3.1 Earth's interior
No one has drilled more than a few kilometres down; we know Earth's interior thanks to the seismic waves sent out by earthquakes. P (primary) waves are push–pull waves that travel through solids and liquids and are the fastest. S (secondary) waves can travel only through solids. On the far side of the Earth from an earthquake there is a wide shadow zone that S waves never reach: proof that the outer core is liquid.
From the outside in, Earth's layers are the crust (about 35 km thick under continents, 5–10 km under oceans), the mantle, the liquid outer core and the solid inner core. The crust and the top of the mantle form the lithosphere, which is broken into plates moving over the partly molten asthenosphere below. Convection currents in the mantle and sinking ocean crust pulling the plate behind it are the main drivers.
3.2 Plate boundaries
At diverging boundaries, magma rises and new crust forms: the Mid-Atlantic Ridge and the East African Rift are examples. At converging boundaries, an ocean plate sinks beneath a continent (subduction), creating deep ocean trenches, volcanic mountains and great earthquakes (the Andes, Japan). When two continents collide, fold mountains such as the Himalayas rise.
At transform boundaries, plates slide past each other; crust is neither made nor destroyed, but stress builds up and is released suddenly in large earthquakes. The North Anatolian Fault is a right-lateral strike-slip fault about 1,500 km long between the Anatolian and Eurasian plates. The East Anatolian Fault is left-lateral. As the Arabian plate pushes north, Anatolia escapes westward.
3.3 Earthquake magnitude and intensity
Magnitude measures the energy released by an earthquake and is logarithmic: each 1-unit increase means a wave amplitude 10 times larger on the seismograph and about 32 times more energy. Today the most reliable measure for large earthquakes is the moment magnitude (Mw). Intensity describes an earthquake's effects at a particular place (damage, shaking felt); the same earthquake has different intensities depending on distance and ground conditions.
On 6 February 2023, two great earthquakes struck the East Anatolian Fault Zone about 9 hours apart (Mw about 7.8 and 7.5). The ruptures ran for hundreds of kilometres, and soft alluvial ground amplified the shaking and increased the damage. The disaster showed again how vital ground surveys, earthquake-resistant building codes and inspection are.
FormulaEnergy ratio ≈ 32^(M₁ − M₂) ≈ 10^(1.5 × (M₁ − M₂)) · Amplitude ratio = 10^(M₁ − M₂)
Key terms
- Lithosphere
- The rigid outer layer of crust and upper mantle, broken into plates.
- Subduction
- A dense ocean plate sinking beneath another plate.
- Strike-slip fault
- A fault where blocks slide horizontally past each other.
- Moment magnitude (Mw)
- A magnitude based on fault area and slip — the most reliable for large earthquakes.
End-of-unit problems
Solve each one yourself first, then open the solution.
Roughly how many times more energy does an Mw 7.0 earthquake release than an Mw 5.0? How many times larger is the wave amplitude on a seismograph?
Show the solution
The difference is 2 units. Energy: 32² ≈ 1,000 times (more exactly 10^(1.5 × 2) = 10³ = 1,000). Amplitude: 10² = 100 times. A 7 sounds only "a bit bigger" than a 5, but it releases a thousand times more energy.
At a station, the P wave arrives 20 seconds before the S wave. If P waves travel at 6 km/s and S waves at 3.5 km/s, roughly how far away is the earthquake?
Show the solution
Let the distance be d: d/3.5 − d/6 = 20. d × (1/3.5 − 1/6) = d × (0.2857 − 0.1667) = d × 0.119 = 20 → d ≈ 168 km. With distances from three stations, the epicentre is where the three circles meet (triangulation).
Why does the North Anatolian Fault neither create nor destroy crust, yet still produce very large earthquakes? For someone standing north of the fault, which way does the southern block move?
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The NAF is a transform (strike-slip) boundary: the plates slide side by side, so crust is neither added nor lost. When the fault locks, stress builds for years; when the rock's strength is exceeded it slips suddenly in a large earthquake. On a right-lateral fault the far block moves to the observer's right: for someone to the north, the southern (Anatolian) block moves west.
In the same earthquake, two neighbourhoods at the same distance from the epicentre suffer light damage on bedrock and heavy damage on an old lake bed. Explain using magnitude and intensity.
Show the solution
Magnitude is a single value for the earthquake, the same for both neighbourhoods. Intensity is the local effect and depends on the ground: loose, water-saturated lake sediments amplify seismic waves, lengthen the shaking and can even liquefy. That is why intensity and damage are higher there — and why ground surveys and proper foundations are vital.