Véronique Farra

Teleseismic imaging of subducting lithosphere and Moho offsets beneath western Tibet

Teleseismic images suggest that the Tarim plate plunges ∼45°S, down to ∼300 km depth, beneath NW Tibet. The 410 km discontinuity shallows by ∼10 km under the plateau, implying ∼100°C cooler upper mantle. The deepest Moho on record (∼90 km) lies under W Qiangtang. It rises abruptly by ∼20 and ∼10 km beneath the Altyn Tagh Fault and Bangong Suture, respectively. Vp/Vs ratios are normal, except in the Yecheng flexural basin and deep under the south Karakax volcanics (∼1.92). W Kunlun’s Neogene tectonics are simply accounted for by oblique subduction of lithospheric mantle beneath an upward-extruding thrust wedge of the Tarim crust.

Complex lithospheric structure under the central Baltic Shield from surface wave tomography

Complex lithospheric structure under the central Baltic Shield from surface wave tomography

Lithospheric and upper mantle stratifications beneath Tibet: New insights from Sp conversions

[1] We assess the upper mantle structure under the Tibetan plateau from a S-to-P converted waves receiver functions study. Contrary to the Ps receiver functions blurred by multiples from Moho to 350 km depth, the Sp are better suited for imaging at these depths. The Moho is clearly recovered and often exhibits a complex and warped structure. The upper crust is marked by a significant low velocity zone in the southern part of the plateau, probably associated with partial melt, which vanishes north of the Bangong suture. In the upper mantle, between the Moho and the 660 km discontinuity, four stratification levels are identified. The strongest converter at a depth ranging between 120 to 180 km corresponds to the bottom of a low shear-wave velocity layer imaged by surface wave inversion.

First-order ray tracing for qP waves in inhomogeneous, weakly anisotropic media

ABSTRACTWe propose approximate ray-tracing equations for qP-waves propagating in smooth, inhomogeneous, weakly anisotropic media. For their derivation, we use perturbation theory, in which deviations of anisotropy from isotropy are considered to be the first-order quantities. The proposed ray-tracing equations and corresponding traveltimes are of the first order. Accuracy of the traveltimes can be increased by calculating a secondorder correction along first-order rays.The first-order ray-tracing equations for qP-waves propagating in a general weakly anisotropic medium depend on only 15 weak-anisotropy parameters (generalization of Thomsen’s parameters). The equations are thus considerably simpler than the exact ray-tracing equations. For higher-symmetry anisotropic media the equations differ only slightly from equations for isotropic media. They can thus substitute for the traditional isotropic ray tracers used in seismic processing. For vanishing anisotropy, the first-order ray-tracing equations reduce ...

Properties of the zeroth-, first-, and higher-order approximations of attributes of elastic waves in weakly anisotropic media

Use of the perturbation theory in the study of attributes of elastic waves propagating in weakly anisotropic media leads to approximate but transparent and simple formulas, which have many applications in forward and inverse wave modeling. We present and study such formulas. We show that all studied attributes depend on elements of a matrix linearly dependent on parameters of a medium. We study this dependence with the goal to understand which parameters of the medium, and in which combinations, affect individual wave attributes. Alternative auxiliar vector bases, in which the matrix can be specified, are proposed and studied. The vector bases offer alternative specifications of polarization vectors of qS waves. One of the important observations is that the higher-order (n > or = 2) perturbation formulas for qS waves are obtained separately for qS1 and qS2 waves. We also study effects of the use of the perturbation theory on the accuracy of the determination of the acoustical axes in weakly anisotropic media. We show that longitudinal directions in the first-order approximation are identical with actual ones. In singular directions, however, the first-order formulas provide directions, which may deviate from the exact ones, or they may even indicate false singular directions. Again, the above-mentioned matrix depending linearly on the parameters of the medium plays a central role in this study.

