Wadati method as a simple tool to study seismically active fault zones: a case study from the West-Bohemia/Vogtland region,central Europe

The ratio of P- to S-wave velocities, V_P/V_S, is an important parameter characterizing rock composition and fluid saturation. We have studied properties of the ratio in the West-Bohemian seismically active region, using data from the earthquake swarm which occurred here in 2008. The earthquake swarm was well recorded by 23 seismic stations from epicentral distances less than 25 km. We selected a subset of 158 events with local magnitudes between 1.5 and 3.8. Applying the Wadati method to the measured arrival times of P and S waves, we arrived at an average value of V_P/V_S =1.68 ± 0.01. This differs a little from the value of V_P/V_S = 1.70, which is routinely used for earthquake locations in the region at present. Moreover, it was recognized that the points in the Wadati graphs for some stations were systematically deviated from the mean straight lines. In particular, the stations with the largest positive deviations (above the mean straight lines) are situated close to the Mariánské Lázně Fault and to some intensive mofettes. Further analyses revealed reduced P- and S-wave velocities along the seismic rays toward these anomalous stations. In our opinion, the seismic waves arriving at the anomalous stations probably propagated along a fault or another zone of weakness. In this way, our results support the hypothesis that the Mariánské Lázně Fault is a deep-seated fault continuing down to the seismically active zone of local earthquakes. From a general point of view, this study demonstrates that even some narrow structural anomalies in the crust, such as fault zones, can be recognized by the simple Wadati method if data from a dense seismic network are available.     

Imaging the Mariánské Lázně Fault (Czech Republic) by 3-D ground-penetrating radar and electric resistivity tomography

Geodynamic activity in the area of West Bohemia is typified by the occurrence of earthquake swarms, Quaternary volcanism and high flux of mantle-derived CO₂. The highest swarm activity occurs beneath the eastern edge of the Cheb basin, which is delineated by the NW-SE trending morphologically pronounced Mariánské Lázn?? Fault (MLF) controlling the formation of the basin. The previous trenching survey across the MLF zone has identified several fault strands with possible Quaternary activity. In this paper we present the results of the geophysical survey focused to trace the faults signatures in geophysical sections and to build an image of near surface tectonics. The method of electric resistivity tomography (ERT) along two profiles parallel to the trench identified a strong resistivity contrast between the bodies of sandy gravels in the middle and conductive clayey sands to the west and weathered crystalline basement to the east. The 2-D ground penetration radar (GPR) sections show direct correlation of reflections with lithological boundaries identified in the trench. As expected, the GPR signal amplitudes increase with the resistivities found in the ERT sections. Two of the four faults identified in the trench are indicated in the resistivity and GPR sections. A 3-D GPR measurement has identified a spot of high amplitudes elongated parallel to the MLF trend, which coincides with the high resistivity body. To improve the signal-to-noise ratio of the time slices we stacked the GPR time slices within vertically homogeneous blocks. This provided a contrast image of the sand-gravel body including its boundaries in three dimensions. The detailed analysis of the 3-D GPR cube revealed additional fault that limits the highly reflective sands and appears to be offset by another younger fault. Our results suggest a complex fault pattern in the studied area, which deserves a further study.     

Depth-Recursive Tomography of the Bohemian Massif at the CEL09 Transect—Part B: Interpretation

