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Now showing items 1 - 14 of 14

  • GRACE gravity data help constraining seismic models of the 2004 Sumatran earthquake

    Cambiotti, G.   Bordoni, A.   Sabadini, R.   Colli, L.  

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  • Residual polar motion caused by coseismic and interseismic deformations from 1900 to present

    Cambiotti, G.   Wang, X.   Sabadini, R.   Yuen, D.A.  

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  • On the response of the Earth to a fault system: its evaluation beyond the epicentral reference frame

    Cambiotti, G.   Sabadini, R.  

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  • Joint estimate of the rupture area and slip distribution of the 2009 L’Aquila earthquake by a Bayesian inversion of GPS data

    Cambiotti, G.   Zhou, X.   Sparacino, F.   Sabadini, R.   Sun, W.  

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  • Joint estimate of the rupture area and slip distribution of the 2009 L'Aquila earthquake by a Bayesian inversion of GPS data

    Cambiotti, G.   Zhou, X.   Sparacino, F.   Sabadini, R.   Sun, W.  

    Usually, when inverting geodetic data to estimate the slip distributions on a fault, the area is made large enough to more than cover the rupture zone, with regularization producing regions of large slip with very small slip over the rest of the surface. We have developed a new inverse method which assumes that nonzero slip is confined to a rectangular region, and which jointly estimates, using Bayesian methods, the boundaries of this region as well as the slip distribution within it, using a smoothing parameter also determined as part of the inversion. Synthetic tests show that our method can successfully image deeper slip regions not resolved by previous methods, and does not produce spurious regions of nonzero slip. We apply our method to coseismic displacements measured by GPS for the 2009 L'Aquila earthquake, first determining the orientation of the fault assuming a simplified model with uniform slip, and then determining probability density functions for the location, length, and width of the rupture area and for the slip distribution. The standard deviation of slip is about 10 cm and describes a normal-faulting earthquake with a maximum slip of 88 +/- 11 cm and seismic moment of 3.32(-0.29)(+0.30) x 10(18) Nm.
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  • Modelling strains and stresses in continuously stratified rotating neutron stars

    Giliberti, E.   Cambiotti, G.   Antonelli, M.   Pizzochero, P. M.  

    We introduce a Newtonian model for the deformations of a compressible, autogravitating, and continuously stratified neutron star. The present framework can be applied to a number of astrophysical scenarios as it allows us to account for a great variety of loading forces. In this first analysis, the model is used to study the impact of a frozen adiabatic index in the estimate of rotation-induced deformations: we assume a polytropic equation of state for the matter at equilibrium but, since chemical reactions may be slow, the perturbations with respect to the unstressed configuration are modelled by using a different adiabatic index. We quantify the impact of a departure of the adiabatic index from its equilibrium value on the stressed stellar configuration and we find that a small perturbation can cause large variations both in displacements and strains. As a first practical application, we estimate the strain developed between two large glitches in the Vela pulsar showing that, starting from an initial unstressed configuration, it is not possible to reach the breaking threshold of the crust, namely to trigger a starquake. In this sense, the hypothesis that starquakes could trigger the unpinning of superfluid vortices is challenged and, for the quake to be a possible trigger, the solid crust must never fully relax after a glitch, making the sequence of starquakes in a neutron star an history-dependent process.
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  • Power-law Maxwell rheologies and the interaction between tectonic and seismic deformations

    Cambiotti, G.   Rigamonti, S.   Splendore, R.   Marotta, A. M.   Sabadini, R.  

    In a lithosphere where dislocation creep dominates the steady-state flow and the viscosity is stress-dependent, the equilibrium between tectonic stress and strain rate is broken after an earthquake due to the sudden coseismic stress change. The imbalance between tectonic stress and strain rate manifests itself during the post-seismic phase and, when seismic stress is comparable or smaller than tectonic stress, it affects post-seismic deformation via an effective anisotropy along the principal axes of the tectonic stress tensor. This issue is herein discussed within the framework of post-seismic models based on power-law Maxwell rheologies and, in the limit case of seismic stress much smaller than tectonic stress, we obtain a first-order approximation of the rheology which results into a linear anisotropic Maxwell model and we find that the effective anisotropy is associated to a two-modal relaxation characterized by the Maxwell time and the Maxwell time divided by the power-law index. Thus, as far as the steady-state flow within the lithosphere is dominated by dislocation creep, linear isotropic viscoelastic rheologies, like Newtonian Maxwell and Burgers models, represent a severe oversimplification which does not account for the physics of post-seismic deformation. This new physics is discussed characterizing the stress state of the ductile layers of the lithosphere before and after the earthquake for normal, inverse and strike mechanisms and for a variety of continental seismogenic zones and thermal models. We show that the first-order approximation of the power-law Maxwell rheology is valid for a quite wide range of small and moderate earthquakes. The most restrictive upper bounds of the seismic magnitude (which hold for the hottest thermal model here considered, with lithospheric thickness of H = 80 km and surface heat flux of Q = 70 mW m(-2)) occur for normal and inverse earthquakes and are 5.6 or 6.3 for a lower crust of wet diorite or felsic granulite, and 6.5 for a mantle of wet olivine. The upper bounds increase by about 0.3-0.4 for strike earthquakes and by more than 1.0 for the cold thermal model (H = 200 km and Q = 50 mW m(-2)).
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  • Compressible viscoelastodynamics of a spherical body at long timescales and its isostatic equilibrium

    Cambiotti, G.   Klemann, V.   Sabadini, R.  

