---
title: "Earthquakes"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 20
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/20-earthquakes
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 20 — Earthquakes

On the morning of 26 December 2004, under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) off the north-western tip of Sumatra, in Indonesia, the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) broke along a [fault](#def-g7-earthquakes-fault): it was one of the largest [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) ever measured. On the nearby [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Sumatra, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) rose in waves up to $51\,\mathrm{m}$ high, which swept as far as $5\,\mathrm{km}$ inland. Over the next hours the waves crossed the Indian [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) and struck the [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of fifteen countries, as far away as Africa. About 230 000 people died. There was, at the time, no warning system in the Indian [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

![A village near the coast of Sumatra a week after the tsunami of 26 December 2004. U.S. Navy photograph by Philip A. McDaniel (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/img-a5c8c967333c.jpg)

*A village near the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Sumatra a week after the [tsunami](#def-g7-earthquakes-tsunami) of 26 December 2004. U.S. Navy photograph by Philip A. McDaniel (public domain, via Wikimedia Commons).*

How do [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) break? How do scientists find, within minutes, where an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) struck and how large it was? And why did the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), hours later, rise on [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) where nobody had felt anything?

**You already know.**

An *[earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake)* is a sudden shaking of the ground, caused by [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that break and slip, deep underground ([Chapter 8](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#ch-g4-volcanoes-earthquakes-first-look)). The outer shell of the Earth is made of [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate), and 92 % of the large [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) happen less than $300\,\mathrm{km}$ from a [plate boundary](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary) ([Chapter 18](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#ch-g7-plate-tectonics)). When an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) strikes, waves travel through the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the whole Earth: a faster kind (the P waves) and a slower kind (the S waves); by timing them, scientists discovered the [core](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) ([Chapter 16](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#ch-g6-inside-the-earth)).

## 20.1 Rocks that break

The [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) move a few centimetres a year, but their edges are rough: they stick together while the rest of the [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) keeps moving. The [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) near the edge bend a little, like a ruler you push at both ends, and store up energy. When the push finally overcomes the friction, the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) unstick all at once and slip: that sudden slip is the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake).

**Definition 20.1 (Fault).**

A *fault* is a break in the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) along which the two blocks on either side have moved with respect to each other. During an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) the blocks slip suddenly along the fault; the larger the fault and the slip, the larger the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake).

A [plate boundary](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary) is made of many [faults](#def-g7-earthquakes-fault), and most of the world’s [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) happen on them. But [faults](#def-g7-earthquakes-fault) also exist inside the [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate), and some of them can break too. When a large [fault](#def-g7-earthquakes-fault) breaks near the surface, the slip can be seen on the ground: a step in a field, a road cut in two, a fence or a row of trees shifted sideways.

![Steps in the ground left by the 1983 Borah Peak earthquake, in Idaho (United States), seen from the air: along the fault, one side dropped down with respect to the other, and the step crosses the road at the top. Photograph: R. E. Wallace, U.S. Geological Survey (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/img-8db0c1e670a3.jpg)

*Steps in the ground left by the 1983 Borah Peak [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), in Idaho (United States), seen from the air: along the [fault](#def-g7-earthquakes-fault), one side dropped down with respect to the other, and the step crosses the road at the top. Photograph: R. E. Wallace, U.S. Geological Survey (public domain, via Wikimedia Commons).*

**Definition 20.2 (Focus and epicentre).**

The *focus* of an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) is the place underground, on the [fault](#def-g7-earthquakes-fault), where the slip starts. The *epicentre* is the point on the Earth’s surface directly above the focus.

The [focus](#def-g7-earthquakes-focus) is often a few kilometres to a few tens of kilometres deep. Of the 7 031 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 6 or more recorded between 1976 and 2025, 5 561 — 79 % — had their [focus](#def-g7-earthquakes-focus) less than $70\,\mathrm{km}$ down. But 483 started more than $300\,\mathrm{km}$ down, and the deepest $688\,\mathrm{km}$ down: these deep [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) happen where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another, inside the sinking [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate).

A large [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) rarely comes alone. It is often followed by smaller ones in the same place, the *aftershocks*, for weeks, months and even years. Sometimes smaller [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) come before it, but nobody can tell they were warnings until the large one has happened: scientists cannot predict [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake). They can only say which [faults](#def-g7-earthquakes-fault) will break again one day, and help people prepare.

