Carene Larmat

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.

Time reversal location of glacial earthquakes

In 2003, Ekstrom et al. reported the detection and location of a new class of earthquakes occurring in the polar regions of the Earth. The proposed source mechanism involves large and sudden sliding motions of glaciers, which gave the name “glacial earthquakes” to these events. In this study we localize some of these earthquakes with a time reversal mirror (TRM) algorithm, which, contrary to ordinary back projection methods, does not involve testing each possible source location. In TRM localization, an earthquake is located on the basis of only one 3-D spectral element simulation of seismic wave propagation by using the full complexity of recorded data as simultaneous time-reversed sources. We show that on the basis of this approach, even glacial earthquakes with a faint signal can be correctly localized and that the pattern of the time-reversed wavefield is coherent with the motion of glaciers down their valley.

In situ characterization of shallow elastic nonlinear parameters with Dynamic Acoustoelastic Testing

In situ measurement of the elastic nonlinear site response is advantageous to provide optimal information for prediction of strong ground motion at a site. We report the first implementation of a technique known as Dynamic Acoustoelastic Testing (DAET) in situ with the ultimate goal of developing a new approach for site characterization. DAET has shown promising results at the laboratory scale for the study of nonlinear elasticity of Earth materials, detailing the full nonlinear elastic properties of the studied sample. We demonstrate the feasibility of DAET in situ and compare it to other methods (nonlinear resonance spectroscopy, wave amplitude dependence of propagation velocity, and wave distortion). Nonlinear elastic properties are characterized by DAET with the advantage of providing a local assessment compared to other methods, here at a depth of 4 m to 5 m. A vertical dynamic strain amplitude of 5 ×10−5 produces a relative change in compressional wave modulus of 6%. We measure an effective parameter of quadratic elastic nonlinearity of order −103, the same order of magnitude measured at the laboratory scale in rocks and in packs of unconsolidated glass beads. Hysteresis is observed in the variation in soil elasticity as a function of the instantaneous dynamic strain that evolves as the dynamic strain amplitude is increased from 9 ×10−7 to 5 ×10−5.

Energy current imaging method for time reversal in elastic media

An energy current imaging method is presented for use in locating sources of wave energy during the back propagation stage of the time reversal process. During the back propagation phase of an ideal time reversal experiment, wave energy coalesces from all angles of incidence to recreate the source event; after the recreation, wave energy diverges in every direction. An energy current imaging method based on this convergence/divergence behavior has been developed. The energy current imaging method yields a smaller spatial distribution for source reconstruction than is possible with traditional energy imaging methods.

On the validation of seismic imaging methods: Finite frequency or ray theory?

We investigate the merits of the more recently developed finite-frequency approach to tomography against the more traditional and approximate ray theoretical approach for state of the art seismic models developed for western North America. To this end, we employ the spectral element method to assess the agreement between observations on real data and measurements made on synthetic seismograms predicted by the models under consideration. We check for phase delay agreement as well as waveform cross-correlation values. Based on statistical analyses on S wave phase delay measurements, finite frequency shows an improvement over ray theory. Random sampling using cross-correlation values identifies regions where synthetic seismograms computed with ray theory and finite-frequency models differ the most. Our study suggests that finite-frequency approaches to seismic imaging exhibit measurable improvement for pronounced low-velocity anomalies such as mantle plumes.

Apparent Explosion Moments from Rg Waves Recorded on SPE

Abstract Seismic moments for the first four chemical tests making up phase I of the Source Physics Experiments (SPE) are estimated from 6‐Hz Rg waves recorded along a single radial line of geophones under the assumption that the tests are pure explosions. These apparent explosion moments are compared with moments determined from the reduced displacement potential method applied to free‐field data. Light detection and ranging (lidar) observations, strong ground motions on the free surface in the vicinity of ground zero, and moment tensor inversion results are evidence that the fourth test SPE‐4P is a pure explosion, and the moments show good agreement, 8×10 10   N·m for free‐field data versus 9×10 10   N·m for Rg waves. In stark contrast, apparent moments for the first three tests are smaller than near‐field moments by factors of 3–4. Relative amplitudes for the three tests determined from Rg interferometry using SPE‐4P as an empirical Green’s function indicate that radiation patterns are cylindrically symmetric within a factor of 1.25 (25%). This fact assures that the apparent moments are reliable even though they were measured on just one azimuth. Spallation occurred on the first three tests, and ground‐based lidar detected permanent deformations. As such, the source medium suffered late‐time damage. Destructive interference between Rg waves radiated by explosion and damage sources will reduce amplitudes and explain why apparent moments are smaller than near‐field moments based on compressional energy emitted directly from the source.

