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Earth and Space Sciences · 术语表

什么是 Focus and epicentre？

又称： focus · epicentre

Definition 20.2 Primary & Middle School Earth and Space Sciences · 第 20 章 — Earthquakes

The focus of an earthquake is the place underground, on the fault, where the slip starts. The epicentre is the point on the Earth’s surface directly above the focus.

A fault, the focus and the epicentre (a drawing, not to scale). The two blocks slip past each other along the fault (red); the slip starts at the focus, and waves spread out from it in all directions, like ripples on a pond. The epicentre (star) is on the surface, straight above the focus.

示例

Example 20.5 (Three circles)

Three stations record the same earthquake. The delays are $10\,\mathrm{s}$ at station M1, $11.25\,\mathrm{s}$ at M2 and $9.75\,\mathrm{s}$ at M3. The distances are $8 \times 10 = 80\,\mathrm{km}$ , $8 \times 11.25 = 90\,\mathrm{km}$ and $8 \times 9.75 = 78\,\mathrm{km}$ . The three circles, drawn below, meet at a single point: the epicentre.

Example 20.11 (From Sumatra to Sri Lanka)

Colombo, the largest city of Sri Lanka, is about $1830\,\mathrm{km}$ from the epicentre of the 2004 earthquake. At $800\,\mathrm{km}/\mathrm{h}$ , a tsunami covers this distance in $1830 \div 800 \approx 2.3$ hours, that is about 2 hours and 17 minutes. Nobody in Sri Lanka felt the earthquake: the wave came without any warning but the sea itself.

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