Earthquakes
The ground beneath you is under constant stress. For years, sometimes centuries, the rock holds. Then, in seconds, it breaks and releases all that stored energy. In this lesson you'll learn how earthquakes are caused, how we measure them, why the 1989 Newcastle earthquake shocked Australia, and how to stay safe when the ground starts shaking.
Printable Worksheets
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Q1 · An earthquake happens underground. How does the damage reach the surface? What travels up from the focus?
Q2 · Australia is on a stable tectonic plate, so why did the 1989 Newcastle earthquake kill 13 people?
● Know
- The difference between focus (hypocenter) and epicentre
- The three types of seismic waves and their properties
- The Richter and Moment Magnitude scales
● Understand
- Why surface waves cause the most damage
- How the logarithmic scale works for earthquake magnitudes
- Why Newcastle 1989 caused so much damage for its magnitude
● Can do
- Identify the focus, epicentre, and wave types on a diagram
- Describe the "drop, cover, hold" safety response
- Explain why intraplate earthquakes can be unexpectedly damaging
- Earthquake
- Focus
- Epicentre
- Seismograph
- Moment magnitude
- Point on Earth's surface directly above where the earthquake originates
- Sudden release of energy in Earth's crust causing the ground to shake
- The standard scale now used to measure earthquake size
- Underground point where an earthquake originates
- Instrument that records ground movement during an earthquake
The ground beneath you is under constant stress, tectonic forces push and pull at rock layers continuously. For years, sometimes centuries, the rock holds. Then, in seconds, it breaks and releases all that energy as an earthquake. The ground shakes. Buildings fall. In 30 seconds, cities can be destroyed.
Key anatomy:
- Focus (hypocenter) = the point underground where the earthquake originates, where rock first breaks.
- Epicentre = the point on the surface directly above the focus. Usually the location of maximum shaking.
Three types of seismic waves travel outward from the focus:
| Wave type | Motion | Speed | Travel through | Damage |
|---|---|---|---|---|
| P-wave (Primary) | Compressional, push-pull | Fastest (~6 km/s) | Solids AND liquids | Little |
| S-wave (Secondary) | Shear, side-to-side | Slower (~3–4 km/s) | Solids ONLY | Moderate |
| Surface wave | Rolling/shaking of ground surface | Slowest | Earth's surface only | Most destructive |
At a seismograph station: P-waves arrive first, S-waves second, surface waves last. The gap between P and S arrival times increases with distance, seismologists use this to calculate how far away the earthquake was.
- P-waves: fastest, travel through all materials, arrive first, least damage.
- S-waves: slower, solids only, arrive second, moderate damage.
- Surface waves: slowest, arrive last, MOST destructive.
- P–S time gap grows with distance → used to locate earthquakes.
- P-wave
- S-wave
- Surface wave
- Travels through solids only; arrives second
- Slowest; causes the most building damage
- Fastest; travels through solids AND liquids; arrives first
A seismograph records ground movement as waves arrive. Two scales measure earthquake size:
- Richter Scale (1935): logarithmic, each whole number increase = 10× greater ground motion amplitude, but ~31× more energy released. Worked well for small-to-medium quakes.
- Moment Magnitude Scale (MMS): the current standard. More accurate for large earthquakes. Both scales give similar numbers for small-to-medium quakes.
| Magnitude | Description | Effect |
|---|---|---|
| < 2.0 | Micro | Not felt |
| 2.0–4.0 | Minor | Felt slightly; rarely damage |
| 5.0–6.0 | Moderate | Felt widely; damage to weak buildings |
| 7.0+ | Major | Serious damage over large areas |
| 9.0+ | Mega-earthquake | Catastrophic; 2011 Japan Tōhoku = 9.1 |
Logarithmic warning: A magnitude 7 earthquake releases approximately 31 × 31 ≈ 1000 times more energy than a magnitude 5. The numbers look small but the scale is enormous.
On 28 December 1989, a magnitude 5.6 earthquake struck Newcastle, NSW. It lasted about 30 seconds. The results:
- 13 people killed, Australia's deadliest 20th-century natural disaster.
- 160 injured; more than 300 buildings demolished.
- Estimated $4 billion in damage (1989 dollars).
Why was a "moderate" earthquake so devastating?
- Building standards: Before 1989, Australian buildings weren't required to be earthquake-resistant. NSW architects assumed Australia was too far from plate boundaries to need it.
