Earthquakes and tectonic plates: why the ground shakes
Follow the movement, then locate the source
Tectonic plates move slowly, yet an earthquake can shake the ground suddenly. Connect the two time scales: slow loading → sudden fault slip → seismic waves. Then separate the earthquake's starting point from the shaking felt at a particular place.
1. Plates move; faults can lock
The lithosphere is Earth's rigid outer shell: the crust and the uppermost mantle. It is divided into tectonic plates that move relative to one another. A fault is a fracture or zone of fractures along which rocks have moved.
At a locked part of a fault, friction resists sliding while surrounding rocks continue to deform. Stored elastic strain energy builds up. When the fault suddenly slips, some of that energy travels outward as seismic waves. Their passage makes the ground shake. This explains many tectonic earthquakes; it does not mean every fault is continuously locked or every earthquake has the same cause.
2. Three ways neighbouring plates move
| Boundary | Relative plate movement |
|---|---|
| Convergent | Toward one another |
| Divergent | Away from one another |
| Transform | Sideways past one another |
Earthquakes occur at all three types. Many are concentrated near plate boundaries, where deformation is common. But plate interiors also have earthquakes: stresses can reactivate faults within a plate. A boundary map is a useful pattern, not a line outside which earthquakes are impossible.
3. Focus and epicentre: two different positions
The focus, or hypocentre, is where rupture starts within Earth. The epicentre is the point on the surface directly above it. Rupture can spread along a much larger part of the fault. The epicentre is not the entire source area, and the strongest shaking or greatest damage need not occur exactly there.
Work through the sequence rather than imagining two whole plates jumping at once:
- Slow movement loads the rocks around a locked fault; friction resists sliding there.
- The rocks deform elastically and store strain energy.
- Sudden slip starts at the focus and rupture spreads along part of the fault.
- Seismic waves carry away some of the released energy and shake the ground; the event does not necessarily remove all stress.
4. One event, different shaking
Magnitude describes the size of an earthquake at its source. Intensity describes shaking and its effects at a particular location. One earthquake can therefore have different intensities in different towns. Distance from the rupture, depth and local ground conditions help explain those differences. Damage also depends on how structures are built.
Suppose Town A reports stronger shaking than Town B during the same event. That is evidence of different local shaking, not two different earthquakes or a magnitude that changes from town to town. Magnitude estimates can be revised as more recordings become available; that is a separate issue.
5. Forecasting is not a date promise
Scientists can estimate earthquake probabilities over an area and time period. They cannot currently make reliable predictions specifying the exact time, place and magnitude of a future earthquake. An early-warning system detects an earthquake after it has begun and may alert places before strong shaking reaches them. Early warning is not advance prediction of an earthquake that has not started.
Five practice checks
- A fault stays locked while nearby rocks slowly deform. What can explain a later sudden earthquake?
Show answer
Correct answer: B
Friction can resist sliding while elastic strain accumulates. Sudden slip releases energy; seismic waves transmit some of it.
- Rupture starts at point F, 15 km below the ground. Point E is on the surface directly above F. Which labels are correct?
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Correct answer: C
The focus is the underground starting point of rupture. The epicentre is the surface point vertically above it; 15 km is an illustrative depth.
- Two neighbouring plates slide horizontally past one another. Which boundary does the sketch represent?
Show answer
Correct answer: A
A transform boundary has relative sideways motion. Divergent plates separate; convergent plates approach one another.
- During one earthquake, Town A experiences stronger shaking than Town B. Which interpretation is best?
Show answer
Correct answer: B
Intensity describes local shaking and effects. Distance and ground conditions can differ; the earthquake's source size does not change with the observer's location.
- A system detects rupture that has already started and sends an alert before strong shaking reaches a distant town. What is this?
Show answer
Correct answer: C
Early warning uses detection after an earthquake starts. It is different from reliably predicting the exact time, place and magnitude of a future event.
Sources
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