In the accompanying paper (Part A), depth-recursive tomography was applied to the CEL09 refraction data. A deblurred P-wave velocity image was obtained down to a depth of 20 km. This paper (Part B) is devoted to the interpretation of the upper- and middle-crustal structures of the Bohemian Massif imaged in the CEL09 section. Because of inherent ambiguity of the refraction method in imaging low-velocity zones, other well-known results based on other geophysical data sets are also used to independently verify the interpreted velocity features. Comparison with the density and velocity models previously obtained indicates that the presented P-wave velocity image has superior resolution revealing or verifying a number of geological features. The prominent lateral velocity changes encountered in the CEL09 pattern across the imaged crustal section were used to delineate the main terranes and deep regional fault zones such as the Kru?né hory Fault, the SW continuation of the Litomě?ice Fault Zone, the West and Central Bohemian Shear Zones, the Blanice-Rodl Fault, the P?ibyslav-Vitis Fault and the Boskovice-Diendorf Fault. The 450-km-long CEL09 transect reveals seven major deeply rooted high-velocity (HV) anomalies identified as Variscan massifs intruded near or within these deep fault zones. They form buried ridges mostly parallel to the SW-NE trending Variscan strike. Their discovery allows new insights into a number of phenomena such as the West Bohemian earthquake swarms, the Saxothuringian paradox, the character of the Saxothuringian-Barrandian contact zone, the detachment surface due to the slab of the Saxothuringian crust subducting beneath the Teplá-Barrandian zone in the Devonian, the depth extent of the Mariánské-Lázně Complex (MLC) as an equivalent unit of the Zone Erbendorf-Vohenstrauss (ZEV), the subsidence of the Barrandian syncline, the root zones of the Central and South Bohemian Plutons, the accretionary wedge formed along the Moravo-Moldanubian suture and its link with the Gföhl terrane, the Carpathian foreland relief and the subsidence observed in the Vienna Basin.     

Structural characteristics of epicentral areas in Central Europe: study case Cheb Basin (Czech Republic)

The earthquake distribution pattern of Central Europe differs systematically from the neighbouring areas of NW and southern Europe regarding the fault plane kinematics. Within a belt between the French Massif Central and the northern part of the Bohemian Massif (1000 km) sinistral faulting along N-S zones dominates on the contrary to the Alps and their foreland with common bookshelf shears. One of the prominent N-S structures is the Regensburg-Leipzig-Rostock Zone (A) with several epicentral areas, where the main seismic center occurs in the northern Cheb Basin (NW Bohemia). The study demonstrates new structural results for the swarm-quake region in NW-Bohemia, especially for the Nový Kostel area in the Cheb Basin. There the N-S-trending newly found Počatky-Plesná zone (PPZ) is identical with the main earthquake line. The PPZ is connected with a mofette line between Hartušov and Bublák with evidence for CO₂ degassing from the subcrustal mantle. The morphologically more prominent Mariánské Lázně fault (MLF) intersects the PPZ obliquely under an acuate angle. In the past the MLF was supposed to be the tectonic structure connected with the epicentral area of Nový Kostel. But evidence from the relocated hypocentres along the PPZ (at 7–12 kms depth) indicate that the MLF is seismically non-active. Asymmetric drainage patterns of the Cheb Basin are caused by fault related movement along Palaeozoic basement faults which initiate a deformation of the cover (Upper Pliocene to Holocene basin filling). The PPZ forms an escarpment in Pliocene and Pleistocene soft rock and is supposingly acting as an earthquake zone since late Pleistocene time. The uppermost Pleistocene of 0.12–0.01 Ma deposited only in front of the fault scarp dates the fault activity. The crossing faults envelope crustal wedges under different local stress conditions. Their intersection line forms a zone beginning at the surface near Nový Kostel, dipping south with increasing depth, probably down to about 12 km. The intersection zone represents a crustal anomaly. There fault movements can be blocked up and peculiar stress condition influence the behaviour of the adjacent crust. An ENE-WNW striking dextral wrench fault was detected which is to expect as kinematic counterpart to the ca. N-S striking sinistral shear zones. Nearly E-W striking fracture segments were formerly only known as remote sensing lineaments or as joint density zones. The ENE shear zone is characterized by a set of compressional m-scale folds and dm-scale faults scattered within a 20 m wide wrench zone. It is built up of different sets of cleavage-like clay plate pattern of microscopical scale. The associated shear planes fit into a Riedel shear system. One characteristic feature are tiny channels of micrometer scale. They have originated after shear plane bending and are the sites of CO₂ mantle degassing.     

