North Dipping Research Articles

Geomorphic evidence for the geometry and slip rate of a young, low-angle thrust fault: Implications for hazard assessment and fault interaction in complex tectonic environments

We present surface evidence and displacement rates for a young, active, low-angle (∼20°) reverse thrust fault in close proximity to major population centers in southern California (USA), the Southern San Cayetano fault (SSCF). Active faulting along the northern flank of the Santa Clara River Valley displaces young landforms, such as late Quaternary river terraces and alluvial fans. Geomorphic strain markers are examined using field mapping, high-resolution lidar topographic data, 10Be surface exposure dating, and subsurface well data to provide evidence for a young, active SSCF along the northern flank of the Santa Clara River Valley. Displacement rates for the SSCF are calculated over 103–104 yr timescales with maximum slip rates for the central SSCF of 1.9+1.0−0.5 mm yr−1 between ∼19–7 ka and minimum slip rates of 1.3+0.5−0.3 mm yr−1 since ∼7 ka. Uplift rates for the central SSCF have not varied significantly over the last ∼58 ka, with a maximum value of 1.7+0.9−0.6 mm yr−1 for the interval ∼58–19 ka, and a minimum value of 1.2±0.3 mm yr−1 since ∼7 ka. The SSCF is interpreted as a young, active structure with onset of activity at some time after ∼58 ka. The geometry for the SSCF presented here, with a ∼20° north dip in the subsurface, is the first interpretation of the SSCF based on geological field data. Our new interpretation is significantly different from the previously proposed model-derived geometry, which dips more steeply at 45–60° and intersects the surface in the middle of the Santa Clara River Valley. We suggest that the SSCF may rupture in tandem with the main San Cayetano fault. Additionally, the SSCF could potentially act as a rupture pathway between the Ventura and San Cayetano faults in large-magnitude, multi-fault earthquakes in southern California. However, given structural complexities, including significant changes in dip and varying Holocene displacement rates along strike, further work is required to examine the possible mechanism, likelihood, and frequency of potential through-going ruptures between the Ventura and San Cayetano faults. Confirmation of the SSCF in a previously well-studied area, such as southern California, demonstrates that identification of young faults is critical for accurate seismic hazard assessment. We suggest that many young, active faults remain undetected in other structurally complex and tectonically active regions globally, and that significant seismic hazards can be overlooked.

New geoelectrical characterization of a continental collision zone in the Central – Eastern Pyrenees: Constraints from 3-D joint inversion of electromagnetic data

Continent-continent collisions are responsible for the formation of large mountain ranges like the Himalayas and the Alps and play a primary role in the development of the continents. The continental collision between the Iberian and European plates during the Alpine Orogeny resulted in the formation of the Pyrenees. In this study new electromagnetic data from the Eastern Pyrenees were complemented with older data from the Central Pyrenees, constraining the physical and geological processes at the eastern end of the Pyrenean mountain range. The electrical resistivity distribution beneath the Central-Eastern Pyrenees was characterized by means of three-dimensional (3-D) joint inversion of three electromagnetic datasets: (1) the MT impedance tensor (Z), (2) the geomagnetic transfer function (T), and (3) the inter-station horizontal magnetic transfer function (H). The main finding was the non-continuity to the east of the major conductive anomaly observed previously beneath the Central and West-Central Pyrenees related to partial melting of the Iberian subducted lower crust. Lower amounts of water (related to the presence of muscovite and biotite) in the subducted lower crust beneath the Eastern Pyrenees were suggested to explain the lack of partial melting in this part of the mountain range. The electrical resistivity model also revealed higher electrical resistivity values for the lithospheric mantle beneath the Eastern Pyrenees than beneath the Central Pyrenees, thus supporting the hypothesis of an heterogeneous Iberian plate inherited from the Variscan Orogeny. A less clear signature was the lateral variation along the strike direction of the lithosphere-asthenosphere boundary beneath the Eastern Pyrenees (relatively flat, between 110 km and 140 km depth) and the Central Pyrenees (north dipping, between 80 km and 120 km depth beneath the Iberian Plate and between 110 km and 160 km depth beneath the European plate), supporting the hypothesis of a missing lithospheric root beneath the Eastern Pyrenees.

