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Author(s): C. J. Cline II (corresponding author) [1]; U. H. Faul [2]; E. C. David [1]; A. J. Berry [1]; I. Jackson [1] Lateral variations of seismic wave speeds and [...]

Seismic site response of discontinuous permafrost is discussed. The presence of frozen ground in soil deposits can significantly affect their dynamic response due to stiffer conditions characterized by higher shear-wave velocities compared to unfrozen soils. Both experimental and numerical investigations were conducted to examine the problem. The experimental program included a series of 1g shaking table tests on small-scale models. Nonlinear numerical analyses were performed employing FLAC software. The numerical model was verified using the obtained experimental results. Parametric simulations were then conducted using the verified model to study variations of the free-field spectral accelerations (on top of the frozen and unfrozen soil blocks) with the scheme of frozen-unfrozen soil, and to determine the key parameters and their effects on seismic site response. Results show that spectral accelerations were generally higher in frozen soils than in unfrozen ones. It was found that the shear-wave velocity of the frozen soil as well as the assumed geometry of the blocks and their spacing have a significant impact on the site response. Key words: site response analysis, cold regions, discontinuous permafrost, shaking table, numerical modelling. La reponse sismique du site a pergelisol discontinu est abordee. La presence de sol gele dans les depots du sol peut affecter de maniere significative leur reponse dynamique due a des conditions plus rigides caracterisees par les vitesses des ondes de cisaillement plus eleve par rapport aux sols degeles. A la fois les reconnaissances experimentales et numeriques ont ete menees pour examiner le probleme. Le programme experimental comprenait une serie d'essais de tables de secouement de 1g sur les modeles a petite echelle. Les analyses numeriques non lineaires ont ete effectuees en utilisant le logiciel FLAC. Le modele numerique a ete verifie par les resultats experimentaux obtenus. Les simulations parametriques ont ensuite ete realisees en utilisant le modele verifie afin d'etudier les variations des accelerations spectrales a champ libre (au-dessus des blocs de terre congeles et degeles) avec le schema de sol gele-degele, et determiner les parametres cles et leur effet sur la reponse sismique du site. Les resultats montrent que les accelerations spectrales etaient generalement plus elevees dans les sols geles que dans les sols degeles. Il a ete constate que la vitesse des ondes de cisaillement dans le sol gele ainsi que la geometrie supposee des blocs et leur espacement ont un impact significatif sur la reponse du site. [Traduit par la Redaction] Mots-cles : analyse de la reponse du site, regions froides, pergelisol discontinu, table de secouement, modelisation numerique., Introduction Surface seismic ground motions are significantly influenced by local site conditions--e.g., surficial soil or bedrock conditions, depth and geometry of the sedimentary basin, topography--and by the characteristics of the [...]

Large earthquakes nucleate at tectonic plate boundaries, and their occurrence within a plate's interior remains rare and poorly documented, especially offshore. The two large earthquakes that struck the northeastern Indian [...]

Large earthquakes trigger very small earthquakes globally during passage of the seismic waves and during the following several hours to days (1-10), but so far remote aftershocks of moment magnitude [...]

The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary--from an environment of continental collision to an island arc subduction zone (1,2). On [...]

The determination of the chemical composition of Earth's lower mantle is a long-standing challenge in earth science. Accurate knowledge of sound velocities in the lower-mantle minerals under relevant high-pressure, high-temperature conditions is essential in constraining the mineralogy and chemical composition using seismological observations (1), but previous acoustic measurements were limited to a range of low pressures and temperatures. Here we determine the shear-wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle using Brillouin scattering spectroscopy (2). The mineralogical model that provides the best fit to a global seismic velocity profile (1) indicates that perovskite constitutes more than 93 per cent by volume of the lower mantle, which is a much higher proportion than that predicted by the conventional peridotitic mantle model. It suggests that the lower mantle is enriched in silicon relative to the upper mantle, which is consistent with the chondritic Earth model. Such chemical stratification implies layered-mantle convection with limited mass transport between the upper and the lower mantle., It is widely accepted, on the basis of penological evidence, that at least Earth's uppermost mantle, perhaps down to the top of the mantle transition zone, has a peridotitic (pyrolitic) [...]

