Vertical Fault Research Articles

Impact of in situ stress and fault reactivation on seal integrity in the East Irish Sea Basin, UK

Abstract Despite having been affected by several stages of exhumation during the Cretaceous and Cenozoic, the contemporary stress state of the East Irish Sea Basin (EISB) is poorly characterised. As the basin is mature in terms of exploitation of hydrocarbons, future exploration beyond the conventional Sherwood Sandstone Group reservoir (Triassic) necessitates a greater understanding of the in situ stress field, while proposed natural gas storage and carbon sequestration schemes also require detailed stress field information. Using petroleum well data, the in situ stress field of the EISB has been characterised to assess the mechanical seal integrity. A strike-slip stress regime most-likely prevails in the basin, meaning the Maximum Horizontal Stress (SHmax) is the greatest of the principal stresses. Interpretation of stress orientation data suggests that SHmax is oriented 152° ± 12°, consistent with mean stress orientations across the wider region associated with plate boundary forces. Some degree of structural control appears to influence the orientation of SHmax, with orientations locally aligned sub-parallel to major Permo-Triassic basin-bounding faults. Fault reactivation risk is evaluated through modelling the pore pressure increase required to induce failure on pre-existing faults. Vertical faults striking 30° from SHmax are optimally-oriented to become reactivated under elevated pore pressure conditions. For any project relying on an element of fault seal for the containment of buoyant fluids at the average reservoir depth of 800 m, pore pressure increase should be less than 3.3 MPa to avoid reactivating pre-existing optimally-oriented faults. Higher pressure increases would be required to initiate reactivation of faults with other orientations. Vertical faults striking perpendicular to SHmax are least likely to become reactivated, and in the absence of halite, seal integrity would instead be limited by caprock strength and capillary-entry pressure. Major faults affecting the basin have been analysed for their slip tendency (ratio of shear to normal stress), which provides an indication of their susceptibility to become reactivated. Although the analysis is limited due to lack of an accurate 3D representation of the fault network, the results suggest that many of the fault orientations observed in the EISB exhibit high slip tendencies, including N–S striking faults to the north and west of the East Deemster Fault, where the SHmax orientation is NW–SE. Faults striking perpendicular to SHmax, such as the Lagman Fault, are least likely to become reactivated due to higher normal stresses that inhibit frictional sliding, while faults striking parallel or very close to SHmax also exhibit low slip tendency as they are not subjected to significant shear stresses.

Integrated study on the topographic and shallow subsurface expression of the Grote Brogel Fault at the boundary of the Roer Valley Graben, Belgium

The Grote Brogel Fault (GBF) is a major WNW-ESE striking normal fault in Belgium that diverges westward from the NW-SE striking western border fault system of the Roer Valley Graben. The GBF delimits the topographically higher Campine Block from the subsiding Roer Valley Graben, and is expressed in the Digital Terrain Model (DTM) by relief gradients or scarps. By integrating DTM, Electrical Resistivity Tomography (ERT), Cone Penetration Test (CPT) and borehole data, we studied the Quaternary activity of the GBF and its effects on local hydrogeology. In the shallow subsurface (<50m) underneath these scarps, fault splays of the GBF were interpreted on newly acquired ERT profiles at two investigation sites: one on the eastern section and the other on the western section, near the limit of the visible surface trace of the fault. Borehole and CPT data enabled stratigraphic interpretations of the ERT profiles and thereby allowed measuring vertical fault offsets at the base of Pleistocene fluvial deposits of up to 12m. Groundwater measurements in the boreholes and CPTs indicate that the GBF acts as a hydrologic boundary that prevents groundwater flow from the elevated footwall towards the hangingwall, resulting in hydraulic head differences of up to 12.7m. For the two investigation sites, the hydraulic head changes correlate with the relief gradient, which in turn correlates with the Quaternary vertical offset of the GBF. ERT profiles at the eastern site also revealed a local soft-linked stepover in the shallow subsurface, which affects groundwater levels in the different fault blocks, and illustrates the complex small-scale geometry of the GBF.