![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.](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/fig-5778d27e1779.svg)

*A [fault](#def-g7-earthquakes-fault), the [focus](#def-g7-earthquakes-focus) and the [epicentre](#def-g7-earthquakes-focus) (a drawing, not to scale). The two blocks slip past each other along the [fault](#def-g7-earthquakes-fault) (red); the slip starts at the [focus](#def-g7-earthquakes-focus), and waves spread out from it in all directions, like ripples on a pond. The [epicentre](#def-g7-earthquakes-focus) (star) is on the surface, straight above the [focus](#def-g7-earthquakes-focus).*

## 20.2 Recording an earthquake

**Definition 20.3 (Seismometer and seismogram).**

A *seismometer* is an instrument that records the shaking of the ground. Its record, a wiggly line drawn against time, is a *seismogram*.

The idea of a [seismometer](#def-g7-earthquakes-seismometer) is simple. A frame is fixed firmly to the ground; a heavy weight hangs from it on a spring. When the ground shakes, the frame shakes with it, but the heavy weight, held only by its spring, stays almost still. The instrument records the movement of the frame with respect to the still weight: that is the movement of the ground. In the drawing below, a pen fixed to the mass draws it on paper wrapped around a turning drum.

![Left: the principle of a seismometer (a drawing). Right: a seismogram drawn for this book (not a real record): the line is calm, then the faster waves arrive, then, ten seconds later, the slower and larger ones.](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/fig-5ae63d050203.svg)

*Left: the principle of a [seismometer](#def-g7-earthquakes-seismometer) (a drawing). Right: a [seismogram](#def-g7-earthquakes-seismometer) drawn for this book (not a real record): the line is calm, then the faster waves arrive, then, ten seconds later, the slower and larger ones.*

### How far away was the earthquake?

A [seismogram](#def-g7-earthquakes-seismometer) tells more than “the ground shook”. The faster waves always arrive first and the slower ones after. Close to the [focus](#def-g7-earthquakes-focus), they arrive almost together; far away, the slower waves fall more and more behind, just as during a [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) the thunder lags farther behind the lightning when the [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) is far away. So the delay between the two arrivals tells the distance to the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake).

How many kilometres for each second of delay? It depends on the speeds of the two waves in the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) they cross. With the speeds of the reference model of the Earth ([Chapter 16](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#ch-g6-inside-the-earth)), each second of delay means about $7\,\mathrm{km}$ in the upper [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) and $9\,\mathrm{km}$ in the lower [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers). In the exercises of this chapter we use the round value of $8\,\mathrm{km}$ for each second.

**Method 20.4 (Finding the epicentre with three stations).**

1. On the [seismogram](#def-g7-earthquakes-seismometer) of each station, read the delay between the arrival of the faster waves and the arrival of the slower ones.
2. Turn each delay into a distance (here: $8 \times$ the delay in seconds, in kilometres).
3. On a [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) , draw around each station a circle whose radius is its distance. The [epicentre](#def-g7-earthquakes-focus) lies somewhere on each circle.
4. With two circles, there are usually two possible points; the third circle passes through only one of them: that is the [epicentre](#def-g7-earthquakes-focus) .

**Example 20.5 (Three circles).**

Three stations record the same [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-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](#def-g7-earthquakes-focus).

![Each circle is centred on a station (triangle), with a radius equal to that station’s distance from the earthquake. The three circles meet at only one point, the epicentre (star).](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/fig-eba5fed6865b.svg)

*Each circle is centred on a station (triangle), with a radius equal to that station’s distance from the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake). The three circles meet at only one point, the [epicentre](#def-g7-earthquakes-focus) (star).*

![Seismometers in a quiet underground room, set on a concrete pier that rests on the rock, far from roads and machines (AI-generated illustration of a generic scene).](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/img-63c13ab7607b.jpg)

*[Seismometers](#def-g7-earthquakes-seismometer) in a quiet underground room, set on a concrete pier that rests on the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), far from roads and machines (AI-generated illustration of a generic scene).*

## 20.3 How large? Magnitude and intensity

Two different numbers describe an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), and they are often confused.

**Definition 20.6 (Magnitude).**

The *magnitude* of an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) measures its size: the energy released at the source. It is worked out from the [seismograms](#def-g7-earthquakes-seismometer). Each [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) has a single magnitude, written with one decimal, such as 6.3 or 9.1.