Geodynamic pattern of the West Bohemia region based on permanent GPS measurements

In West Bohemia in the period of 2003–2005 five permanent GPS stations were established to detect local movement trends. Their mutual position changes were determined from time series of GPS observations and were associated with seismic, gravity, and geo-scientific data related to the geodynamics of the West Bohemian region. Knowledge of local physical processes based on spatial and time earthquake occurrences, focal mechanisms of main events, stress and strain fields set up a tool for recent seismotectonic analyses. The permanent GPS measurements bring independent effective phenomenon, direct monitoring of site movements. The movements detected by our GPS stations evidenced WSW-ENE extension with subsiding trends in the western part of the Cheb Basin and the Smrčiny Mts. Besides, there were monitored dextral movements along the Mariánské Lázně tectonic fault zone (MLF). A comparison of results with previous data formed a presumption that an antithetic stress pattern has to exist inside the inner part of the MLF tectonic zone. This antithetic stress can explain the coexistence of dextral and sinistral movements on individual tectonic elements in the West Bohemian area.     

Derived gravity field of the seismogenic upper crust of SE Germany and West Bohemia and its comparison with seismicity

The gravity field of the seismogenic upper crust was derived from the Bouguer gravity map by applying the Butterworth high-pass filter in the wave-number domain. The cutoff wavelength of the filter was 110 km, to pass the gravity signals of structures within the 18 km thick seismogenic layer. The derived residual gravity map reveals potential stress concentrating structures, which may cause seismicity provided they lie within the existing zones of weakness. Furthermore we derived a shaded relief map of the horizontal gravity gradient, which highlighted the tectonic lines accompanied by density contrast. The directional analysis of this map shows three dominant strike directions. The most prominent one is “the Hercynian” NW-SE strike direction represented by the Franconian Line, the Gera-Jáchymov Fault Zone and the Elbe Zone. The second dominant strike is the Rhenisch NNE-SSW trending represented by the Upper Rhine Graben Zone, Rheinsberg-Heldburg Line and several Proterozoic volcanic belts in the Teplá-Barrandien Unit. The third pronounced trending of the ENE-WSW direction is represented by the Erzgebirge and Eger Graben gravity low. The N-S trending Rostock-Leipzig-Regensburg Zone (Pritzwalk-Naab Lineament) is not distinctly reflected in the derived gravity maps, although many fault segments have a meridian direction. The relative reactivation potential of some pre-existing fault systems identified in the gravity map was studied with respect to the wide range of the recent stress configuration determined in the West Bohemia/Vogtland region. The resulting diagrams show that the steep NNW-SSE to N-S faults (represented by some segments of the Mariánské Lázně Fault Zone) are oriented favourably for reactivation. On the contrary, the orientation of the ENE-WSW faults limiting the Eger Graben (Litoměřice Fault, etc.) is unfavourable for reactivation for all dip values.     

Control of paths of quaternary volcanic products in western Bohemian Massif by rejuvenated Variscan triple junction of ancient microplates

Our objective is to look for deep paths of Cenozoic volcanism and migration routes of active mantle volatiles through the lithosphere of the western Bohemian Massif. We show that the rejuvenated junction of three mantle domains, delimited by different orientation of seismic anisotropy and belonging to originally separated microplates — the Saxothuringian (ST), Moldanubian (MD) and Teplá-Barrandian (TB) — can provide the easiest upward routes of fluids through the deep lithosphere. Geographic distribution of mantle-fluid escapes at the surface suggests fluid migration through the ductile lower crust and through partly open faults in the rigid upper crust, which is locally detached and shifted from its lower part and from the mantle lithosphere. Present-day escapes of mantle-derived helium and CO₂ concentrate mainly in two tectonically different crust edifices — in the Cheb Basin (CHB) and in an allochtonous block called the Mariánské Lázně Complex (MLC). Crystalline basement of the CHB developed above the Variscan ‘triple junction’ of the mantle lithosphere domains. The basement was extended during the Cenozoic and dissected by systems of faults into small partly sunken blocks. Thanks to buoyancy the mantle fluids migrate upwards along the lithosphere junction into the faulted basement of the CHB. The highest CO₂ flow and the highest ³He/⁴He ratios are observed at intersections of major normal faults and along the southern boundary of the Smrčiny (Fichtelgebirge) granite Pluton. The fluid escapes are separated from the earthquake swarm epicentres. Routes of the fluids to the MLC are longer and more complicated. Surface escapes tap the mantle fluids mainly from the Mariánské Lázně Fault (MLF) and from the tectonic boundaries along which the MLC block of the TB lower crust was thrust over the ST complexes. Hypocentres of earthquake swarms of the two major focal areas at Novy Kostel and Lazy, located mainly at depths of 6–13 km, reside either in granite or in underlying gneiss, while the escapes of mantle fluids follow major faults or boundaries of crystalline units outside the Smrčiny and Karlovy Vary granite Plutons. We suggest that primarily those parts of faults in the upper crust, which is strengthened by granite magmatism and rigid enough to selectively accumulate stresses, are seismoactive. On the other hand, other parts of the faults tapping ascending mantle volatiles are ‘lubricated’ by the fluids and secondary mineralogical changes, and thus they cannot accumulate sufficient stresses to be released by earthquakes. A comparison of the most probable paths of the mantle fluids with the space-time distribution of the Novy Kostel hypocentres does not seem to support the model of the earthquake swarms triggered by pressurized fluids of mantle origin.     

