What are Earthquakes?| Impacts, causes, and Intensity

what are earthquakes

This article will help you understand what are earthquakes, the epicenter of an earthquake, elastic rebound theory, where do earthquakes occur, and how often.

What are Earthquakes?

  • The shaking or trembling caused by the sudden release of energy
  • Usually associated with faulting or breaking of rocks
  • Continuing adjustment of position results in aftershocks

What is the Elastic Rebound Theory?

The elastic rebound theory explains how energy is stored in rocks;

  • Rocks bend until the strength of the rock is exceeded,
  • Rupture occurs and the rocks quickly rebound to an undeformed shape,
  • Energy is released in waves that radiate outward from the fault.
Elastic Rebound Theory

The Focus and Epicenter of an Earthquake

The point within Earth where faulting begins is the focus or hypocenter,

However, the point directly above the focus on the surface is the epicenter

what are earthquakes
The Focus and Epicenter of an Earthquake

How are earthquakes recorded?

Seismographs record earthquake events. They are carefully engineered instruments that serve the application of recording earthquakes’ magnitudes. The instruments are built to precision and are, therefore, quite sensitive to motion.


At convergent boundaries, focal depth increases along a dipping seismic zone called a Benioff zone.

what are earthquakes
Pictorial Representation of Earthquake

Where Do Earthquakes Happen and How Often?

80% of all earthquakes occur in the circum-Pacific belt;

  • Most of these result from convergent margin activity,
  • 15% occur in the Mediterranean-Asiatic belt,
  • Remaining 5% occur in the interiors of plates and on spreading ridge centers.
  • More than 150,000 quakes strong enough to be felt are recorded each year.
Locations of the natural hazard

The most recent case is the 2021 Earthquake of Haiti which was severe enough to be titled the deadliest earthquake of the year. Furthermore, approximately 2000 deaths till now recorded and many injured. Moreover, more than 83,000 buildings were damaged. According to UNICEF, 1.2 million people are affected.

What are Economics and Societal Impacts of Earthquakes?

what are earthquakes?
Damage in Oakland, CA, 1989
  • Building collapse
  • Fire
  • Tsunami
  • Ground failure

What are Seismic Waves?

Response of material to the arrival of energy fronts released by rupture

There are two types of Seismic Waves:

  • Body waves
    • P and S
  • Surface waves
    • R and L

Body Waves: P and S waves

Body waves

P or primary waves

  • fastest waves
  • travel through solids, liquids, or gases
  • compressional wave, material movement is in the same direction as wave movement

S or secondary waves

  • slower than P waves
  • travel through solids only
  • shear waves – move material perpendicular to wave movement
A diagrammatical representation of body waves

Surface Waves: R and L waves

Surface Waves

  • Travel just below or along the ground’s surface
  • Slower than body waves; rolling and side-to-side movement
  • Especially damaging to buildings
what are earthquakes
Surface Waves

How is an Earthquake’s Epicenter Located?

Seismic wave behavior

  • P waves arrive first, then S waves, then L and R
  • Average speeds for all these waves is known
  • After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.
Wave Comparison

How is an Earthquakes’ Epicenter Located?

Time-distance graph showing the average travel times for P- and S-waves of earthquakes. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves.

Time-Distance Graph
  1. Three seismograph stations are needed to locate the epicenter of an earthquake
  2. A circle where the radius equals the distance to the epicenter is drawn
  3. The intersection of the circles locates the epicenter
The epicenter of an Earthquake

How are the Size and Strength of Earthquakes Measured?


a subjective measure of the kind of damage done and people’s reactions to it

isoseismal lines identify areas of equal intensity

Modified Mercalli Intensity Map1994 Northridge, CA earthquake, magnitude 6.7

The following is an abbreviated description of the 12 levels of Modified Mercalli intensity:

  1. Not felt except by a very few under especially favorable conditions.
  2. Felt only by a few persons at rest, especially on upper floors of buildings.
  3. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
  4. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
  5. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
  6. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
  7. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
  8. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
  9. 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.
  10. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
  11. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
  12. Damage total. Lines of sight and level are distorted. Objects thrown into the air.
Measuring EQs

Magnitude of Earthquakes

Richter scale measures the total amount of energy released by an earthquake; independent of intensity

The amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

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