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We report on a seismic metamaterial experiment in a pine-tree forest environment where the dense collection of trees behaves as subwavelength coupled resonators for surface seismic waves. For the METAFORET experiment, more than 1000 seismic sensors were deployed over a 120 m × 120 m area to study the properties of the ambient and induced seismic wavefield that propagates in the ground and in trees. The goal of the experiment was to establish a link between seismic-relevant scales and microscale and mesoscale studies that pioneered the development of metamaterial physics in optics and acoustics. The first results of the METAFORET experiment show the presence of frequency band gaps for Rayleigh waves associated with compressional and flexural resonances of the trees, which confirms the strong influence that a dense collection of trees can have on the propagation of seismic waves.

We apply shear wave elastography, a medical imaging technique that uses ultrafast ultrasound to analyze rupture nucleation. In order to better understand friction and rupture processes which are responsible for earthquake nucleation, we build a laboratory experiment that reproduces the stick-slip behavior of a seismic fault for soft elastic materials. The rupture process is generated by controlling a driving slab through a motor in synchronization with the ultrasound acquisition system. As an analog to an asperity on a fault, a patch of sand grains constitutes the stick-slip behavior. In contrast to classical seismological methods, we are able to follow the wave propagation inside the sample and thus highly resolve the near and intermediate terms of the elastic wavefield. First results confirm the previously proven existence of super-shear as well as sub-Rayleigh fronts for the in-plane shearing mode. We anlayze the rupture speed distribution on the fault and resolve the extension of the asperity as well as the main nucleation zone. While the dynamics for different ruptures in one experiment vary, the radiated far-field of different ruptures appears more stable. Finally, we present a gel-on-gel sliding setup which allows control of the bimaterial effect present in the previous setups. [ABSTRACT FROM AUTHOR]

An underwater acoustic barrier based upon forward scattering in a Time-Reversal Mirror (TRM) was experimentally demonstrated for the first time in 2000 by Song et al.. The barrier consisted of a TRM, a vertical receive array (VRA) and a co-located probe source working at 3,500 Hz in an ocean waveguide near the western coast of Italy. In April 2003 further barrier tests were performed by applying for the first time at sea a new method that provided the capability to focus at different depths by using only the TRM transducers without the complication of additional probe sources. An echo repeater towed by R/V Alliance crossed the barrier, emulating the field forward scattered by a possible intruder insonified by the TRM. The presence of a target between the time reversal mirror and the focus can be detected if it significantly disturbs the quiescent region. A normal mode code is used to model the sound propagation in a waveguide. This is applied to predict the unperturbed and perturbed focused acoustic field measurements conducted at sea. Model-data comparison suggests that target detection performance is reasonably predictable using a numerical propagation model. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]