Geology and chemistry of Variscan-type pegmatite systems (SE Germany) – With special reference to structural and chemical pattern recognition of felsic mobile components in the crust

The geology of pegmatite systems encompasses lithology, shape, and structure while the chemistry of major and trace elements is indicative of the ore composition; both are the “pillars” of the CMS classification scheme (Chemical composition-Mineral assemblage-Structural geology) for barren and rare-metal pegmatites, including their granitic affiliates. The term Variscan-type has been coined to describe a style of formation linked to the ensialic orogens and a timebound mineralization sandwiched between the Caledonides and the Alpides. The primary formation covers the time from the Neoproterozoic through the Permian and ends with a hydrothermal phase waning eventually in the supergene alteration and is subdivided into three stages: (1) from diatectic to metatectic gneisses, (2) from metapegmatites, metamorphic pegmatoids to thrusting, (3) from the crust to the mantle and from barren to rare metal pegmatites. This evolution is characterized by a retrograde metamorphism from HP/MP to LP regimes. The tabular and stock-like pegmatitic, aplitic and granitic rocks in autochthonous and allochthonous units are grouped into 8 types (A–H) based on the above qualifiers of the CMS scheme. On a large scale, felsic mobilizates are accumulated by mimetic (facsimile) crystallization in anticlines with the most effective traps encountered where the directions of great circle plunges cut each other at almost right angle (stereonet analysis). The term “mobilizates” is used to describe felsic mobile components in the crust which migrated to a different extent from the site of their formation. On a small scale, where southward-dipping planar architectural elements are cut across by deep-seated lineaments the “temperature depression” of the retrograde system occurs and rare-metal pegmatites are located. This subhorizontal plane is correlated with a gently dipping Moho and vertical lineamentary fault zones with bulges of the Moho (chemical contour map analysis). Spider diagrams whose element contents are normalized to a reference paragneiss are categorized into 4 chemical patterns: (1) circular patterns (= metamorphic mobilizates, magmatic mobilizates), (2) necking-down patterns (= different degrees of fractionation), (3) lens-shaped patterns (= wall rock alteration), (4) stellate pattern (= different degrees of fractionation and mixing of fluids). The marker assemblages among the major elements are: Si-Fe-P: metamorphic to magmatic (sub)crustal mobilizates, K-Na-Al: metamorphic mobilizates, Ti-Mg: restites of metamorphic and magmatic mobilizates, Ca: remnant in the exocontact of pegmatitic systems, Mn: marker of depth-pressure The marker assemblages among the minor elements are: As-Bi: HT hydrothermal-metamorphic fluids, Cu-Ni-Mo: hydrothermal-deep-seated +(ultra)basic sources, U-Zn: hydrothermal-deep-seated sources, Pb: LT hydrothermal, Nb-Ba-Rb: pegmatitic fractionation-Ba (early)-Rb (late), Zr: restites of metamorphic mobilization + fractionation, REE: metamorphic mobilizates. The marker to discriminate hypogene and supergene kaolinization are: (1) hypogene (Ca- Mg out, Zn-Cu-Bi-Rb-Nb in), (2) supergene (Zr-Ti in).

Subduction Initiation With Vertical Lithospheric Heterogeneities and New Fault Formation

AbstractHow subduction initiates with mechanically unfavorable lithospheric heterogeneities is important and rarely studied. We investigate this with a geodynamic model for the Puysegur Incipient Subduction Zone (PISZ) south of New Zealand. The model incorporates a true free surface, elasto‐visco‐plastic rheology and phase changes. Our predictions fit the morphology of the Puysegur Trench and Ridge and the deformation history on the overriding plate. We show how a new thrust fault forms and evolves into a smooth subduction interface, and how a preexisting weak zone can become a vertical fault inboard of the thrust fault during subduction initiation, consistent with two‐fault system at PISZ. The model suggests that the PISZ may not yet be self‐sustaining. We propose that the Snares Zone (or Snares Trough) is caused by plate coupling differences between shallower and deeper parts, the tectonic sliver between two faults experiences strong rotation, and low‐density material accumulates beneath the Snares Zone.

