The Earthquake

1. The magnitude of an earthquake is measured by :

The magnitude of an earthquake is measured by :

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Tiltmeter
Richter scale
Seismograph
Seismometer
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UPSC CISF-AC-EXE – 2023
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The magnitude of an earthquake, which represents the energy released at the earthquake’s source, is measured using scales like the Richter scale or, more commonly today, the Moment Magnitude Scale.
– **Richter scale (B):** An early scale used to measure the magnitude of earthquakes based on the amplitude of the largest seismic wave recorded.
– **Seismograph (C) / Seismometer (D):** These are instruments used to detect and record the ground motion caused by seismic waves from an earthquake. The data recorded by a seismograph is used to calculate the magnitude and locate the earthquake, but the instrument itself measures motion, not magnitude directly.
– **Tiltmeter (A):** An instrument used to measure very small changes in the inclination of the ground surface, often used to monitor volcanic activity or ground deformation.
While the Richter scale is well-known, the Moment Magnitude Scale is now the preferred method for measuring earthquake magnitude globally, especially for larger earthquakes, as it provides a more accurate estimate of the total energy released. Both are scales used to quantify magnitude, distinct from the instruments used to record seismic waves.

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2. Consider the following statements: 1. The point where energy is rele

Consider the following statements:

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  • 1. The point where energy is released is called hypocentre.
  • 2. The point on the surface nearest to the focus is called epicentre.

Which of the statements given above is/are correct?

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1 only
2 only
Both 1 and 2
Neither 1 nor 2
This question was previously asked in
UPSC CBI DSP LDCE – 2023
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The correct option is C.
– Statement 1 is correct: The hypocentre (also known as the focus) is the point within the Earth where an earthquake rupture starts and energy is released.
– Statement 2 is correct: The epicentre is the point on the Earth’s surface directly above the hypocentre (focus), and it is usually the point where the earthquake effects are strongest. It is indeed the point on the surface nearest to the focus.
Seismic waves radiate outwards from the hypocentre. The intensity of the earthquake is typically highest near the epicentre.

3. Consider the following statements: 1. In a seismograph, P waves are

Consider the following statements:

  • 1. In a seismograph, P waves are recorded earlier than S waves.
  • 2. In P waves, the individual particles vibrate to and fro in the direction of wave propagation whereas in S waves, the particles vibrate up and down at right angles to the direction of wave propagation.

Which of the statements given above is/are correct?

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1 only
2 only
Both 1 and 2
Neither 1 nor 2
This question was previously asked in
UPSC IAS – 2023
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The correct answer is C) Both 1 and 2.
Statement 1 is correct. P waves (Primary waves) are compressional waves that travel fastest through the Earth’s layers. S waves (Secondary waves) are shear waves and travel slower than P waves. Therefore, P waves arrive at a seismograph before S waves from the same earthquake event.
Statement 2 is correct. P waves are longitudinal waves, meaning the particles of the medium vibrate back and forth in the direction of wave propagation. S waves are transverse waves, meaning the particles vibrate perpendicular (at right angles) to the direction of wave propagation.
P waves can travel through solids, liquids, and gases, while S waves can only travel through solids. This difference in behavior was crucial in understanding the Earth’s internal structure, particularly the liquid outer core which stops S waves.

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4. Tsunami causes devastation near the coast of the sea as the speed of t

Tsunami causes devastation near the coast of the sea as the speed of the sonic waves generated at the epicentre is

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directly proportional to the depth of the sea
inversely proportional to the depth of the sea
directly proportional to the distance between the coast and epicentre
inversely proportional to the distance between the coast and epicentre
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UPSC CAPF – 2011
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The correct option is A) directly proportional to the depth of the sea.
The speed of a tsunami wave (a shallow-water wave in terms of wavelength relative to depth) is determined by the formula v = sqrt(g * d), where ‘v’ is speed, ‘g’ is the acceleration due to gravity, and ‘d’ is the depth of the water. This formula shows that the wave speed is directly proportional to the square root of the depth. Therefore, in deeper water, tsunamis travel faster, and as they approach the shallower coast, they slow down.
Although the relationship is precisely proportional to the square root of the depth, among the given options, “directly proportional to the depth of the sea” is the correct representation that speed increases with depth. In the open ocean, where the depth is great, tsunamis can travel at hundreds of kilometers per hour. Near the coast, the decrease in depth causes the wave to decelerate significantly, leading to the stacking up of energy and a dramatic increase in wave height as it breaks upon the shore.

