Reactivation of deep faults beneath Southern Apennines: evidence from the 1990–1991 Potenza seismic sequences

We relocate the 1990–1991 Potenza (Southern Apennines belt, Italy) sequences and calculate focal mechanisms. This seismicity clusters along an E–W, dextral strike–slip structure. Second-order clusters are also present and reflect the activation of minor shears. The depth distribution of earthquakes evidences a peak between 14 and 20 km, within the basement of the subducting Apulian plate. The analysed seismicity does not mirror that of Southern Apennines, which include NW–SE striking normal faults and earthquakes concentrated within the first 15 km of the crust. We suggest that the E–W faults affecting the foreland region of Apennine propagate up to 25 km of depth. The Potenza earthquakes reflect the reactivation of a deep, preexisting fault system. We conclude that the seismotectonic setting of Apennines is characterized by NW–SE normal faults affecting the upper 15 km of the crust, and by E–W deeper strike–slip faults cutting the crystalline basement of the chain.     

Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.     

大姚—姚安地区强震孕震机理——NW向褶皱节理的构造强化与活动

以21世纪初滇中大姚—姚安一带接连发生的4次Ms6.0~6.5级强震为例,通过分析该地区构造类型及其演化历史,结合川滇块体的现今运动特征和构造应力场,提出滇中构造区运动模型和特殊的孕震模式,探讨褶皱节理与地震活动的关系。研究表明,这些地震序列均有规律地沿北西方向展布,具有高度一致的沿北西向节理面右旋走滑的力学破裂机制。该地区断裂不发育,而是以北西向中生代褶皱构造为主。野外调查发现,该地区广泛发育与褶皱伴生的北西向纵向节理及北东向横向节理,前者较后者更为发育。这些节理密集成带,呈不等间距排列,带宽30~50 m,带内节理密度20~30条/m。节理面上发育有方解石脉和辉长岩脉,同时发育挤压片理化带、新鲜的水平擦痕和松软的断层泥,说明这些节理不仅与深部地壳有关联,而且近期有着新活动迹象。分析认为,在滇中块体南南东运动的背景下,这些地震是在现今北北西向挤压应力场作用下,北西向纵向节理发生构造强化、贯通、破裂的过程中产生的。"活节理"与活动断裂、活动褶皱等构造一样,是地球上广泛存在的活动构造之一。在特殊的构造环境和特定的构造应力场作用下,节理构造会演变为一种构造强化带或活化带,进而成为一种特殊的孕震构造。     

Stress tensor and focal mechanisms along the Dead Sea fault and related structural elements based on seismological data

The Dead Sea fault is among the largest active strike–slip fault of the world. This study is focused on the southern part of this fault, from the Sea of Galilee to the Gulf of Aqaba, as monitored mainly by the Jordanian and Israeli seismic networks. The data of arrival times and polarities allowed relocation of earthquakes with a better azimuthal coverage and computation of focal mechanisms. This last step has been realized by inverting the polarities to determine a unique stress tensor for the region and the compatible focal mechanisms. Inversion with different subsets of the data set, based on tectonic regionalization, has also been performed to evaluate the impact of each cluster of earthquakes on the global solution. The population of focal mechanisms is clearly dominated by strike–slip events, with the notable exception of a cluster of earthquakes, south of the Dead Sea, which displays several normal focal mechanisms. This last cluster forces σ₁ to be vertical and σ₂ to be horizontal. A large number of fault planes, however, are close to the vertical, inhibiting the action of the vertical component of the stress tensor, and acting like under strike–slip stress regime. We observed a good agreement between the location of the earthquakes and the active faults, based on geological data. In addition, there is a good agreement between the fault plane solutions and the orientation of the active faults.     