One or Two Early Cretaceous Arc Systems in the Lhasa Terrane, Southern Tibet

AbstractSpatial and temporal variations of arc‐related magmatism are key to determining the subduction polarity of incompletely preserved arc systems. Petrographic, geochronological, geochemical, and isotope data of Early Cretaceous volcanic rocks from the northern Lhasa Terrane around Yanhu indicate south dipping subduction of the Bangong Tethys. Two distinct calc‐alkaline magmatic successions are recognized: older medium‐K basalts and Mg‐rich andesites (131–116 Ma), and younger high‐K basalts and trachyandesites (116–110 Ma). The medium‐K basalts exhibit a typical arc signature, whereas the medium‐K andesites show higher MgO contents relative to arc andesites. The medium‐K series are interpreted as partial melting of a metasomatized mantle wedge source at lower pressure and greater water activity in generating the Mg‐rich andesites. The high‐K series are characterized by enrichments in highly incompatible elements and are considered as low‐degree partial melting of asthenosphere mantle source that was previously metasomatized. All samples show arc‐related signatures, which indicate the development of what we term the Baingoin‐Yanhu arc in the northern Lhasa Terrane. This observation, in combination with the distribution of Early Cretaceous arc magmatism across the Lhasa Terrane, which prior to subsequent deformation had a width of at least 600 km, requires the existence of two arc systems flanking the Lhasa Terrane and related to opposed oceanic plate subduction: north dipping subduction of the Neo‐Tethys and south dipping subduction of the Bangong Tethys. Compositional change from medium‐K to high‐K calc‐alkaline volcanism around Yanhu records changing mantle geodynamics, which we infer to reflect rollback and breakoff of the south dipping Bangong Tethyan slab.

A Uniform Orogen‐parallel Extension System of the Shear Zones in the Tongbai‐Dabie Orogenic Belt, Central China

AbstractLarge‐scale magmatism affected the Tongbai‐Dabie orogenic belt during post‐collisional lateral tectonic extension in the Cretaceous, which was suggested to account for the widespread deformation and migmatization in the Tongbai‐Dabie complexes. However, it cannot explain the most deformations in the shear zones. The northwest‐southeast shear zones are developed around or wrapped the Tongbai‐Dabie complexes. They play an important role for the interpretation of the tectonic evolution of the Tongbai‐Dabie orogenic belt. By a systematically observation and description of the geometry and kinematics of these shear zones, we found that the shear zones to the north dip NE and have a uniform sinistral shear sense, the shear zone to the south dips SW and has a uniform dextral shear sense, and the shear zones at the core are sub‐horizontal and have a uniform top‐to‐NW sense of shear. Combining with the comparison of previous and our geochronological studies, we interpret these associations as indicating that these shear zones were originally a single, more flat‐lying and sub‐horizontal shear zone with a uniform top‐to‐NW shear sense before the folding‐doming of the Tongbai‐Dabie complexes and suggest that the Tongbai‐Dabie orogenic belt experienced a uniform top‐to‐NW orogen‐parallel extension in the ductile lithosphere before the widespread magmatism in the Cretaceous.

Crustal Structure and Evolution of the Eastern Himalayan Plate Boundary System, Northeast India