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5. Which one of the following can be considered as an initial warning of

Which one of the following can be considered as an initial warning of an approaching Tsunami ?

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Thundering noise
Squally winds and rainfall
Rapid withdrawal of water away from the beach
Rapid landward movement of water
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UPSC CAPF – 2010
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The correct option is C) Rapid withdrawal of water away from the beach.
The most common natural warning sign of an approaching tsunami, particularly for the first wave, is a sudden and significant recession of the sea from the coastline, exposing the seabed. This happens because the trough (low point) of the tsunami wave often reaches the shore before the crest (high point). This is an immediate signal that a large wave is approaching and coastal areas should be evacuated.
A thundering noise can sometimes be heard as the tsunami wave breaks, but the water receding is often the first visible sign. Squally winds and rainfall are associated with meteorological phenomena like cyclones, not tsunamis which are primarily caused by underwater geological events. Rapid landward movement of water *is* the tsunami arriving, not a warning beforehand.

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6. Which one of the following statements about earthquake waves is not

Which one of the following statements about earthquake waves is not correct?

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P waves move faster and are the first to arrive at the surface.
P waves can travel through gaseous, liquid and solid materials.
Seismographs located beyond 145° from epicentre can record the arrival of P waves.
P waves have maximum area covered under its shadow zone.
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UPSC NDA-2 – 2022
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Let’s examine each statement about earthquake waves:
A) P waves (Primary waves) are the fastest seismic waves and are the first to reach any point on the Earth’s surface or within it. This statement is correct.
B) P waves are compressional waves and can travel through solid, liquid, and gaseous mediums. This statement is correct.
C) Seismographs located beyond 145° from the epicenter *can* record P waves, but these are the P waves that have been refracted by the Earth’s core. There is a P-wave shadow zone between approximately 105° and 145° where direct P waves are not recorded due to refraction. Beyond 145°, refracted P waves arrive. This statement is correct.
D) The shadow zone for P waves is the area between ~105° and ~145°. S waves (Secondary waves) cannot travel through the liquid outer core, resulting in an S-wave shadow zone that starts at ~105° and extends to the entire hemisphere beyond that. Therefore, the S-wave shadow zone is much larger than the P-wave shadow zone. This statement is incorrect.
The key difference in shadow zones is that P waves are refracted by the core (creating a partial shadow zone), while S waves are stopped by the liquid outer core (creating a larger, complete shadow zone beyond 105°).
Earthquake waves provide valuable information about the Earth’s interior structure. The existence and size of the shadow zones for P and S waves were crucial in determining the presence of a liquid outer core.

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7. Which of the following statements regarding earthquakes is/are correct

Which of the following statements regarding earthquakes is/are correct ?

  • 1. The point on the Earth’s surface directly above the focus is called the epicentre of the earthquake.
  • 2. Earthquakes generate Primary and Secondary waves that radiate outward from the earthquake focus.
  • 3. Deep-focus earthquakes are likely to cause more damage than shallow-focus earthquakes.

Select the answer using the code given below :

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1 only
1 and 2 only
2 and 3 only
1, 2 and 3
This question was previously asked in
UPSC NDA-1 – 2024
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Statements 1 and 2 are correct, while statement 3 is incorrect.
Statement 1 accurately defines the epicentre as the point on the Earth’s surface directly above the focus (hypocentre), which is the point where the earthquake originates within the Earth. Statement 2 is correct as earthquakes generate various types of seismic waves, including Primary (P) waves and Secondary (S) waves (body waves), which propagate outwards from the focus. Statement 3 is incorrect; deep-focus earthquakes generally cause less damage at the surface than shallow-focus earthquakes of similar magnitude because the seismic waves travel a greater distance and lose energy before reaching the surface.
Earthquake magnitude measures the energy released at the source, while intensity measures the shaking and damage caused at a specific location. Surface waves typically cause the most damage.