Superimposed brittle structures in the late-orogenic extension of the Northern Apennine: results from the Carrara area (Alpi Apuane, NW Tuscany)

This paper presents data on the geometric and kinematic features of brittle faults in the Carrara area (Alpi Apuane, NW Tuscany, Italy), providing the first paleostress determination in a key region of the Northern Apennine. Meso-and microfault analyses point out the presence of brittle structures formed during a polyphase deformation and characterized by an older stage (DS1), with an interfering system of strike slip and normal faulting, followed by the DS2 stage developing normal faults locally showing apenninic trend (NW/SE). As a whole, the brittle structures are related to the late Pliocene–middle Pleistocene evolution of the Thyrrenian side of the Northern Apennine.     

Effects of the hydrothermal circulation on the strain field of the Campanian Plain (southern Italy)

Focal mechanisms of earthquakes and fault‐slip data have been collected to constrain the strain regime acting in the hydrothermal zone and surrounding areas of the Campanian Plain (southern Italy), a NW–SE elongated structural depression. The NW–SE striking faults bounding the depression move in response to a NE–SW striking regional extension. Within the depression, an extended hydrothermal circulation occurs related to the Vesuvius, Campi Flegrei and Ischia active volcanoes. In this zone, the strike of the extension is N–S. Results from a finite element model constrained by the collected data show that the presence of a lower rigidity zone due to the hydrothermal circulation may explain (a) the observed deflection of the direction of regional extension, and (b) why large magnitude earthquakes occur at the boundaries of the hydrothermal zone and not along the faults delimiting the structural depression.     

山西太古代——中生代构造应力场

本文采用历史分析与力学分析相结合的原则,研究山西太古代至中生代五次主要构造运动的形变特征、主应力方向及分布规律。中太古代晚期（阜平运动）以SSW向挤压为主;晚太古代末期（五台运动）、早元古代末期（吕粱运动）和侏罗纪（燕山运动）受SE—SEE向挤压;白垩纪（四川运动）以SSW向挤压为特征。五次构造运动的最大主压应力均呈近水平方向。     

南黄海盆地中部隆起构造特征及其成因机制

通过选取南黄海盆地中部隆起内部地震反射清晰、构造特征明显的典型地震剖面,开展精细的构造解释,系统梳理了南黄海盆地中部隆起的构造样式特征,识别出挤压(滑脱、高角度逆冲、对冲/背冲)、走滑(正花状、y字型)、伸展(铲式正断层)等多种构造组合样式.首次提出在中部隆起内部发育2条NW-SE向走滑断层.在此基础上,结合区域应力场特征和深部地球动力学背景,明确了中部隆起构造样式的发育期次、成因机制和构造演化历程.研究结果表明:(1)滑脱构造主要位于中部隆起北部,滑脱面位于志留系底部的泥页岩.滑脱构造应力机制来源于三叠纪末印支运动时期华北板块与下扬子板块之间的碰撞造山作用;(2)高角度逆冲主要位于中部隆起南部,其应力机制来源于早侏罗世燕山运动早期,古太平洋板块初始高速、低角度NW向俯冲;(3)走滑断层主要表现为具有压扭特征的正花状构造,位于中部隆起东南部、中西部,对应于早白垩世时期,古太平洋板块低角度俯冲由NW向转变为NNW向引起的左旋剪切作用,中国东部郯庐断裂在该时期亦表现为左旋剪切特征;(4)伸展正断表现为铲式正断层特征,发育在中部隆起南北边界,即在中部隆起与南黄海盆地南部坳陷、北部坳陷的接触部...     

Active faults pattern and interplay in the Azerbaijan region (NW Iran)

Northwest Iran is dominated by two main sets of active strike slip faults that accommodate oblique convergence between the Arabian and Iranian Plates. The best known are the right-lateral North-Tabriz, Qoshadagh, Maragheh and Zagros (Main Recent) strike slip Faults. This work reports that these dominant NW–SE to E–W striking faults are conjugate to smaller, NNE–SSW striking, left-lateral faults with minor dip slip component. All of these active faults displace Precambrian rock units, which suggests that they root in the crystalline basement of the NW Iranian microcontinent. Coulomb stress variance during co-seismic rupture along one of these faults may cause reactivation of the other faults. The minor set of left-lateral fault is therefore important to introduce in seismic risk assessment.     

