Definition

An earthquake is a natural phenomenon characterized by the sudden shaking or movement of the Earth's surface. This is caused by the release of energy stored in the Earth's crust, which creates seismic waves that propagate through the Earth. Earthquakes can range from minor tremors that are barely felt to powerful quakes that cause significant destruction and loss of life.

Some Terminologies

Summary of Terms

1. Hypocentre (Focus)

   • The exact point within the Earth where an earthquake originates.
   • Located at a specific depth below the Earth's surface.

2. Epicentre

   • The point on the Earth's surface directly above the hypocentre.
   • Often the location with the most intense earthquake effects.

3. Focus

   • Another term for the hypocentre.
   • The initial point of rupture and energy release during an earthquake.

4. Wave Front

   • A surface over which the phase of a seismic wave is constant.
   • Represents the leading edge of seismic energy as it travels through the Earth.

5. Fault Scarp

   • A steep slope or cliff formed by the vertical displacement of the ground along a fault.
   • Visible evidence of ground movement during an earthquake.

Definition of Seismology

Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. It encompasses the study of the causes and effects of earthquakes, the behavior of seismic waves, and the methods to measure and analyze these waves. Seismologists utilise specialized instruments to record and interpret the data from seismic waves, providing insights into the Earth's internal structure and contributing to the understanding and mitigation of earthquake hazards.

Types of Seismic Waves

Seismic waves are energy waves generated by the sudden release of energy in the Earth's crust, typically due to fault movement. These waves travel through the Earth and can be detected by seismographs. Seismic waves are broadly categorized into two main types: Body Waves and Surface Waves.

1. Body Waves: Body waves travel through the interior of the Earth. They are divided into two types: Primary (P) waves and Secondary (S) waves.

   • Primary (P) Waves

     • Description: P waves are compressional waves that cause particles in the material they pass through to move back and forth in the same direction as the wave is moving.
     • Speed: P waves are the fastest type of seismic wave and, therefore, the first to be recorded by seismographs.
     • Propagation: They can travel through both solids and liquids.
     • Characteristics: Due to their high speed and ability to move through various media, P waves are crucial for seismologists to determine the location of an earthquake's epicenter.

   • Secondary (S) Waves

     • Description: S waves are shear waves that cause particles to move perpendicular to the direction of wave propagation.
     • Speed: S waves are slower than P waves and are the second type of wave to be detected.
     • Propagation: They can only travel through solids and are stopped by liquids.
     • Characteristics: S waves provide important information about the Earth's interior, especially in determining the composition and physical state of materials.

2. Surface Waves: Surface waves travel along the Earth's surface. They are slower than body waves but can be more destructive due to their larger amplitudes and longer duration. Surface waves are divided into two types: Love waves and Rayleigh waves.

   • Love Waves

     • Description: Love waves are shear waves that cause horizontal shaking of the ground, perpendicular to the direction of wave propagation.
     • Speed: They travel faster than Rayleigh waves but slower than body waves.
     • Propagation: Confined to the Earth's surface, Love waves result from the horizontal shear motion of the ground.
     • Characteristics: Love waves are particularly damaging to structures because they cause horizontal ground motion.

   • Rayleigh Waves

     • Description: Rayleigh waves cause both vertical and horizontal ground motion, in an elliptical rolling fashion, similar to ocean waves.
     • Speed: They are the slowest among the seismic waves.
     • Propagation: These waves travel along the surface and can penetrate the Earth's layers to some extent, but their energy dissipates with depth.
     • Characteristics: Rayleigh waves are often responsible for the heaving and rolling motion felt during an earthquake, contributing significantly to the damage to buildings and infrastructure.

Causes of Earthquakes

Earthquakes are primarily caused by the movement of tectonic plates, the large slabs of rock that make up the Earth's crust. The main causes include:

1. Tectonic Plate Movements

   • Fault Lines: Earthquakes often occur along fault lines, which are fractures in the Earth's crust where blocks of rock move past each other. The most well-known fault line is the San Andreas Fault in California.
   • Plate Boundaries: There are three main types of plate boundaries where earthquakes commonly occur:
     • Divergent Boundaries: Plates move apart, creating new crust (e.g., mid-ocean ridges).
     • Convergent Boundaries: Plates move towards each other, causing one plate to subduct beneath another (e.g., the Pacific Ring of Fire).
     • Transform Boundaries: Plates slide past each other horizontally (e.g., the San Andreas Fault).

2. Volcanic Activity

   • Earthquakes can be triggered by volcanic activity, where the movement of magma exerts pressure on the surrounding rock, causing it to fracture.

3. Human Activities

   • Induced Seismicity: Activities such as mining, reservoir-induced seismicity (due to the filling of large reservoirs), geothermal energy extraction, and hydraulic fracturing (fracking) can also cause earthquakes.

Magnitude of Earthquakes

Magnitude is a quantitative measure of the energy released at the source of the earthquake. It is a logarithmic scale, meaning each whole number increase on the scale represents a tenfold increase in measured amplitude and approximately 31.6 times more energy release. The two most commonly used magnitude scales are the Richter Scale and the Moment Magnitude Scale (Mw).

1. Richter Scale: Developed by Charles F. Richter in 1935, the Richter scale measures the amplitude of seismic waves. It is a logarithmic scale, where each increase of 1.0 represents a tenfold increase in wave amplitude and roughly 31.6 times more energy release.

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2. Moment Magnitude Scale (Mw): The Moment Magnitude Scale has largely replaced the Richter scale for large, distant, or deep earthquakes. It measures the total energy released by an earthquake, which provides a more accurate representation, especially for larger quakes.

Intensity of Earthquakes

Intensity measures the effects and severity of an earthquake's shaking at specific locations. It is a qualitative assessment based on observations of the earthquake's impact on people, structures, and the Earth's surface. The most widely used scale to describe earthquake intensity is the Modified Mercalli Intensity (MMI) Scale.

1. Modified Mercalli Intensity (MMI) Scale
   • The MMI scale ranges from I (not felt) to XII (total destruction).
   • It provides a subjective measure based on observed effects, such as damage to buildings, changes in the landscape, and people's reactions.
   • The MMI scale is divided into 12 levels of intensity, each described by specific criteria:
     • I (Not Felt): Not felt except by a very few under especially favorable conditions.
     • II (Weak): Felt only by a few persons at rest, especially on upper floors of buildings.
     • III (Weak): Felt quite noticeably by people indoors, especially on upper floors of buildings, but many do not recognize it as an earthquake.
     • IV (Light): Felt indoors by many, outdoors by few. Some awakened at night; dishes, windows, doors disturbed; walls make creaking sound.
     • V (Moderate): Felt by nearly everyone; many awakened. Some dishes, windows broken; unstable objects overturned.
     • VI (Strong): Felt by all; many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
     • VII (Very Strong): Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures.
     • VIII (Severe): Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures.
     • IX (Violent): Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
     • X (Extreme): Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
     • XI (Extreme): Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
     • XII (Extreme): Damage total. Lines of sight and level are distorted. Objects thrown into the air.

Differences Between Magnitude and Intensity

• Magnitude:

  • Objective measure of the energy released at the earthquake's source.
  • Consistent value regardless of location.
  • Quantitative and based on instrumental data.
  • Commonly reported using the Richter scale or Moment Magnitude Scale.

• Intensity:

  • Subjective measure of the earthquake's effects at specific locations.
  • Varies based on distance from the epicenter, local building structures, and geological conditions.
  • Qualitative and based on observations.
  • Described using the Modified Mercalli Intensity (MMI) Scale.