Gradient-index surface acoustic metamaterial for steering omnidirectional ultra-broadband seismic waves

Elastic phononic crystals (PnCs) and metamaterials show promising prospects in seismic wave pro-tection. Hitherto, most existing solutions for this purpose use PnCs' bandgaps against seismic waves (mainly Rayleigh waves). However, due to the lack of available large-bandgap PnCs at low frequencies so far, their operating bandwidths are relatively small. The bandwidth will be even smaller for omnidirectional protection since the full bandgaps are always smaller than the directional bandgaps. Here, to realize ideal protection against seismic Rayleigh waves, we propose a gradient-index seismic metamaterial based on a surface acoustic PnC. Constructed using common building materials (e.g, steel and concrete), this seismic metamaterial is robust, convenient to implement, and can protect buildings from ultra-broadband seismic Rayleigh waves omnidirectionally. For example, such a metamaterial with a lattice constant of 6 meters can suppress 15-30 dB Rayleigh waves in a frequency range from 4 Hz to 20 Hz.(c) 2022 Elsevier Ltd. All rights reserved.