Earthquake Causes, Effects and Measurement

Earthquakes are sudden, violent shaking of the ground caused by the release of energy stored in rocks. They are the most widespread tectonic hazard, affecting every continent and causing some of the deadliest natural disasters in human history. This lesson examines how earthquakes occur, the types of seismic waves they produce, how they are measured, and the range of effects they cause. This knowledge is essential for understanding the case studies that follow.

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How Earthquakes Occur

Earthquakes are caused by the sudden release of stress that has built up in the Earth's crust over time. The process follows a clear sequence:

1. Stress accumulation: At plate boundaries, tectonic plates are constantly moving, but friction between the plates can lock them together. The plates continue to be pushed by convection currents, but they cannot move freely. Stress (elastic strain energy) builds up in the rocks over years, decades or centuries.

2. Rupture: When the accumulated stress exceeds the strength of the rocks, the rocks fracture and the plates suddenly jerk past each other. The point underground where the rock fractures and the earthquake originates is called the focus (or hypocentre).

3. Energy release: The stored energy is released as seismic waves that radiate outward from the focus in all directions, causing the ground to shake.

4. Surface effects: The point on the Earth's surface directly above the focus is called the epicentre. This is usually (but not always) where the shaking is most intense.

Focus Depth and Its Significance

The depth of the focus has a major influence on the earthquake's impact:

Focus Depth	Classification	Characteristics
0–70 km	Shallow focus	Most common; usually the most destructive because seismic energy has less distance to travel before reaching the surface. Most earthquake deaths are caused by shallow-focus earthquakes
70–300 km	Intermediate focus	Less common; energy dissipates more before reaching the surface, so shaking is usually less intense
300–700 km	Deep focus	Least common; occur at subduction zones where oceanic crust descends deep into the mantle. Can still be felt over large areas but usually cause less surface damage

Exam Tip: A shallow-focus earthquake of moderate magnitude can be far more destructive than a deep-focus earthquake of greater magnitude. The 2010 Haiti earthquake (Mw 7.0, focus depth 13 km) was devastating precisely because it was so shallow — the energy reached the surface with minimal dissipation.

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Seismic Waves

Earthquakes produce three types of seismic waves, each with different properties:

P-Waves (Primary Waves)

• Fastest seismic waves — travel at approximately 5–8 km/s through rock
• Compression waves — particles vibrate back and forth in the same direction as the wave travels (like pushing and pulling a slinky)
• Can travel through solids, liquids and gases
• Arrive first at seismograph stations (hence "primary")
• Cause relatively little damage

S-Waves (Secondary Waves)

• Slower than P-waves — travel at approximately 3–5 km/s through rock
• Shear waves — particles vibrate at right angles to the direction of wave travel (like shaking a rope side to side)
• Can only travel through solids (they cannot pass through the Earth's liquid outer core — this is how scientists discovered the outer core is liquid)
• Arrive second at seismograph stations
• Cause more damage than P-waves because the side-to-side motion is more destructive to buildings

Surface Waves (L-Waves / Love and Rayleigh Waves)

• Slowest waves but cause the most damage
• Travel along the Earth's surface rather than through the interior
• Produce the most violent ground shaking, including both side-to-side and rolling motions
• Two types: Love waves (horizontal shaking) and Rayleigh waves (rolling, elliptical motion similar to ocean waves)
• Their amplitude (size) decreases with depth, so they primarily affect structures at the surface

Wave Type	Speed	Motion	Travel Through	Damage Potential
P-waves	Fastest (~5-8 km/s)	Back-and-forth (compression)	Solids, liquids, gases	Low
S-waves	Medium (~3-5 km/s)	Side-to-side (shear)	Solids only	Moderate
Surface waves	Slowest	Rolling + side-to-side	Along surface only	Highest

Exam Tip: The fact that S-waves cannot travel through liquids was crucial evidence for the layered structure of the Earth. When S-waves from an earthquake disappear on the far side of the Earth, it creates an S-wave shadow zone — proving that the outer core is liquid.

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Measuring Earthquakes

Earthquakes are measured using two main scales:

The Richter Scale