Three-dimensional gravimetric modelling to detect the deep structure of the region Vogtland/NW-Bohemia

The occurrence of swarm earthquakes in the Vogtland/NW-Bohemia area results probably from the physical interactions of fluids, the stress field and the geometry of the geological units. Therefore the present study aims at the development of a 3-D density model of the region with a vertical range of 35 km. A new Bouguer anomaly map is presented containing about 17 000 gravity data points. Prominent Bouguer anomalies are produced by the granites of Eibenstock and Karlovy Vary (low with −75 mGal), the metabasites near Mariánzké Lázně (high with 5 mGal) and the Münchberg Gneiss Massif (gravity high of Hof with 10 mGal). The geometry of the internal model structures correspond to geological units and, thus, the modelled gravity fits well the observed Bouguer anomaly. The 3-D gravimetric modellings indicate detailed geometries of the geological settings. With regard to the periodic occurrence of swarm earthquakes in the Vogtland region the existence of an upwelling mantle or a magmatic body is investigated. Precise information only can be given, if the vertical extension of the near surface bodies is known.     

Geological interpretation of gravity profiles through the Karlovy Vary granite massif (Czech Republic)

We examined the shape of the Late Variscan Karlovy Vary granite massif located south of the Ohre/Eger graben in Northern Bohemia by reinterpretation of existing gravity data on two perpendicular profiles. The granite body of about 360 km² total outcrop size has the elongation ratio 0.35 with the major axis trending NE-SW. The SW part of the body was crossed in the nineties by the seismic profile 9HR which localized the bottom of granites in a depth of about 10 km. We used this value as a reference datum in our gravity profiles. We positioned one of our profiles along the seismic profile 9HR and the other one perpendicularly, i.e. parallel with the elongation of the outcrop surface. We interpret the shape of the main granite body in the vicinity of Karlovy Vary as a continuous desk whose floor is horizontal (or subhorizontal) and varies along its whole extension about a depth of 10 km. This thickness is approximately identical with that of the Saxothuringian nappes imaged by seismic reflection. The near surface upper contact of the granite body is mildly inclined, and outward dipping. It changes to steep sides or inward inclined contacts in deeper levels. The Lesny-Lysina (Kynžvart) massif is a separate granite body about 324 km thick, not continuously connected with the main Karlovy Vary massif. The gravity curve suggests that granites often enclose in their endocontact large blocks of country metasediments or metabasites the existence of which is partly evidenced by their outcrops outside the line of the profile. The granite body is found density-homogenous. Minor density differences between granite varieties are caused mainly by more intense hydrothermal alterations in younger suite granites. We interpret vertical conduits for the ascent of granitic magmas to be parallel to the Jáchymov-Gera and Ohře (Eger) lineaments or the Mariánské Lázně fault zone as indicated by the elongation of some outcrops. However, they are not clearly imaged from the gravity data. The effect of the depression of the Sokolov basin along the faults parallel with the Ohře (Eger) lineament is shallow and it is not indicated by any change in the floor depth of the granite body. Comparison of the seismicity distribution suggests that the hypocenters occur mostly outside of the granite bodies or near their contact with the country rock.     

