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21.
Evidence for lithospheric detachment in the central Andes from local earthquake tomography 总被引：4，自引：1，他引：3
Data from three temporary seismic networks were merged for tomographic inversion. Although the deployments did not coincide in time, spatial overlap was achieved by re-occupying existing sites. Travel times and t operators of about 1600 earthquakes were inverted for 3D models of νp, νp/νs and P-wave attenuation (Qp− 1). All three attributes provide a consistent image of the entire subduction zone on a lithospheric scale. The tomographic images reveal low velocities and high attenuation in the crust and mantle underlying the Western Cordillera and most of the Puna plateau, indicative of weak rheology and mostly asthenospheric mantle. In contrast, forearc and eastern foreland are characterized by high Qp values, corresponding to cold temperatures in accordance with thermal models. In the backarc, between 23°S and 24°S, a high velocity, high Qp structure beneath the Eastern Cordillera and eastern Puna is interpreted as detaching continental lithosphere that has been thickened in the orogenic process. South of this structure, the mantle is characterized by low velocities, high νp/νs ratios, and low Qp values. Here it is believed that lithosphere originally underlying Andean crust has already been removed. This is supported by new estimates of crustal thickness and volcanic activity. 相似文献
22.
Yiannis Tsapras Rachel A. Street Keith Horne Alan Penny Fraser Clarke Hans Deeg Francisco Garzon Simon Kemp Maria Rosa Zapatero Osorio Alejandro Oscoz Abad Santiago Madruga Sanchez Carlos Eiroa Alcione Mora Antxon Alberdi rew Cameron John K. Davies Roger Ferlet Carol Grady Allan W. Harris Javier Palacios reas Quirrenbach Heike Rauer Jean Schneider Dolf de Winter Bruno Merin Enrique Solano 《Monthly notices of the Royal Astronomical Society》2001,325(3):1205-1212
In 1998 the EXPORT team monitored microlensing event light curves using a charge-coupled device (CCD) camera on the IAC 0.8-m telescope on Tenerife to evaluate the prospect of using northern telescopes to find microlens anomalies that reveal planets orbiting the lens stars. The high airmass and more limited time available for observations of Galactic bulge sources make a northern site less favourable for microlensing planet searches. However, there are potentially a large number of northern 1-m class telescopes that could devote a few hours per night to monitor ongoing microlensing events. Our IAC observations indicate that accuracies sufficient to detect planets can be achieved despite the higher airmass. 相似文献
23.
Santiago Torres Enrique García-Berro reas Burkert Jordi Isern 《Monthly notices of the Royal Astronomical Society》2001,328(2):492-500
In this paper we analyse the consequences in the white dwarf population of a hypothetical merger episode in our Galactic disc. We have studied several different merging scenarios with our Monte Carlo simulator. For each one of these scenarios we have derived the main characteristics of the resulting white dwarf population and we have compared them with the available observational data, namely the white dwarf luminosity function and the kinematic properties of the white dwarf population. Our results indicate that very recent (less than ∼6 Gyr ago) and massive (∼16 per cent of the mass of our Galaxy) merger episodes are quite unlikely in view of the available kinematical properties of the disc white dwarf population. Smaller merger episodes (of the order of ∼4 per cent of the mass of our Galaxy) are, however, compatible with our current knowledge of those kinematical properties. Finally, we prove that the white dwarf luminosity function is quite insensitive to such a merger episode. 相似文献
24.