AbstractWe use data from 24 broadband seismographs located south of the Eastern Himalayan plate boundary system to investigate the crustal structure beneath Northeast India. P wave receiver function analysis reveals felsic continental crust beneath the Brahmaputra Valley, Shillong Plateau and Mikir Hills, and mafic thinned passive margin transitional crust (basement layer) beneath the Bengal Basin. Within the continental crust, the central Shillong Plateau and Mikir Hills have the thinnest crust (30 ± 2 km) with similar velocity structure, suggesting a unified origin and uplift history. North of the plateau and Mikir Hills the crustal thickness increases sharply by 8–10 km and is modeled by ∼30∘ north dipping Moho flexure. South of the plateau, across the ∼1 km topographic relief of the Dawki Fault, the crustal thickness increases abruptly by 12–13 km and is modeled by downfaulting of the plateau crust, overlain by 13–14 km thick sedimentary layer/rocks of the Bengal Basin. Farther south, beneath central Bengal Basin, the basement layer is thinner (20–22 km) and has higher Vs (∼4.1 km s−1) indicating a transitional crystalline crust, overlain by the thickest sedimentary layer/rocks (18–20 km). Our models suggest that the uplift of the Shillong Plateau occurred by thrust faulting on the reactivated Dawki Fault, a continent margin paleorift fault, and subsequent back thrusting on the south dipping Oldham Fault, in response to flexural loading of the Eastern Himalaya. Our estimated Dawki Fault offset combined with timing of surface uplift of the plateau reveals a reasonable match between long‐term uplift and convergence rate across the Dawki Fault with present‐day GPS velocities.

Neoproterozoic‐Early Paleozoic Peri‐Pacific Accretionary Evolution of the Mongolian Collage System: Insights From Geochemical and U‐Pb Zircon Data From the Ordovician Sedimentary Wedge in the Mongolian Altai

AbstractNeoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical, and zircon U‐Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mountains. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke, and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U‐Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian‐Ordovician and culminating at circa 500 Ma. We propose that the Ordovician succession forms an “Altai sedimentary wedge,” the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan‐Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (Early Cambrian) to the development of the “Ikh‐Mongol Magmatic Arc System” and the giant Altai sedimentary wedge above a north dipping subduction zone (Late Cambrian‐Ordovician). Such a dynamic process resembles the tectonic evolution of the peri‐Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri‐Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro‐Ordovician geodynamic evolution of the Mongolian collage system.

The deep structure and reactivation of the Kyrgyz Tien Shan: Modelling the past to better constrain the present

In the Tien Shan belt, Cenozoic to active deformation is guided by structures and rheological contrasts partly inherited from the Paleozoic Variscan orogeny and reactivated by the India-Asia collision. Cenozoic deformation net estimates are constrained by lithospheric scale geological cross-sections, which rely on reconstructions of the Paleozoic Central Asian Orogenic Belt (CAOB). However, several geodynamic scenarii have been proposed for its formation including a south or a north dipping subduction, which are still debated due to lack of constraints. Here, we designed numerical experiments to test different hypotheses about the initial geometry and the effective rheology of the paleo-sutures and especially the one located between the Tien Shan and the Tarim basin, where most Cenozoic deformation localizes. The different geometries of Paleozoic structures are used as input variables in the thermo-mechanical models, which are then run forward in time. After a finite amount of shortening, the structures that develop self-consistently out of the proposed heterogeneities are then compared to the current Cenozoic finite and active deformations. We find that a crustal south-dipping suture zone lying on a resistant lithospheric mantle best explains the localization of the deformation, the current geometry of the structures and the Moho depth variations. Using the model results, we propose a mechanically consistent depth-interpolated crustal cross-section of the Kyrgyz Tien Shan, which incorporates both geological and geophysical data.

Magnetotelluric imaging across the tectonic structures in the eastern segment of the Central Indian Tectonic Zone: Preserved imprints of polyphase tectonics and evidence for suture status of the Tan Shear