The Earth's magnetic field is generated by a dynamo in the liquid iron core, which convects in response to cooling of the overlying rocky mantle. The core freezes from the innermost surface outward, growing the solid inner core and releasing light elements that drive compositional convection (1-3). Mantle convection extracts heat from the core at a rate that has enormous lateral variations (4). Here we use geodynamo simulations to show that these variations are transferred to the inner-core boundary and can be large enough to cause heat to flow into the inner core. If this were to occur in the Earth, it would cause localized melting. Melting releases heavy liquid that could form the variable-composition layer suggested by an anomaly in seismic velocity in the 150 kilometres immediately above the inner-core boundary (5-7). This provides a very simple explanation of the existence of this layer, which otherwise requires additional assumptions such as locking of the inner core to the mantle, translation from its geopotential centre (7,8) or convection with temperature equal to the solidus but with composition varying from the outer to the inner core (9). The predominantly narrow downwellings associated with freezing and broad upwellings associated with melting mean that the area of melting could be quite large despite the average dominance of freezing necessary to keep the dynamo going. Localized melting and freezing also provides a strong mechanism for creating seismic anomalies in the inner core itself, much stronger than the effects of variations in heat flow so far considered (10)., The core responds passively to the non-uniform heat flow imposed by the mantle: it plays a purely passive role in this coupled convective system. Variations in heat flux around the [...]

It has been shown that the Earth's inner core has an axisymmetric anisotropic structure with seismic waves traveling ~3% faster along polar paths than along equatorial directions. Hemispherical anisotropic patterns of the solid Earth's core are rather complex, and the commonly used hexagonal-close-packed iron phase might be insufficient to account for seismological observations. We show that the data we collected are in good agreement with the presence of two anisotropically specular east and west core hemispheres. The detected travel-time anomalies can only be disclosed by a lattice-preferred orientation of a body-centered-cubic iron aggregate, having a fraction of their [111] crystal axes parallel to the Earth's rotation axis. This is compelling evidence for the presence of a body-centered-cubic Fe phase at the top of the Earth's inner core. elastic anisotropy | Fe-bcc/-hcp | PKiKP/PKIKP waves | molecular dynamics www.pnas.org/cgi/doi/10.1073/pnas.1004856107

West Antarctica and adjacent seafloor have topographic elevations 0.5-1.2 km greater than expected from models of lithospheric age and crustal structure. Ocean crust near New Zealand has no equivalent depth anomaly, but tectonic subsidence histories from Campbell Plateau petroleum wells show anomalously high subsidence rates during the Paleogene, and total subsidence 0.5-0.9 km greater than expected from rift basin models. Geophysical and geochemical anomalies suggest that upward mantle flow supports the anomalous topography beneath Antarctica, and the Campbell Plateau subsidence history indicates that topographic support mechanisms were long lived (>80 Ma) and recoverable over a period of ~30 m.y. as plate motions moved New Zealand from Antarctica. We construct models of Late Cretaceous and Cenozoic mantle flow with a slab graveyard and upwelling above that is initially rooted at 1000-1500 km depth. Our models match topography and subsidence history anomalies, and are consistent with mantle seismic wave speed anomalies and the geoid. We suggest that when thermally driven slab downwelling ceased ca. 100 Ma, low-density material that was fertilized within a broad zone in the lower mantle during the previous ~400 m.y. of Gondwana subduction was released and able to rise. Mantle upwelling from depths of 700-1500 km, lasting for periods of ~100-200 m.y., with enriched chemistry related to the prior subduction history may be a general process that follows subduction death, and has not previously been recognized. doi: 10.1130/G30613.1