Right Lateral Shear and Rotation in the Northeast of the Arabian-Iranian Collision Zone

Accommodation of continental convergence by crustal thickening and lateral transport is mainly featured as strike-slip faulting along the trends roughly orthogonal to the orientation of plate convergence. This style of faulting will affect seismicity of the involving areas which can be proved in low seismic zones by determining regional stress pattern using numerical methods. Accordingly, the stress distribution and deformation pattern of the South Sanandaj-Sirjan zone in the northeastern part of the Iranian-Arabian collision zone is investigated here using a three dimensional mechanical model. The modeled area is bounded between the Zagros thrust fault on the west and Dehshir-Baft fault in the east. The model is composed of three layers: the upper two layers represent the upper brittle and lower ductile crust of the collided continent and the lowest layer represents the lithospheric mantle. The upper crust behaves as an elastic material while the lower crust is considered as a non-Newtonian viscous fluid layer. The lithospheric mantle is taken as a low-viscosity material which is not allowed to move in any direction relative to the overlying layers. The Zagros thrust fault was treated with two different dip values saying 90o and 45o but Dehshir-Baft fault was modeled as a vertical fault and allowed to have a dextral movement regarding to the existing evidence. The driving mechanism applied to the western side of the model was chosen considering two different approaches including a kinematic approach (the Arabian-Eurasian convergence velocity; 35 mm/yr) and a dynamic approach (an external boundary force equal to 3.55E+17 N). The resulted stress field indicates an orogen-parallel component of right lateral shear along the Zagros fault implying a rotational deformation pattern within the modeled region that suggests a stress partitioning in the study area. The pattern also indicates a stress accumulation towards the south which could be a reason for the regional seismic quiescence between the two seismic Zagros thrust and Dehshir-Baft faults. Based on the present modeling results, it seems that high stress localization on the boundary faults can be a support of block structure approach or quasi-rigid blocks deformation within the study area. The resultant patterns of stress and displacement fields are generally totally comparable with plate boundary shear zones and have been proven by field data.

Finite element analysis of the propagation of Earth’s surface deformation as a consequence of normal dip-slip offshore fault rupture

This paper addresses the deformation response of seabed sands subject to underlying normal fault movement. This problem is relevant to the design of overlying offshore structures and subsea oil/gas pipelines connecting offshore platforms to the shoreline. The mechanism of the fault propagation in overlying seabed deposits is examined using 2D finite element modeling. Abaqus© is used as a numerical platform in modeling this complex problem, while accounting for nonlinear soil behavior with strain softening. Different dip angles and vertical fault displacements of up to 10% of the soil layer thickness were considered. The results include the effect of the relative density of the seabed sands and the soil layer thickness on the extent and magnitude of ground surface deformations. The required bedrock displacement/offset for the rupture to reach the surface and the length and location of the distorted zone are also investigated. Based on the parametric analyses and results presented in this paper, observations related to the potential magnitudes and extents of surface deformations for various conditions of seabed densities and thicknesses are provided. These would be of importance in determining likely effects of distortion/loading on pipelines and offshore structures crossing the fault zone.

Finite-element 3D modeling of stress patterns around a dipping fault

Stresses in a block around a dipping fracture simulating a damage zone of a fault are reconstructed by finite-element modeling. A fracture corresponding to a fault of different lengths, with its plane dipping at different angles, is assumed to follow a lithological interface and to experience either compression or shear. The stress associated with the destruction shows an asymmetrical pattern with different distances from the highest stress sites to the fault plane in the hanging and foot walls. As the dip angle decreases, the high-stress zone becomes wider in the hanging wall but its width changes negligibly in the foot wall. The length of the simulated fault and the deformation type affect only the magnitude of maximum stress, which remains asymmetrical relative to the fault plane. The Lh/Lf ratio, where Lh and Lf are the widths of high-stress zones in the hanging and foot walls of the fault, respectively, is inversely proportional to the fault plane dip. The arithmetic mean of this ratio over different fault lengths in fractures subject to compression changes from 0.29 at a dip of 80° to 1.67 at 30°. In the case of shift displacement, ratios are increasing to 1.2 and 2.94, respectively.Usually they consider vertical fault planes and symmetry in a damage zone of faults. Following that assumption may cause errors in reconstructions of stress and fault patterns in areas of complex structural setting. According geological data, we know the structures are different and asymmetric in hanging and foot walls of fault. Thus, it is important to quantify zones of that asymmetry. The modeling results have to be taken into account in studies of natural faults, especially for practical applications in seismic risk mapping, engineering geology, hydrogeology, and tectonics.

