Seismic holes for controlling surface waves

The concept of controlling seismic waves is in progress and first full-scale experiments had confirmed the interest which we can have for this new axis of research. The possibility of imagining a seismic 2D or 3D metamaterial made of a grid of inclusions or empty boreholes in the soil, interacting with a part of the earthquake signal is becoming realistic. Here we show the dynamic response of a meter-scale grid of vertical empty cylindrical holes, under the excitation of a 50 hz source. The idea of seimic metamaterial is flowing from results obtained with breakwater device for waves propagating over sea and ocean as a potential application of photonic crystal at the meter-scale domain [1]. Non-overtopping dykes for ocean waves can be also envisaged with meter scale invisibility carpets for water waves [2]. Then the possibility of imagining seismic 2D device in the soil became realistic. We distinguish long period surface waves (low frequency, i.e. < 1 Hz) travelling along the Earth’s surface, on the crust, and short ones (< 10 Hz) mainly generated in case of site effects described above or by human activities at the Earth surface. An important fact is the low value of surface wave velocity, generated by natural seismic source in superficial and under-consolidated recent subgrade: sometimes less than 100 m/s. In these soft soil layers, wavelengths of induced surface waves are shorter than direct P and S waves ones: from few meters to hundreds of meters. This order of wavelength is similar to those of buildings so we can expect building’s resonance in case of earthquakes [3]. The objective is to conceive seismic metamaterials whose size could be close to those of the building project (figure 1). Figure 1: Concept of structured soil with vertical elements. These elements could have two roles: increasing the bearing capacity of the soil and interacting favorably with seismic vibrations. In this paper, we present a dynamic interpretation of results [3] obtained thank to a meter-scale grid of vertical empty cylindrical holes, excited in vibration by a monochromatic 50 hz source (figure 2). Figure 2: Grid of holes and seismic sensors during the seismic test of Grenoble in 2012 [1]. REFERENCES1. Hu, X. and C.T. Chan. “Refraction of water waves by periodic cylinder arrays”, Phys. Rev. Lett. 95, 154501, 2005.2. G. Dupont, O. Kimmoun, B. Molin, S. Guenneau and S. Enoch, Numerical and experimental study of an invisibilitycarpet in a water channel, Phys. Rev. E (in press).3. Brûlé, S., E. H. Javelaud, S. Enoch, and S. Guenneau. “Experiments on Seismic Metamaterials: Molding SurfaceWaves”, Phys. Rev. Lett. 112, 133901, 2014.