**Definition 20.7 (Intensity).**

The *intensity* of an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) at a place describes how strongly the ground shook there, from what people felt and from the damage they saw. It is written in Roman numerals, from I (not felt) to XII (total destruction). One [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) has many intensities: a different one in each place.

The magnitude scale climbs fast. Each step of 1 in [magnitude](#def-g7-earthquakes-magnitude) means waves about 10 times larger on the [seismogram](#def-g7-earthquakes-seismometer) and about 32 times more energy released. An [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 8 releases about $32 \times 32 \approx 1000$ times more energy than one of [magnitude](#def-g7-earthquakes-magnitude) 6.

**Method 20.8 (Comparing two magnitudes).**

To compare the energy of two [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), count the whole steps between their [magnitudes](#def-g7-earthquakes-magnitude), and multiply by 32 once for each step: 1 step, 32 times; 2 steps, about 1000 times; 3 steps, about 32 000 times.

**Example 20.9 (Magnitude 9 and magnitude 7).**

The 2004 [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) off Sumatra had a [magnitude](#def-g7-earthquakes-magnitude) of 9.1. An [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 7.1 is two steps smaller: it released about $32 \times 32 = 1024$, so about 1000 times, less energy.

The largest [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) ever measured all happened where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another: [magnitude](#def-g7-earthquakes-magnitude) 9.5 in Chile on 22 May 1960, 9.2 in Alaska in 1964, 9.1 off Sumatra in 2004 and 9.1 off the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Japan on 11 March 2011. Large [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) are rare, and the larger they are, the rarer, as the chart shows: each step up in [magnitude](#def-g7-earthquakes-magnitude), about ten times fewer.

![Earthquakes per year, on average over 1976–2025, by magnitude. The vertical scale is special: each line is ten times the one below it. Data: U.S. Geological Survey earthquake catalogue (7 031 earthquakes of magnitude 6 or more).](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/fig-c50298f154d7.svg)

*[Earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) per year, on average over 1976–2025, by [magnitude](#def-g7-earthquakes-magnitude). The vertical scale is special: each line is ten times the one below it. Data: U.S. Geological Survey [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) catalogue (7 031 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 6 or more).*

The [intensity scale](#def-g7-earthquakes-intensity) has no formula: it is a list of what happens. The table gives it in short form.

| **[intensity](#def-g7-earthquakes-intensity)** | **what is felt and seen** |
| --- | --- |
| I | not felt, except by a very few people |
| II | felt only by a few people at rest, especially upstairs; hanging objects may swing |
| III | felt indoors; it feels like a passing truck |
| IV | felt indoors by many; dishes, windows and doors rattle |
| V | felt by nearly everyone, many woken up; some dishes and windows broken |
| VI | felt by all, many frightened; some heavy furniture moved; slight damage |
| VII | little damage in well-built buildings, much in poorly built ones; some chimneys broken |
| VIII | much damage in ordinary buildings, some partly collapse; chimneys and walls fall |
| IX | great damage even in strong buildings; buildings shifted off their foundations |
| X | most buildings destroyed with their foundations; rails bent |
| XI | few buildings left standing; bridges destroyed |
| XII | damage total; objects thrown into the air |

The [intensity](#def-g7-earthquakes-intensity) is highest, usually, near the [epicentre](#def-g7-earthquakes-focus), and decreases with distance. But it also depends on the ground: solid [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) shakes the least, soft [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) the most. A town built on soft ground can be shaken harder than another town closer to the [epicentre](#def-g7-earthquakes-focus) but built on [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

![After the Hebgen Lake earthquake, Montana (United States), August 1959: the road along the lake slumped into the water, and a lodge ended up almost under water. Damage like this is the mark of a high intensity. Photograph: I. J. Witkind, U.S. Geological Survey (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/img-51effd49af2b.jpg)

*After the Hebgen [Lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), Montana (United States), August 1959: the road along the [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) slumped into the water, and a lodge ended up almost under water. Damage like this is the mark of a high [intensity](#def-g7-earthquakes-intensity). Photograph: I. J. Witkind, U.S. Geological Survey (public domain, via Wikimedia Commons).*

