Earthquake Theory and Fieldwork

1) Earthquake theory

(i) What is an earthquake?

    

An earthquake is a sudden movement of the earth’s crust caused by the release of stress accumulated along tectonic plate boundaries or fault lines.

(ii) Why do earthquakes form?

The Earth’s surface is divided into pieces which all interlock each other like a jigsaw. 

These plates aren’t stationary. Below them there is hot liquid magma which makes plates move. When the plates move because of the hot magma underneath and make contact with another plate, energy is produced. The energy produced can be felt on the Earth’s surface through seismic waves. This is what we call an earthquake.

(iii) How do earthquakes form?

 The plates can interact with each other in four ways:

Boundary 

Plates Involved 

What Happens

Result 

Example

Destructive

Oceanic and Continental 

The denser oceanic plate subducts the continental plate causing hot molten rock to form and rise. This creates a volcano.

Volcano, Earthquakes and fold mountains

Nazca and South American Plates

Constructive

Oceanic and Oceanic 

Two plates move away from each other forcing hot magma to rise from underneath. 

Volcano and Earthquakes

North American and Eurasion Plates – Iceland

Collision 

Continental and Continental 

This is where two continental plates collide together and neither can sink hence forcing material upwards.

Earthquake and fold mountains 

Indian and Eurasion plates – Himalayas

Conservative 

Any 

Two plates slide past each other usually getting stuck. This builds pressure and when they finally release the pressure there is sudden movement.

Earthquake 

North American and Pacific Plates – San Andres Fault

(iv) Where are earthquakes usually found in the world?

The take place all over the world. They don’t just occur on plate boundaries, they also occur on minor faults. 80% of all tectonic activity take place on the boundary of the pacific plate. This includes; the Philippines islands, New Zealand, and Japan.

2. Earthquake fieldwork

(i) How can we measure the magnitude of an earthquake?

There are two ways of measuring the magnitude of an earthquake (i) the Mercalli scale and (ii) the Richter scale.

Mercalli scale

The Mercalli intensity scale is a numerical and subjective way of measuring the magnitude of earthquakes. It is a scale between 1-12; 1 being nothing being felt and 12 total destruction. The scale isn’t used very much today because it isn’t accurate because different will have different experiences which will reflect the different scores given to different earthquakes.

Richter scale 

The Richter scale is a numerical and objective way of measuring the magnitude of earthquakes. It is a scale between 1-10; it has a logarithm scale i.e. an earthquake measure 4 on the Richter scale is 10 times as big as one measuring 3.  This is the most popular way of predicting earthquakes today. It works by using seismometers to measure the seismic energy released.

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