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Sci Tech
Quake-proof design, construction
LARGE STRAIN energy released during an earthquake travels as seismic waves in all directions through the Earth's layers, reflecting and refracting at each interface. These waves are of two types - body waves and surface waves; the latter are restricted to near the Earth's surface. Body waves consist of Primary Waves (P-waves) and Secondary Waves (S-waves), and surface waves consist of Love waves and Rayleigh waves. Under P-waves, material particles undergo extensional and compressional strains along direction of energy transmission, but under S-waves, oscillate at right angles to it.
Love waves cause surface motions similar to that by S-waves, but with no vertical component. Rayleigh wave makes a material particle oscillate in an elliptic path in the vertical plane (with horizontal motion along direction of energy transmission).
P-waves are fastest, followed in sequence by S-, Love and Rayleigh waves. For example, in granites, P- and S-waves have speeds 4.8 km/sec and 3 km/sec, respectively. S-waves do not travel through liquids. They in association with effects of Love waves cause maximum damage to structures by rocking motion on the surface.
When P- and S-waves reach the Earth's surface, most of their energy is reflected back. Some of this energy is returned back to the surface by reflections at different layers of soil and rock. Shaking is more severe (about twice as much) at the Earth's surface than at substantial depths. This is often the basis for designing structures buried underground for smaller levels of acceleration than those above the ground.
The instrument that measures earthquake shaking, a seismograph, has three components — sensor, recorder and timer.
Principle on which it works is explicitly reflected in the early seismograph a pen attached at the tip of an oscillating simple pendulum (a mass hung by a string from a support) marks on a chart paper that is held on a drum rotating at a constant speed.
A magnet around the string provides required damping to control amplitude of oscillations. Pendulum mass, string, magnet and support together constitute the sensor; drum, pen and chart paper constitute the recorder; and the motor that rotates the drum at constant speed forms the timer.
One such instrument is required in each of the two orthogonal horizontal directions. For measuring vertical oscillations, the string pendulum is replaced with a spring pendulum oscillating about a fulcrum.
Some instruments do not have a timer device (i.e., the drum holding the chart paper does not rotate). They provide only the maximum extent (or scope) of motion. For this reason they are called seismoscopes.
The analog instruments have evolved over time, but today, digital instruments with modern computer technology are more common. The digital instrument records ground motion in memory of the microprocessor that is in-built in the instrument.
C.V.R.Murty
IIT, Kanpur
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