bangladesh

The present study has been carried out for core and log based petrophysical evaluation of subsurface reservoir sandstones considering at least one well from each of the Kailas Tila, Titas, Bakhrabad and Shahbazpur gas fields of Bangladesh. Core analysis results show that average core porosity, permeability and pore throat values are 20%, 209 mD and 44020 Å, respectively. These measured values support that the sandstone reservoirs are categorized as good quality reservoirs. Core porosity values usually exceed thin section porosity. Different cross-plots indicate that permeability is largely dependent on porosity. Permeability and porosity are also dependent on textural parameters that include size, shape, sorting and matrix of the reservoir sandstones. It reveals from both thin section study and different cross plots that diagenetic cements are the main controlling factors of the reservoirs. Quantitative determination of the volume of cements shows that cements have inverse correlation with porosity and permeability. Log based petrophysical parameters include shale volume, porosity, water and hydrocarbon saturation, permeability, moveability index and bulk volume of water. The average values of the mentioned parameters are 20%, 22%, 26%, 74%, 110 mD, 0.28 and 0.05, respectively. The most important parameters are porosity and permeability, which indicate that log porosity exceeds core porosity and log permeability significantly lags behind core permeability. The study also reveals that 23 gas zones covering total thickness of 385 m sandstones mostly posses good quality reservoirs except few moderate quality reservoirs.