Shallow reflection seismic evidence of tectonic activity in the Cheb Basin,NW Bohemia

With a high-resolution seismic survey, we targeted the shallow structural features of the Cheb Basin, Northwest Bohemia. The aim of this study was to evaluate these features’ relation to seismic activity below the Cheb Basin with the first reflection seismic image of parts of the basin. We recorded a densely sampled, 3200 m long seismic survey stretching from Kopanina towards SE, into the basin. From the recordings of 170 shots at 192 geophones with 384-m total spread, we obtained a reflection seismic image down to the basement at 300-m depth, supplemented by a shallow tomographic velocity model for the uppermost 40 m. Strong lateral velocity contrasts indicate Quaternary river terraces, into which the Plesná river incises. We observe reverse faults in the lower basin sediments, which we interpret as signs of a push-up structure related to the N-S oriented Po?átky-Plesná Zone (PPZ). However, we do not observe any vertical fault offsets in the younger sedimentary layers, which suggests that any normal or reverse faulting must be older than 20 Ma. The image agrees well with previous lithostratigraphic borehole data and previous sedimentological and tectonic models. Our explanation for the formation of the escarpment at the eastern bank of the Plesná valley, previously interpreted as outcrop of the PPZ, relies solely on incision of the Plesná river into weak sediments.     

Tectonosedimentary Evolution of the Cheb Basin (NW Bohemia,Czech Republic) between Late Oligocene and Pliocene: A Preliminary Note

A working model of tectono-sedimentary evolution is proposed for the Cheb Basin, a polyhistory sedimentary basin formed between the late Oligocene and Pliocene by reactivation of basement fracture systems in the northwestern part of the Bohemian Massif. The basin is located at the intersection of the Ohe (Eger) Graben structural domain, characterized by dominance of NE-striking graben systems in present-day geology, and the NW-striking Cheb-Domalice Graben, a major strike-slip – dominated structure in Western Bohemia. The first significant depositional episode in the Cheb Basin coincides with the deposition of late Oligocene-Miocene clastics in the whole extensional system of the Ohe Graben, controlled by E-W – trending depocenters. The main structural feature of the Cheb Basin region at that time was a palaeohigh caused by a NW- trending accommodation zone separating minor E-W – trending depocentres. The second, late Pliocene, episode of sedimentation occurred under a very different kinematic regime than the Oligo-Miocene rift basin evolution. During this time, the present-day structure of the Cheb Basin and the Cheb-Domalice Graben formed as a consequence of sinistral displacement on the Mariánské Lázn Fault Zone. Reactivation of this strike-slip fault zone led to the formation of a horsetail splay of oblique-extensional faults at the northern termination of the Mariánské Lázn Fault Zone, which contained the present-day Cheb Basin.     

地震折射和电阻率法在水库坝址勘察中的应用

本文以笔者参与的某水库工程勘察为例,介绍了如何根据地质地球物理特征及浅层地震折射波法和高密度电阻率法的原理,选择合适的工作方法和技术参数.并结合典型探测资料,阐述了浅层地震折射波法和高密度电阻率法用于调查覆盖层厚度、基岩风化分带及断裂构造带的位置和产状等工程地质问题中的有效性.     

A magnetotelluric profile of the Wind River Thrust, Wyoming

Data from ten magnetotelluric (MT) stations over the Wind River Uplift and adjacent basins are interpreted with constraints from the Consortium for Continental Reflection Profiling (COCORP) seismic reflection data and from gravity data. The MT data reveal the general configuration of the conductive basins and resistive uplifts; low resistivity zones are interpreted as faults which correspond to those visible in the COCORP sections.

The Wind River Thrust Fault is modelled as a conductive zone that can be traced to a depth of at least 20 km, and the crust beneath the Green River Basin is about 40 km thick.

The modelled constant dip of the Wind River Thrust is consistent with a tectonic model of lateral compressive stress.     