This paper presents a frequency domain analysis of a circular tunnel with piecewise liners subjected to seismic waves. In our model the surrounding medium of the tunnel is considered as a linear elastic medium and described by the dynamic elasticity theory, while piecewise liners and connecting joints are treated as curved beams and described by a curved beam theory. Scattered wave field in the surrounding elastic medium are obtained by the wave function expansion approach. The governing equations for vibrations of a curved beam are discretized by the general differential quadrature method. We use domain decomposition methods to establish the global discrete dynamic equations for piecewise liners. Boundary least squares collocation methods, based on the continuity conditions of stresses and displacements between surrounding soil and the piecewise liners, are used to determine the response of the liners and the surrounding medium. Numerical results conclude that the presence of the joints significantly changes the distributions of the tunnel internal force, and dramatically increase shear forces and moment of the tunnel liners around joints. [DOI: 10.1115/1.2827362] Keywords: tunnel liner, curved beam, General Differential Quadrature (GDQ) method, the frequency domain

To clarify the relationship between earthquakes and electrical phenomena, signals of three parameters were observed continuously at Sennan in Akita Prefecture, Japan: ground acceleration (GAC), earth potential difference (EPD), and electrostatic field in the atmosphere (EFA). A large metal plate was used as an electrostatic antenna to observe EFA variation. For this paper, we used the digital natural observation (D-NOB) method as a signal processing technique and found clear EPD and EFA waveforms during seismic wave propagation. Our observation and examination results show that GAC causes EPD variation. Subsequently, the EPD variation induces EFA variation. Index Terms--Earth potential difference (EPD), earthquake, electromagnetic observation, electrostatic field in the atmosphere (EFA), ground acceleration (GAC).

The aim of this paper is to discuss the existing scientific literature in order to gather all the available information dealing with the origin and the nature of the ambient seismic noise wavefield. This issue is essential as the use of seismic noise is more and more popular for seismic hazard purposes with a growing number of processing techniques based on the assumption that the noise wavefield is predominantly consisting of fundamental mode Rayleigh waves. This survey reveals an overall agreement about the origin of seismic noise and its frequency dependence. At frequencies higher than 1 Hz, seismic noise systematically exhibits daily and weekly variations linked to human activities, whereas at lower frequencies (between 0.005 and 0.3 Hz) the variation of seismic noise is correlated to natural activities (oceanic, meteorological...). Such a surface origin clearly supports the interpretation of seismic noise wavefield consisting primarily of surface waves. However, the further, very common (though hidden) assumption according which almost all the noise energy would be carried by fundamental mode Rayleigh waves is not supported by the few available data: no 'average' number can though be given concerning the actual proportion between surface and body waves, Love and Rayleigh waves (horizontal components), fundamental and higher modes (vertical components), since the few available investigations report a significant variability, which might be related with site conditions and noise source properties. Keywords: seismic noise wavefield; microseisms; microtremors; surface waves; seismic hazard

The propagation of seismic waves in block two- and three-dimensional media is investigated. These media are composed of identical cells in which there are several fluid blocks and one elastic [...]

We have used paleomagnetism to estimate the maximum moment magnitude ([M.sub.w]) of earthquakes in the east Ventura Basin and San Fernando Valley, in metropolitan Los Angeles. Magnetic declinations show differential rotation between crustal blocks with linear dimensions of 10-20 km, similar to the thickness of the seismogenic layer. The maximum magnitude of an earthquake based on blocks of this size is [M.sub.w] = 6.8, comparable to the 1971 San Fernando and 1994 Northridge earthquakes and consistent with paleoseismic trenching and surface ruptures of the 1971 earthquake. The paleomagnetic results suggest that the blocks have retained their configuration while moving relative to each other for the past ~0.8 m.y. Therefore, it is unlikely that in this area multiple blocks combined to trigger much larger shocks during this period, in contrast to adjacent regions where events with [M.sub.w] > 7 have been postulated on the basis of paleoseismic excavations. Keywords: paleomagnetism, block rotations, earthquake magnitude, seismic moment.