A new tectonic model for Abu-Dabbab seismogenic zone (Eastern Desert,Egypt): evidence from field-structural,EMR and seismic data

Abu-Dabbab area is the most active seismic zone in the central Eastern Desert of Egypt, where seismic activities are daily recorded. The reported earthquakes are microearthquakes of local magnitudes (ML < 2.0). A spatial distribution of these microearthquakes shows that the earthquakes of the area follow an ENE–WSW trending pattern, which is nearly perpendicular to the Red Sea Rift. Focal mechanisms of different fault styles were recognized with dominant normal faulting (with a strike-slip component) events characterized by focal depths greater than 7 km and reverse ones of shallower focal depths. Several lines of evidence indicating that the brittle-ductile transition zone underlies the Abu-Dabbab area occurs at a relatively shallow depth (10–12 km) and it is acting as a low-angle normal shear zone (LANF). Field-structural, EMR and seismic data (this study) reveal that the maximum compressive stress (σ1) in the area is perturbed from the regional NW–SE direction to ENE–WSW orientation. This stress rotation is evidently akin to the reactivation of the crustal scale Najd Fault System (NFS), where such reactivation is attributed to the ongoing activity/opening of the Red Sea. Our tectonic model proposes that the continuous activity on the brittle-ductile transition zone including the LANF led to stress localization, which triggering a brittle deformation in the upper crustal-levels and associated shallow dipping thrusts. Such bimodal tectonic model suggests that the deep earthquakes are owing to the tectonic movement on the LANF (transtension), whereas the shallow earthquakes are related to a brittle deformation inside the fault blocks of the upper crust (transpression). Deformation creep along this zone didn’t permit continuous accumulation of strain and hence reduce the possible occurrence of large earthquakes.     

An Alleghenian orocline: the Asturian Arc,northwestern Spain

The Asturian Arc was produced in the Early Permian by a large E–W dextral strike–slip fault (North Iberian Megashear) which affected the Cantabrian and Palentian zones of the northeastern Iberian Massif. These two zones had previously been juxtaposed by an earlier Kasimovian NW–SE sinistral strike–slip fault (Covadonga Fault). The occurrence of multiple successive vertical fault sets in this area favoured its rotation around a vertical axis (mille-feuille effect). Along with other parallel faults, the Covadonga Fault became the western margin of a proto-Tethys marine basin, which was filled with turbidities and shallow coal-basin successions of Kasimovian and Gzhelian ages. The Covadonga Fault also displaced the West Asturian Leonese Zone to the northwest, dragging along part of the Cantabrian Zone (the Picos de Europa Unit) and emplacing a largely pelitic succession (Palentian Zone) in what would become the Asturian Arc core. The Picos de Europa Unit was later thrust over the Palentian Zone during clockwise rotation. In late Gzhelian time, two large E–W dextral strike–slip faults developed along the North Iberian Margin (North Iberian Megashear) and south of the Pyrenean Axial Zone (South Pyrenean Fault). The block south of the North Iberian Megashear and the South Pyrenean Fault was bent into a concave, E-facing shape prior to the Late Permian until both arms of the formerly NW–SE-trending Palaeozoic orogen became oriented E–W (in present-day coordinates). Arc rotation caused detachment in the upper crust of the Cantabrian Zone, and the basement Covadonga Fault was later resurrected along the original fault line as a clonic fault (the Ventaniella Fault) after the Arc was completed. Various oblique extensional NW–SE lineaments opened along the North Iberian Megashear due to dextral fault activity, during which numerous granitic bodies intruded and were later bent during arc formation. Palaeomagnetic data indicate that remagnetization episodes might be associated with thermal fluid circulation during faulting. Finally, it is concluded that the two types of late Palaeozoic–Early Permian orogenic evolution existed in the northeastern tip of the Iberian Massif: the first was a shear-and-thrust-dominated tectonic episode from the Late Devonian to the late Moscovian (Variscan Orogeny); it was followed by a fault-dominated, rotational tectonic episode from the early Kasimovian to the Middle Permian (Alleghenian Orogeny). The Alleghenian deformation was active throughout a broad E–W-directed shear zone between the North Iberian Megashear and the South Pyrenean Fault, which created the basement of the Pyrenean and Alpine belts. The southern European area may then be considered as having been built by dispersal of blocks previously separated by NW–SE sinistral megashears and faults of early Stephanian (Kasimovian) age, later cut by E–W Early Permian megashears, faults, and associated pull-apart basins.     

