Extensional neotectonics around the bend of the Western/Central Alps: an overview

The Western Alps’ active tectonics is characterized by ongoing widespread extension in the highest parts of the belt and transpressive/compressive tectonics along its borders. We examine these contrasting tectonic regimes using a multidisciplinary approach including seismotectonics, numerical modeling, GPS, morphotectonics, fieldwork, and brittle deformation analysis. Extension appears to be the dominant process in the present-day tectonic activity in the Western Alps, affecting its internal areas all along the arc. Shortening, in contrast, is limited to small areas located along at the outer borders of the chain. Strike-slip is observed throughout the Alpine realm and in the foreland. The stress-orientation pattern is radial for σ3 in the inner, extensional zones, and for σ1 in the outer, transcurrent/tranpressional ones. Extensional areas can be correlated with the parts of the belt with the thickest crust. Quantification of seismic strain in tectonically homogeneous areas shows that only 10–20% of the geodesy-documented deformation can be explained by the Alpine seismicity. We propose that, Alpine active tectonics are ruled by isostasy/buoyancy forces rather than the ongoing shortening along the Alpine Europe/Adria collision zone. This interpretation is corroborated by numerical modeling. The Neogene extensional structures in the Alps formed under increasingly brittle conditions. A synthesis of paleostress tensors for the internal parts of the West-Alpine Arc documents major orogen-parallel extension with a continuous change in σ3 directions from ENE–WSW in the Simplon area, to N–S in the Vanoise area and to NNW–SSE in the Briançon area. Minor orogen-perpendicular extension increases from N to S. This second signal correlates with the present-day geodynamics as revealed by focal-plane mechanisms analysis. The orogen-parallel extension could be related to the opening of the Ligurian Sea during the Early-Middle Miocene and to compression/rotation of the Adriatic indenter inducing lateral extrusion.     

Subduction zone geometry and stress patterns in the tyrrhenian sea

Joint hypocenter determination is performed for intermediate and deep earthquakes of the Tyrrhenian Sea region.This analysis allowed us to obtain a catalogue of 70 well-located events in this peculiar Benioff zone, which is characterized by quite low seismic activity, compared to the Pacific deep earthquake regions. The method used for the analysis is that ofFrohlich (1979), a variant of the successive approximation technique, which allows use of a great number of events and stations but saves computer memory. The results show a spoon-shaped Benioff zone, dipping NW in the Tyrrhenian Sea to 500km depth. 32 reliable fault-plane solutions have been determined using these new earthquake locations, confirming the predominance of down-dip compression in the central part of the slab and more complex motion along the borders of the zone, as previously suggested byGasparini et al. (1982).     

Seismotectonic Model for 1966 Xingtai Earthquake Swarm——Nucleation of Newly Generated Fault or Friction Stick-slip Along a Preexisting Active Fault

Volume 15,Number 275INTRODUCTIONThe Xln抄al eafthquake swarrn occurred In Shulu basin In the western part ofNorih China PlainIncludes Malan M6．8 eafthquake In hn纽M Countyon Marchs,1966,Don驯aug M6．7 and M7．2earthquakes In Ningln County on March 22,1966,Balchekou M6．2 ealthquake In Shulu County onMarch 26,1966,and啊…ac M6．0 earthquake in Juhi County on March 29 1966．They aredlstnbutedin an elliptical area in N35”E direction with long axisd劝out 110 km and short axis o…     

太原－临汾地区地震构造反潜在震源区划分

在搜集了目前太原－临汾地区的地震构造特征和地震活动资料基础上，分析总结了该地区的大震和强震发生的地质构造、地震和地球物理场等条件。根据地震的发生条件，以及近几年来对潜在震源区划分的一些新认识，对太原－临汾地区的潜在震源区作了重新划分，并将结果与《中国地震烈度区划图（１９９０）》中的相应方案作了对比分析。     

核电厂选址中的近场地震构造调查

讨论了在核电厂选址阶段近场范围内地震构造调查的主要内容和方法,包括近场区地质构造环境及第四纪构造活动分区、断裂活动性鉴定、深部构造环境以及地震发震构造的确定等,并对其中一些关键问题作了较为详细的论述     

Fault-generated mountain fronts in the central apennines (Central Italy): Geomorphological features and seismotectonic implications

The morphotectonic framework of the Central Apennines is given by faulted blocks bounded by normal faults, mostly trending NW–SE, NNW–SSE and NE–SW, which cut previous compressive structures. Such a structural setting is consistent with the focal mechanisms of the earthquakes which often occur in this area. In this paper, three lithologically different normal fault-generated mountain fronts are analysed in order to assess the relations between their geomorphic features and active tectonics. They border the Norcia depression (Sibillini Mts, Umbria), the Amatrice–Campotosto plateau (Laga Mts, Lazio) and the Fucino basin (Marsica Mts, Abruzzi). The Norcia depression is bounded by a N20°W trending normal fault to the east and by a parallel antithetic fault to the west. The main fault has a 1000 m throw and gives rise to a wide fault escarpment, characterized by: (1) sharp slope breaks due to low angle gravity faults; (2) important paleolandslides; and (3) several fault scarplets on the piedmont belt affecting Quaternary deposits. The Amatrice–Campotosto plateau is delimited by the western slope of Mt Gorzano which runs along a N20°W trending normal fault having a 1500m throw. Minor parallel faults dislocate Quaternary landforms. Large-scale massmovements also occur here. The Fucino basin was struck by the 1915 Avezzano earthquake (I=XI MCS) which produced extensive surface faulting along two parallel NW trending normal fault escarpments on the eastern border of the basin. There is paleoseismic evidence including buried gravity graben in Late Glacial gravels and tectonic dip-slip striations on Holocene calcitic crusts covering bedrock normal fault planes. These data suggest that active extensional tectonics plays a major role in the slope morphogenesis of the Central Apennines and they indicate the importance of geomorphic analysis in seismic zonation of this area.     