The Central Indian Tectonic Zone (CITZ), a major tectonic feature extending across the Indian sub-continent, was formed by the accretion of the cratonic domains in the north and south blocks of peninsular India during the Proterozoic. It is characterized with prominent shear/fault zones and granulite belts that carry the evolutionary history of the central Indian region. This study presents the two-dimensional resistivity structure of the hitherto least studied eastern segment of the mega lineament in the Indian subcontinent. Magnetotelluric (MT) data along the Rajnandgaon-Bareli profile, which extends from the southern margin (Central Indian Shear-CIS) to the northern limit (Narmada-Son Lineament-NSL) of the CITZ, were used to investigate the resistivity character of the crust and shallow upper mantle. The study imaged conductive to moderately conductive structures, which show correlation to the various shear/faults and granulitic belts mapped in the region, interspersed in a resistive background. An upper crustal isolated conductor was observed below the Bhandra – Balaghat Granulite belt, which is exposed along the CIS. A vertical moderately conductive structure noticed in the middle to upper mantle depths under the Ramakona-Katangi Granulite (RKG) belt and this is interpreted as fault/shear zone that acted as the channel/pathway for the emplacement of parental magma of the RKG rocks. A more interesting feature in the MT model is a north dipping conductive horizon extending from the upper crust to the Moho. This feature shows further extension into the upper mantle as a subvertical moderately conductive zone and can be traced onto the surface mark of the Tan Shear, which suggests suture status to this prominent shear zone within the CITZ. Resistivity section also shows mid-lower crustal conductive/moderately conductive horizons under the Deccan trap and NSL, which are produced due to the large-scale magma intrusions into the crust during the Cretaceous-Tertiary Deccan volcanism through the pre-existing weak zones in the CITZ. The conductive to moderately conductive features of the MT model are indicative of the complex tectonic history of the region that range from accretion processes started in the Paleoproterozoic, followed by multiple metamorphic and deformation cycles in different geological times, to the Late Cretaceous volcanism.

Source Parameters of 1st April 2015 Chamoli Earthquake (Mw 4.8) vis-à-vis Seismotectonics of the Region

Abstract The paper presents a detailed analysis of 1st April 2015 earthquake, whose epicenter (30.16° N, 79.28° E) was located near Simtoli village of Chamoli district, Uttarakhand. The focal depth is refined to 7 km by the grid search technique using moment tensor inversion. The source parameters of the earthquake as estimated by spectral analysis method suggested the source radius of ~1.0 km, seismic moment as 1.99E+23 dyne-cm with moment magnitude (Mw) of 4.8 and stress drop of 69 bar. The fault plane solution inferred using full waveform inversion indicated two nodal planes, the northeast dipping plane having strike 334° and dip 5° and the southwest dipping plane with dip 86° and strike 118°. The parallelism of the nodal plane striking 334° with dip 5° as indicated in depth cross sections of the tectonic elements suggested the north dipping Main Boundary Thrust (MBT) to be the causative fault for this earthquake. Spatio-temporal distribution of earthquakes during the period 1960-2015 showed seismic quiescence during 2006-2010 and migration of seismicity towards south.

Geometry of the Main Himalayan Thrust and Moho beneath Satluj valley, northwest Himalaya: Constraints from receiver function analysis

AbstractCrustal configuration beneath the Satluj valley region of the northwest Himalaya has been studied with the help of receiver function analysis of teleseismic earthquakes recorded by 18 broadband seismological stations. These stations were located on diverse geotectonic units from the Himalayan foreland basin in the south to the Tethyan Himalaya (TH) in the north. A gentle north dipping structure of the Main Himalayan Thrust (MHT) is imaged between the Sub and Higher Himalaya in contrast to the reported ramp structure of the MHT beneath the Garhwal and Nepal Himalaya. The ramp structure is, however, identified further north, beyond the South Tibetan Detachment in Satluj valley. The depth of the MHT varies from ~16 to 27 km across the Sub, Lesser, and Higher Himalaya, and it increases to ~38 km beneath the TH forming a ramp. This is significantly a different structure of the MHT beneath the Satluj valley, which is attributed to the effect of underthrusting Delhi‐Hardwar Ridge, a transverse structure to the Himalayan arc. Conspicuously, no strong or large earthquake is observed during 1964–2015 in this segment of the Himalayan Seismic Belt. The RF modeling, on the other hand, shows ~44 km crustal thickness beneath the Himalayan Frontal Thrust (HFT), and it gradually increases to ~62 km beneath the TH. Low shear wave velocity (~0.8–1.8 km s−1) is observed in the uppermost 3–4 km of the crust beneath the stations near the HFT, which may be the effect of the sedimentary column of the Indo‐Gangetic Plain.

