Unit 8:  Internal dynamics of the Earth                                    1   2   3

3.2. Earthquakes

Earthquakes are sudden movements of the surface layers of the Earth. They occur when large rocky masses inside the Earth’s crust move past each other.

The movement of these rocky masses releases a large amount of energy, and this can be sudden, violent and destructive.

a) Elements of an earthquake

     An earthquake has the following elements:

       - The hypocentre (or focus) is the point in the Earth’s crust where

          the earthquake starts and it produces seismic waves.

          At an earthquakes hypocentre the rocks break up, which causes

          the ground to move violently, and energy is released.

       - The epicenter is the point on the Earth’s surface that is directly

          above the hypocentre. The epicentre is the place where the seismic waves

          first reach the Earth’s surface and the effects of the earthquake are felt

          most intensely.

       - The seismic waves are vibrations that extend in all directions

          from the hypocentre. When they reach the surface, they produce catastrophes.

b) Measuring of an earthquake

     Seismographs are the devices that geologists used to detect and record

     earthquakes. A seismograph consists of a hanged weight with a pen which draws

     on a roll of paper located below.

     During the shake, the base of the seismograph and the roll of paper move

     but not the weight, so that the pen draws squiggly lines on the paper,

     creating a record of the earthquake. This graph is called a seismogram.

Animation: How a seismograph works (Norton)

We can use two different parameters to measure earthquakes:

- The magnitude of an earthquake tells us how much energy is released

  during a tremor. We use a 9-degrees scale (Richter scale) to measure

  the magnitude of an earthquake. On this scale, each number represents

  a release of energy that is 10 times more than the previous number.

- The intensity of an earthquake is determined by studying how much

  destruction it causes. To express intensity we compare the various levels

  of destruction caused by an earthquake with a 12-degree scale

  of descriptions (Mercalli scale). This scale is not very reliable because

  the destruction caused depends on factors like the quality of the constructions.

Animation: Mecalli scale (elearning)

c) Danger of earthquakes

    An earthquake is one of the natural disasters that can cause most death

    and destruction, mainly because they can provoke buildings collapse,

    landslides, floods, fires, etc. If the earthquake occurs under the sea floor,

    tsunamis can destroy coastal areas.

    As it is extremely difficult to predict earthquakes, there are some measures

    to prevent earthquakes disasters. The main ones are:

        - Elaborate earthquake risk maps that indicate areas of high risk.

        - Build earthquake-resistant buildings.

        - Develop civil protection programmes for catastrophic situations.

        - Inform people about the measure they should adopt during and after

           an earthquake.

3.3. Indicate the differences between:

a. Hypocentre - Epicentre

b. Magnitude of an earthquake - Intensity of an earthquake.

3.4. Listen and identify which of the two terms of each couple is been described:

Fukusihima (Linguaframe)