THE PALEOSEISMIC SURFACE RUPTURE AT SOUTH OF CENTRAL ALTYN TAGH FAULT AND ITS TECTONIC IMPLICATION

In this study, we described a 14km-long paleoearthquakes surface rupture across the salt flats of western Qaidam Basin, 10km south of the Xorkol segment of the central Altyn Tagh Fault, with satellite images interpretation and field investigation methods. The surface rupture strikes on average about N80°E sub-parallel to the main Altyn Tagh Fault, but is composed of several stepping segments with markedly different strike ranging from 68°N~87°E. The surface rupture is marked by pressure ridges, sub-fault strands, tension-gashes, pull-apart and faulted basins, likely caused by left-lateral strike-slip faulting. More than 30 pressure ridges can be distinguished with various rectangular, elliptical or elongated shapes. Most long axis of the ridges are oblique(90°N~140°E)to, but a few are nearly parallel to the surface rupture strike. The ridge sizes vary also, with heights from 1 to 15m, widths from several to 60m, and lengths from 10 to 100m. The overall size of these pressure ridges is similar to those found along the Altyn Tagh Fault, for instance, south of Pingding Shan or across Xorkol. Right-stepping 0.5~1m-deep gashes or sub-faults, with lengths from a few meters to several hundred meters, are distributed obliquely between ridges at an angle reaching 30°. The sub-faults are characterized with SE or NW facing 0.5~1m-high scarps. Several pull-apart and faulted basins are bounded by faults along the eastern part of the surface rupture. One large pull-apart basins are 6~7m deep and 400m wide. A faulted basin, 80m wide, 500m long and 3m deep, is bounded by 2 left-stepping left-lateral faults and 4 right-stepping normal faults. Two to three m-wide gashes are often seen on pressure ridges, and some ridges are left-laterally faulted and cut into several parts, probably owing to the occurrence of repetitive earthquakes. The OSL dating indicates that the most recent rupture might occur during Holocene.
Southwestwards the rupture trace disappears a few hundred meters north of a south dipping thrust scarp bounding uplifted and folded Plio-Quaternary sediments to the south. Thrust scarps can be followed southwestward for another 12km and suggest a connection with the south Pingding Shan Fault, a left-lateral splay of the main Altyn Tagh Fault. To the northeast the rupture trace progressively veers to the east and is seen cross-cutting the bajada south of Datonggou Nanshan and merging with active thrusts clearly outlined by south facing cumulative scarps across the fans. The geometry of this strike-slip fault trace and the clear young seismic geomorphology typifies the present and tectonically active link between left-lateral strike-slip faulting and thrusting along the eastern termination of the Altyn Tagh Fault, a process responsible for the growth of the Tibetan plateau at its northeastern margin. The discrete relation between thrusting and strike-slip faulting suggests discontinuous transfer of strain from strike-slip faulting to thrusting and thus stepwise northeastward slip-rate decrease along the Altyn Tagh Fault after each strike-slip/thrust junction.     

INFLUENCE OF NUMBER AND SPACING OF SENSORS ON THE EFFECTIVENESS OF SEISMIC ARRAYS*

Ideally, geophones would be placed in a noiseless environment, in which case there would be no reason to resort to arrays of geophones. If the noise is such that an array is required, the objective of the array is to enhance the signal-to-noise ratio and thus to maximize the intelligence that can be derived from a given signal. The design of the array will be a function of the signal characteristics, of the direction and velocity of the noise in the bandpass of the signal, and of the site geology. It has been demonstrated previously that in a practical sense the optimum array processing is represented by precise beam forming, by which we mean simple time-delay and summation. Increasing the number N of sensors within a given area decreases the inter-element spacing and may increase the coherency between noise samples at adjacent sensors, thus yielding poorer results compared to √N improvement one expects to get if the noise is uncorrelated. Increasing the number of sensors by proportionately increasing the area is liable to result in signal deterioration, also yielding an unfavorable comparison to √N improvement in signal-to-noise. These two effects, together with economical factors, combine to limit the number of sensors that can be used. Although the data on which our conclusions are reached were drawn from earthquake seismology, the principles involved are equally applicable to exploration seismology and to other geophysical measurements in which arrays of sensors are required.     

La turbulence isentropique latérale dans le tourbillon circulaire

Résumé La turbulence latérale étant considérée comme un mouvement d'agitation à deux dimensions dans les surfaces isentropiques, on établit successivement le tenseur deReynolds et le taux de production de l'énergie cinétique de turbulence.

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Summary Assuming lateral mixing as a two-dimensional eddy motion in isentropic surfaces, theReynolds' tensor and the rate of production of kinetic energy of turbulence are established.

