NCEIV42 0-0812

Question 1

Lesson 42

Answer 1

Lesson 42 Recording an earthquake

Question 2

An earthquake c

Answer 2

An earthquake comes like a thief in the night, without warning.

Question 3

It was necessar

Answer 3

It was necessary, therefore, to invent instruments that neither slumbered nor slept.

Question 4

Some devices we

Answer 4

Some devices were quite simple.

Question 5

One, for instan

Answer 5

One, for instance, consisted of rods of various lengths and thicknesses with would stand up end like ninepins.

Question 6

When a shock ca

Answer 6

When a shock came, it shook the rigid table upon which these stood.

Question 7

If it were gent

Answer 7

If it were gentle, only the more unstable rods fell.

Question 8

If it were seve

Answer 8

If it were severe, they all fell.

Question 9

Thus the rods,

Answer 9

Thus the rods, by falling, and by the direction in which they fell, recorded for the severe, they all fell.

Question 10

Thus the rods,

Answer 10

Thus the rods, by falling, and by the direction in which they fell, recorded for the slumbering scientist the strength of a shock that was too weak to waken him, and the direction from which it came.

Question 11

But instruments

Answer 11

But instruments far more delicate than that were needed if any really serious advance was to be made.

Question 12

The ideal to be

Answer 12

The ideal to be aimed at was to devise an instrument that could record with a pen on paper, the movements of the ground or of the table as the quake passed by.

Question 13

While I write m

Answer 13

While I write my pen moves, but the paper keeps still.

Question 14

With practice,

Answer 14

With practice, no doubt, I could in time learn to write by holding the pen still while the paper moved.

Question 15

That sounds a s

Answer 15

That sounds a silly suggestion, but that was precisely the idea adopted in some of the early instruments (seismometers) for recording earthquake waves.

Question 16

But when table,

Answer 16

But when table, penholder and paper are all moving, how is it possible to write legibly? The key to a solution of that problem lay in an everyday observation.

Question 17

Why does a pers

Answer 17

Why does a person standing in a bus or train tend to fall when a sudden start is made? It is because his feet move on , but his head stays still.

Question 18

A simple experi

Answer 18

A simple experiment will help us a little further.

Question 19

Tie a heavy wei

Answer 19

Tie a heavy weight at the end of a long piece of string.

Question 20

With the hand t

Answer 20

With the hand to and from and around but not up and string so that the weight nearly touches the ground.

Question 21

Now move the ha

Answer 21

Now move the hand to and from and around but not up and down.

Question 22

It will be foun

Answer 22

It will be found that the weight a piece of string.

Question 23

With the hand h

Answer 23

With the hand held high in the air, hold the string so that the weight nearly touches the ground.

Question 24

Now move the ha

Answer 24

Now move the hand to and fro and around but not up and down.

Question 25

It will be foun

Answer 25

It will be found that ten weight moves but slightly or not at all.

Question 26

Imagine an eart

Answer 26

Imagine an earthquake shock shaking the floor, the paper, you and your hand.

Question 27

In the midst of

Answer 27

In the midst of all this movement, the weight and the pen would be still.

Question 28

But as the pape

Answer 28

But as the paper moved from side to side under the pen point, its movement would be recorded in ink upon its surface.

Question 29

It was upon thi

Answer 29

It was upon this principle that the first instruments were made, but while the drum was being shaken, the line that the pen was drawing wriggled from side to side.

Question 30

The apparatus t

Answer 30

The apparatus thus described, however, records only the horizontal component of the wave movement, which is, in fact, much more complicated.

Question 31

If we could act

Answer 31

If we could actually see the path described by a particle, such as a sand grain in the rock, it would be more like that of a bluebottle path described by a particle, such as a sand grain in the rock, it would be more like that of a bluebottle buzzing round the room; it would be up and down, to and fro and from side to side.

Question 32

Instruments hav

Answer 32

Instruments have been devised and can be so placed that all three elements can be recorded in different graphs.

Question 33

When the instru

Answer 33

When the instrument is situated at more than 700 miles from the earthquake center, the graphic record shows three waves arriving one after at short intervals.

Question 34

The first recor

Answer 34

The first records the arrival of longitudinal vibrations.

Question 35

The second mark

Answer 35

The second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first.

Question 36

These two have

Answer 36

These two have travelled through the earth.

Question 37

It was from the

Answer 37

It was from the study of these that so much was learnt about the interior of the earth.

Question 38

The third, or m

Answer 38

The third, or main.

Question 39

The third, or m

Answer 39

The third, or main wave, is the slowest and has travelled round the earth through the surface rocks.