Eric Clévédé

Time‐reversal imaging of seismic sources and application to the great Sumatra earthquake

The increasing power of computers and numerical methods (like spectral element methods) allows continuously improving modelization of the propagation of seismic waves in heterogeneous media and the development of new applications in particular time reversal in the three-dimensional Earth. The concept of time-reversal (hereafter referred to as TR) was previously successfully applied for acoustic waves in many fields like medical imaging, underwater acoustics and non destructive testing. We present here the first application at the global scale of TR with associated reverse movies of seismic waves propagation by sending back long period time-reversed seismograms. We show that seismic wave energy is refocused at the right location and the right time of the earthquake. When TR is applied to the Sumatra-Andaman earthquake (26 Dec. 2004), the migration of the rupture from the south towards the north is retrieved. Therefore, TR is potentially interesting for constraining the spatio-temporal history of complex earthquakes.

Tarapacá intermediate‐depth earthquake (Mw 7.7, 2005, northern Chile): A slab‐pull event with horizontal fault plane constrained from seismologic and geodetic observations

[1] A large (Mw 7.7) intermediate-depth earthquake occurred on 13 June 2005 in the Tarapaca region of the northern Chile seismic gap. Source parameters are inferred from teleseismic broadbands, strong motions, GPS and InSAR data. Relocated hypocenter is found at

Seismic waveform modeling and surface wave tomography in a three-dimensional Earth: asymptotic and non-asymptotic approaches

98 km depth within the subducting slab. The 21-days aftershock distribution, constrained by a postseismic temporary array, indicates a sub-horizontal fault plane lying between the planes of the double seismic zone and an upper bound of the rupture area of 60 km  30 km. Teleseismic inversion shows a slab-pull down dip extension mechanism on a nearly horizontal plane. Total seismic and geodetic moments are

The COSY Project: verification of global seismic modeling algorithms

5.5 Â 10 20 N.m, with an averaged slip of 6.5 m from geodesy. The earthquake rupture is peculiar in that the effective velocity is slow, 3.5 Km.s A1 for a high stress-drop, 21 –30 MPa. We propose that rupture was due to the reactivation by hydraulic embrittlement of a inherited major lithospheric fault within the subducting plate. The stress-drop suggests that the region of the slab between planes of the double seismic zone can sustain high stresses. Citation: Peyrat, S., et al. (2006), Tarapaca intermediate-depth earthquake (Mw 7.7, 2005, northern Chile): A slab-pull event with horizontal fault plane constrained from seismologic and geodetic observations, Geophys.

New refinements in attenuation measurements from free-oscillation and surface-wave observations

Abstract We investigate the impact of the theoretical limitations brought by asymptotic methods on upper-mantle tomographic models deduced from long-period surface wave data (period >80 s), by performing a synthetic test using a non-asymptotic formalism. This methodology incorporates the effects of back and multiple forward scattering on the wave field by summing normal modes computed to third order of perturbations directly in the 3D Earth, and models the sensitivity to scatterers away from the great-circle path. We first compare the methods we used for the forward problem, both theoretically and numerically. Then we present results from the computation of 7849 synthetic Love waveforms in an upper mantle model consisting of two heterogeneities with power up to spherical harmonic degree 12. The waveforms are subsequently inverted using a 0th order asymptotic formalism (equivalent to a path-average approximation in the surface waves domain). We show that the main structures are retrieved, but that the theoretical noise on the output model is of the same order as the noise due to the path-coverage and a priori constraints.

Prompt gravity signal induced by the 2011 Tohoku-Oki earthquake

Abstract Progress in determining the details of the global 3-D seismic velocity structure requires the ability to accurately model seismic wave propagation (e.g., travel times, waveforms, etc.) through heterogeneous 3-D Earth models. While for spherically symmetric models (quasi-) analytical solutions are available for the verification of numerical algorithms, this is not the case for general heterogeneous models. It is therefore desirable to establish global 3-D test models and verified reference seismograms, which allow us to assess the accuracy of numerical algorithms quantitatively. Prior to a workshop held on this issue at the 1997 IASPEI Meeting, a 3-D test model was handed out to various groups and long-period synthetic seismograms were returned. This workshop was the initiation of the COmparison of global SYnthetic seismogram techniques (COSY) Project, which aims at establishing a WWW page (http://www.geophysik.uni-muenchen.de/COSY), where the test models and seismograms as well as some of the algorithms can be accessed. In this paper, we study the accuracy of and compare solutions from different numerical methods for a spherically symmetric model and the 3-D test model. The algorithms compared use the normal-mode method, the Direct Solution Method (DSM), a direct evaluation of the Greens function for spherically symmetric media (GEMINI), and the finite-difference (FD) method. Our 3-D test model is a perturbation to the spherically symmetric background model (PREM) based on a (scaled) temperature field from numerical modeling of mantle convection. The model displays many features in common with recent seismic tomographic images. We suggest that in addition to (future) 3-D reference Earth models, verified reference synthetic seismograms should be established for use by the seismological community.

