What this lesson teaches
I can explain how tectonic processes decide the magnitude of an earthquake and how it is measured.
Syllabus K335, Topic 4.2, Key Question 1. How do tectonic processes affect the magnitude of earthquakes?
Make a guess
Two towns are the same distance from an earthquake's epicentre. One is built on soft mud, the other on hard rock. Which shakes more?
- The town on hard rock, because rock carries waves better.
- Both shake the same. One earthquake has one magnitude.
- The town on soft mud.
Show the answer
The town on soft mud.
Soft ground amplifies seismic waves, so shaking is stronger there even though the earthquake and the distance are the same.
An earthquake happens when rock suddenly slips along a fault; magnitude measures its size, but shaking and damage vary by place.
Moving plates bend and strain the rock. Friction keeps a fault locked while stress builds up. When the stress beats the friction, the rock slips suddenly. The stored energy is released as seismic waves. The point underground where slipping starts is the focus. The point on the surface directly above it is the epicentre.
Seismic waves travel through Earth and make the ground shake. Seismometers record this shaking. Scientists use the records to find where the earthquake was and how big it was.
Magnitude measures the size of an earthquake and the energy it released. The Richter scale (ML) measures local earthquakes from the size of recorded waves. It is less reliable for very big ones. Moment magnitude (Mw) is better for large earthquakes. It uses how much fault area slipped, how far it slipped and how stiff the rock is.
One earthquake has one magnitude, although early estimates may be revised. But shaking differs from place to place, depending on distance, depth and ground type. Neither scale counts damaged buildings, and losses also depend on the people and buildings in the way. Magnitude alone cannot tell you the death toll.
Put these in order
Put the steps of an earthquake in order.
- The waves reach the surface and shake the ground, most strongly near the epicentre.
- Stored energy is released as seismic waves.
- Stress builds until it is greater than the friction holding the fault.
- Moving plates bend and strain the rock along a fault.
- The rock slips suddenly at the focus.
Show the answer
- Moving plates bend and strain the rock along a fault. Friction keeps the fault locked, so the stress has nowhere to go and builds up.
- Stress builds until it is greater than the friction holding the fault. The rock can no longer hold, so it slips suddenly.
- The rock slips suddenly at the focus. The energy stored in the bent rock is released all at once.
- Stored energy is released as seismic waves. The waves travel out through the Earth in every direction.
- The waves reach the surface and shake the ground, most strongly near the epicentre.
- Focus
- Where the rock first slips, underground.
- Epicentre
- The surface point directly above the focus.
- Moment magnitude
- Better than Richter for large earthquakes.
Step by step
Stress builds
A locked fault resists while the rock bends.
The fault slips
Stored energy is released as seismic waves.
The ground shakes
Distance and ground type affect local shaking.
Real examples to use
Tohoku, Japan, 2011
A magnitude 9.0 earthquake struck off north-east Japan when the Pacific Plate slipped under the plate carrying Japan. It was one of the largest earthquakes ever recorded.
Haiti and Chile, 2010
Haiti's magnitude 7.0 earthquake killed more than 220,000 people by UN estimates. Chile's magnitude 8.8 earthquake released about 500 times more energy but killed fewer than 800. Magnitude alone does not decide the damage.
Worked example: Same earthquake, different shaking
A station on soft mud may record stronger shaking than one on hard rock at the same distance. This is still one earthquake with one magnitude. The difference comes from the local ground.
Watch out for this
The epicentre is the underground point where the earthquake starts.
That is the focus. The epicentre is on the surface, directly above it.
Check your understanding
Which statement correctly separates magnitude from local impact?
- Magnitude is measured from the number of buildings destroyed.
- Every place feels the same shaking, because there is one magnitude.
- One earthquake has one magnitude but uneven shaking.
Show the answer
One earthquake has one magnitude but uneven shaking.
Yes. Distance, depth and ground type change the shaking.