2014年5月云南盈江M_S5.6、M_S6.1地震发震构造分析

2014年5月云南省盈江县先后发生MS5.6、MS6.1地震,为确定它们的发震构造及其所反映的区域活动构造格局,笔者围绕该区开展了地震烈度调查、活动构造遥感解译、地质构造及构造地貌野外调查、震源机制解及余震分布资料分析等工作。调查与分析表明,两次地震的宏观震中均位于盈江县勐弄乡麻栗坡村附近,但发震断层明显不同。前者为NE走向左旋走滑的昔马—盘龙山断裂,后者为近SN向右旋走滑的苏典断裂。历史地震资料显示,盈江地区的地震活动多以5~6级的中-强震为主,并具有明显的群发性和沿SN向断层迁移的特征。在实皆断裂及滇西内弧带的共同作用下,腾冲地块内以大盈江断裂为界,北部主要发育近SN向右旋走滑断裂,南部则以NE向左旋走滑断裂为主,其中近SN向断层晚第四纪活动性更强。     

Tectonic structures across the East African Rift based on the source parameters of the 20 May 1990 M7.2 Sudan earthquake

Earthquakes in Kenya are common along the Kenya Rift Valley because of the slow divergent movement of the rift and hydrothermal processes in the geothermal fields. This implies slow but continuous radiation of seismic energy, which relieves stress in the subsurface rocks. On the contrary, the NW-SE trending rift/fault zones such as the Aswa-Nyangia fault zone and the Muglad-Anza-Lamu rift zone are the likely sites of major earthquakes in Kenya and the East African region. These rift/fault zones have been the sites of a number of strong earthquakes in the past such as the M _w = 7.2 southern Sudan earthquake of 20 May 1990 and aftershocks of M _w = 6.5 and 7.1 on 24 May 1990, the 1937 M _s = 6.1 earthquake north of Lake Turkana close to the Kenya-Ethiopian border, and the 1913 M _s = 6.0 Turkana earthquake, among others. Source parameters of the 20 May 1990 southern Sudan earthquake show that this earthquake consists of only one event on a fault having strike, dip, and rake of 315°, 84°, and ?3°. The fault plane is characterized by a left-lateral strike slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment M _o = 7.65 × 10¹⁹ Nm, and moment magnitude, M _w = 7.19 (?7.2). The fault rupture started 15 s earlier and lasted for 17 s along a fault plane having dimensions of ?60 km × 40 km. The average fault dislocation is 1.1 m, and the stress drop, ?σ, is 1.63 MPa. The distribution of historical earthquakes (M _w ≥ 5) from southern Sudan through central Kenya generally shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW–SE alignment of epicenters is characterized by earthquakes of local magnitude M _l ≤ 4.0, except the 1928 Subukia earthquake (M _s = 6.9) in central Kenya. This NW–SE alignment of epicenters is consistent with the trend of the Aswa-Nyangia Fault Zone, from southern Sudan through central Kenya and further southwards into the Indian Ocean. We therefore conclude that the NW–SE trending rift/fault zones are sites of strong earthquakes likely to pose the greatest earthquake hazard in Kenya and the East African region in general.     