    The problem of compressibility in modelling of viscoelastic deformations of planetary bodies is still a topic under discussion. Studies facing this topic discuss the error when considering a stratification of layers with constant material parameters. But homogeneous compressible layers imply that the initial state is not stable. So, any perturbation method applied to this type of state results in an ill-posed problem, evident in a denumerable infinite set of modes in the spectral representation of the solution. Based on the analytic solution of Cambiotti and Sabadini, we discuss any violation from the stable Adams-Williamson condition to result in unphysical behaviour where we concentrate here on the consequences for the horizontal displacement and deformation within the mantle due to surface loading. This focus motivates to revisit the Longman paradox, which discusses the boundary conditions for a compressible fluid core.
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  • The 2011 Tohoku-Oki earthquake GCMT solution from the GOCE model of the Earth's crust

    Sabadini, R.   Cambiotti, G.  

    Space gravity missions allow us to make a step ahead in the physics of large earthquakes, M-w higher than 8.5, thanks to the gravity signal from mass rearrangement within the crust and lithospheric mantle and from the ocean water washed away from the epicentral region by co-seismic displacement of the ocean bottom. Although designed to detect the time dependent and static components of the gravity field, the Gravity Recovery And Climate Experiment (GRACE) and the Gravity and steady state Ocean Circulation Explorer (GOCE) space missions play a complementary role in retrieving this new physics by sampling the co-seismic gravity signal and the thickness of the crust, the latter of importance to determine the synthetic expression of the former within realistic, dislocation Earth's models. We present a novel procedure for estimating the global Centroid Moment Tensor (CMT) solution, which provides the principal seismic source parameters (hypocentre and moment tensor) of the 2011 Tohoku earthquake that relies solely on space gravity data from GRACE and GOCE. Increasing the GOCE crustal thickness from 13.0, to 16.5 and 20.0 km, corresponding to the error bounds, the former and the latter values, of the regional value of 16.5 km, the epicentre for the best model moves by about 20 km roughly in the SE direction and the magnitude Mw decreases from 9.19 +/- 0.11 for the thinner crust to 9.07 +/- 0.11, the latter concordant with the CMT solution from teleseismic waves.
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  • Time-dependent geoid anomalies at subduction zones due to the seismic cycle

    Cambiotti, G.   Sabadini, R.   Yuen, D. A.  

    We model the geoid anomalies excited during a megathrust earthquake cycle at subduction zones, including the interseismic phase and the contribution from the infinite series of previous earthquakes, within the frame of self-gravitating, spherically symmetric, compressible, viscoelastic Earth models. The fault cuts the whole 50 km lithosphere, dips 20 degrees, and the slip amplitude, together with the length of the fault, are chosen in order to simulate an M-w =3D 9.0 earthquake, while the viscosity of the 170 km thick asthenosphere ranges from 10(17) to 10(20) Pa s. On the basis of a new analysis from the Correspondence Principle, we show that the geoid anomaly is characterized by a periodic anomaly due to the elastic and viscous contribution from past earthquakes and to the back-slip of the interseismic phase, and by a smaller static contribution from the steady-state response to the previous infinite earthquake cycles. For asthenospheric viscosities from 10(17)-10(18) to 10(19)-10(20) Pa s, the characteristic relaxation times of the Earth model change from shorter to longer timescales compared to the 400 yr earthquake recurrence time, which dampen the geoid anomaly for the higher asthenospheric viscosities, since the slower relaxation cannot contribute its whole strength within the interseismic cycle. The geoid anomaly pattern is characterized by a global, time-dependent positive upwarping of the geoid topography, involving the whole hanging wall and partially the footwall compared to the sharper elastic contribution, attaining, for a moment magnitude M-w =3D 9.0, amplitudes as high as 6.6 cm for the lowermost asthenospheric viscosities during the viscoelastic response compared to the elastic maximum of 3.8 cm. The geoid anomaly vanishes due to the back-slip of the interseismic phase, leading to its disappearance at the end of the cycle before the next earthquake. Our results are of importance for understanding the post-seismic and interseismic geoid patterns at subduction zones.
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  • GRACE gravity data help constraining seismic models of the 2004 Sumatran earthquake

    Cambiotti, G.   Bordoni, A.   Sabadini, R.   Colli, L.  

