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Measuring Earthquakes

Akoranga 18 Mahi Tuatahi

  1. Outline why earthquakes occur.
  2. Explain what prevents tectonic plates from moving in a smooth, constant manner.
  3. Define the term fault.
  4. Explain the difference between an earthquake’s focus and epicenter.
  5. What happens to wet, loose soil during an earthquake?


  1. __Outline why earthquakes occur.__
    Earthquakes occur when two or more tectonic plates move against each other.
  2. __Explain what prevents tectonic plates from moving in a smooth, constant manner.__
    Friction between the two plates prevents constant movement.
  3. __Define the term fault.__
    A crack in the Earth’s crust due to TP movement
  4. __Explain the difference between an earthquake’s focus and epicenter.__
    The focus is the place underground where the earthquake occurs. The epicenter is on the surface directly above the focus.
  5. __What happens to wet, loose soil during an earthquake?__

Te Whāinga Ako

  1. How earthquakes are measured

Write the date and te whāinga ako in your book

The Pacific and Australian Plates are locked together in the South Island along the Alpine Fault. These two plates are pushing into each other in a transform (right-lateral strikeslip) fault – this is a major 600 km transform fault, which also causes uplift, forming the Southern Alps.


Elastic potential energy builds up over a period of time, and eventually the rock cannot withstand any more strain. This is due to the fault moving $30mm$ per year. The energy is released as the plates move, releasing a huge amount of energy in an earthquake.

Fault Lines


Earthquake Waves

Three types of waves move through Earth’s surface when an earthquake occurs



Also known as primary waves are the first waves to hit as they move the fastest ($8kms^{-1}$). They squeeze and stretch the rock.



Also known as secondary waves. These waves move at $5kms^{-1}$ and make the ground shake up and down as well as from side to side.



Travelling at $4kms^{-1}$ are the last waves to hit. These waves do the most damage as they travel near the surface of the crust.


FeaturesP- WavesS - WavesL - Waves
Full NamePrimarySecondaryLong
ActionPush / pullUp / down / sidewaysRolling
Travelling SpeedSpeed 5 - 13 km/sSpeed 3 - 8 km/sSpeed 3 - 4 km/s
Damage LevelMinorModerateMajor
In WaterPass throughAbsorbedPass through
LocationIn crustIn crustSurface only

Measuring Earthquakes

Earthquakes are measured on two different scales:

Mercalli Scale

Earthquake intensity is determined using:

  1. Strength of earthquake
  2. Distance from epicenter
  3. Nature of surface materials
  4. Building design

The Mercalli scale does not give a true indication of the actual strength of an earthquake because the amount of damage done to different places will largely depend on the type of materials used and the degree of construction of buildings and structures.


Richter Scale




Challenge: Build a Seismograph

Open Google Classroom on your device and find the Build a Seismograph task!