Neotectonics of the Dinarides–Pannonian Basin transition and possible earthquake sources in the Banja Luka epicentral area

This study provides evidence for post-5 Ma shortening in the transition area between the Dinarides fold-and-thrust belt and the Pannonian Basin and reviews possible earthquake sources for the Banja Luka epicentral area (northern Bosnia and Herzegovina) where the strongest instrumentally recorded earthquake (M_L 6.4) occurred on 27 October 1969. Geological, geomorphological and reflection seismic data provide evidence for a contractional reactivation of Late Palaeogene to Middle Miocene normal faults at slip rates below 0.1 mm/a. This reactivation postdates deposition of the youngest sediments in the Pannonian Basin of Pontian age (c. 5 Ma). Fault plane solutions for the main 1969 Banja Luka earthquake (M_L 6.4) and its largest foreshock (M_L 6.0) indicate reverse faulting along ESE–WNW-striking nodal planes and generally N–S trending pressure axes. The spatial distribution of epicentres and focal depths, analyses of the macroseismic field and fault-plane solutions for several smaller events suggest on-going shortening in the internal Dinarides. Seismic deformation of the upper crust is also associated with strike-slip faults, likely related to the NE–SW trending, sinistral Banja Luka fault. Possibly, this fault transfers contraction between adjacent segments of the Dinarides thrust system. The study area represents the seismically most active region of the Dinarides apart from the Adriatic Sea coast and the bend zone around Zagreb. We propose that on-going thrusting in the internal Dinarides thrust system takes up a portion of the current Adria–Europe convergence.     

A ten year Moment Tensor database for Western Greece

Moment Tensors (MTs) provide important information for seismotectonic, stress distribution and source studies. It is also important as a real time or near real time information in shakemaps, tsunami warning, and stress transfer. Therefore a reliable and rapid MT computation is a routine task for modern seismic networks with broadband sensors and real-time digital telemetry. In this paper we present the database of Moment Tensor solutions computed during the last ten years in Western Greece by the University of Patras, Seismological Laboratory (UPSL). The data from UPSL broad band network were used together with the ISOLA Moment Tensor inversion package for routine MT calculation. The procedures followed and the comparison of UPSL derived solutions with the ones provided by other agencies for Western Greece region are presented as well. The Moment Tensor database includes solutions for events in the magnitude range 2.8–6.8 and provides a unique insight into the faulting characteristics of Western Greece. Moreover it paves the way for detailed studies of stress tensor and stress transfer. The weak events' Moment Tensor included in UPSL's database are important for the comprehension of local seismotectonics and reveal the role of minor faults, which may be critical in seismic hazard estimation.     

首都圈地区的地壳厚度及泊松比

利用2002~2003年中国地震局地质研究所台阵实验室以唐山大震区为中心布设的40个流动宽频带地震台站和首都圈数字台网的107个固定台站的远震数据,采用接收函数叠加搜索方法测定了首都圈地区地壳厚度和平均泊松比.综合利用首都圈数字地震台网的宽频带和短周期台站,以及流动地震台阵的观测数据,使我们的结果具有较前人更好的空间分辨率,为我们研究首都圈地壳的变形及其与地震的关系提供了重要的数据平台.结果表明: (1)首都圈地区地壳厚度和泊松比具有明显的横向分块特征,并与断层切割的地质块体有较好的相关性,地壳厚度变化形成了由涵盖北京—三河—唐山的NE向地壳厚度变化过渡带与张家口—北京—天津的NW向壳幔界面凹陷带构成的交汇构造,后者与所谓的张家口—渤海地震带基本吻合,而且本文给出的深部构造背景与首都圈地区的NE向和NW向地震带的交汇特征相吻合;(2)首都圈地区地壳厚度具有较大的差异,区域内地壳厚度的变化达15 km.其中,台站SZJ下方地壳厚度达到43.8 km,而台站BDH下方地壳厚度仅为28.8 km,总体上研究区西北侧的张家口—怀来地区的地壳厚度较大(~40 km),而唐山以东地区地壳较薄(28~32 km);(3) 研究区地壳的平均泊松比值为0.26左右,其最大值偏离泊松介质（σ=0.25）21%以上,而最小值偏离标准泊松比值9.6%,北京周边地区被高泊松比的介质环绕,而唐山东侧为低泊松比介质,地壳泊松比的分布特征反映了华北克拉通裂解过程中地幔物质的侵入;(4)研究区中强地震大多发生在地壳厚度和泊松比变化的陡变带,且偏于低泊松比的一侧,首都圈地震的成因仅考虑由于板块作用引起的水平应力场是不够的,有必要充分重视由于上地幔变形引起的地壳垂直变形和上地幔物质侵入造成的地壳变形运动与热效应.