New images of the crustal structure beneath eastern Tibet from a high-density seismic array

An east–west trending, high-density seismic array was deployed along the eastern margin of the Tibetan Plateau to investigate its eastward expansion. The 160 km long array spans the Ruoergai basin, Minjiang fault, Minshan Mountains, Tazang fault, and West Qinling. The array included 330 short-period seismographs spaced at 500 m intervals, which recorded teleseismic 3-component waveforms over a one month period. P-wave receiver functions calculated from 35 teleseismic events provided an image of crustal structure. The results show a massive thrust nappe structure around the Minshan Mountains and beneath the Minjiang fault. We suggest that this nappe formed after the closure of the Paleo-Tethyan ocean. The resultant Triassic thrusting contributed to partial uplift of the eastern Ruoergai basin and the Minshan Mountains in middle-to-late Miocene time. Receiver function images show that the Tazang fault is a crustal-scale rupture cutting across the Moho. The western Tazang fault appears as a nearly vertical strike-slip fault accommodating left lateral shear at the terminus of the eastern Kunlun fault. After clockwise rotation from an approximate east–west orientation to a nearly north–south orientation, the eastern Tazang fault became a west-dipping thrust fault, which caused crustal thickening beneath the Minshan Mountains and the West Qinling. Our results suggest that late Cenozoic uplift of the eastern margin of the plateau is produced by eastward overthrusting and crustal shortening, processes that absorbed slip along the Tazang and Kunlun faults.

Genesis and evolution of the Watukosek fault system in the Lusi area (East Java)

Detailed analysis of two-dimensional seismic lines acquired in the NE Java basin has been performed to unravel the subsurface geology of the region around the Lusi mud eruption. This work revealed the existence of a system consisting of a complex set of faults, here called the Watukosek fault system, forming triangular deformation zones converging at the top of the early Miocene Carbonates. This system continues downwards with vertical individual fault segments, often bordering the steep margins of the carbonate platforms. The analysis of data includes the interpretation of seismic lines, regional structural data inferred from basement gravity maps and present-day main direction of stress. Results suggest that a possible rotation of stress direction from N40E-S40W to N-S occurred during the post-Miocene history of the Java back arc tectonic evolution. The Watukosek fault system was first generated as a tensional lineament during the E-W sinistral transpressive strain, which involved the basement. In this phase, synthetic and antithetic Riedel faults formed, the former controlling the NW-SE orientation of the structural highs represented by Oligo-Miocene carbonate platforms. As a consequence of the rotation of the main principal stress direction to a N-S direction, the Watukosek fault and similar parallel lineaments became sinistral Riedel shears, developing intense triangular deformation zones. Based on the stratigraphic position of gentle anticlinal deformations with axis corresponding to the N40E-S40W oriented triangular deformation zones, the transpressive strain linked to the N-S main stress compression occurred likely in the Late Pliocene-Early Pleistocene.The detailed examination of a) stratigraphy at the wells BJP-1 and Porong-1 as well as b) the seismo-stratigraphic architecture of the entire succession in the study area, allowed a new subsurface interpretation and revision of the stratigraphic units below Lusi. The thick Early Miocene Tuban Formation is found sandwiched between the coheval Upper Kalibeng Formation and the Early Miocene Carbonate of the Kujung Formation, which in turn overlain the older Ngimbang Formation.

Static elastic deformation in an orthotropic half-space with rigid boundary model due to non-uniform long strike slip fault

The solution of static elastic deformation of a homogeneous, orthotropic elastic uniform half-space with rigid boundary due to a non-uniform slip along a vertical strike-slip fault of infinite length and finite width has been studied. The results obtained here are the generalisation of the results for an isotropic medium having rigid boundary in the sense that medium of the present work is orthotropic with rigid boundary which is more realistic than isotropic and the results for an isotropic case can be derived from our results. The variations of displacement with distance from the fault due to various slip profiles have been studied to examine the effect of anisotropy on the deformation. Numerically it has been found that for parabolic slip profile, the displacement in magnitude for isotropic elastic medium is greater than that for an orthotropic elastic half-space, but, in case of linear slip, the displacements in magnitude for an orthotropic medium is greater than that for the isotropic medium.

