2.1 Tectonic processes affecting magnitude of earthquakes

Question 1

How do tectonic plates result in earthquakes

Answer 1

• Earthquakes refer to the shaking of the Earth’s ground due to the sudden release of energy in the Earth’s lithosphere.
• Earthquakes generally occur along plate boundaries, which contain systems of deep fractures called faults.
• occur when:
  1. Rock masses on either side of a fault are pushed by tectonic forces.
  2. Friction causes them to get locked, and stress builds up.
  3. When the stress exceeds the strength of the fault, the rocks snap or suddenly move to a new position.
  4. This sudden movement causes seismic waves to be released, resulting in ground shaking.
• The point in the earth’s crust where seismic waves are released is known as focus
• The focus is the origin of the earthquake.
• The point on the Earth’s surface directly above the focus is called the epicentre
• Shaking is generally felt most strongly near the epicentre.

Question 2

How are earthquakes measured?

Answer 2

• using seismometers.
• These are sensitive instruments that detect ground vibrations and determine the magnitude of an earthquake.
• The greater the seismic energy released during an earthquake, the greater the magnitude.
• Scientists use the following scales to rate the magnitudes of earthquakes:
  1. Richter Scale (ML)
  2. Moment Magnitude Scale (Mw).

Question 3

Richter Scale (ML)

Answer 3

• The Richter Scale calculates earthquake magnitude using the height of the largest wave recorded on seismometers.
• Thus, earthquake magnitude is measured based on the maximum seismic intensity reached, rather than the total seismic energy released throughout the earthquake.
• The scale is numbered from 1 to 10
• Scale is logarithmic - an earthquake of magnitude 6 releases about 32 times more energy than a magnitude 5 earthquake.
• However, the Richter Scale has a limitation—it would rate an earthquake with a single drastic spike in wave energy as having a higher magnitude than a long earthquake with many large, intense waves.
• This underestimates longer earthquakes which release more overall energy by rating them as having lower magnitudes even though they are likely to do more damage.
• Hence, the Richter Scale is no longer commonly used, except for small, local earthquakes.

Question 4

Moment Magnitude Scale

Answer 4

• rates earthquake magnitude based on the total energy released during the earthquake instead of just the largest wave like the Richter Scale.
• hence, it is generally more accurate, especially in measuring earthquakes of magnitude 8 and above.
• Scientists have adjusted the magnitudes of past earthquakes which were initially measured using the Richter Scale.
  Example:
• Prince William Sound Earthquake in Alaska, USA, in 1964 was initially rated 8.6 on the Richter Scale, but was later adjusted to 9.2 based on the Mw scale.
• the Mw scale is also logarithmic - an earthquake of magnitude 6 releases about 32 times more energy than a magnitude 5 earthquake.