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The distribution of plate motion between multiple fault strands and how this distribution may evolve remain poorly understood, despite the key implications for seismic hazards. The North Anatolian Fault in northwest Turkey is a prime example of a multistranded continental transform. Here we present the first constraints on late Quaternary slip rates on its northern branch across the Cinarcik Basin in the eastern Marmara Sea. We use both deep penetration and high-resolution multichannel seismic reflection data with a stratigraphic age model to show that a depocenter has persisted near the fault bend responsible for that transform basin. Successively older depocenters have been transported westward by fault motion relative to Eurasia, indicating a uniform right-lateral slip rate of 18.5 mm/yr over the last 500,000 years, compared to overall GPS rates (23-24 mm/yr). Thus, the northern branch has slipped at a nearly constant rate and has accounted for most of the relative plate motion between Eurasia and Anatolia since ~0.5 Ma.

The deployment of a worldwide network for infrasound detection requires numerical methods for modeling these signals over long distances. A ray theoretical approach appears robust and efficient. It furthermore allows a straightforward interpretation of recorded phases. We have developed a three-dimensional Hamiltonian ray tracing for modeling linear acoustic waves in the atmosphere. Propagating over distances superior to 500 km requires the curvature of the Earth to be considered, which is achieved by using spherical coordinates. High atmospheric winds are properly handled through a modified Hamiltonian. These winds as well as sound velocity can change significantly during long-lasting propagations; these variations are also included in our modeling. Finally, the amplitude of infrasonic signals is computed by concomitantly solving for paraxial rays and assessing the evolution of the ray tube thus defined. We present the theory for this atmospheric infrasound modeling and some simple applications that establish its robustness and potential.

International audience; Active volcanoes in the Vanuatu archipelago permanently generate infrasonic waves. Their monitoring over 1 year exhibits clear seasonal trends in the direction of arrival of the detected signals. From summer to winter the azimuth variation reaches 15°. This deviation is essentially due to the reversibility of the zonal stratospheric wind with season which strongly affects the deflection of the ray direction. A three-dimensional (3-D) ray-tracing modeling roughly explains the observed seasonal trend in the azimuth variation but underestimates its amplitude. The discrepancy between the measurements and the results of simulation is explained by undervalued wind speeds in the upper atmosphere. Infrasonic observations are used as input of an inversion scheme for adjusting the vertical structure of the wind in the upper stratosphere and mesosphere. It is shown that the mesospheric zonal winds are underestimated by at least 20 m/s throughout the year with differences reaching 50 m/s.

[1] The easternmost segment of the Nankai trough remained unruptured during the 1944 and 1946 earthquakes that affected the rest of the trench. It is therefore a zone of seismic gap that also undergoes a peculiar tectonic regime due to its proximity with the Izu collision zone. We investigate this area with densely sampled active seismic data. One hundred ocean bottom seismometers, spaced only 1 km apart, were deployed along a line perpendicular to the trough. A two-step tomographic approach, designed for this type of acquisition, is applied: first, a long-wavelength velocity model is built by a linearized inversion of first arrival travel times, its resolution is assessed, and its reliability is confirmed through a semiglobal approach in which a large range of starting models is tested; second, this tomographic model is used to exploit late reflected arrivals with a prestack depth migration, the result of which is validated by classically modeling refraction and reflection travel times in the derived structural model. Thus both velocities and discontinuities are mapped in our model with a minimum of data interpretation and a priori knowledge. Our results bring new insights on the deep crustal zones of the margin and its evolution. The existence of presently inactivated major thrusts is very clearly evidenced in the backstop. A general timing for its evolution and deformation is proposed. Farther seaward, the presence of a subducted ridge beneath the accretionary wedge is confirmed. Its compressive structure and origin are demonstrated. It exhibits a thickened lower crust which nature is discussed. Besides, strong analogies between this “Paleo-Zenisu” subducting ridge and the Zenisu ridge, observed farther south, are also emphasized, bringing evidences for a steady state crustal deformation pattern.

[1] Classical active seismic methods fail to sharply image the earth’s deep crust. We present the first crustal-scale application of 2-D full waveform inversion based on dense ocean bottom seismic data to investigate the Eastern Nankai subduction system (Japan). This approach allows to quantify seismic velocities up to an unprecedented degree of resolution. Results reveal compressive tectonic features within both the subducting oceanic crust and the backstop. At depth, velocity anomalies along major faults and structural discontinuities bring evidence for the presence of fluids and weakened material and also for a possible coseismic slip partitioning structure. INDEX TERMS: 0902 Exploration Geophysics: Computational methods, seismic; 3025 Marine Geology and Geophysics: Marine seismics (0935); 8010 Structural Geology: Fractures and faults; 8105 Tectonophysics: Continental margins and sedimentary basins (1212). Citation: Dessa, J.-X., S. Operto, S. Kodaira, A. Nakanishi, G. Pascal, J. Virieux, and Y. Kaneda (2004), Multiscale seismic imaging of the eastern Nankai trough by full waveform inversion, Geophys. Res. Lett., 31, L18606, doi:10.1029/2004GL020453.

[1] Ray tracing for sonic wave propagation in a two-dimensional atmosphere structure is performed in the presence of spatially variable wind by using an Hamiltonian approach. Paraxial ray tracing is deduced by first-order perturbations. Using standard atmospheric data or illustrative models, we test numerically the Hamiltonian approach. The ray tracing allows accurate estimation of trajectories and associated travel-times as well as stable and accurate amplitude variations along a ray. Lateral variations of both sonic and wind velocity are important for quantitative evaluation of acoustic waves. Gradients of the wind field must be taken into account for precise quantification of ray trajectories and, consequently, for the localization of ground impacts, a key element for any micro-barometric recording station.

[1] On November 14, 2001, a strong earthquake occurred on the high plateau of western China. At a distance of 1800 km from the epicenter, coherent infrasonic waves were detected for more than one hour. Using both an inverse location procedure and a complete simulation of the radiated pressure field, distant source regions are accurately located. The seismo-acoustic coupling along a mountain belt is clearly demonstrated. Such an event offers an opportunity to validate velocity models in the atmosphere as well as to improve the understanding of the amplification of ground displacement caused by the topography.