Structural analysis of aeromagnetic data from the Rainy River Block, Wabigoon subprovince, Minnesota, USA and Ontario, Canada: Strain partitioning along a Neoarchean terrane boundary and implications for mineral exploration

High-resolution aeromagnetic data are used to constrain the structural architecture and mineral potential of the Neoarchean Rainy River Block (RRB)—a sigma-shaped wedge of till-covered volcano-plutonic rocks that form the southern margin the Wabigoon subprovince and span the international border between northern Minnesota, USA and western Ontario, Canada. Reduced-to-pole transformed, upward continued and derivative enhanced aeromagnetic data show that the RRB can be divided into three structural domains: the Rainy Lake wrench zone (RLWZ), Border zone (BZ), and Oaks belt (OB). Strain intensity decreased from east (strongly deformed RLWZ) to west (moderately deformed OB). The RLWZ experienced superimposed folding that resulted in complex rock geometries and sigmoidal patterns in the aeromagnetic data. A large northeast trending syncline, partially represented by an aeromagnetic low that is cut by similarly oriented shear zones and faults, comprises the BZ. The OB is interpreted to occupy a regional-scale strain shadow and is composed of a relatively well preserved and magnetically heterogeneous volcanic pile and subvolcanic intrusion that have been overturned and are steeply north dipping. As the RRB is fault-bounded terrane boundary and hosts numerous mafic intrusions, it was a regional zone of synvolcanic and syntectonic paleo-permeability and, therefore, has potential for hosting orogenic gold and volcanogenic massive sulfide (VMS) deposits. However, linear fault geometries throughout most of the RRB were not conducive to formation of large orogenic gold deposits and pervasive deformation in the RLWZ may have compromised preservation of hypogene VMS mineralized zones. Thus, the structurally preserved OB is the most prospective area for hosting VMS deposits in the RRB, whereas the BZ and surrounding rocks are prospective for hosting orogenic gold deposits near a ∼30km long restraining bend in the Quetico Fault.

Comparative study of two tsunamigenic earthquakes in the Solomon Islands: 2015 Mw 7.0 normal‐fault and 2013 Santa Cruz Mw 8.0 megathrust earthquakes

AbstractThe July 2015 Mw 7.0 Solomon Islands tsunamigenic earthquake occurred ~40 km north of the February 2013 Mw 8.0 Santa Cruz earthquake. The proximity of the two epicenters provided unique opportunities for a comparative study of their source mechanisms and tsunami generation. The 2013 earthquake was an interplate event having a thrust focal mechanism at a depth of 30 km while the 2015 event was a normal‐fault earthquake occurring at a shallow depth of 10 km in the overriding Pacific Plate. A combined use of tsunami and teleseismic data from the 2015 event revealed the north dipping fault plane and a rupture velocity of 3.6 km/s. Stress transfer analysis revealed that the 2015 earthquake occurred in a region with increased Coulomb stress following the 2013 earthquake. Spectral deconvolution, assuming the 2015 tsunami as empirical Green's function, indicated the source periods of the 2013 Santa Cruz tsunami as 10 and 22 min.

Rapid spatiotemporal variations in rift structure during development of the Corinth Rift, central Greece

AbstractThe Corinth Rift, central Greece, enables analysis of early rift development as it is young (<5 Ma) and highly active and its full history is recorded at high resolution by sedimentary systems. A complete compilation of marine geophysical data, complemented by onshore data, is used to develop a high‐resolution chronostratigraphy and detailed fault history for the offshore Corinth Rift, integrating interpretations and reconciling previous discrepancies. Rift migration and localization of deformation have been significant within the rift since inception. Over the last circa 2 Myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift, but this transition did not occur synchronously along strike. Isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620–340 ka, characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at ~30 m/kyr. Since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin, demonstrated by lateral growth of discrete depocenters at ~40 m/kyr. A single central depocenter formed by circa 130 ka, indicating full fault linkage. These results indicate that rift localization is progressive (not instantaneous) and can be synchronous once a rift border fault system is established. This study illustrates that development processes within young rifts occur at 100 kyr timescales, including rapid changes in rift symmetry and growth and linkage of major rift faults.