## 20.4 Tsunamis

**Definition 20.10 (Tsunami).**

A *tsunami* is a series of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) waves set off by a sudden movement of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor — usually an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), sometimes an [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) — which can cross whole [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) and [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) the [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

When a [fault](#def-g7-earthquakes-fault) under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) breaks, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor moves suddenly up or down, and lifts or lowers the whole column of water above it. The water spreads out in waves. Out in the deep [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), a [tsunami](#def-g7-earthquakes-tsunami) is surprising: it is rarely more than a metre high, and ships do not even notice it pass beneath them. But it is fast: over $800\,\mathrm{km}/\mathrm{h}$, as fast as a jet plane. As it reaches shallow water near the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), it slows down to 30 or $50\,\mathrm{km}/\mathrm{h}$ — still faster than anyone can run — and grows taller. Most [tsunamis](#def-g7-earthquakes-tsunami) are less than $3\,\mathrm{m}$ high at the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water); the largest pass $30\,\mathrm{m}$. The first wave is not always the largest, and the waves can keep coming for hours.

![A tsunami crossing the ocean (a drawing, not to scale: the height of the wave is greatly exaggerated). It is low and very fast over the deep ocean; it slows down and grows in shallow water near the coast.](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/fig-26d193988fb8.svg)

*A [tsunami](#def-g7-earthquakes-tsunami) crossing the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) (a drawing, not to scale: the height of the wave is greatly exaggerated). It is low and very fast over the deep [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); it slows down and grows in shallow water near the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).*

**Example 20.11 (From Sumatra to Sri Lanka).**

Colombo, the largest city of Sri Lanka, is about $1830\,\mathrm{km}$ from the [epicentre](#def-g7-earthquakes-focus) of the 2004 [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake). At $800\,\mathrm{km}/\mathrm{h}$, a [tsunami](#def-g7-earthquakes-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](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake): the wave came without any warning but the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) itself.

**Remark 20.12 (The warning the sea gives).**

On a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), three signs can announce a [tsunami](#def-g7-earthquakes-tsunami): a strong or long [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake); the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) behaving strangely — a sudden [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), or the water drawing back far from the shore, showing the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor like a very low tide; or a loud roar coming from the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). During the shaking, protect yourself as you learned (drop, cover, hold on). Then do not wait for an official warning: go as high, or as far inland, as you can, at once. A [tsunami](#def-g7-earthquakes-tsunami) from a nearby [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) can arrive within minutes.

## 20.5 Exercises

**Exercise 20.1 ★.**

Match each word with its meaning: [fault](#def-g7-earthquakes-fault); [focus](#def-g7-earthquakes-focus); [epicentre](#def-g7-earthquakes-focus); [seismometer](#def-g7-earthquakes-seismometer); [magnitude](#def-g7-earthquakes-magnitude); [intensity](#def-g7-earthquakes-intensity). (a) The place underground where the slip starts. (b) How strongly the ground shook at one place. (c) A break in the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) along which the two sides have moved. (d) The point on the surface above the [focus](#def-g7-earthquakes-focus). (e) The size of an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), one number for each [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake). (f) An instrument that records the shaking of the ground.

**Solution of Exercise 20.1.**

(a) [focus](#def-g7-earthquakes-focus); (b) [intensity](#def-g7-earthquakes-intensity); (c) [fault](#def-g7-earthquakes-fault); (d) [epicentre](#def-g7-earthquakes-focus); (e) [magnitude](#def-g7-earthquakes-magnitude); (f) [seismometer](#def-g7-earthquakes-seismometer).

**Exercise 20.2 ★.**

The [focus](#def-g7-earthquakes-focus) of an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) lies $30\,\mathrm{km}$ straight below a village. (a) Where is the [epicentre](#def-g7-earthquakes-focus)? (b) How far is the village from the [focus](#def-g7-earthquakes-focus)? (c) A second village is $40\,\mathrm{km}$ from the [epicentre](#def-g7-earthquakes-focus). Will its [intensity](#def-g7-earthquakes-intensity) usually be higher or lower than in the first village? Why?

**Solution of Exercise 20.2.**

(a) At the village itself: the [epicentre](#def-g7-earthquakes-focus) is the point of the surface straight above the [focus](#def-g7-earthquakes-focus). (b) $30\,\mathrm{km}$, straight down. (c) Usually lower: the shaking weakens with distance. (It also depends on the ground: on soft [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), the second village could still shake hard.)

