Factors affecting severity of earthquakes: – focus and epicentre

Earthquake and Volcanic Hazards and Impacts

Factors Affecting the Severity of Earthquakes

When an earthquake happens, its impact on people and buildings depends on two key things: the focus (the point underground where the energy is released) and the epicentre (the point on the surface directly above the focus). Think of the focus as a hidden spring that suddenly snaps, and the epicentre as the tip of a hammer that feels the shock.

Focus (Depth of the Earthquake)

• Shallow focus (0–70 km): ⚡️ The shaking is strong because the energy has less distance to travel. Buildings feel it more, and damage is usually higher.
• Intermediate focus (70–300 km): 🌐 The shaking is weaker than shallow ones, but still noticeable over a wide area.
• Deep focus (300–700 km): 🌌 The energy spreads out, so the surface shaking is gentler, but the quake can be felt over a very large region.

Epicentre (Distance from the Focus)

The epicentre is the point on the ground that is directly above the focus. The closer you are to the epicentre, the stronger the shaking you feel. Imagine standing under a giant drum: the closer you are to the center, the louder the sound. The same principle applies to earthquakes.

Magnitude vs. Intensity

Magnitude measures the total energy released (e.g., $M_w = \frac{2}{3}\log_{10}E - 6.07$), while intensity describes the shaking felt at a specific location (e.g., the Modified Mercalli Intensity scale). A deep quake with a high magnitude might have a lower intensity at a nearby epicentre than a shallow quake of lower magnitude.

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