Coseismic slip distribution of the 2015 Mw7.8 Gorkha, Nepal, earthquake from joint inversion of GPS and InSAR data for slip within a 3‐D heterogeneous Domain

AbstractWe derive a coseismic slip model of the 2015 Mw7.8 Gorkha earthquake on the basis of GPS and line‐of‐sight displacements from ALOS‐2 descending interferograms, using Green's functions calculated with a 3‐D finite element model (FEM). The FEM simulates a nonuniform distribution of elastic material properties and a precise geometric configuration of the irregular topographical surface. The rupturing fault is modeled as a low‐angle and north dipping surface within the Main Frontal Thrust along the convergent margin of the Himalayas. The optimal model that inherits heterogeneous material properties provides a significantly better solution than that in a homogenous domain at the 95% confidence interval. The best fit solution for the domain having a nonuniform distribution of material properties reveals a rhombus‐shaped slip zone of three composite asperities. Slip is primarily concentrated at a depth of 15 km with both dip‐slip (maximum 6.54 m) and strike‐slip (maximum 2.0 m) components, giving rise to a geodetic‐based moment of 1.09 × 1021 Nm in general agreement with the seismological estimate. The optimal relative weights among GPS and interferometric synthetic aperture radar (InSAR) are deduced from a new method, MC‐HVCE which combines a Monte Carlo search and a Helmert Method of Variance Components Estimation. This method determines the relative weights in a systemic approach which preserves the intrinsic solution smoothness. The joint solution is significantly better than those inverted from each individual data set. This methodology allows us to integrate multiple data sets of geodetic observations with seismic tomography, in an effort to achieve a better understanding of seismic ruptures within crustal heterogeneity.

Architectural evidence of dune collapse in the Navajo Sandstone, Zion National Park, Utah

The Canyon Overlook Trail of Zion National Park follows an outcrop of Navajo Sandstone, which displays a uniquely well-exposed assemblage of features associated with failure of the lee face of a large eolian dune, and run-out over an expanse of interdune sediments downwind of that bedform. Exposed features include dramatic folds in the interdune succession and a stacked series of thrust sheets incorporating both interdune and overlying dune deposits. Thrust surfaces display consistent strikes, parallel to those of undeformed foresets, and incorporate zones of brittle failure and fluid deformation, including folds overturned in the direction of foreset dip. These features correspond to predictions made by a previous researcher's model of dune collapse, formulated from less fortuitously exposed architectures in the Navajo Sandstone. Unlike the previous model, however, this site preserves distinct indications that the bulk of deformed material accumulated above the level of the contemporary interdune surface, in an aggradational succession.Paleotopographic reconstruction, based on preserved facies relationships at this site, indicates the presence of a large dune, partially encroached upon a well-developed wet interdune succession, made up of two half-meter carbonate mud layers, separated by a meter of medium-grained sand. Trapping of pore water pressure between these mud layers during liquefaction reduced shear strength in this interval, facilitating the collapse of the lee face of the upwind dune into the interdune area, and transmitted resultant shear forces to distal portions of the interdune expanse, in the shallow subsurface. Shear failure developed along bedding planes in the horizontally laminated carbonate muds, which provided both lubrication of the shear surfaces and structural support for the preservation of coherent thrust sheets during production of an imbricated succession of shear zones in the toe portion of the slump. Individual shear surfaces exposed in this outcrop extend for up to 50m along strike and dip north up to 55°. Upturned mud layers in the toe of the slump resisted deflation, promoting preservation of an irregular interdune topography, over which the reorganized dune ultimately advanced.