Time‐reversal in seismology.

This talk is a review of the use and history of time‐reversal in seismology. time‐reversal has been developed independently in the field of acoustics and in geophysics exploration, without the two communities being aware of the fact, as testifies the lack of the cross‐references in early papers. The elegance of time‐reversal is that it thrives with complexity. Seismologists have to deal with complex signal transmitted through the earth. Moreover, earthquakes are complex sources, involving different episodes of slip along the fault. Early in the development of time‐reversal in seismology, emphasis has been put on the need of accurate numerical schemes to back‐propagate the wavefield and the need of relatively dense data. In addition to presenting several of the landmark applications of time‐reversal in seismology, several of our results will be outlined. In the last decade, our group has studied the potential of TR with the unique approach of combining laboratory experiments with application to seismology ...

Apparent Explosion Moments from Rg Waves Recorded on SPEApparent Explosion Moments from Rg Waves Recorded on SPE

Abstract Seismic moments for the first four chemical tests making up phase I of the Source Physics Experiments (SPE) are estimated from 6‐Hz Rg waves recorded along a single radial line of geophones under the assumption that the tests are pure explosions. These apparent explosion moments are compared with moments determined from the reduced displacement potential method applied to free‐field data. Light detection and ranging (lidar) observations, strong ground motions on the free surface in the vicinity of ground zero, and moment tensor inversion results are evidence that the fourth test SPE‐4P is a pure explosion, and the moments show good agreement, 8×10 10   N·m for free‐field data versus 9×10 10   N·m for Rg waves. In stark contrast, apparent moments for the first three tests are smaller than near‐field moments by factors of 3–4. Relative amplitudes for the three tests determined from Rg interferometry using SPE‐4P as an empirical Green’s function indicate that radiation patterns are cylindrically symmetric within a factor of 1.25 (25%). This fact assures that the apparent moments are reliable even though they were measured on just one azimuth. Spallation occurred on the first three tests, and ground‐based lidar detected permanent deformations. As such, the source medium suffered late‐time damage. Destructive interference between Rg waves radiated by explosion and damage sources will reduce amplitudes and explain why apparent moments are smaller than near‐field moments based on compressional energy emitted directly from the source.

Apparent Explosion Moments fromRgWaves Recorded on SPE

Abstract Seismic moments for the first four chemical tests making up phase I of the Source Physics Experiments (SPE) are estimated from 6‐Hz Rg waves recorded along a single radial line of geophones under the assumption that the tests are pure explosions. These apparent explosion moments are compared with moments determined from the reduced displacement potential method applied to free‐field data. Light detection and ranging (lidar) observations, strong ground motions on the free surface in the vicinity of ground zero, and moment tensor inversion results are evidence that the fourth test SPE‐4P is a pure explosion, and the moments show good agreement, 8×10 10   N·m for free‐field data versus 9×10 10   N·m for Rg waves. In stark contrast, apparent moments for the first three tests are smaller than near‐field moments by factors of 3–4. Relative amplitudes for the three tests determined from Rg interferometry using SPE‐4P as an empirical Green’s function indicate that radiation patterns are cylindrically symmetric within a factor of 1.25 (25%). This fact assures that the apparent moments are reliable even though they were measured on just one azimuth. Spallation occurred on the first three tests, and ground‐based lidar detected permanent deformations. As such, the source medium suffered late‐time damage. Destructive interference between Rg waves radiated by explosion and damage sources will reduce amplitudes and explain why apparent moments are smaller than near‐field moments based on compressional energy emitted directly from the source.

3D Geophysical Modeling Validation and UQ for Ground-­based NEM

This Powerpoint presentation illustrates several aspects considered in 3D high-resolution modeling approaches for geophysical analysis.