**Exercise 20.3 ★.**

[Magnitude](#def-g7-earthquakes-magnitude) or [intensity](#def-g7-earthquakes-intensity)? (a) “The [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) measured 6.8.” (b) “In our street, all the dishes fell and a chimney broke.” (c) “It was VI in the town and IV in the villages of the [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).” (d) “It released 32 times more energy than last year’s.”

**Solution of Exercise 20.3.**

(a) [Magnitude](#def-g7-earthquakes-magnitude) (a number with one decimal). (b) [Intensity](#def-g7-earthquakes-intensity) (what was seen in one place). (c) [Intensity](#def-g7-earthquakes-intensity) (Roman numerals, different in two places). (d) [Magnitude](#def-g7-earthquakes-magnitude): the energy released, one step of [magnitude](#def-g7-earthquakes-magnitude) more.

**Exercise 20.4 ★.**

True or false? Correct the false ones. (a) Scientists can tell a week in advance when an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) will strike. (b) In the open [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), a [tsunami](#def-g7-earthquakes-tsunami) is a huge wall of water. (c) One [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) has a single [magnitude](#def-g7-earthquakes-magnitude) but many intensities. (d) Most large [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) happen near [plate boundaries](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary).

**Solution of Exercise 20.4.**

(a) False: scientists cannot predict [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake); they only know which [faults](#def-g7-earthquakes-fault) will break again one day. (b) False: in the open [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) a [tsunami](#def-g7-earthquakes-tsunami) is rarely more than a metre high; it grows near the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). (c) True. (d) True: 92 % of the [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 6 or more happen less than $300\,\mathrm{km}$ from a [plate boundary](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary).

**Exercise 20.5 ★★.**

Use the chart of [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) per year. (a) About how many [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 6 to 6.9 happen in a year? Of [magnitude](#def-g7-earthquakes-magnitude) 7 to 7.9? (b) How many times fewer are the second than the first? (c) Add the three bars from 7 upwards: about how many [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 7 or more happen in a year? (d) The chart says 0.04 a year for [magnitude](#def-g7-earthquakes-magnitude) 9 or more. How many is that in 50 years?

**Solution of Exercise 20.5.**

(a) About 127 a year of [magnitude](#def-g7-earthquakes-magnitude) 6 to 6.9; about 13 of [magnitude](#def-g7-earthquakes-magnitude) 7 to 7.9. (b) $126.7 \div 13.12 \approx 9.7$: about 10 times fewer. (c) $13.12 + 0.76 + 0.04 = 13.92$: about **14** a year. (d) $0.04 \times 50 = 2$: two in 50 years (Sumatra 2004 and Japan 2011).

**Exercise 20.6 ★★.**

Use the method for comparing [magnitudes](#def-g7-earthquakes-magnitude). (a) How many times more energy does an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 7 release than one of [magnitude](#def-g7-earthquakes-magnitude) 6? (b) And one of [magnitude](#def-g7-earthquakes-magnitude) 8 than one of [magnitude](#def-g7-earthquakes-magnitude) 6? (c) How many times larger are its waves on the [seismogram](#def-g7-earthquakes-seismometer), for the same distance, between [magnitude](#def-g7-earthquakes-magnitude) 6 and 8?

**Solution of Exercise 20.6.**

(a) One step: 32 times. (b) Two steps: $32 \times 32 = 1024$, about 1000 times. (c) Each step makes the waves 10 times larger: $10 \times 10 =
100$ times larger.

**Exercise 20.7 ★★.**

Use $8\,\mathrm{km}$ for each second of delay between the faster and the slower waves. (a) A station records a delay of $5\,\mathrm{s}$. How far away was the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake)? (b) Another records $25\,\mathrm{s}$. (c) An [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) struck $120\,\mathrm{km}$ from a station: what delay will the station read? (d) Why can one station alone not tell where the [epicentre](#def-g7-earthquakes-focus) is?

**Solution of Exercise 20.7.**

(a) $8 \times 5 = 40\,\mathrm{km}$. (b) $8 \times 25 = 200\,\mathrm{km}$. (c) $120 \div 8 = 15\,\mathrm{s}$. (d) One station gives only a distance, not a direction: all the points at that distance form a circle around the station, and the [epicentre](#def-g7-earthquakes-focus) could be anywhere on it.

