We derive 3‐D tomographic maps of the Earth's mantle, CMB and outer core, from seismic P, PcP, PKPbc, PKPdf travel time data, based on the bulletins of the International Seismological Centre (1964–1995), after source relocation by Antolik et al. [2001] and phase re‐identification by Engdahl et al. [1998] . Maps of the CMB derived independently from core‐reflected (PcP) or core‐refracted (PKP) phases are not well correlated. We attempt to explain this discrepancy, and study the radial coherence of whole‐Earth tomographic images, to identify possible trade‐offs between CMB undulations and velocity anomalies in the mantle or outer core. Imaged velocity anomalies in the lowermost mantle are anticorrelated with the topography of the CMB; likewise, imaged lateral heterogeneities in the outer core are correlated with the topography of the CMB. This, together with the study of Piersanti et al. [2001] , suggests that the core anomalies might not be entirely fictitious.
SUMMARY We investigate the temporal changes of crustal velocity associated to the seismic sequence of 2016–2017, which struck central Italy with a series of moderate to large earthquakes. We cross-correlate continuous recordings of 2 yr of ambient seismic noise from a network of 28 stations within a radius of 90 km around Amatrice town. We then map the spatio-temporal evolution of the velocity perturbations under the effect of subsequent earthquakes. Coinciding with each of the three main shocks of the sequence we observe a sudden drop of seismic velocity which tends to quickly recover in the short term. After the end of the strongest activity of the sequence, the coseismic velocity changes display gradual healing towards pre-earthquake conditions following a quasi-linear trend, such that by the end of 2017 about 75 per cent of the perturbation is recovered. The spatial distribution of the velocity drop fluctuates with time, and the area that shows the most intense variations beyond the ruptured fault system elongates in the NE direction. This zone roughly corresponds to a region of foredeep sedimentary deposits consisting of highly hydrated and porous sandstones, which respond to the passage of seismic waves with increased pore pressure and crack number, leading to a reduction of the effective relative velocity.
<p>We conduct joint tomographic inversions of P and S travel time observations to obtain models of delta v_P and delta v_S in the entire mantle. We adopt a recently published method which takes into account the geodynamic coupling between mantle heterogeneity and core-mantle boundary (CMB) topography by viscous flow, where sensitivity of the seismic travel times to the CMB is accounted for implicitly in the inversion (i.e. the CMB topography is not explicitly inverted for). The seismic maps of the Earth's mantle and CMB topography that we derive can explain the inverted seismic data while being physically consistent with each other. The approach involved scaling P-wave velocity (more sensitive to the CMB) to density anomalies, in the assumption that mantle heterogeneity has a purely thermal origin, so that velocity and density heterogeneity are proportional to one another. On the other hand, it has sometimes been suggested that S-wave velocity might be more directly sensitive to temperature, while P heterogeneity is more strongly influenced by chemical composition. In the present study, we use only S-, and not P-velocity, to estimate density heterogeneity through linear scaling, and hence the sensitivity of core-reflected P phases to mantle structure. Regardless of whether density is more closely related to P- or S-velocity, we think it is worthwhile to explore both scaling approaches in our efforts to explain seismic data. The similarity of the results presented in this study to those obtained by scaling P-velocity to density suggests that compositional anomaly has a limited impact on viscous flow in the deep mantle.</p>
8 Convective flow in the mantle can be thought of (and modeled) as exclusively driven 9 by density heterogeneities in the mantle itself, and the resulting lateral variations 10 in the Earth’s gravity field. With this assumption, and a model of mantle rhe11 ology, a theoretical relationship can be found between 3-D mantle structure and 12 flow-related quantities that can be measured on the Earth’s surface, like free-air 13 gravity anomalies. This relationship can be used to set up an inverse problem, 14 with 1-D mantle viscosity as a solution. In the assumption that seismic velocity 15 anomalies be of purely thermal origin, and related to density anomalies by a simple 16 scaling factor, we invert the large-scalelength component of the above-mentioned 17 measurements jointly with seismic observations (waveforms and/or travel times) to 18 derive an accurate 5-layer spherically symmetric model of upperand lower-mantle 19 viscosity. We attempt to account for non-uniqueness in the inverse problem by ex20 ploring the solution space, formed of all possible radial profiles of Earth viscosity, 21 by means of a non-deterministic global optimization method: the evolutionary algo22 rithm (EA). For each sampled point of the solution space, a forward calculation is 23 conducted to determine a map of gravity anomalies, whose similarity to GRACE is 24 then measured; the procedure is iterated to convergence, according to EA criteria. 25 The robustness of the inversion is tested by means of synthetic tests, indicating that 26 our gravity data set is able to constrain less than 6 radial layers, each with uniform 27 viscosity. Independently of the tomographic model or the scaling factor adopted to 28 convert seismic velocity into density structure, the EA optimization method finds 29 viscosity profiles characterized by low-viscosity in a depth range corresponding to 30 the transition zone, and relatively uniform elsewhere. 31