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8. Which of the following statements is/are correct ? 1. Hypocenter is

Which of the following statements is/are correct ?

  • 1. Hypocenter is the point on the surface of the Earth, nearest to the focus.
  • 2. Velocity of earthquake waves is higher in denser materials.
  • 3. P waves move faster and are the first to arrive at the surface of the Earth.

Select the correct answer using the code given below :

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1 and 2
2 and 3
1 and 3
3 only
This question was previously asked in
UPSC NDA-1 – 2023
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The correct option is B.
Statement 1 is incorrect because the hypocenter is the point of origin of the earthquake *within* the Earth, while the epicenter is the point *on the surface* directly above the hypocenter. Statement 2 is generally correct as seismic wave velocity increases with the rigidity and density of the medium. Statement 3 is correct; P waves (primary or compressional waves) are faster than S waves (secondary or shear waves) and arrive first.
1. Hypocenter (or focus) is the point below the Earth’s surface where the earthquake originates. The epicenter is the point on the Earth’s surface directly above the hypocenter.
2. Seismic wave speeds depend on the elastic properties and density of the material. Wave speed is proportional to the square root of the ratio of an elastic modulus (like bulk modulus or shear modulus) to density. While density increases with depth, the elastic moduli generally increase faster, leading to an overall increase in wave speed with depth in the Earth’s mantle and core.
3. P waves are longitudinal waves that can travel through solids, liquids, and gases. S waves are transverse waves that can only travel through solids. Because P waves involve compression and expansion, they travel faster than S waves, which involve shearing motion.

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9. Point of Origin of Earthquake Wave is known as

Point of Origin of Earthquake Wave is known as

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Epicentre.
Focus.
Photosphere.
Seismic Zone.
This question was previously asked in
UPSC NDA-1 – 2021
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The point of origin of an earthquake *inside* the Earth is known as the focus or hypocenter. Earthquake waves (seismic waves) radiate outwards from this point.
The Focus (or Hypocenter) is the actual point within the Earth’s crust where an earthquake originates, due to the rupture of rocks along a fault line. The energy released at the focus propagates as seismic waves.
The Epicentre is the point on the Earth’s surface located directly above the focus. It is usually where the earthquake shaking is strongest and damage is often greatest. A Seismic Zone is an area where earthquakes are known to occur frequently. Photosphere is the visible surface of the Sun.

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10. Consider the following statements relating to Richter scale: 1. It w

Consider the following statements relating to Richter scale:

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  • 1. It was devised in 1935 by Charles F. Richter
  • 2. It describes the quantity of energy released by a single earthquake
  • 3. Richter Scale has no upper limit

Which of the statements given above is/are correct?

1 only
1 and 2 only
2 and 3 only
1, 2 and 3
This question was previously asked in
UPSC NDA-1 – 2019
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All three statements regarding the Richter scale are considered correct in the context of this question.
– Statement 1: The Richter magnitude scale was indeed devised by Charles F. Richter in 1935, along with Beno Gutenberg.
– Statement 2: The Richter scale measures the magnitude of an earthquake, which is directly related to the energy released. While the scale is based on the amplitude of seismic waves, magnitude is a logarithmic measure from which the quantity of energy released can be estimated.
– Statement 3: Theoretically, the Richter scale does not have an upper limit in its mathematical definition. Although practical measurement limitations existed with the original instruments for very large earthquakes, the scale itself isn’t bounded.
The Richter scale is a logarithmic scale; each whole number increase in magnitude represents a tenfold increase in wave amplitude and approximately a 32-fold increase in the energy released. For large earthquakes, the Moment Magnitude Scale (Mw) is now preferred as it is more directly related to the total energy released and does not suffer from saturation effects like the Richter scale at high magnitudes.

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