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Zusammenfassung Auf Grund der Annahme, daß der Gleitaustausch als zweidimensionale Austauschbewegung in isentropen Flächen angesehen werden kann, werden der Tensor vonReynolds und der Betrag der kinetischen Turbulenzenergie abgeleitet, die in der Einheit von Volumen und Zeit erzeugt wird.

     

Hydrogeophysical approach for contamination assessment in NamSon landfill,Hanoi, Vietnam

Self potential (SP) and electrical resistivity tomography (ERT) methods are used together with the results of groundwater samples hydrogeochemical analysis to assess the impact of the water leak from the landfill garbage site at NamSon located in Northern Hanoi on causing pollution to the surrounding environment and affecting geological structure. Selected survey area covers an area of 180 × 300 m lying in the low land of the NamSon site with a slope ranging about 8 m in direction NW–SE. There are three geophysical measurements lines denoted as T1, T2 and T3. Processing 180 SP data points has allowed to draw maps of equipotential epoch in the two periods in 2015 and 2016. The maps show four zones of SP positive anomalies with maximum amplitudes of about +20 mV where the groundwater flow direction is downward and five zones of SP negative anomalies with minimum values in a range from ?180 to ?260 mV where the groundwater flow direction is upward. Resistivity values of the subsurface layers of soils and rocks have been aquired from 2D inverse model for measuring ERT in March 2015 and March 2016. The results of the ERT allowed to define the low resistivity in the range 15–20 Ωm related to leachate plume from NamSon landfill site. Results of the physico-chemical analysis of groundwater samples from the existing six boreholes show increases in concentration of the measured pollutant parameters indicating contamination of the groundwater as a result of solid waste leachate accumulation. This result is affirmative evidence for the survey results by geophysical technique. The rapid decrease in quality of groundwater over the last year is probably due to the influence of the leachate from the NamSon landfill site.     

Surface complexation effects on phosphate adsorption to ferric iron oxyhydroxides along pH and salinity gradients in estuaries and coastal aquifers

Non-conservative behavior of dissolved inorganic phosphate (DIP) in estuaries is generally ascribed to desorption from iron and aluminum (hydr)oxides with increasing salinity. Here, we assess this hypothesis by simulating the reversible adsorption of phosphate onto a model oxide (goethite) along physico-chemical gradients representative of surface and subsurface estuaries. The simulations are carried out using a surface complexation model (SCM), which represents the main aqueous speciation and adsorption reactions of DIP, plus the ionic strength-dependent coulombic interactions in solution and at the mineral-solution interface. According to the model calculations, variations in pH and salinity alone are unlikely to explain the often reported production of DIP in surface estuaries. In particular, increased aqueous complexation of phosphate by Mg²⁺ and Ca²⁺ ions with increasing salinity is offset by the formation of ternary Mg-phosphate surface complexes and the drop in electrical potential at the mineral-water interface. However, when taking into account the downstream decrease in the abundance of sorption sites, the model correctly simulates the observed release of DIP in the Scheldt estuary. The sharp increase in pH accompanying the admixing of seawater to fresh groundwater should also cause desorption of phosphate from iron oxyhydroxides during seawater intrusion in coastal aquifers. As for surface estuaries, the model calculations indicate that significant DIP release additionally requires a reduction in the phosphate sorption site density. In anoxic aquifers, this can result from the supply of seawater sulfate and the subsequent reductive dissolution of iron oxyhydroxides coupled to microbial sulfate reduction.     

Recent plumbing system of the Krakatau volcano revealed by teleseismic earthquake distribution

Spatial and temporal analysis of global seismological data 1964–2005 reveals a distinct teleseismic earthquake activity producing a columnar-like formation in the continental wedge between the Krakatau volcano at the surface and the subducting slab of the Indo-Australian plate. These earthquakes occur continuously in time, are in the body-wave (m _b) magnitude range 4.5–5.3 and in the depth range 1–100 km. The Krakatau earthquake cluster is vertical and elongated in the azimuth N30°E, suggesting existence of a deep-rooted fault zone cutting the Sunda Strait in the SSW-NNE direction. Possible continuation of the fault zone in the SW direction was activated by an intensive 2002/2003 aftershock sequence, elongated in the azimuth of N55°E. Beneath the Krakatau earthquake cluster, an aseismic gap exists in the Wadati-Benioff zone of the subducting plate at the depths 100–120 km. We interpret this aseismic gap as a consequence of partial melting inhibiting stress concentration necessary to generate stronger earthquakes, whereas the numerous earthquakes observed in the overlying lithospheric wedge beneath the volcano probably reflect magma ascent in the recent plumbing system of the Krakatau volcano. Focal depth of the deepest events (~100 km) of the Krakatau cluster constrains the location of the primary magma generation to greater depths. The ascending magmatic fluids stress fault segments within the Sunda Strait fault zone and change their friction parameters inducing the observed tectonic earthquakes beneath Krakatau.