We present preliminary results of an iterative inversion of seismic and density data for the 3-D thermochemical structure of the upper mantle. Our approach relies on a mineral physics model based on current knowledge of material properties at high pressure (P) and temperature (T). The phase equilibria and the elastic properties are computed by using a recent thermodynamical model covering a six oxides (NCFMAS) system. Anelastic properties are implemented with a P, T and frequency dependent law based on available mineral physics knowledge. The model predicts values of physical parameters (e.g., shear velocity, density) as function of pressure (or depth), temperature and composition. Equilibrium compositions or mixtures of different compositions (e.g., MORB and Harzburgite) can be considered. First, we interpret available seismic models for temperature, assuming given compositions. For each model, we predict density and viscosity structure. Second, we compute the geoid kernels considering the average viscosity profiles of each model and perturb the 3-D density model(s). Density variations from the starting models are assumed to be due to lateral variations in composition. Consistently with the origin of such anomalies through melting extraction, we start by assuming only variations along a compositional axis that goes from harzburgite to MORB. We iterate the procedure until convergence. The inversion is implemented using a parametrization in spherical harmonics, a global scale basis which allows a clear analysis of the results in terms of relative contribution of different harmonic degrees (or wavelengths). Although lateral variations in viscosity are not accounted for in the inversion, we evaluate their effects with a forward approach, using the numerical code STAGYY. The synthetics geoids are computed with instantaneous flow calculations and compared with observations. We also use extreme physical laws for the most uncertain material parameters, i.e. viscosity and anelasticity, in order to assess their role on the outcome of the inversion. In general, we found that lateral thermal variations can explain most of the data. Including gravity data helps to determine lateral variations in composition. At a global scale, the dychotomy between continental and oceanic regions clearly emerges. Also, the large temperature variations between continents and oceans down to 300km produce large viscosity variations. In turn, these play a not negligible role on the geoid anomalies at spherical harmonics between 4 and 16 degrees. Including higher frequency seismic data, comparing the results with available petrological information and adding more complexities into the compositional parametrization (e.g., water effects) can help to better resolve the thermochemical anomalies at a regional scale.
Using a spherical model of postseismic deformation, for the first time we have computed the global contribution of large earthquakes to the relative sealevel variations in the twentieth century. We have found that great earthquakes have the overall tendency to produce a sealevel rise, and that they affect the measurements taken at those tide‐gauge sites that are commonly employed to obtain global estimates of sealevel rise. Though on a global scale most of the signal is associated with thrust events, on a regional scale the effects of great transcurrent earthquakes cannot be neglected. Depending on the viscosity of the asthenosphere, the contribution of earthquakes to the long‐term sealevel changes amounts to at least 0.1 mm/yr. Thus, the climate‐driven long‐term sealevel changes deduced by tide‐gauge observations may be slightly, but not negligibly, overestimated.
We present an overview of the potential of active monitoring techniques to investigate the many factors affecting the concentration of radon in houses. We conducted two experiments measuring radon concentration in 25 apartments in Rome and suburban areas for two weeks and in three apartments in the historic center for several months. The reference levels of 300 and 100 Bq/m3 are overcome in 17% and 60% of the cases, respectively, and these percentages rise to 20% and 76% for average overnight radon (more relevant for residents’ exposure). Active detectors allowed us to identify seasonal radon fluctuations, dependent on indoor-to-outdoor temperature, and how radon travels from the ground to upper floors. High levels of radon are not limited to the lowest floors when the use of heating and ventilation produces massive convection of air. Lifestyle habits also reflect in the different values of gas concentration measured on different floors of the same building or in distinct rooms of the same apartment, which cannot be ascribed to the characteristics of the premises. However, the finding that high residential radon levels tend to concentrate in the historic center proves the influence of factors such as building age, construction materials, and geogenic radon.
The relative seismic velocity variations possibly associated to large earthquakes can be readily monitored via cross-correlation of seismic noise. In a recently published study, more than 2 yr of continuous seismic records have been analysed from three stations surrounding the epicentre of the 2009 April 6, Mw 6.1 L'Aquila earthquake, observing a clear decrease of seismic velocities likely corresponding to the co-seismic shaking. Here, we extend the analysis in space, including seismic stations within a radius of 60 km from the main shock epicentre, and in time, collecting 5 yr of data for the six stations within 40 km of it. Our aim is to investigate how far the crustal damage is visible through this technique, and to detect a potential post-seismic recovery of velocity variations. We find that the co-seismic drop in velocity variations extends up to 40 km from the epicentre, with spatial distribution (maximum around the fault and in the north–east direction from it) in agreement with the horizontal co-seismic displacement detected by global positioning system (GPS). In the first few months after L'Aquila earthquake, the crust's perturbation in terms of velocity variations displays a very unstable behaviour, followed by a slow linear recovery towards pre-earthquake conditions; by almost 4 yr after the event, the co-seismic drop of seismic velocity is not yet fully recovered. The strong oscillations of the velocity changes in the first months after the earthquake prevent to detect the fast exponential recovery seen by GPS data. A test of differently parametrized fitting curves demonstrate that the post-seismic recovery is best explained by a sum of a logarithmic and a linear term, suggesting that processes like viscoelastic relaxation, frictional afterlip and poroelastic rebound may be acting concurrently.