Rolando Dünner reas Reisenegger rés Meza † Pablo A. Araya Hernán Quintana 《Monthly notices of the Royal Astronomical Society》2007,376(4):1577-1587
An exponentially expanding Universe, possibly governed by a cosmological constant, forces gravitationally bound structures to become more and more isolated, eventually becoming causally disconnected from each other and forming so-called 'island universes'. This new scenario reformulates the question about which will be the largest structures that will remain gravitationally bound, together with requiring a systematic tool that can be used to recognize the limits and mass of these structures from observational data, namely redshift surveys of galaxies. Here we present a method, based on the spherical collapse model and N -body simulations, by which we can estimate the limits of bound structures as observed in redshift space. The method is based on a theoretical criterion presented in a previous paper that determines the mean density contrast that a spherical shell must have in order to be marginally bound to the massive structure within it. Understanding the kinematics of the system, we translated the real-space limiting conditions of this 'critical' shell to redshift space, producing a projected velocity envelope that only depends on the density profile of the structure. From it we created a redshift-space version of the density contrast that we called 'density estimator', which can be calibrated from N -body simulations for a reasonable projected velocity envelope template, and used to estimate the limits and mass of a structure only from its redshift-space coordinates. 相似文献
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26.
Cracks play a very important role in many geotechnical issues and in a number of processes in the Earth’s crust. Elastic waves can be used as a remote sensing tool for determining crack density. The effect of varying crack density in crystalline rock on the P- and S-wave velocity and dynamic elastic properties under confining pressure has been quantified. The evolution of P- and S-wave velocity were monitored as a suite of dry Westerly granite samples were taken to 60, 70, 80 and 90 % of the unconfined uniaxial strength of the sample. The damaged samples were then subjected to hydrostatic confining pressure from 2 MPa to 200 MPa to quantify the effect of varying crack density on the P- and S-wave velocity and elastic properties under confining pressure. The opening and propagation of microcracks predominantly parallel to the loading direction during uniaxial loading caused a 0.5 and 6.3 % decrease in the P- and S-wave velocity, respectively. During hydrostatic loading, microcracks are closed at 130 MPa confining pressure. At lower pressures the amount of crack damage in the samples has a small but measureable effect. We observed a systematic 6 and 4 % reduction in P- and S-wave velocity, respectively, due to an increase in the fracture density at 2 MPa confining pressure. The overall reduction in the P- and S-wave velocity decreased to 2 and 1 %, respectively, at 50 MPa. The elastic wave velocities of samples that have a greater amount of microcrack damage are more sensitive to pressure. Effective medium modelling was used to invert elastic wave velocities and infer crack density evolution. Comparing the crack density results with experimental data on Westerly granite samples shows that the effective medium modelling used gave interpretable and reasonable results. Changes in crack density can be interpreted as closure or opening of cracks and crack growth. 相似文献
27.
2009年9月30日,印度尼西亚巴东市发生7.6级地震。之后,印度尼西亚又接连遭受了几次强震袭击,如2009年12月23日6.1级、2010年3月6日7.1级、2010年4月7日7.8级和2010年5月9日7.4级地震。2010年6月16日一天之内,印度尼西亚又接连发生了3次地震,震级分别为6.3级、7.0级和6.3级。2010年2月,由John McCloskey领军的一个科学家团队就2009年9月巴东地震致函英国《自然—地球科学》杂志(Nature Geoscience),题为:The September 2009 Padang Earthquake。信中提出警告:印度尼西亚苏门答腊沿海地区或将发生引发巨大海啸的强烈地震,其灾害程度足以造成与2004年南亚大海啸相匹敌的惨重伤亡,提醒居民要为更加强烈的地震来袭做好准备。为此,"Nature Geoscience"特发表了一篇社论,题为:Earthquakes off Sumatra(见本期译文"苏门答腊近海地震")。下边是John McCloskey等致"Nature Geoscience"的信函译文。 相似文献
28.
Chiara Mastropietro reas Burkert Ben Moore 《Monthly notices of the Royal Astronomical Society》2009,399(4):2004-2020
We use high-resolution N -body/smoothed particle hydrodynamics (SPH) simulations to study the hydrodynamical interaction between the Large Magellanic Cloud (LMC) and the hot halo of the Milky Way. We investigate whether ram pressure acting on the satellite's interstellar medium can explain the peculiarities observed in the H i distribution and the location of the recent star formation activity.
