By Croor Singh
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In a recent study, researchers from the Indian Institute of Science, Bengaluru, King Saud University, Saudi Arabia, and Bhabha Atomic Research Centre, Mumbai, have reported a method to better identify building sites with soil that could be susceptible to damage from earthquakes. The findings of the study were published in the journal PLOS One.
Soil, which is the uppermost layer of the Earth's crust, is made of layers of varying thickness, culminating in what is called bedrock. The energy released in an earthquake vibrates this bedrock, and these vibrations propagate upwards. The different layers of soil between the bedrock and the surface are akin to springs of different stiffnesses—hard rocky soil layers act like stiff springs which vibrate only as much as (and in sync with) the bedrock, while softer soil layers act like weak springs, and can thus undergo much bigger displacements. If buildings aren't designed to account for the nature of the soil where they are built, they can be structurally damaged due to these oscillations.
The stiffness of a layer of soil is estimated by measuring how fast shear waves move through it. Shear waves are waves in which oscillations are perpendicular to the direction of wave propagation, akin to how the ripples in a pond move outwards when you throw a stone, although the water at any point moves up and down. Shear waves move faster through stiffer soil than through loose soil.
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