The Shillong Plateau and the great 1897 Assam earthquake

AbstractPrevious analysis of triangulation data of the Survey of India concluded that the great 1897 Assam earthquake occurred on a south dipping fault near the northern edge of the Shillong Plateau, which was named the Oldham fault. This attribution has been questioned on geological and geodetic grounds. We refine the triangulation data, adding recently discovered observations, and demonstrate that they require average slip of 25 ± 5 m on a fault that dips south at ∼40° beneath the plateau. The best fitting solution to the geodetic observations gives a rupture length of 79 km. However, the Chedrang fault, immediately to the west of the Oldham fault, appears to have slipped as a subvertical tear fault during or shortly after the 1897 earthquake, with over 10 m of down‐to‐the‐west normal‐sense slip. This observation suggests that the western end of the main rupture approached within a few kilometers of the Chedrang fault, giving a length of 95 km for the rupture. This range of parameters gives a magnitude 8.15<Mw<8.35 for the earthquake. The triangulation data cannot be satisfied by slip on the north dipping faults that border the southern edge of the Shillong Plateau nor by slip on a south dipping fault that has been postulated in the Brahmaputra Valley. GPS velocities show that up to 5 mm/yr of shortening is taken up across the plateau and its borders; this suggests, via moment‐frequency relations, that the interval between great earthquakes in the region is several thousand years but that earthquakes of magnitude 7 or greater should occur roughly once per century.

Seismotectonic analysis of the 2013 seismic sequence at the western Corinth Rift

This study focuses on a series of small intraplate earthquakes that took place during May–August 2013 on the southwestern coast of the Corinth Rift (Central Greece), a few km southeast of Aigion city. The Corinth Rift is one of the most seismically active parts of the Mediterranean. We analyzed more than 1500 events with 0.4≤ML≤3.7, the major part of which was recorded by a dense local network. The seismicity is densely clustered in a volume of dimensions ∼4×2×6km3, aligned in a N110° direction and at depths ranging between 6 and 12km. Precisely relocated hypocenters and reliably constrained focal mechanisms indicate north dipping planar faults with an average dip of ∼60°. Stress inversion of focal mechanisms implies that the dominant local stress field is extensional in a N5° direction, in good agreement with geodetic observations. The swarm evolved in two phases, with a spatiotemporal migration of epicenters from the eastern toward the western part of the rupture zone, while slip distribution appears homogeneous over the eastern part and strongly inhomogeneous in the western part. These two phases also produced different results in scaling relations such as the Gutenberg–Richter law, the Modified Omori Formula and the Epidemic Type of Aftershock Sequence model. Similar results from other studies have been reported and correlated with a fluid driven mechanism, however further research is required to strengthen this hypothesis for the purposes of this study.

Control of mechanical stratigraphy on bed-restricted jointing and normal faulting: Eagle Ford Formation, south-central Texas

Outcrops of the middle Eagle Ford Formation in south-central Texas reveal well-developed joint networks in subhorizontal competent carbonate (chalk) beds and less well developed networks in interlayered incompetent calcareous mudrock beds. Northeast-striking bed-perpendicular joints in competent beds have the longest trace lengths and are abutted by northwest-striking joints. All observed joints terminate vertically in incompetent beds. Normal faults are common but less abundant than joints; dominantly dip north, northwest, or southeast; and are abutted by the joint sets and, thus, predated jointing. The faults cut multiple competent and incompetent beds, providing vertical connectivity across mechanical layering. Products of hybrid and shear failure, the dip of these faults is steep through competent beds and moderate through incompetent beds, resulting in refracted fault profiles with dilation and calcite precipitation along steep segments. Fluid inclusions in fault zone calcite commonly contain liquid hydrocarbons. Rare two-phase fluid inclusions homogenized between about (1) 40 and 58°C, and (2) 90 and 100°C, suggesting trapping of aqueous fluids at elevated temperatures and depths on the order of 2 km (6562 ft). Fluid inclusion and stable isotope geochemistry analyses suggest that faults transmitted externally derived fluids. These faults likely formed at depths equivalent to portions of the present-day oil and gas production from the Eagle Ford play in south Texas. Faults connect across layering and provide pathways for vertical fluid movement within the Eagle Ford Formation, in contrast to vertically restricted joints that produce bed-parallel fracture permeability. These observations elucidate natural fractures and induced hydraulic fracturing within the Eagle Ford Formation.

