1,721,108 research outputs found
Radial profiles of seismic attenuation in the upper mantle based on physical models
No full textThermally activated, viscoelastic relaxation of the Earth's materials is responsible for intrinsic attenuation of seismic waves. Seismic observations have been used to define layered radially symmetric attenuation models, independent of any constraints on temperature and composition. Here, we interpret free-oscillation and surface wave attenuation measurements in terms of physical structures, by using the available knowledge on the physical mechanisms that govern attenuation at upper-mantle (<400 km) conditions. We find that observations can be explained by relatively simple thermal and grain-size structures. The 1-D attenuation models obtained do not have any sharp gradients below 100km, but fit the data equally well as the seismic models. The sharp gradients which characterize these models are therefore not required by the data. In spite of the large sensitivity of seismic observations to temperature, a definitive interpretation is limited by the unknown effects of pressure on anelasticity. Frequency dependence of anelasticity, as well as trade-offs with deeper attenuation structure and dependence on the elastic background model, are less important. Effects of water and dislocations can play an important role as well and further complicate the interpretation. Independent constraints on temperature and grain size expected around 100km depth, help to constrain better the thermal and grain-size profiles at greater depth. For example, starting from a temperature of 1550 K at 100 km and assuming that the seismic attenuation is governed by the Faul & Jackson's (2005) mechanism, we found that negative thermal gradients associated with several cm grain sizes (assuming low activation volume) or an adiabatic gradient associated with ∼1 cm grain size, can explain the data. A full waveform analysis, combining the effects on phase and amplitude of, respectively, elasticity and anelasticity, holds promise for further improving our knowledge on the average composition and thermal structure of the upper mantle. © 2008 The Authors Journal compilation © 2008 RAS
Three-dimensional radial anisotropic structure of the North American upper mantle from inversion of surface waveform data
No full textInsights into the nature of the transition zone from physically constrained inversion of long-period seismic data
No full textImposing a thermal and compositional significance to the outcome of the inversion of seismic data facilitates their interpretation. Using long-period seismic waveforms and an inversion approach that includes constraints from mineral physics, we find that lateral variations of temperature can explain a large part of the data in the upper mantle. The additional compositional signature of cratons emerges in the global model as well. Above 300 km, we obtain seismic geotherms that span the range of expected temperatures in various tectonic regions. Absolute velocities and gradients with depth are well constrained by the seismic data throughout the upper mantle, except near discontinuities. The seismic data are consistent with a slower transition zone and an overall faster shallow upper mantle, which is not compatible with a homogenous dry pyrolite composition. A gradual enrichment with depth in a garnet-rich component helps to reduce the observed discrepancies. A hydrated transition zone would help to lower the velocities in the transition zone, but it does not explain the seismic structure above it. © 2007 by The National Academy of Sciences of the USA
On the computation of long period seismograms in a 3-D earth using normal mode based approximations
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Recommended from our members
Investigating deep and shallow earth structures using different seismological approaches
Full text linkInvestigating the elastic structure at multiple scales of the Earth is crucial for understanding the composition and dynamics of the Earth. Different seismological approaches to extract the information contained in seismic waveforms are presented here. The work consists of three topics: (I) applications of source stacking followed by cross-correlation, (II) modeling seismic anisotropy in the lower mantle, and (III), temporal variation of near-surface seismic velocity and anisotropy in Hokkaido.I. Accurate synthetic seismic wavefields can now be computed in 3D earth models using the spectral element method, which helps improve resolution in full-waveform global tomography. However, computational costs are still a challenge. These costs can be reduced by implementing a source stacking method (Capdeville et al. 2003), in which multiple earthquake sources are simultaneously triggered in only one teleseismic SEM simulation. One drawback of this approach is the perceived loss