Steel Sheet Piles – Applications and Elementary Design Issues

High-intensity housing having been carried out in town’s centres causes that many complex issues related to earthworks and foundations must be resolved. Project owners are required to ensure respective number of parking bays, which in turn demands 2-3 storeys of underground car parks. It is especially difficult to fulfil in dense buildings of old town areas where apart from engineering problems, very stringent requirements of heritage conservator supervision are also raised. The problems with ensuring stability of excavation sidewalls need to be, at the same time, dealt with analysis of foundations of neighbouring structures, and possible strengthening them at the stages of installing the excavation protection walls, progressing the excavations and constructing basement storeys. A separate problem refers to necessity of constructing underground storeys below the level of local groundwater. This requires long-term lowering of water table inside excavation while at possibly limited intervention in hydrological regime beyond the project in progress. In river valleys such “hoarding off” the excavation and cutting off groundwater leads to temporary or permanent disturbances of groundwater run-off and local swellings. Traditional way to protect vertical fault and simultaneously to cut-off groundwater inflow consists in application of steel sheet pilings. They enable to construct monolithic reinforced concrete structures of underground storeys thus ensuring both their tightness and high rigidity of foundation. Depending on situation, steel sheet pilings can be in retrieving or staying-in-place versions. This study deals with some selected aspects of engineering design and fabrication of sheet piling for deep excavations and underground parts of buildings.

The role of post-collisional strike-slip tectonics in the geological evolution of the late Neoproterozoic volcano-sedimentary Guaratubinha Basin, southern Brazil

The Guaratubinha Basin is a late Neoproterozoic volcano-sedimentary basin included in the transitional-stage basins of the South American Platform. The aim of this study is to investigate its tectonic evolution through a detailed structural analysis based on remote sensing and field data. The structural and aerogeophysics data indicate that at least three major deformational events affected the basin. Event E1 caused the activation of the two main basin-bounding fault zones, the Guaratubinha Master Fault and the Guaricana Shear Zone. These structures, oriented N20-45E, are associated with well-defined right-lateral to oblique vertical faults, conjugate normal faults and vertical flow structures. Progressive transtensional deformation along the two main fault systems was the main mechanism for basin formation and the deposition of thick coarse-grained deposits close to basin-borders. The continuous opening of the basin provided intense intermediate and acid magmatism as well as deposition of volcaniclastic sediments. Event E2 characterizes generalized compression, recorded as minor thrust faults with tectonic transport toward the northwest and left-lateral activation of the NNE-SSW Palmital Shear Zone. Event E3 is related to the Mesozoic tectonism associated with the South Atlantic opening, which generated diabase dykes and predominantly right-lateral strike-slip faults oriented N10-50W. Its rhomboidal geometry with long axis parallel to major Precambrian shear zones, the main presence of high-angle, strike-slip or oblique faults, the asymmetric distribution of geological units and field evidence for concomitant Neoproterozoic magmatism and strike-slip movements are consistent with pull-apart basins reported in the literature.

Active transpressional tectonics in the Andean forearc of southern Peru quantified by10Be surface exposure dating of an active fault scarp

AbstractOur understanding of the style and rate of Quaternary tectonic deformation in the forearc of the Central Andes is hampered by a lack of field observations and constraints on neotectonic structures. Here we present a detailed analysis of the Purgatorio fault, a recently recognized active fault located in the forearc of southern Peru. Based on field and remote sensing analysis (Pléiades DEM), we define the Purgatorio fault as a subvertical structure trending NW‐SE to W‐E along its 60 km length, connecting, on its eastern end, to the crustal Incapuquio Fault System. The Purgatorio fault accommodates right‐lateral transpressional deformation, as shown by the numerous lateral and vertical plurimetric offsets recorded along strike. In particular, scarp with a 5 m cumulative throw is preserved and displays cobbles that are cut and covered by slickensides. Cosmogenic radionuclide exposure dating (10Be) of quartzite cobbles along the vertical fault scarp yields young exposure ages that can be bracketed between 0 to 6 ka, depending on the inheritance model that is applied. Our preferred scenario, which takes in account our geomorphic observations, implies at least two distinct rupture events, each associated with ~3 and ~2 m of vertical offset. These two events plausibly occurred during the last thousand years. Nevertheless, an interpretation invoking more tectonic events along the fault cannot be ruled out. This work affirms crustal deformation along active faults in the Andean forearc of southern Peru during the last thousand years.