Due to the present nearly edge-on orientation of the disc with respect to the orbital motion, compression at the leading edge can explain the high density region observed in H i at the south-east border. In the case of a face-on disc (according to Mastropietro the LMC was moving almost face-on before the last perigalactic passage), ram pressure directed perpendicular to the disc produces a clumpy structure characterized by voids and high density filaments that resemble those observed by the Parkes H i survey. As a consequence of the very recent edge-on motion, the Hα emission is mainly concentrated on the eastern side where 30 Doradus and most of the supergiant shells are located, although some Hα complexes form a patchy distribution on the entire disc. In this scenario, only the youngest stellar complexes show a progression in age along the leading border of the disc. 相似文献
Due to the present nearly edge-on orientation of the disc with respect to the orbital motion, compression at the leading edge can explain the high density region observed in H i at the south-east border. In the case of a face-on disc (according to Mastropietro the LMC was moving almost face-on before the last perigalactic passage), ram pressure directed perpendicular to the disc produces a clumpy structure characterized by voids and high density filaments that resemble those observed by the Parkes H i survey. As a consequence of the very recent edge-on motion, the Hα emission is mainly concentrated on the eastern side where 30 Doradus and most of the supergiant shells are located, although some Hα complexes form a patchy distribution on the entire disc. In this scenario, only the youngest stellar complexes show a progression in age along the leading border of the disc. 相似文献
29.
Seb Oliver Robert G. Mann Ruth Carballo Alberto Franceschini Michael Rowan-Robinson Maria Kontizas Anastasios Dapergolas Evanghelos Kontizas Aprajita Verma David Elbaz Gian Luigi Granato Laura Silva Dimitra Rigopoulou J. Ignacio Gonzalez-Serrano Steve Serjeant reas Efstathiou Paul P. van der Werf 《Monthly notices of the Royal Astronomical Society》2002,332(3):536-548
We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 6.7 and 15 μm with the ISOCAM instrument on board the Infrared Space Observatory ( ISO ). The final map in each band was constructed by the co-addition of four independent rasters, registered using bright sources securely detected in all rasters, with the absolute astrometry being defined by a radio source detected at both 6.7 and 15 μm. We sought detections of bright sources in a circular region of radius 2.5 arcmin at the centre of each map, in a manner that simulations indicated would produce highly reliable and complete source catalogues using simple selection criteria. Merging source lists in the two bands yielded a catalogue of 35 distinct sources, which we calibrated photometrically using photospheric models of late-type stars detected in our data. We present extragalactic source count results in both bands, and discuss the constraints that they impose on models of galaxy evolution, given the volume of space sampled by this galaxy population. 相似文献
30.
Elena D'Onghia reas Burkert Giuseppe Murante Sadegh Khochfar 《Monthly notices of the Royal Astronomical Society》2006,372(4):1525-1530
The processes are investigated by which gas loses its angular momentum during the protogalactic collapse phase, leading to disc galaxies that are too compact with respect to the observations. High-resolution N -body/SPH simulations in a cosmological context are presented including cold gas and dark matter (DM). A halo with quiet merging activity since redshift z ∼ 3.8 and with a high-spin parameter is analysed that should be an ideal candidate for the formation of an extended galactic disc. We show that the gas and the DM have similar specific angular momenta until a merger event occurs at z ∼ 2 with a mass ratio of 5:1. All the gas involved in the merger loses a substantial fraction of its specific angular momentum due to tidal torques and dynamical friction processes falls quickly into the centre. In contrast, gas infall through small subclumps or accretion does not lead to catastrophic angular momentum loss. In fact, a new extended disc begins to form from gas that was not involved in the 5:1 merger event and that falls in subsequently. We argue that the angular momentum problem of disc galaxy formation is a merger problem: in cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1 are expected to occur in almost all galaxies. We suggest that energetic feedback processes could in principle solve this problem, however only if the heating occurs at the time or shortly before the last substantial merger event. Good candidates for such a coordinated feedback would be a merger-triggered starburst or central black hole heating. If a large fraction of the low angular momentum gas would be ejected, late-type galaxies could form with a dominant extended disc component, resulting from late infall, a small bulge-to-disc ratio and a low baryon fraction, in agreement with observations. 相似文献