2.1 Tectonic Processes affecting the Magnitude of Earthquakes

How do tectonic processes affect the magnitude of earthquakes?

What are earthquakes?

• 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 faults 

How do tectonic processes result in an earthquake?

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 built up 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

  ∴ results in ground shaking

Shaking is generally felt most strongly at the

• epicentre (first line of impact)

What are the features of an earthquake? (FYI check TB)

Focus

• the point in the Earth’s crust where seismic waves are released

Epicenter 

• the point on the Earth’s surface directly above the focus 

What is the relationship between an earthquake and the focus + epicentre?

Focus of an earthquake

• may be deep in the crust or

• shallow near Earth’s surface

  • generally, shallow-focus earthquakes do the most damage

    → cuz of their closeness to the surface

    → 0-70km (distance from focus to epicentre)

  • a deeper focus results in

    • weaker earthquake experience on Earth’s surface

    • seismic waves take longer to travel

      → would have lost the majority of their energy by then

      → 70-700km

      ∴ less damage

How are earthquakes measured?

via 

• seismometers 

  • sensitive instruments that detect ground vibrations 

    → determining the magnitude of an earthquake

• the greater the seismic energy released during an earthquake

  • the greater the magnitude 

scales used to rate the magnitude of earthquakes

• Richter Scale (ML)

• Moment Magnitude Scale (Mw)

How is the Richter Scale used to rate the magnitudes of earthquakes?

calculates earthquake magnitude using the

• height of the largest wave recorded on seismometers

∴ earthquake magnitude is measured based on (FYI)

• maximum seismic intensity reached

• rather than total seismic energy released through the entire earthquake

the scale is

• numbered from 1 - 10

  • 10 being the largest magnitude

• logarithmic

  • earthquake of magnitude 6 releases about 32x more energy than magnitude 5 earthquake

  • magnitude 1 - frequent earthquakes

  • magnitude >6 - powerful but less frequent earthquakes

Limitations

• rates earthquakes with a single drastic spike in wave energy as having a higher magnitude

  • than a long earthquake with many large, intense waves

∴ underestimates longer earthquakes which

• releases more overall energy

by rating them as having lower magnitudes though they are

• likely to do more damage

∴ The Richter Scale is no longer commonly used

• except for small, local earthquakes

TLDR - Richter scale measures local magnitude of earthquakes

How is the moment magnitude scale used to rate the magnitudes of earthquakes?

rates earthquake magnitude based on the 

• total energy released during the earthquake

• estimates the total energy released during an earthquake 

∴ generally more accurate in measuring earthquakes of magnitude ≥8 (than the Richter scale)

• scientists have adjusted the magnitudes of past earthquakes

  • which were initially measured by the Richter scale

the scale is 

• logarithmic

  • an earthquake of magnitude 6 releases about 32x more energy than a magnitude 5 earthquake

  • magnitude 1 - very frequent earthquakes

  • magnitude >6 - very powerful but less frequent earthquakes