**Exercise 20.8 ★★.**

Use the [intensity](#def-g7-earthquakes-intensity) table. Give the [intensity](#def-g7-earthquakes-intensity) for each report. (a) “Felt by nearly everyone; we were woken up and two glasses broke.” (b) “Only my grandmother, resting upstairs, felt it; the lamp swung.” (c) “Walls and chimneys fell; several houses partly collapsed.” (d) The same [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) gave reports (a) and (c). Explain how this is possible.

**Solution of Exercise 20.8.**

(a) V. (b) II. (c) VIII. (d) The [intensity](#def-g7-earthquakes-intensity) changes from place to place: report (c) comes from a place close to the [epicentre](#def-g7-earthquakes-focus), or built on soft ground; report (a) from a place farther away, or on solid [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). The [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) still has only one [magnitude](#def-g7-earthquakes-magnitude).

**Exercise 20.9 ★★.**

Of the 7 031 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 6 or more of 1976–2025, 5 561 had their [focus](#def-g7-earthquakes-focus) less than $70\,\mathrm{km}$ deep. (a) How many had a deeper [focus](#def-g7-earthquakes-focus)? (b) What percentage is that? Round to the nearest whole number. (c) The deep ones happen where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another. Explain why they can start so deep there.

**Solution of Exercise 20.9.**

(a) $7031 - 5561 = 1470$. (b) $1470 \div 7031 \approx 0.209$: about **21 %**. (c) The sinking [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) goes down deep into the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers). It is still rigid enough to break, so [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) can start inside it, hundreds of kilometres down.

**Exercise 20.10 ★★.**

Malé, the capital of the Maldives, is about $2500\,\mathrm{km}$ from the [epicentre](#def-g7-earthquakes-focus) of the 2004 [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake). (a) At $800\,\mathrm{km}/\mathrm{h}$, how long did the [tsunami](#def-g7-earthquakes-tsunami) take to reach it? Give hours and minutes. (b) How much later did it arrive than at Colombo (2 hours 17 minutes)? (c) With such delays, why could a warning system have saved many lives?

**Solution of Exercise 20.10.**

(a) $2500 \div 800 = 3.125$ hours: 3 hours and $0.125 \times 60 = 7.5$ minutes, about 3 hours 8 minutes. (b) 3 h 8 min $-$ 2 h 17 min $=$ 51 minutes later. (c) The wave took hours to arrive, while a large [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) is located within minutes: a warning could have sent people to high ground long before the wave came.

**Exercise 20.11 ★★★.**

A [tsunami](#def-g7-earthquakes-tsunami) crosses $400\,\mathrm{km}$ of deep [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) at $800\,\mathrm{km}/\mathrm{h}$, then the last $20\,\mathrm{km}$ of shallow water at $40\,\mathrm{km}/\mathrm{h}$. (a) How many minutes does it take for the deep part? (b) For the shallow part? (c) How long is the whole journey? (d) What is surprising in your answers? (e) Why does the wave grow taller as it slows down?

**Solution of Exercise 20.11.**

(a) $400 \div 800 = 0.5$ hour: 30 minutes. (b) $20 \div 40 = 0.5$ hour: 30 minutes. (c) 60 minutes. (d) The last $20\,\mathrm{km}$ take as long as the first $400\,\mathrm{km}$: the wave slows down enormously in shallow water. (e) The front of the wave slows down while the water behind it still comes fast: the water piles up, and the wave grows taller.

**Exercise 20.12 ★★★.**

(a) How many times more energy does an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 9 release than one of [magnitude](#def-g7-earthquakes-magnitude) 6? (b) Each year there are about 127 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of [magnitude](#def-g7-earthquakes-magnitude) 6 to 6.9. Suppose each of them releases the energy of a [magnitude](#def-g7-earthquakes-magnitude) 6. How many years of them does it take to release the energy of one [magnitude](#def-g7-earthquakes-magnitude) 9 [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake)? (c) What does this tell you about the share of the energy carried by the very largest [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake)?

**Solution of Exercise 20.12.**

(a) Three steps: $32 \times 32 \times 32 = 32\,768$, about **33 000** times. (b) $32\,768 \div 127 \approx 258$: about 260 years of all the magnitude-6 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of the world. (c) Almost all the energy is released by the few very largest [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake): a single [magnitude](#def-g7-earthquakes-magnitude) 9 outweighs centuries of magnitude-6 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake).