渭河断裂深、中、浅和近地表显示

渭河断裂是渭河盆地一条重要的隐伏断裂,对渭河盆地的形成和发展乃至盆地内的地震活动都具有一定的控制作用。文中基于横跨渭河断裂的深地震反射、浅层地震反射、钻孔勘探和槽探等勘探方法取得的探测结果,从深部、中部、浅部以及近地表4个深度,给出了渭河断裂的呈现特点,即最深切割层位、不同深度的产状变化、错距大小、断裂的最新活动时代和活动期次等     

代县盆地可控音频大地电磁浅层电性结构探测

为研究代县盆地及其断裂的空间展布以及深部的延展情况,东南跨过五台山北麓断裂带,西北至恒山山前黄土丘陵区,布置了一条NW向穿过代县盆地的可控音频大地电磁(CSAMT)测深剖面,全长12.55km.共完成可控源音频大地电磁测点246个,观测频率为8533.333 ～1.333333Hz.视电阻率、相位曲线具有明显的分段特征...     

NEW DISCOVERY OF RESHUI-TAOSTUO RIVER FAULT IN DULAN,QINGHAI PROVINCE AND ITS IMPLICATIONS

The 40km-long, NEE trending Reshui-Taostuo River Fault was found in the southern Dulan-Chaka highland by recent field investigation, which is a strike-slip fault with some normal component. DEM data was generated by small unmanned aerial vehicle(UAV)on key geomorphic units with resolution<0.05m. Based on the interpretation and field investigation, we get two conclusions:1)It is the first time to define the Reshui-Taostuo River Fault, and the fault is 40km long with a 6km-long surface rupture; 2)There are left-handed dislocations in the gullies and terraces cut by the fault. On the high-resolution DEM image obtained by UAV, the offsets are(9.3±0.5) m, (17.9±1.5) m, and(36.8±2) m, measured by topographic profile recovery of gullies. The recovery measurements of two terraces present that the horizontal offset of T₁/T₀ is(18.2±1.5) m and the T₂/T₁ is (35.8±2) m, which is consistent with the offsets from gullies. According to the historical earthquake records, a M5 3/4 earthquake on April 10, 1938 and a M_S5.0 earthquake on March 21, 1952 occurred at the eastern end of the surface rupture, which may be related to the activity of the fault. By checking the county records of Dulan and other relevant data, we find that there are no literature records about the two earthquakes, which is possibly due to the far distance to the epicenter at that time, the scarcity of population in Dulan, or that the earthquake occurred too long ago that led to losing its records. The southernmost ends of the Eastern Kunlun Fault and the Elashan Fault converge to form a wedge-shaped extruded fault block toward the northwest. The Dulan Basin, located at the end of the wedge-shaped fault block, is affected by regional NE and SW principal compressive stress and the shear stress of the two boundary faults. The Dulan Basin experienced a complex deformation process of compression accompanying with extension. In the process of extrusion, the specific form of extension is the strike-slip faults at each side of the wedge, and there is indeed a north-east and south-west compression between the two controlling wedge-shaped fault block boundary faults, the Eastern Kunlun and Elashan Faults. The inferred mechanism of triangular wedge extrusion deformation in this area is quite different from the pure rigid extrusion model. Therefore, Dulan Basin is a wedge-shaped block sandwiched between the two large-scale strike-slip faults. Due to the compression of the northeast and southwest directions of the region, the peripheral faults of the Dulan Basin form a series of southeast converging plume thrust faults on the northeast edge of the basin near the Elashan Fault, which are parallel to the Elashan Fault in morphology and may converge with the Elashan Fault in subsurface. The southern marginal fault of the Dulan Basin(Reshui-Taostuo River Fault)near the Eastern Kunlun fault zone is jointly affected by the left-lateral strike-slip Eastern Kunlun Fault and the right-lateral strike-slip Elashan Fault, presenting a left-lateral strike-slip characteristic. Meanwhile, the wedge-shaped fault block extrudes to the northwest, causing local extension at the southeast end, and the fault shows the extensional deformation. These faults absorb or transform the shear stress in the northeastern margin of the Tibet Plateau. Therefore, our discovery of the Dulan Reshui-Taostuo River Fault provides important constraints for better understanding of the internal deformation mode and mechanism of the fault block in the northeastern Tibetan plateau. The strike of Reshui-Taostuo River Fault is different from the southern marginal fault of the Qaidam Basin. The Qaidam south marginal burial fault is the boundary fault between the Qaidam Basin and the East Kunlun structural belt, with a total length of ~500km. The geophysical data show that Qaidam south marginal burial fault forms at the boundary between the positive gravity anomaly of the southern East Kunlun structural belt and the negative gravity anomaly gradient zone of the northern Qaidam Basin, showing as a thrust fault towards the basin. The western segment of the fault was active at late Pleistocene, and the eastern segment near Dulan County was active at early-middle Pleistocene. The Reshui-Taostuo River Fault is characterized by sinistral strike-slip with a normal component. The field evidence indicates that the latest active period of this fault was Holocene, with a total length of only 40km. Neither remote sensing image interpretation nor field investigation indicate the fault extends further westward and intersects with the Qaidam south marginal burial fault. Moreover, it shows that its strike is relatively consistent with the East Kunlun fault zone in spatial distribution and has a certain angle with the burial fault in the southern margin of Qaidam Basin. Therefore, there is no structural connection between the Reshui-Taostuo River Fault and the Qaidam south marginal burial fault.     