Interseismic coupling, seismic potential, and earthquake recurrence on the southern front of the Eastern Alps (NE Italy)

AbstractHere we use continuous GPS observations to document the geodetic strain accumulation across the South‐Eastern Alps (NE Italy). We estimate the interseismic coupling on the intracontinental collision thrust fault and discuss the seismic potential and earthquake recurrence. We invert the GPS velocities using the back slip approach to simultaneously estimate the relative angular velocity and the degree of interseismic coupling on the thrust fault that separates the Eastern Alps and the Venetian‐Friulian plain. Comparison between the rigid rotation predicted motion and the shortening observed across the area indicates that the South‐Eastern Alpine thrust front absorbs about 70% of the total convergence between the Adria and Eurasia plates. The coupling is computed on a north dipping fault following the continuous external seismogenic thrust front of the South‐Eastern Alps. The modeled thrust fault is currently locked from the surface to a depth of ≈10 km. The transition zone between locked and creeping portions of the fault roughly corresponds with the belt of microseismicity parallel and to the north of the mountain front. The estimated moment deficit rate is 1.3 ± 0.4 × 1017Nm/yr. The comparison between the estimated moment deficit and that released historically by the earthquakes suggests that to account for the moment deficit the following two factors or their combination should be considered: (1) a significant part of the observed interseismic coupling is released aseismically and (2) infrequent “large” events with long return period (> 1000 years) and with magnitudes larger than the value assigned to the largest historical events (Mw≈ 6.7).

Three‐dimensional lithospheric electrical structure of Southern Granulite Terrain, India and its tectonic implications

AbstractThe crustal as well as the upper mantle lithospheric electrical structure of the Southern Granulite Terrain (SGT) is evaluated, using the magnetotelluric (MT) data from two parallel traverses: one is an ~ 500 km long N‐S trending traverse across SGT and another a 200 km long traverse. Data space Occam 3‐D inversion was used to invert the MT data. The electrical characterization of lithospheric structure in SGT shows basically a highly resistive (several thousands of Ohm meters) upper crustal layer overlying a moderately resistive (a few hundred Ohm meters) lower crustal layer which in turn is underlain by the upper mantle lithosphere whose resistivity shows significant changes along the traverse. The highly resistive upper crustal layer is interspersed with four major conductive features with three of them cutting across the crustal column, bringing out a well‐defined crustal block structure in SGT with individual highly resistive blocks showing correspondence to the geologically demarcated Salem, Madurai, and Trivandrum blocks. The 3‐D model also brought out a well‐defined major crustal conductor located in the northern half of the Madurai block. The electrical characteristics of this south dipping conductor and its close spatial correlation with two of the major structural elements, viz., Karur‐Oddanchatram‐Kodaikanal Shear Zone and Karur‐Kamban‐Painavu‐Trichur Shear Zone, suggest that this conductive feature is closely linked to the subduction‐collision tectonic processes in the SGT, and it is inferred that the Archean Dharwar craton/neoproterozoic SGT terrain boundary lies south of the Palghat‐Cauvery shear zone. The results also showed that the Achankovil shear zone is characterized by a well‐defined north dipping conductive feature. The resistive block adjoining this conductor on the southern side, representing the Trivandrum block, is shown to be downthrown along this north dipping crustal conductor relative to the Madurai block, suggesting a northward movement of Trivandrum block colliding against the Madurai block. The lithospheric upper mantle electrical structure of the SGT up to a depth of 100 km may be broadly divided into two distinctly different segments, viz., northern and southern segments. The northern lithospheric segment, over a major part, is characterized by a thick resistive upper mantle, while the southern one is characterized by a dominantly conductive medium suggesting a relatively thinned lithosphere in the southern segment.