Seismic Waves

What Are Seismic Waves?

The energy that travels under the surface layer of the earth and causes an earthquake is called seismic waves. A crack starts to open on a pre-existing point or line of weakness deep underground when an earthquake takes place. Stress builds over the surface of the earth as the crack grows on to become larger and larger. This energy causing the earthquake is known as seismic waves. Seismic waves transfer energy without moving material. 

The crack grows with a speed of 2 to 3 km/sec. The level or size of the earthquake also depends on the area on which it takes place. The magnitude, that is, the size of the waves depends on the level of break or slip that has taken place under the surface. The process of cracks and slips taking place is known as Rupture. 

So, the elastic waves that are formed are because of the rupturing that takes place deep underground and continues to grow at a very fast pace. The speed of this growth depends on their nature and the properties of the earth. Here is a fact for you: as we go deeper and deeper into the surface of the earth, the seismic waves found there are of higher density, pressure, and velocity.

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Types of Earthquake Waves And Their Effects

Let us first categorize the S-waves based on the medium that they travel in, namely: 

1. Body waves 

2. Surface waves

The waves that take place under the surface or through the earth are called body waves. On the other hand, the waves that occur on the surface of the earth are called surface waves.

Body Waves or ‘Through The Earth’ Waves Are Further Divided Into Two: 

  • P waves (for Primary waves)  

  • S waves (for Secondary Waves)

Primary waves are faster in pace, and Secondary waves are slower in pace.
Surface waves earthquakes are very significant, too, as when they grow, they bring destruction to the surface of the earth where all the buildings and people live. It is mostly the energy formed by surface waves that can knock down big buildings.

P Waves (Primary Waves) 

P waves are the fastest seismic waves of all and thus called Primary ones. P waves grow or travel at a speed of 5 kilometers per sec through the earth’s crust. P waves are the first ones to reach any particular location or point when an earthquake occurs. The waves have a tendency to flow through all three i.e., solids, liquids, and gases. The materials that they flow through experience a force or energy that slightly pulls them apart and pushes them together. The same energy is experienced by a building when an earthquake occurs. 

S Waves (Secondary Waves) 

S waves are the second-fastest seismic waves and thus called Secondary. The speed at which the S waves travel is almost half the speed of Primary Waves. S waves are the ones to reach any location after the primary waves when an earthquake occurs. Unlike Primary Waves, Secondary Waves make the material go through an up and down shaking movement from the sides when it flows through them. Unlike P waves, S waves can travel through rocks only. 

Types of Seismic Waves


Particle Motion

Other Characteristics

Primary Waves

Compression (Pushes and pulls) in the same direction of the wave propagating 

1. Travels the fastest. 

2. Arrives first at the seismograph. 

3. Travel in the linear direction. 

4. It can travel in solids, liquids, and gases. 

Secondary Waves

Alternating motions perpendicular to the direction of the wave 

1. Travels only through solids.

2. Travels at half the speed of P waves. 

3. Travel in a transversal direction.

Surface Waves

Motion parallel to the earth’s surface

1. Largest at the surface and decrease with depth.

2. Dispersive in nature.

How Do Seismographs Record Earthquakes?

Seismographs are instruments used to record earthquakes. The seismograph is mounted on the surface of the earth, and when there are tremors, the entire unit shakes. However, it is also attached mass on the spring, which does not shake. This mass has inertia and hence, remains in the same place. When the seismograph starts shaking under this mass, the device records the relative motion between itself and the rest of the instrument (which is shaking) and records the ground motion. 

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FAQ (Frequently Asked Questions)

1.  Do Other Planets Have Earthquakes? If Yes, What Are They Called?

It depends on the geological properties of the planet. A very hot interior is required for such a process to take place (like earth’s crust). Not much exploration has been done on all the planets but, the sign of such activity is found on Mars and Venus in the form of volcanoes, and it is followed with lava flowing from these landforms subsequent to the earthquake. But since during the process of recording, the activity has already ceased, making the exact reasons for such activity are yet to be established. Generally, the planet's name comes before the term quake to signify the location of the quake. A fun fact for you, recently, a Marsquake of 7 magnitudes was recorded!

2. What Happens to The Sea or Ocean If An Earthquake Occurs?

Such an activity is called a submarine earthquake. When the same process of earthquake takes place at the bottom of the water bodies, it is called a submarine earthquake. The only major thing that happens if there is a submarine earthquake is that a Tsunami takes place. Earthquakes at the bottom of the sea or ocean create a series of waves that result in a destructive tsunami. Submarine Earthquakes can also cause damage to communication cables (submarine) and bring disturbance in internet connections. Submarine earthquake-prone areas fall mostly in Asia.