- Intraplate setting: The earthquake occurred away from a plate boundary, due to stress buildup within the Indo-Australian Plate. It was unexpected.
- Soil amplification: Soft soil in Newcastle amplified the shaking compared to solid rock.
What changed: After Newcastle, Australian building codes were updated to include seismic design requirements across the whole country. Emergency preparedness planning improved significantly.
What to do in an earthquake: Drop, cover, hold. Get under a sturdy table and hold on. Don't run outside, falling glass and debris are more dangerous than staying put. Stay away from windows.
After an earthquake, a seismograph records P-waves arriving 40 seconds before S-waves. P-waves travel at 6 km/s. Predict how you could use the P–S arrival time difference to estimate how far away the earthquake was.
How close was your prediction?
Good, the P–S time gap is the key tool for calculating distance to an earthquake.
Key idea: P and S waves travel at different speeds, so the time gap between their arrivals grows with distance. Use three stations to triangulate the epicentre.
For each group, identify which term does NOT belong and explain why:
- Group A: P-wave, S-wave, Surface wave, Tsunami wave
- Group B: Focus, Epicentre, Fault, Seismogram
- Group C: Richter Scale, Moment Magnitude Scale, Beaufort Scale, Seismograph
- Group D: Newcastle 1989, Tōhoku 2011, Black Summer fires, Kobe 1995
A seismograph at a station records the following arrivals after an earthquake: P-waves at 10:00:00 AM, S-waves at 10:00:40 AM, surface waves at 10:01:20 AM.
Answer these questions in your workbook:
- What is the P–S time gap?
- Which wave type arrives last?
- Which wave type will cause the most damage at the surface?
- If P-waves travel at 6 km/s, approximately how far was the earthquake from this station? (Hint: P-waves take the same number of seconds as the P–S gap, but at 6 km/s.)
Earlier you were asked: Australia is on a stable tectonic plate, so why did the 1989 Newcastle earthquake kill 13 people?
Now that you've worked through the lesson, write a fuller answer. Use the words intraplate, building codes, focus and magnitude in your response.
Q1. Explain the difference between the focus and epicentre of an earthquake. (2 marks)
Q2. Describe the three types of seismic waves produced by an earthquake, including which type arrives first and which is most destructive. (4 marks)
Q3. Why were Australians surprised by the 1989 Newcastle earthquake? What has been done to prevent similar levels of damage in future? (3 marks)
Answers
▾MCQ 1
B The epicentre is the point on Earth's surface directly above the focus (underground origin point). It is usually, but not always, the location of maximum shaking.
MCQ 2
C Surface waves travel along Earth's surface, causing the rolling and shaking motion that topples buildings. They are the slowest but carry the most destructive energy at the surface.
MCQ 3
C Each whole magnitude step represents ~31× more energy. Two steps (5 to 7) means 31 × 31 ≈ 1000× more energy. The number difference looks small but the actual energy difference is huge.
MCQ 4
B Before 1989, Australian building codes did not require earthquake-resistant construction. The assumption was that Australia was too far from plate boundaries to experience damaging earthquakes, which the Newcastle earthquake disproved.
MCQ 5
A S-waves (shear waves) require particles to transmit shear stress. Liquids cannot do this because their particles slide freely past each other. S-waves are therefore blocked by liquids and can only travel through solids.
Short Answer 1
Model answer: The focus (hypocenter) is the underground point where an earthquake originates, where rock first breaks along a fault and energy is released. The epicentre is the point on Earth's surface directly above the focus. The epicentre is usually the site of the most intense shaking because seismic waves have the shortest distance to travel to reach it from the focus.
Short Answer 2
Model answer: P-waves (primary) are compressional waves that travel fastest (~6 km/s) through solids and liquids, they arrive first but cause little damage. S-waves (secondary) are shear waves that travel more slowly and through solids only, they arrive second and cause moderate damage. Surface waves are the slowest, arriving last, but travel along Earth's surface and cause the most destructive shaking, responsible for most building collapse.
Short Answer 3
Model answer: Australians were surprised because Australia is on a stable part of the Indo-Australian Plate, far from major plate boundaries. Few expected a damaging intraplate earthquake. The destruction was so severe partly because buildings in NSW had not been designed to withstand seismic shaking. After 1989, the Australian Building Code was updated to include earthquake-resistant construction requirements nationwide, applying to new buildings across all states and territories.