## 20.6 Problem: Where did the ground shake?

**Problem 20.1.**

Weekend problem — three stations record an earthquake under the sea, the epicentre is found on the map, and the town of Port Hale is warned before the tsunami arrives

An [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) strikes under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) of an invented [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). Three [seismometer](#def-g7-earthquakes-seismometer) stations, A, B and C, stand on land; the town of Port Hale stands on an island ([map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) below; one square is $20\,\mathrm{km}$). In this region, each second of delay between the faster and the slower waves means $8\,\mathrm{km}$. The stations record:

| **station** | **faster waves arrive** | **slower waves arrive** |
| --- | --- | --- |
| A | 10:15:25 | 10:15:45 |
| B | 10:15:16 | 10:15:29 |
| C | 10:15:19 | 10:15:34 |

**Part I — Three stations.**

1. For each station, find the delay between the two arrivals, in seconds.
2. Which station is closest to the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) ? How can you tell without any calculation?
3. Find the distance from each station to the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) .
4. Station A’s distance alone does not say where the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) was. Describe all the places that are at this distance from A.

![](https://one-course.com/images/onecourse/chapters/earth-1/g7-earthquakes/fig-ab91d3244e66.svg)

**Part II — Finding the epicentre.**

5. One square is $20\,\mathrm{km}$ . How many squares is the radius of each circle?
6. Place the corner O at the bottom left of the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) . Station C is 3 squares east and 14.5 squares north of O; going 6 squares ( $120\,\mathrm{km}$ ) due east of C, you reach a point E. Check on the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) that E is also on the circles of A and B: it is the [epicentre](#def-g7-earthquakes-focus) . How many squares east and north of O is it?
7. Is the [epicentre](#def-g7-earthquakes-focus) on land or under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ? Why does that matter?
8. The [magnitude](#def-g7-earthquakes-magnitude) was 7.4. How many times more energy than a [magnitude](#def-g7-earthquakes-magnitude) 6.4 [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) did it release?

**Part III — The wave reaches the coast.**

9. On the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) , Port Hale is 12 squares from the [epicentre](#def-g7-earthquakes-focus) . How many kilometres is that?
10. At $800\,\mathrm{km}/\mathrm{h}$ , how many minutes does the [tsunami](#def-g7-earthquakes-tsunami) take to reach Port Hale?
11. Locating the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) and sending the warning takes 5 minutes after the shaking starts. At what time after the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) does the warning reach Port Hale?
12. How many minutes do the people of Port Hale have to reach high ground before the [tsunami](#def-g7-earthquakes-tsunami) arrives?

**Solution of Problem 20.1.**

**1.** A: $45 - 25 = 20$ s. B: $29 - 16 = 13$ s. C: $34 - 19 =
15$ s.

**2.** Station B: its delay is the shortest, and the shorter the delay, the closer the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake). (Its faster waves also arrived first.)

**3.** A: $8 \times 20 = 160\,\mathrm{km}$. B: $8 \times 13 =
104\,\mathrm{km}$. C: $8 \times 15 = 120\,\mathrm{km}$.

**4.** All the points $160\,\mathrm{km}$ from A: a circle of radius $160\,\mathrm{km}$ centred on A. The [epicentre](#def-g7-earthquakes-focus) is somewhere on it.

**5.** A: $160 \div 20 = 8$ squares. B: $104 \div 20 = 5.2$ squares. C: $120 \div 20 = 6$ squares.

**6.** E is $3 + 6 = 9$ squares east and 14.5 squares north of O, out at [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) between the three stations; the circles of A and B pass through it.

**7.** Under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). A sudden movement of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor can set off a [tsunami](#def-g7-earthquakes-tsunami): the [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and the island have to be warned.

**8.** One step of [magnitude](#def-g7-earthquakes-magnitude): about 32 times more energy.

**9.** $12 \times 20 = 240\,\mathrm{km}$.

**10.** $240 \div 800 = 0.3$ hour, and $0.3 \times 60 = 18$ minutes.

**11.** 5 minutes after the [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake).

**12.** $18 - 5 = 13$: the people of Port Hale have about **13 minutes** to reach high ground. (The wave slows down in the shallow water near the island, which gives a little more time, but nobody should count on it: they leave at once.)