    The analysis of Gravity Recovery and Climate Experiment (GRACE) Level 2 data time series from the Center for Space Research (CSR) and GeoForschungsZentrum (GFZ) allows us to extract a new estimate of the co-seismic gravity signal due to the 2004 Sumatran earthquake. Owing to compressible self-gravitating Earth models, including sea level feedback in a new self-consistent way and designed to compute gravitational perturbations due to volume changes separately, we are able to prove that the asymmetry in the co-seismic gravity pattern, in which the north-eastern negative anomaly is twice as large as the southwestern positive anomaly, is not due to the previously overestimated dilatation in the crust. The overestimate was due to a large dilatation localized at the fault discontinuity, the gravitational effect of which is compensated by an opposite contribution from topography due to the uplifted crust. After this localized dilatation is removed, we instead predict compression in the footwall and dilatation in the hanging wall. The overall anomaly is then mainly due to the additional gravitational effects of the ocean after water is displaced away from the uplifted crust, as first indicated by de Linage et al. (2009). We also detail the differences between compressible and incompressible material properties. By focusing on the most robust estimates from GRACE data, consisting of the peak-to-peak gravity anomaly and an asymmetry coefficient, that is given by the ratio of the negative gravity anomaly over the positive anomaly, we show that they are quite sensitive to seismic source depths and dip angles. This allows us to exploit space gravity data for the first time to help constraining centroid-momentum-tensor (CMT) source analyses of the 2004 Sumatran earthquake and to conclude that the seismic moment has been released mainly in the lower crust rather than the lithospheric mantle. Thus, GRACE data and CMT source analyses, as well as geodetic slip distributions aided by GPS, complement each other for a robust inference of the seismic source of large earthquakes. Particular care is devoted to the spatial filtering of the gravity anomalies estimated both from observations and models to make their comparison significant.
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  • New insights into mantle convection true polar wander and rotational bulge readjustment

    Cambiotti, G.   Ricard, Y.   Sabadini, R.  

    The Earth's rotation axis is constantly tracking the main inertia axis of the planet that evolves due to internal and surface mass rearrangements. This motion called True Polar Wander (TPW) is due to mantle convection on the million years time scale. Most studies assumed that on this long time scale the planet readjusts without delay and that the Earth's rotation axis and the Maximum Inertia Direction of Mantle Convection (MID-MC) coincide. We herein overcome this approximation that leads to inaccurate TPW predictions and we provide a new treatment of Earth's rotation discussing both analytical and numerical solutions. We obtain an average TPW rate in the range [0.5 degrees-1.5 degrees] Myr(-1) and a sizeable offset of several degrees between the rotation axis and the MID-MC. This is in distinct contrast with the general belief that these two axes should coincide or that the delay of the readjustment of the rotational bulge can be neglected in TPW studies. We thus clarify a fundamental issue related to mantle mass heterogeneities and to TPW dynamics. (C) 2011 Elsevier B.V. All rights reserved.
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  • Incompressible analytical models for spinning-down pulsars

    Giliberti, E.   Antonelli, M.   Cambiotti, G.   Pizzochero, P. M.  

    We study a class of Newtonian models for the deformations of non-magnetised neutron stars during their spin-down. All the models have an analytical solution which allows to easily grasp the dependence of the strain on the star's main physical quantities, such as radius, mass, and crust thickness. We first use the model proposed by Franco, Link, and Epstein that depicts the star as made of a fluid core and an elastic crust with the same density, to compare the response to a decreasing centrifugal force on stars having different masses and equations of state. We find that the strain angle is peaked at the equator and its maximum value decreases as a function of the mass. Afterwards, we introduce a second, more refined, model in which the core and the crust have different densities, and the gravitational potential of the deformed body is self-consistently accounted for. The strain angle is still a decreasing function of the stellar mass, but now its maximum value is typically peaked at the poles and is larger (by a factor of four) than the corresponding value in the one-density model. Finally, within the present analytic approach, we evaluate the impact of the Cowling approximation: when the perturbations of the gravitational potential are neglected, we find an underestimation of the centrifugal effect on the star, since the strain angle is about 40% of the one obtained with the complete model.
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  • A source model for the great 2011 Tohoku earthquake (M-w=9.1) from inversion of GRACE gravity data

    Cambiotti, G.   Sabadini, R.  

    The co-seismic slip distribution of the 2011 Tohoku megathrust earthquake is constrained from GeoForschungsZentrum (GFZ) Gravity Recovery and Climate Experiment (GRACE) Level 2 data time series and our self-gravitating, compressible 1-D Earth model. After spatial localization of space gravity data in the surrounding of the U.S. Geological Survey (USGS) epicenter by means of orthogonal Slepian functions, we estimate the long-wavelength co-seismic gravity signature. The pattern is bipolar: the positive pole off-shore in the Pacific ocean (+3.61 mu Gal) and the negative pole in the northern Japan and Japan sea (-8.6 mu Gal). Inversion of the GRACE data resolves average features of finite fault models: the total seismic moment (5.3 +/- 1 x 10(22) Nm, corresponding to M-w = 9.1 in agreement with the centroid-moment-tensor solution), the rake angle (87 degrees +/- 9 degrees), and the mean position of the slip distribution on the fault plane. We obtain that the mean depth of the rupture is 17.1 +/- 5 kin, just below the Moho discontinuity, although we cannot exclude that the rupture also extended to shallower crustal layers and deeper within the lithospheric mantle due to the poor resolution of the along-dip dimension of the fault. (C) 2012 Elsevier B.V. All rights reserved.
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