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How the Ground Shakes During Earthquakes?

How Ground Shakes During Earthquakes

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Ground shaking is the consequence of movement of seismic waves through the ground, which ranges from quite gentle in small earthquakes to considerably violent in large earthquakes. Ground shaking move buildings from side to side and up and down. The shaking that any structure experiences depends on how far it is from the fault and the soil under and around it.

Seismic waves travel faster through hard rock than through softer rock for instance soil and sand. However, as the waves pass from harder to softer rocks, they slow and their strength increases, so shaking is more intense where the ground is softer. Nonetheless, even buildings on hard rock can also experience intense shaking and damage if they are close to the surface rapture.

Not all seismic waves are the same. There are several different types and each type has a unique way of moving. Commonly, seismic wave is composed of two major waves namely body waves and surface waves.

Understanding the behavior and influences of these waves would clarify ground shaking of the ground during an earthquake. This cannot be achieved without utilization of proper instruments, such as seismograph, to record various waves of earthquakes. These waves are different in speed (arrival at specific location), amplitude, and carry various level of energy. 

Types of Seismic Waves

Large strain energy released during an earthquake as seismic waves travels in all directions through layers of the Earth, reflecting and refracting at each interface. These waves are of two major types:

1. Surface waves

It travels over the surface of the earth, and it consist of Rayleigh wave and love waves.

1.1 Rayleigh waves

Rayleigh waves or ground roll travel like ocean waves over the surface of the Earth. It moves the ground surface up and down. They cause most of the shaking at the ground surface during an earthquake.

Fig. 1: Rayleigh Waves

1.2 Love waves 

Love waves are fast and move the ground from side to side.

Fig. 2: Love Waves

2. Body waves

Body waves travel through the Earth. It consists of Primary Waves (P-waves) and Secondary Waves (S-waves)

2.1 P-waves 

P-waves are the fastest type of seismic wave. As P-waves travel, the surrounding rock is repeatedly compressed and then stretched along direction of energy transmission.

Fig. 3: P waves

2.2. S-waves arrive

S-waves arrive after P-waves because they travel more slowly and it does not travel through liquids. The material particles shifted up and down or side to side as the wave travels through it.

S-waves in association with effects of Love waves cause maximum damage to structures by their racking motion on the surface in both vertical and horizontal directions.

When P- and S-waves reach the surface of the Earth, 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.

Fig. 4: S waves

Instruments for Earthquake Detection and Measurements

A seismograph, which is also known as seismometer, is an instrument used to detect and record earthquakes. It consists of sensor, recorder, and timer. The principle on which it works is simple. A pen attached to a mass hung by a string from a support, Fig. 5, marks on a chart paper that is held on a drum rotating at a constant speed.

Fig 5: Seismograph

A magnet around the string provides required damping to control the amplitude of oscillations. The pendulum mass, string, magnet and support together constitute the sensor. The drum, pen and chart paper constitute the recorder.

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 as shown in the Fig. 6 is replaced with a spring pendulum oscillating about a fulcrum.

Fig. 6: Seismograph for Recording Vertical Motions

As earthquake occurs, the base of the seismograph moves, but the mass does not. The seismograph records a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. This record is proportional to the motion of the seismometer mass relative to the earth, but it can be mathematically converted to a record of the absolute motion of the ground.

The analog instruments have evolved over time, but today, digital instruments using modern computer technology are more commonly used. The digital instrument records the ground motion on the memory of the microprocessor that is in-built in the instrument.

Strong Ground Motions

Ground shaking is the result of movements caused by seismic waves. The seismic waves, which reach certain point at various instants of time, have different amplitudes and carry different levels energy.

Consequently, the motion at any site on ground is random in nature with its amplitude and direction varying randomly with time. Strong motions that can possibly damage structures can happen with earthquakes in the vicinity or even with large earthquakes at reasonable medium to large distances.

The motion of the ground can be described in terms of displacement, velocity or acceleration. The variation of ground acceleration with time recorded at a point on ground during an earthquake is called an accelerogram, Fig. 7.

The nature of accelerograms may vary depending on energy released at source, type of slip at fault rupture, geology along the travel path from fault rupture to the Earth’s surface, and local soil.

Fig. 7: Typical Accelerograms

Accelerograms carry distinct information regarding ground shaking; peak amplitude, duration of strong shaking, frequency content and energy content. Peak amplitude (peak ground acceleration, PGA) is physically intuitive.

For instance, a horizontal PGA value of 0.6g (= 0.6 times the acceleration due to gravity) suggests that the movement of the ground that can cause a maximum horizontal force on a rigid structure equal to 60% of its weight.

In a rigid structure, all points in it move with the ground by the same amount, and hence experience the same maximum acceleration of PGA. Usually, strong ground motions carry significant energy associated with shaking of frequencies in the range 0.03-30Hz (i.e., cycles per sec).

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