Active faulting offshore SE Spain (Alboran Sea): Implications for earthquake hazard assessment in the Southern Iberian Margin

The southern margin of the Iberian Peninsula hosts the convergent boundary between the European and African Plates. The area is characterised by low to moderate magnitude shallow earthquakes, although large historical events have also occurred. In order to determine the possible sources of these events, we recently acquired swath-bathymetry, TOBI sidescan sonar and high-resolution seismic data on the Almería Margin (Eastern Alboran Sea). The new dataset reveals the offshore continuation of the NE–SW trending Carboneras Fault, a master fault in the Eastern Betic Shear Zone, and its associated structures (N150 and NS faults). These structures are active since they cut the Late Quaternary sedimentary units. The submarine Carboneras Fault zone is 100 km long, 5–10 km wide, and is divided into two N045 and N060 segments separated by an underlapping restraining stepover. Geomorphic features typically found in subaerial strike-slip faults, such as deflected drainage, water gaps, shutter ridges, pressure ridges and “en echelon” folds suggest a strike-slip motion combined with a vertical component along the submarine Carboneras Fault. Considering the NNW–SSE regional shortening axis, a left-lateral movement is deduced for the Carboneras Fault, whereas right-lateral and normal components are suggested for the associated N150 and NS faults, respectively. The offshore portion of this fault is at least twice as long as its onshore portion and together they constitute one of the longest structures in the southeastern Iberian Margin. Despite the fact that present day seismicity in the Almería margin seems to be associated with the N150 to NS faults, the Carboneras Fault is a potential source of large magnitude (M_w ∼7.2) events. Hence, the Carboneras Fault zone could pose a significant earthquake and tsunami hazard to the coasts of Spain and North Africa, and should therefore be considered in any hazard re-evaluation.     

桥头集-东关断裂合肥盆地段活动特征

北西走向的桥头集-东关断裂与郯庐断裂带南段相交切，横跨合肥盆地及其东缘的隆起区，在合肥盆地内呈隐伏状态。本研究跨桥头集-东关断裂合肥盆地段布设3条浅层地震勘探测线，揭示断裂浅部构造特征。解译出的F_P1、F_P2和F_P3断层性质相同，倾向一致，上断点深度相近，反映出桥头集-东关断裂合肥盆地段是一条走向NW、倾向SW、具有逆断分量的断裂。3条浅层地震反射剖面中第四系的底界面反射波组T_Q呈连续近水平展布，表明桥头集-东关断裂两侧的第四系厚度无明显变化。在浅震剖面解译的基础上，布设了1条跨断层钻孔联合剖面，剖面揭露出的第四系没有被错动。结合本区第四系地层发育情况及钻孔地层年龄分析，认为桥头集-东关断裂中更新世以来不活动。根据本文探测结果，尚不能排除桥头集-东关断裂早更新世有一定的弱活动。     

鄂尔多斯北缘断裂托克托段晚第四纪活动特征

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