THE NORTHWARD PROPAGATION OF THE ARC TECTONICS OF THE NORTHEAST PAMIR AND NORTHWEST TARIM BASIN

THE NORTHWARD PROPAGATION OF THE ARC TECTONICS OF THE NORTHEAST PAMIR AND NORTHWEST TARIM BASINThisworkispartoftheresearchproject“themechanismofJiashistrongearthquakeswarmandprediction ofearthquakeriskinthenortheastPamir     

Earthquake evidence for compressive stress in the southeast Australian crust

Earthquakes in SE Australia are usually caused by compressive stresses acting in the crust, and are associated with steeply dipping faults. Sometimes the faulting is predominantly strike‐slip, as for the Bowning earthquakes of 1977 and some of the Dalton/Gunning earthquakes; and sometimes it is high‐angle thrust faulting, as for the 1961 Robertson and 1973 Picton earthquakes. No surface expression of the faults associated with any recent earthquakes in SE Australia has been reported.

The directions of the pressure axes, from all the earthquakes for which focal mechanisms have been determined, do not form a consistent pattern. This suggests that the faulting associated with earthquakes in SE Australia is dominated by the geometry of pre‐existing crustal faults or zones of weakness.

In situ stress measurements have not been made near the epicentral areas of the larger recent earthquakes, because of the absence of competent, near‐surface rocks coupled to the crust. However, in the western part of the Lachlan Fold Belt the in situ stress results indicate that the maximum pressure axis is approximately E‐W. The evidence from the focal mechanisms does not preclude the persistence of this stress regime farther to the east, and a regional compressive stress in the crust with an azimuth of about 120° is consistent with most of the earthquake focal mechanisms and the in situ stress measurements throughout SE Australia.     

Compression along the northern Apennines? Evidence from the Mw 5.3 Monghidoro earthquake

In this paper, we present the seismological data recorded during the deployment of a dense three‐component seismic network installed a few hours after the 2003 Mw 5.3 Monghidoro earthquake, in northern Apennines. The main shock focal solutions derived from polarities distribution and body wave modelling of regional broadband data show a NE–SW striking reverse mechanism. Accurate relative locations of aftershocks and the inversion of focal mechanisms show that earthquakes occurred on a NW‐dipping backthrust within the Adria lithosphere under a NW‐trending horizontal compression. The observed compression is a secondary process possibly explained by differential motion within the Adriatic lithosphere. Fault geometry and kinematics is controlled by pre‐existing structures.     

河套地震带的震源机制类型时空分布特征

基于地质构造背景分析,收集2000年以来发生在河套地震带的M_L≥2.8级地震作为研究对象,综合运用基于P波初动的振幅比方法(APAS)和基于波形拟合的CAP反演方法求出256次地震事件的震源机制解。以断层节面滑动角作为判定指标,分区域给出了河套地震带的震源机制类型空间和时间分布图像,从断层滑动角度呈现河套地震带应力场时空变化过程。结果显示:临河盆地断层节面滑动角主要在水平±20°方向存在优势分布,走滑型特征显著;具体来讲,狼山-色尔腾山山前断裂带、临河断裂、乌拉山山前断裂等主要以纯走滑型地震为主,巴彦乌拉山断裂与磴口-本井断裂之间的区域多分布正走滑型地震。呼包盆地断层节面解虽然也呈现出走滑型为主的特征,但滑动角分布较为离散,优势分布方向不明显;呼包盆地西侧的包头至西山咀凸起一带表现出以走滑为主的小范围震源应力场特征,呼包盆地内部及东侧由于显著的区域垂直差异运动,正断层和逆冲型地震所占比例较大,震源机制类型整体呈现出与构造相依的分布特点。分析认为,2000年以来,河套地震带的应力场存在一定的时空非均匀性变化,研究结果更多表现了河套地震带的震源应力场变化过程,而研究资料时间不够长和震级不够大是引起这种应力场非均匀性暂态特征的主要因素。     

The contemporary state of stress and strain at the western pericline of the Greater Caucasus

