EARTHQUAKES

YEAR 7

SICHUAN EARTHQUAKECHINA 2008

CHINESE INSTRUMENT FOR EARTHQUAKES

1. FORCES IN EARTH’S CRUST An earthquake is the shaking and

trembling that results from the movement of rock in the crust.

The movement of plates creates powerful forces that squeeze or pull the rock in the crust.

These forces are stress. Stress change the shape and the

volume of rock.

Types of stress1. Shearing: stress that pushes a mass

of rock in 2 opposite directions2. Tension: pulls on the crust,

stretching rock so that it becomes thinner in the middle.

3. Compression: squeezes rock until it folds or breaks

• Any change in the volume or shape of Earth’s crust is called deformation.

A fault is a break in the crust where slabs of crust slip past each other.

Faults usually occur along plate boundaries, where the forces of plate motion compress, pull or shear the crust so much that the crust breaks.

There are 3 types: normal (tension) reverse (compression)

strike-slip (shearing),

Hanging wall and Foot wall

STRIKE-SLIP FAULT

REVERSE FAULT

NORMAL FAULT

The forces of plate movement can change Earth’s surface

Anticline Syncline Fault-block mountains Plateau

Fault-block mountains

Plateau :A large area of flat land elevated high above sea level.

2. MEASURING EARTHQUAKES

Most earthquakes begin in the lithosphere.

The focus is the point beneath Earth’s surface where rock that is under stress breaks, triggering an earthquake.

The point on the surface directly above the focus is called the epicenter.

An earthquake produces vibrations called waves.

These waves carry energy in all directions.

Seismic waves are vibrations that travel through Earth carrying the energy released during an Earthquake (from the focus, through Earth’s interior and across the surface)

There are 3 types of seismic waves:1. Primary waves (P): compress and expand

like an accordion. They cause building to contract and expand. Travel through solids and liquids.

2. Secondary waves (S): vibrate from side to side as well as up and down. They shake the ground back and forth. Only travel through solids.

3. Surface waves: when P and S waves reaches the surface, some of them become surface waves. It produce severe ground movements.

SURFACE WAVES

A seismograph records the ground movements caused by seismic waves as they move through the Earth.

The zigzag pattern of lines used to represent an earthquake is called a seismogram. It is produced by a seismograph.

Geologists use seismic waves to locate an earthquakes epicenter.

Seismic waves travel at different speeds. P waves arrive first followed by S waves

To tell how far the epicenter is from the seismograph, scientists measure the difference between the arrival times of P and S waves.

Then they draw at least 3 circles using data from different seismograph set up at stations all over the world. The point where the 3 circles intersect is the location of the epicenter.

0 1000 2000 3000 4000 50000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Difference in arrival time of P and S waves (min)

Distance to Epicenter (km)

Seismic wave arrival times

Magnitude is a measurement of earthquake strength based on seismic waves and movement along faults.

There are 3 scales:1. The Mercalli scale: The level of damage at a given

place2. The Richter scale: Measured by a seismograph3. The Moment Magnitude scale:

Use by geologist today Rating system that estimates the total energy

released by an earthquake Below 5 consider small or cause little damage Above 5 produce great destruction

4. EARTHQUAKE SAFETY

Geologist can determine earthquake risk by locating where faults are active and where past earthquakes have occurred.

Even with data from many sources, geologists can’t predict when and where a quake will strike.

Causes of earthquake damage include

1. Shaking2. Liquefaction3. Aftershocks4. Tsunamis.

SHAKING Shaking by seismic waves can trigger

landslides or avalanches.

It can damage or destroy buildings and bridges, topple utility poles, and fracture gas and water mains.

LIQUEFACTION Occurs when an earthquake turns loose, soft

soil into liquid mud

AFTERSHOCKS earthquake that occurs after a larger

earthquake in the same area

TSUNAMIS When an earthquake

jolts the ocean floor, plate movement causes the ocean floor to rise slightly and push water out of its way. If its strong enough, the water displaced by the quake forms large waves.

EARTHQUAKE SAFETY Best way to protect yourself is to drop, cover

and hold.Indoors: Avoid windows, mirrors, wall

hangings…Outdoors: move to an open area such as a

playground. Avoid vehicles, power lines, trees and buildings. Sit down to avoid being thrown down.

SAFER BUILDINGS New buildings must be made stronger

and more flexible. Older buildings need to be modified.

Base isolated building rests on rubber pads or springs to absorb energy from the quake.

Making utilities safer (flexible joints and valves on pipes)

Earthquake Response of Base-isolated Building

Earthquake Response of Conventional Fixed-Base Building