of resolution at depth, in particular because high-amplitude fundamental mode surface waves dominate the summed waveforms, without the possibility of windowing and weighting as in conventional waveform tomography. This can be addressed by redefining the cost function and computing the cross-correlation wavefield between pairs of stations before each inversion iteration. While the Green’s function between the two stations is not reconstructed as well as in the case of ambient noise tomography, where sources are distributed more uniformly around the globe, this is not a drawback, since the same processing is applied to the 3D synthetics and to the data, and the source parameters are known to a good approximation. By doing so, time windows with large energy arrivals corresponding to fundamental mode surface waves can be separated. Also, applying a weighting scheme can bring out the contribution of overtones and body waves and also can balance the contributions of frequently sampled paths. The approach is computationally very efficient and can help address such questions as model resolution in the presence of noise, and trade-offs between different physical parameters (anisotropy, attenuation, crustal structure etc..) that would be computationally very costly to address adequately when using conventional full waveform tomography based on single-event wavefield computations.II. By assuming a geodynamical scenario of slab subduction and considering the seismic anisotropy in D” is dominated by crystal preferred orientation, the textural evolution and re- resultant elastic properties can be computed, which may help to explain the seismic anisotropy observations (e.g., McNamara et al. 2002, 2003; Wenk et al. 2011). In previous work, Cottaar et al. (2014) calculated the seismic anisotropy produced in a single mineral system, comparing bridgmanite (MgSiO3 perovskite, Pv) to magnesium post-perovskite (pPv), based on elastic properties and slip systems determined from laboratory experiments and theoretical ab-initio computations. More realistic situations are considered in this study: (a) the polycrystal plasticity model (Lebensohn and Tom ́e 1993) which consists Pv/pPv, cubic calcium perovskite (CaSiO3, CaPv), and cubic periclase (MgO), (b) the forward and reverse Pv-pPv phase transitions during the slabs’ subduction and the subsequent upwelling, (c) partial melting in the deepest portions of the slab at the base of upwelling. To validate and compare the results with seismological observations, the spatial distribution of radial anisotropy described by the parameter ξ = (VSH/VSV)2, and shear wave splitting (SWS) directions and strengths are extracted.
III. Here an earthquake-based method (Chen et al. 2017) is applied through interferometry of earthquake coda waves to investigate the temporal changes of elastic properties by using borehole-surface station pairs. The technique provides comprehensive observations not only of S-wave velocity but, additionally, of P-wave and S-wave azimuthal anisotropy between two sensors. The 15-year temporal variations of elastic properties are monitored at the KiK- net station IBUH03 in southern Hokkaido, which experienced the 2003 Mw 8.3 Tokachi-Oki (PGA= ∼ 350cm/s2) and the 2018 Mw 6.7 Hokkaido Eastern Iburi (PGA= ∼ 500cm/s2) earthquakes. The preliminary results show sudden reductions of VSiso and VP after the two major earthquakes considered, and these were subsequently recovered with short-term and long-term recovery rates. Changes in the fast S-wave polarization direction and strength of anisotropy were also detected during the coseismic periods. Furthermore, a clear seasonal trend of elastic parameters can be observed. The results are compared with temporal variations in equivalent water thickness, precipitation, and temperature. A Hudson-Crampin anisotropic effective model is implemented to understand the seasonal crack behavior. The modeling result indicates that crack density and the aspect ratio could play an important role in our observations
Seismic evidence for a steeply dipping reflector-stagnant slab in the mantle transition zone
No full textStudies of seismic tomography have been highly successful at imaging the deep structure of subduction zones. In a study complementary to these tomographic studies, we use array seismology and reflected waves to image a stagnant slab in the mantle transition zone. Using P and S (SH) waves we find a steeply dipping reflector centred at ca. 400 km depth and ca. 550 km west of the present Mariana subduction zone (at 20N, 140E). The discovery of this anomaly in tomography and independently in array seismology (this paper) helps in understanding the evolution of the Mariana margin. The reflector/stagnant slab may be the remains of the hypothetical North New Guinea Plate, which was theorized to have subducted ca. 50 Ma.Deutsche Forschungsgemeinschaf
- …