We collected materials on geological indicators of paleostresses at the western pericline of the Greater Caucasus mega-anticlinorium and within the large transverse flexure-fault zone (Anapa and Dzhiginka zones) limiting this mega-anticlinorium. Based on the data, we reconstructed local stress states in different tectonic zones. The reconstructed local stresses showed a considerable variation of the orientations axes of principal stress near the two zones. In a site adjacent to the flexure-fault zone and located near the western pericline of the Greater Caucasus mega-anticlinorium, the detachment systems of northeastern (NE–SW) strike are determined. Additionally, field structural studies proved elongation in the northwestern (NW–SE) direction. This was also verified by the reconstruction of orientations of minimum compression stress axes (maximum deviatory tension) implemented by cataclastic analysis of structural–kinematic information on the movements of the fault planes (tectonic cracks and minor ruptures). We found a well-expressed multistage regime of the northwestern (NW–SE) tension within the limits of the Semisam anticline. Tension deformations (along the axis of the main folded structure) are manifested in structures of different scales; the values of relative elongation are defined for some of them. At the western pericline of the Greater Caucasus mega-anticlinorium, in the Miocene deposits, a north–south (NNW) compression regime with steep inclinations of axes of maximum compression stresses was identified. In the boundary zone between the Northwestern Caucasus and transverse Kerch–Taman trough, an alteration of the orientations of main axes of normal stresses was found. These changes led to the replacement of horizontal-compression and horizontalshear (with a NE-oriented compression) settings, which are predominant in the Caucasus, with settings of horizontal tension (with steep NNW-oriented compression axes).     

Geomorphic observations from southwestern terminus of Palghat Gap,south India and their tectonic implications

The region around Wadakkancheri, Trichur District, Kerala is known for microseismic activity, since 1989. Studies, subsequent to 2nd December 1994 (M =4.3) earthquake, identified a south dipping active fault (Desamangalam Fault) that may have influenced the course of Bharathapuzha River. The ongoing seismicity is concentrated on southeast of Wadakkancheri and the present study concentrated further south of Desamangalam Fault. The present study identifies the northwestern continuity of NW–SE trending Periyar lineament, which appears to have been segmented in the area. To identify the subtle landform modifications induced by ongoing tectonic adjustments, we focused on morphometric analysis. The NW–SE trending lineaments appear to be controlling the sinuosity of smaller rivers in the area, and most of the elongated drainage basins follow the same trend. The anomalies shown in conventional morphometric parameters, used for defining basins, are also closely associated with the NW–SE trending Periyar lineament/s. A number of brittle faults that appear to have been moved are consistent with the present stress regime and these are identified along the NW–SE trending lineaments. The current seismic activities also coincide with the zone of these lineaments as well as at the southeastern end of Periyar lineament. These observations suggest that the NW–SE trending Periyar lineaments/faults may be responding to the present N–S trending compressional stress regime and reflected as the subtle readjustments of the drainage configuration in the area.     

Geometry and kinematics of recent deformation in the Mondy–Tunka area (south-westernmost Baikal rift zone, Mongolia–Siberia)

We used satellite imagery and field data to investigate the south‐westernmost Baikal rift zone. We focus our study in the Mondy and Ikhe Ukhgun valleys, site of an Mw = 6.9 seismic event in 1950. Surface deformations are observed along the E–W‐trending Mondy strike‐slip fault and along the Ikhe Ukhgun thrust. The Mondy fault system is 80 km long and is composed of four segments 10–15 km long. These segments are characterized by subvertical planes with left‐lateral movements. The Ikhe Ukhgun thrust is 20 km long, dips 40° to the south and shows reverse movement with a left‐lateral component. These observations are consistent with the present‐day regional NNE–SSW compression and with the focal mechanism of the 1950 Mondy earthquake that was recently re‐evaluated. These features, like those observed in the Tunka basin, demonstrate a recent change of regional strain regime from transtension to transpression that we place before the Late Pleistocene.