1D layered velocity models and microseismic event locations: synthetic examples for a case with a single linear receiver array

We discuss various aspects of 1D velocity-model building for application to microseismic data analysis. We generate simple synthetic example data using a widely used single linear array geometry. The synthetic data contain 30 sources with known locations for a reference model based on previous studies of the Barnett shale. We investigate several key factors that should be considered, including selection of the calibration technique, inclusion of a priori information such as lateral heterogeneity and parameter ranges, and choice of algorithm for travel time computations. For the source–receiver geometry considered here, hypocenter location errors (±6 m in X and ±12 m in Z) can result from differently calibrated models only and without including the errors in picked arrival times and polarization estimates. We find that the errors in hypocenter locations are reduced (±3 m in X and ±6 m in Z) when a model calibrated with multiple shots simultaneously is used. Using four different models (vertical fault, dipping layers, channels, and these effects combined), we demonstrate that systematic errors in hypocenter locations can result when a 1D layered model is used in lieu of a laterally heterogeneous subsurface. Finally, we show that event locations from a velocity model calibrated using direct-arrival times are more stable than from a model calibrated with first-arrival times.

A finite difference method for off-fault plasticity throughout the earthquake cycle

We have developed an efficient computational framework for simulating multiple earthquake cycles with off-fault plasticity. The method is developed for the classical antiplane problem of a vertical strike-slip fault governed by rate-and-state friction, with inertial effects captured through the radiation-damping approximation. Both rate-independent plasticity and viscoplasticity are considered, where stresses are constrained by a Drucker–Prager yield condition. The off-fault volume is discretized using finite differences and tectonic loading is imposed by displacing the remote side boundaries at a constant rate. Time-stepping combines an adaptive Runge–Kutta method with an incremental solution process which makes use of an elastoplastic tangent stiffness tensor and the return-mapping algorithm. Solutions are verified by convergence tests and comparison to a finite element solution. We quantify how viscosity, isotropic hardening, and cohesion affect the magnitude and off-fault extent of plastic strain that develops over many ruptures. If hardening is included, plastic strain saturates after the first event and the response during subsequent ruptures is effectively elastic. For viscoplasticity without hardening, however, successive ruptures continue to generate additional plastic strain. In all cases, coseismic slip in the shallow sub-surface is diminished compared to slip accumulated at depth during interseismic loading. The evolution of this slip deficit with each subsequent event, however, is dictated by the plasticity model. Integration of the off-fault plastic strain from the viscoplastic model reveals that a significant amount of tectonic offset is accommodated by inelastic deformation ( ∼ 0.1 m per rupture, or ∼ 10% of the tectonic deformation budget).

Controls on fracture network characteristics of the middle member of the Bakken Formation, Elm Coulee field, Williston Basin, United States

Although most of the production from the Bakken Formation in the Elm Coulee field of the Williston Basin is interpreted to come from matrix permeability, plots of reservoir performance indicators, such as estimated ultimate recovery (EUR), show areas of anomalously high production believed to be caused by secondary (fracture) permeability. Recent studies have alluded to basement tectonics controlling the structure of the overlying sedimentary cover and thus the reservoir performance in the Elm Coulee field, but the exact controlling mechanism has not been demonstrated. This paper describes an antiformal structure overlying a zone of deformation in a seismic data set. The zone of deformation consists of small vertical offset faults displaying evidence of wrench faulting and involves the Bakken petroleum system and deeper units. An analog model is used to guide interpolation and extrapolation of the faults when they are traced through the data cube. With the help of analog models, it is established that the antiformal structure and the associated deformation zone evolved over a stepover in a northwest-southeast–oriented left-lateral–strike-slip system in the rigid basement. This strike-slip system may be secondary to the regional left-lateral Brockton-Froid fault zone, which lies to the north of the study area. The fracture network in the Elm Coulee field is likely to be enhanced around such antiformal structures, providing a plausible explanation for the locations of the EUR sweet spots. Thus, it is proposed that locating such deformation zones forms a potential new exploration trend in this field.

Mapas de anomalias gravimétricas da Cidade Universitária Armando Salles de Oliveira – USP/SP

A Cidade Universitária Armando de Salles Oliveira, da Universidade de São Paulo (CUASO –USP/SP), é considerada um laboratório natural de pesquisas em Ciências da Terra. Ao longo de mais de duas décadas, dados gravimétricos foram obtidos como forma de treinamento de alunos e técnicos, formando um conjunto de 787 pontos distribuídos em 8 km2. O mapa de anomalia ar-livre apresenta forte correlação com a topografia, enquanto o mapa de anomalia Bouguer não apresenta correlação com a topografia e a geologia. Este mapa permitiu identificar um gradiente EW centrado na área, revelando uma estrutura de subsuperfície, modelada, neste mesmo estudo, como uma falha praticamente vertical, com atitude 090/80 N e cerca de 50 m de deslocamento vertical.

