The ground is moving
Earth’s outer layer is broken into giant plates that slowly move a few centimetres each year. Usually, the edges of the plates are stuck together by friction. Stress builds up over years, until suddenly the rocks break and slip. The energy released shakes the ground. This is an earthquake.
There are about 500 000 detectable earthquakes each year. Most are too small to feel. About 100 cause damage.
Faults
A fault is a crack where rocks have moved.
| Fault type | Movement | Where |
|---|---|---|
| Normal fault | Rocks pull apart; one side drops down | Divergent boundaries |
| Reverse (thrust) fault | Rocks push together; one side rides up | Convergent boundaries |
| Strike-slip fault | Rocks slide sideways past each other | Transform boundaries, like the San Andreas Fault |
Focus and epicentre
- The focus is the point underground where the rock breaks.
- The epicentre is the point on the surface directly above it.
Shaking is usually strongest near the epicentre.
Seismic waves
The energy travels as seismic waves.
| Wave | Type | Speed | Travels through | Effect |
|---|---|---|---|---|
| P-waves (primary) | Longitudinal (push-pull) | Fastest; arrive first | Solids and liquids | A sharp jolt |
| S-waves (secondary) | Transverse (side to side) | Slower | Solids only | Stronger shaking |
| Surface waves | Rolling and side-to-side | Slowest | Along the surface | Most damage |
Because S-waves can’t travel through liquid, scientists discovered that Earth’s outer core is liquid: S-waves don’t pass through it.
Measuring earthquakes
A seismograph records ground shaking as a zigzag line called a seismogram. By comparing the arrival times of P- and S-waves at three or more stations, scientists can pinpoint the epicentre.
Magnitude
Magnitude measures the energy released. Scientists today use the moment magnitude scale.
- Each whole number step means about 32 times more energy.
- A magnitude 7 releases about 1000 times more energy than a magnitude 5.
| Magnitude | Effects |
|---|---|
| Under 3 | Usually not felt |
| 4 | Felt; dishes rattle |
| 5 | Some damage to weak buildings |
| 6 | Strong; damage in populated areas |
| 7 | Major; serious damage over large areas |
| 8 and up | Great; can destroy communities near the epicentre |
Intensity is different: it describes how strongly shaking is felt at a particular place, which depends on distance, soil and buildings.
Famous earthquakes
| Earthquake | Year | Magnitude | Notes |
|---|---|---|---|
| Cascadia (off BC and the U.S. Pacific coast) | 1700 | About 9 | Known from Indigenous oral histories, drowned forests and a tsunami recorded in Japan |
| Haida Gwaii, BC | 1949 | 8.1 | Canada’s largest recorded earthquake |
| Chile | 1960 | 9.5 | Largest ever recorded |
| Alaska | 1964 | 9.2 | Caused tsunamis along the Pacific coast |
| Indian Ocean | 2004 | 9.1 | Tsunami killed over 225 000 people |
| Japan (Tōhoku) | 2011 | 9.0 | Tsunami caused a nuclear accident |
| Turkey–Syria | 2023 | 7.8 | Over 50 000 deaths |
Earthquake hazards
- Collapsing buildings and bridges (the biggest cause of deaths)
- Landslides
- Tsunamis when the sea floor shifts
- Liquefaction: wet, sandy ground turns to mush and buildings sink
- Fires from broken gas lines
Staying safe
Before: secure heavy furniture, make an emergency kit, and plan where to meet.
During: Drop, Cover and Hold On. Drop to the ground, take cover under a sturdy table, and hold on. Stay away from windows. If outside, move to an open area. Near the coast, move to high ground after strong shaking because a tsunami may follow.
After: expect aftershocks, check for injuries and gas leaks, and listen for official information.
Engineers design earthquake-resistant buildings with flexible frames, shock-absorbing bases and strong connections. British Columbia has been upgrading its schools to withstand earthquakes.
Common mistakes
- Thinking the magnitude scale is evenly spaced. Each step is about 32 times more energy.
- Thinking you should run outside during shaking. Falling debris is dangerous; drop, cover and hold on.
- Thinking scientists can predict exact earthquakes. They can estimate risks, not exact times.
Try it yourself
- Build towers of blocks or straws on a tray of jelly and shake it. Which designs survive?
- Use a slinky to show P-waves (push-pull) and S-waves (side to side).
- Make an earthquake emergency kit checklist for your home.
Quick recap
- Earthquakes happen when stressed rocks suddenly slip along faults.
- The focus is underground; the epicentre is on the surface above it.
- P-waves, S-waves and surface waves carry the energy.
- Seismographs measure shaking; each magnitude step is about 32 times more energy.
- Drop, Cover and Hold On during an earthquake.