Page 4 - EQTips_Eng

Learning

2 Earthquake Design
Earthquake Tip and

Construction

How the ground shakes?

Seismic Waves
Large strain energy released during an earthquake P-Waves
travels as seismic waves in all directions through the Push and pull
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 (Figure 1). Body waves consist of
Primary Waves (P-waves) and Secondary Waves (S- Extension Compression
waves), and surface waves consist of Love waves and S-Waves Up and down
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 (Figure 2). Love waves
cause surface motions similar to that by S-waves, but
with no vertical component. Rayleigh wave makes a Side to side Direction of
material particle oscillate in an elliptic path in the Energy Transmission
vertical plane (with horizontal motion along direction
of energy transmission). Love Waves Sideways in horizontal plane

Structure

Surface Waves

Rayleigh Waves
Elliptic in vertical plane

Soil
Body
Waves
Fault EQ

Rupture Geologic Strata Figure 2:

Motions caused by Body and Surface Waves

Figure 1: Arrival of Seismic Waves at a Site (Adapted from FEMA 99, Non-Technical
Explanation of the NEHRP Recommended

Provisions)

P-waves are fastest, followed in sequence by S-,
Love and Rayleigh waves. For example, in granites, P- Measuring Instruments
and S-waves have speeds ~4.8 km/sec and The instrument that measures earthquake shaking,
~3.0km/sec, respectively. S-waves do not travel a seismograph, has three components – the sensor, the
through liquids. S-waves in association with effects of recorder and the timer. The principle on which it works
Love waves cause maximum damage to structures by is simple and is explicitly reflected in the early
their racking motion on the surface in both vertical seismograph (Figure 3) – a pen attached at the tip of an
and horizontal directions. When P- and S-waves reach oscillating simple pendulum (a mass hung by a string
the Earth's surface, most of their energy is reflected from a support) marks on a chart paper that is held on
back. Some of this energy is returned back to the a drum rotating at a constant speed. A magnet around
surface by reflections at different layers of soil and the string provides required damping to control the
rock. Shaking is more severe (about twice as much) at amplitude of oscillations. The pendulum mass, string,
the Earth's surface than at substantial depths. This is magnet and support together constitute the sensor; the
often the basis for designing structures buried drum, pen and chart paper constitute the recorder; and
underground for smaller levels of acceleration than the motor that rotates the drum at constant speed
those above the ground. forms the timer.
3