Active tectonic deformation along reactivated faults in Binta basin in Kumaun Himalaya of north India: Inferences from tectono-geomorphic evaluation

We discuss the tectono-geomorphic evaluation of the Binta basin (Lesser Himalaya, India) through quantitative and qualitative morphotectonic analyses at the watershed scale. The basin, drained by the Gagas River and its tributaries, is situated in the zone of the active North Almora Thrust (NAT). Developments of deeply incised V-shaped valleys indicate increased incision capability of streams in response to steepening of hillslope gradients due to neotectonic activity. Prominent features, e.g., off-setting of the NAT by later formed subsidiary faults, deflection of streams, vertical fault scarps, formation of palaeolakes, river ponding and development of fluvial terraces were likely responsible for reformation of the landscape and drainage system due to tectonic perturbations. The morphometric parameters using digital terrain data in the GIS technology and focusing on hydrography (stream length-gradient index, ratio of valley floor width to valley height, transverse topographic symmetry factor and topography (local relief and relief anomaly) verify the rejuvenation of landscape. Anomalously high and low values of stream length-gradient indices of main tributary streams associated with faults and multiple knickpoints along the channel profiles may have been associated with deformational events. Our observations on the tectonic instability of the faulted basin support the existing GPS based studies indicating the maximum crustal shortening and strain rate with a maximum aerial strain of 6-8 x 10(-7) strain/year and deformation of similar to 15 mm/year immediately north of the NAT, and proving that this tectonically active zone is vulnerable for the future earthquakes.

THE 2014 MW 6.9 NORTH AEGEAN TROUGH (NAT) EARTHQUAKE: SEISMOLOGICAL AND GEODETIC EVIDENCE

A strong earthquake (Mw 6.9) on 24 May 2014 ruptured the North Aegean Trough (NAT) in Greece, west of the North Anatolian Fault Zone (NAFZ). In order to provide unbiased constrains of the rupture process and fault geometry of the earthquake, seismological and geodetic data were analyzed independently. First, based on teleseismic long-period P- and SH- waveforms a point-source solution yielded dominantly right-lateral strike-slip faulting mechanism. Furthermore, finite fault inversion of broad-band data revealed the slip history of the earthquake. Second, GPS slip vectors derived from 11 permanent GPS stations uniformly distributed around the meizoseismal area of the earthquake indicated significant horizontal coseismic slip. Inversion of GPS-derived displacements on the basis of Okada model and using the new TOPological INVersion (TOPINV) algorithm permitted to model a vertical strike slip fault, consistent with that derived from seismological data. Obtained results are consistent with the NAT structure and constrain well the fault geometry and the dynamics of the 2014 earthquake. The latter seems to fill a gap in seismicity along the NAT in the last 50 years, but seems not to have a direct relationship with the sequence of recent faulting farther east, along the NAFZ.

Within-Event and Between-Events Ground Motion Variability from Earthquake Rupture Scenarios

Measurement of ground motion variability is essential to estimate seismic hazard. Over-estimation of variability can lead to extremely high annual hazard estimates of ground motion exceedance. We explore different parameters that affect the variability of ground motion such as the spatial correlations of kinematic rupture parameters on a finite fault and the corner frequency of the moment-rate spectra. To quantify the variability of ground motion, we simulate kinematic rupture scenarios on several vertical strike-slip faults and compute ground motion using the representation theorem. In particular, for the entire suite of rupture scenarios, we quantify the within-event and the between-events ground motion variability of peak ground acceleration (PGA) and response spectra at several periods, at 40 stations—all approximately at an equal distance of 20 and 50 km from the fault. Both within-event and between-events ground motion variability increase when the slip correlation length on the fault increases. The probability density functions of ground motion tend to truncate at a finite value when the correlation length of slip decreases on the fault, therefore, we do not observe any long-tail distribution of peak ground acceleration when performing several rupture simulations for small correlation lengths. Finally, for a correlation length of 6 km, the within-event and between-events PGA log-normal standard deviations are 0.58 and 0.19, respectively, values slightly smaller than those reported by Boore et al. (Earthq Spectra, 30(3):1057–1085, 2014). The between-events standard deviation is consistently smaller than the within-event for all correlations lengths, a feature that agrees with recent ground motion prediction equations.