The 2015 Gorkha earthquake: A large event illuminating the Main Himalayan Thrust fault

The 2015 Gorkha earthquake sequence provides an outstanding opportunity to better characterize the geometry of the Main Himalayan Thrust (MHT). To overcome limitations due to unaccounted lateral heterogeneities, we perform Centroid Moment Tensor inversions in a 3-D Earth model for the main shock and largest aftershocks. In parallel, we recompute S-toP and P-to-S receiver functions from the Hi-CLIMB data set. Inverted centroid locations fall within a low-velocity zone at 10–15 km depth and corresponding to the subhorizontal portion of the MHT that ruptured during the Gorkha earthquake. North of the main shock hypocenter, receiver functions indicate a north dipping feature that likely corresponds to the midcrustal ramp connecting the flat portion to the deep part of the MHT. Our analysis of the main shock indicates that long-period energy emanated updip of high-frequency radiation sources previously inferred. This frequency-dependent rupture process might be explained by different factors such as fault geometry and the presence of fluids.

On the shaping factors of the secondary microseismic wavefield

Seismic noise in the period band 3-10 s is known as secondary microseism and it is generated at the ocean surface by the interaction of ocean gravity waves coming from nearly opposite directions. In this paper, we investigate the seismic content of the wavefield generated by a source at the ocean surface and three of the major wavefield shaping factors using the 2D spectral-element method: the ocean-continent boundary, the source site effect and the thickness of seafloor sediments. The seismic wavefield recorded on the vertical component seismograms below the seafloor is mainly composed of the fundamental mode and the first overtone of Rayleigh waves. A mode conversion from the first overtone to the fundamental mode of Rayleigh waves occurs at the ocean-continent boundary. The presence of a continental shelf at the ocean-continent boundary produces a negligible effect on land-recorded seismograms, whereas the source site effect, i.e. the source location with respect to the local ocean depth and sediment thickness, plays the major role. A source in shallow water mostly enhances the fundamental mode of Rayleigh waves, whereas a source in deep water mainly enhances the first overtone of Rayleigh waves. Land-recorded long period signals (T > 6 s) are mostly due to deep water sources, whereas land-recorded short period signals (T > 6 s) are due to sources in relatively shallow water, located close to the shelf break. Seafloor sediments around the source region trap seismic waves reducing the amplitude of land-recorded signals, especially at long periods (T > 6 s).

Ray-theoretical modeling of secondary microseism P waves

Thisworkwas supported byAgence Nationale de la Recherche grant ANR-14-CE01-0012 MIMOSA and grant ANR-10-LABX-19-01 ‘LabexMer’. LG acknowledges support from a Lamont–Doherty Earth Observatory Postdoctoral Fellowship and the Brinson Foundation. MS acknowledges MISTERIOS (CGL2013-48601-C2-1-R).

S velocity reversal in the mantle transition zone

The transition zone differs from the rest of the mantle by high positive P and S wave velocity gradients. However, by applying S receiver function technique to recordings of about 50 globally distributed stations, in 7 regions we obtain evidence of negative discontinuity at a depth around 500 km. Four of these regions correspond to the well known hotspots. No other hotspots are sampled by our observations at this depth. Our modeling suggests that the negative discontinuity is hardly an effect of anisotropy of wadsleyite. More likely, it is caused by isotropic S velocity reduction of around 0.2 km/s. Origin of the low velocity can be related to enhanced water content in wadsleyite.

Comparison of the FORT approximation of the coupling ray theory with the Fourier pseudospectral method

The standard ray theory (RT) for inhomogeneous anisotropic media does not work properly or even fails when applied to S-wave propagation in inhomogeneous weakly anisotropic media or in the vicinity of shear-wave singularities. In both cases, the two shear waves propagate with similar phase velocities. The coupling ray theory was proposed to avoid this problem. In it, amplitudes of the two S waves are computed by solving two coupled, frequency-dependent differential equations along a common S-wave ray. In this paper, we test the recently developed approximation of coupling ray theory (CRT) based on the common S-wave rays obtained by first-order ray tracing (FORT). As a reference, we use the Fourier pseudospectral method (FM), which does not suffer from the limitations of the ray method and yields very accurate results. We study the behaviour of shear waves in weakly anisotropic media as well as in the vicinity of intersection, kiss or conical singularities. By comparing CRT and RT results with results of the FM, we demonstrate the clear superiority of CRT over RT in the mentioned regions as well as the dangers of using RT there.