10 – Normal Modes of the Earth and Planets

This paper deals with improvements in the accuracy and precision of attenuation measurements in the Earth, obtained from observations of surface waves and free oscillations. We focus our study on the observation of the resonant frequencies of the Earth and propose an improved method for deriving attenuation data from the free-oscillation amplitude decays that reduces uncertainties. We test our method on different theoretical seismograms computed for two different Earth models, a spherically symmetric one (the PREM model) and a laterally heterogeneous one (the M84A model). We compute seismograms for: the spheroidal fundamental mode only, the fundamental mode and overtones, without noise and with actual noise. Some biases related to the method are systematically found and corrected. Our conclusion is that if we fulfill some conditions in attenuation measurements by taking into account different parameters, such as the observed resonance period, the relative amplitude of the peak for the particular great-circle path considered, the noise level of the component in the station considered, it is possible to reduce the uncertainty in amplitude measurements. Doing this, we are more confident in free-oscillation measurements than in those of surface waves. It is evident that the presence of noise may increase the mean Q inferred from long-time series if no care is taken relative to the signal-to-noise level. The method is then systematically applied to a dataset of about one thousand vertical seismograms provided by the GEOSCOPE network after various large earthquakes, which occurred from 1982 to 1995. We determine reliable estimates of the mean frequency and attenuation of the fundamental spheroidal mode for the angular order range l=21–51 (period range 175–336 s). Some results are discarded for reasons explained. We compare the consistency of this new dataset with previously published Q observations and commonly used models (PREM, QM1, QL6). We point out the difficulties of obtaining reliable Q factors and show the discrepancies between surface waves and normal modes measurements. It is noted that surface wave attenuation measurements may be strongly affected by the time window used in order to extract each particular surface wave train, but fit normal modes results, in some cases.

Diffraction of long period Rayleigh waves by a slab: effects of mode coupling

Transient gravity changes are expected to occur at all distances during an earthquake rupture, even before the arrival of seismic waves. Here we report on the search of such a prompt gravity signal in data recorded by a superconducting gravimeter and broadband seismometers during the 2011 Mw 9.0 Tohoku-Oki earthquake. During the earthquake rupture, a signal exceeding the background noise is observed with a statistical significance higher than 99% and an amplitude of a fraction of μGal, consistent in sign and order of magnitude with theoretical predictions from a first-order model. While prompt gravity signal detection with state-of-the-art gravimeters and seismometers is challenged by background seismic noise, its robust detection with gravity gradiometers under development could open new directions in earthquake seismology, and overcome fundamental limitations of current earthquake early-warning systems imposed by the propagation speed of seismic waves.

85.16 - Higher Order Perturbation Theory: 3D Synthetic Seismogram Package

This chapter provides an overview of the theory of normal modes and of observations and inversions of normal modes performed since 1961. It describes the gravito-elastic equation and the normal modes of a spherical nonrotating elastic isotropic (SNREI) model. Observing and inverting the normal modes of a planet is one of the most powerful methods for recovering its internal structure. It is also considered the only way to get from seismic data a direct sensitivity to density, including lateral variations, as all other waves are sensitive mostly to the seismic velocities. Normal modes are permanently excited, at the level of the nanogal, too small for their observation on a single record but can sufficiently be enhanced by stacking of at least one years signals. The perturbation theory for the modeling of the asphericity of the Earth and the method of normal mode summation, which allows use of the aspherical modes for the computation of seismograms in a complex Earth are also presented. The chapter considers the theory necessary for the numerical computation of the normal modes of a rotating, aspherical, anelastic, and anisotropic Earth.

Earth's Free Oscillations Excited by the 26 December 2004 Sumatra-Andaman Earthquake

We compute seismograms of the fundamental Rayleigh waves propagating through a slab structure, with either a lateral variation in seismic velocities, or in attenuation. At periods of 100 sec, we show that the phase delay is strongly reduced by the surface waves Fresnel zone, and that coupling must be considered far along the dispersion branch, up to at least l±25. Limiting the coupling to fewer modes produces a signal associated to a ghost structure at the antipode of the slab. We also show that the amplitude perturbations produced by the diffraction and the attenuation of the slab are comparable in size. Future waveform studies, especially those associated to global waveform inversions, must then carefully consider these effects.