What this lesson teaches
I can explain the hazards, impacts and benefits of earthquakes and volcanoes for nature and people.
Syllabus K335, Topic 4.2, Key Question 4. How might tectonic hazards affect the natural and human systems?
Make a guess
After an earthquake, the shaking stops. A day later, a hillside town is still cut off and help cannot arrive. What most likely happened?
- A landslide blocked the only road.
- The earthquake kept shaking the town all day.
- Nothing; earthquakes only cause harm while shaking.
- A tsunami reached the hillside town.
Show the answer
A landslide blocked the only road.
Shaking loosens slopes and triggers landslides. Blocked roads stop rescuers and supplies, so a physical hazard grows into a wider crisis.
Earthquakes can trigger liquefaction, landslides and tsunamis, and damage then spreads through roads, water, power and health services.
Shaking can collapse weak buildings, damage bridges and break pipes. Falling objects and buildings injure or kill people. Damage to water, power and transport networks then disrupts hospitals, communication and businesses far beyond the first damaged site.
Liquefaction is a temporary loss of ground strength. Strong shaking raises the water pressure between grains in loose, waterlogged sediment. The grains then press together less and stop supporting each other well. Buildings may sink or tilt, and buried pipes may shift or float up. The soil has not melted; it depends on the sediment and the water in it.
Shaking can also loosen slopes and trigger landslides. Landslides can bury homes, destroy habitats, block roads and dam rivers. Blocked roads delay rescue. A landslide dam may later burst and cause floods. Slope angle, rock condition and water affect how easily slopes fail.
Put these in order
An earthquake strikes a mountain district. Put the cascade in order.
- A physical hazard has become a wider crisis for the whole district.
- Seismic waves shake a steep, rain-soaked slope.
- Rescue teams, food and medical supplies cannot get through.
- The landslide buries the only road into the district.
- The slope fails and a landslide pours into the valley.
Show the answer
- Seismic waves shake a steep, rain-soaked slope. The shaking loosens rock and soil that the slope could only just hold.
- The slope fails and a landslide pours into the valley. The sliding rock and soil cover whatever lies in their path, including the valley road.
- The landslide buries the only road into the district. With the road gone, people and supplies cannot get in or out.
- Rescue teams, food and medical supplies cannot get through. Injured people wait longer for treatment and shops run out of food, so the harm spreads well beyond the slope.
- A physical hazard has become a wider crisis for the whole district.
A tsunami is a series of long waves caused when a large amount of water is suddenly pushed out of place. An undersea earthquake can cause one if it moves enough seabed, especially up or down. Not every undersea earthquake does. Tsunami waves can cross an ocean and cause dangerous floods and currents on coasts.
Tsunamis damage habitats and buildings, carry debris, spoil water supplies and disrupt ports and livelihoods, sometimes far from the earthquake. Keep the chain clear: earthquake, then secondary hazard, then wider social and environmental losses.
- Liquefaction
- Wet, loose ground loses strength when shaken.
- Tsunami
- Long waves from sudden seabed movement.
- Cascade
- One hazard triggers wider losses.
Step by step
Shaking
An earthquake triggers a landslide.
Road blocked
People and supplies cannot get through.
Help delayed
A physical hazard becomes a wider crisis.
Real examples to use
Tohoku, Japan, 2011
The earthquake set off a tsunami that left about 18,000 people dead or missing. The tsunami also flooded the Fukushima Daiichi nuclear plant, causing a nuclear accident.
Christchurch, New Zealand, 2011
The earthquake of 22 February 2011 killed 185 people. Most died when two office blocks collapsed: 115 in the CTV building and 18 in the PGC building. Shaking also turned wet, sandy ground to liquid (liquefaction), so homes in the eastern suburbs tilted and sank and roads and pipes broke.
Nepal, 2015
A magnitude 7.8 earthquake killed about 9,000 people and set off landslides and an avalanche on Mount Everest.
Worked example: Why wet ground matters
Imagine a riverside district built on loose, waterlogged sand. Strong shaking raises water pressure in the ground, so it loses strength. Buildings tilt and pipes break. The same shaking on solid rock would cause less damage.
Watch out for this
Every undersea earthquake makes a tsunami.
A tsunami needs the seabed to move a large amount of water. Size, depth and the type of movement matter.
Check your understanding
Buildings tilt on wet, loose ground after an earthquake. What best explains this?
- The shaking heats the wet soil until it melts and flows.
- Rising water pressure weakens the grip between grains.
- A tsunami must have hit the buildings and washed the ground away.
Show the answer
Rising water pressure weakens the grip between grains.
Yes. That is liquefaction.