Building Protection From Seismic Waves

By deflecting seismic waves away from structures, this innovative technique known as seismic cloaking has the potential to prevent harmful energy from reaching buildings. A collaborative research effort involving the University of Missouri, the Beijing Institute of Technology, Ningbo University, and Yeshiva University has led to this discovery.

Dr. Guoliang Huang, the Huber and Helen Croft Chair in Engineering at the University of Missouri and a member of the research team, has recently been focusing on utilizing metamaterials for controlling and propagating waves.

Unlike natural materials such as metal, rubber, and glass, which possess properties based on their molecular bonds, artificial metamaterials exhibit unique behaviors due to the patterns of their internal structures, independent of the materials they are made from.

According to Huang, metamaterials enable precise manipulation of energy wave paths, ensuring they travel precisely where intended within a material. This was reported in press materials released by the University of Missouri.

Waveguiding metamaterials have gained traction in various fields including optics, acoustics, and mechanics over the past decade, particularly when engineered using the principles of topology.

Topology, a branch of geometry, classifies shapes as equivalent if they can be stretched and deformed into one another through continuous changes. For instance, a coffee mug and a ring-shaped doughnut are considered equivalent because both have one hole. However, transforming either shape into a pretzel would require discontinuous changes, breaking the equivalence.

The application of topological geometry in physics has proven valuable for studying systems with solutions that exhibit integer-valued quantities, where continuous states are separated by discontinuities.

For instance, the quantum Hall insulator is a 2D film displaying the quantum Hall effect, which causes its resistance to change in a step-like manner as an external magnetic field is continuously varied.

Inspired by these principles, Huang explored the possibility of leveraging topological phenomena on a larger scale. By applying similar principles, he believed it could be feasible to adapt to acoustic waves, mechanical waves, or seismic waves, as the governing equations for these phenomena are remarkably similar.

In 2021, Huang and his team made significant progress by developing a metamaterial capable of precise acoustic steering. This metamaterial allowed internal phonons to travel along predetermined paths within the material without any energy decay. It also facilitated beam splitting, enabling the same sound to be directed to multiple chosen locations within the metamaterial block.

Building upon this success with sound waves, the researchers directed their attention to controlling surface-wave energy. In July, they published new research on a modified metamaterial that could effectively manipulate surface waves.

The research team created an aluminum block with an array of pillar-type resonators interconnected by thin bridges, which acted as wave channels. Each pillar had a length of approximately 1 cm.

The scientists highlighted the promise of the metamaterial with pillar-type resonators, considering its simple structure and fabrication process, as stated in their Science Advances paper.

By adjusting the heights and thicknesses of the bridges, the team successfully steered Rayleigh waves through the array along predetermined paths and controlled the temporal phases of the signals, even in the presence of random fluctuations in the couplings.

Rayleigh waves, which are rolling surface waves associated with earthquakes, can cause substantial damage by displacing structures vertically and horizontally as they propagate through the ground.

The researchers expressed their belief that their work establishes a foundation for engineering applications. By programming couplings in a space-modulated mechanical metasurface, they envision the selective and robust point-to-point transport of surface wave signals.

While the current demonstration focused on a specific frequency band for the surface waves, Huang suggests the possibility of designing active metamaterials. By incorporating "smart materials" that deform in response to feedback, a wider range of scenarios can be accommodated. These smart materials can undergo shape changes, alterations in stiffness, or even phase adjustments in response to electrical or magnetic inputs.

Huang foresees the implementation of a layer of the new metamaterial positioned between the Earth's surface and a building. This metamaterial would effectively redirect seismic waves away from the structure by channeling them through it.

In comparison to conventional seismic protection methods that rely on counterweights or isolating buildings from the ground, the concept of precision waveguiding is still relatively unexplored. Huang aims to leverage the manufacturing simplicity of passive metamaterials to develop cost-competitive solutions for safeguarding against seismic activity.

According to the University of Missouri, the current waveguiding demonstration is in its early stages of development and primarily serves as a foundation for other scientists to build upon and modify as needed.