Earthquakes

Earth Science

What is an Earthquake? A vibration of the Earth produced by a rapid energy

release The point within the earth where an earthquake starts is

called the focusEnergy travels from the focus in all directions- think of a

stone being tossed into a pondStone sets waves of motion in water just as seismic waves

move through earthThe point on the surface of the Earth above the focus is called

the epicenter

The Epicenter The epicenter is the

location directly above the focus

Earthquakes over a 10 yr. period Pacific Ring of fire

Faults Faults are fractures in the Earth where plate

movement has occurred There are three types of forces that create faults.

Compression - squeezesTension - stretchesShear – causes slippage

Each of these three forces can create a different kind of fault.

Earthquake causes Elastic Rebound hypothesis

Rock is bending causing elastic stressEventually the rock will slip and break at the weakest

point (the focus)The vibrations we feel occur as the rock returns to its

original shape

Types of faults Normal Faults- caused by tension forces Reverse Faults- compression Strike-Slip Faults- shear forces

Normal Faults Tension causes rocks to be

pulled apart. Rock above the fault surface

moves downward in relation to rock below the fault surface.

Can help to form mountains such as the Sierra Nevada Mountain Range

Reverse Fault Compression causes rock

to break. Rocks above the fault

surface are forced up and over the rocks below the faults.

Can form mountains such as the Rockies.

Strike-Slip Fault Two of Earth’s plates

move past each other due to shear forces.

There is very little vertical movement

Ex: San Andreas Fault

Earthquake Info Seismic waves – waves

that move through the Earth’s crust

Seismic waves travel outward from the focus of an earthquake

Surface Waves- these travel along Earth’s outer layer

Body waves- travel through Earth’s interior Primary Waves (P) – waves

that travel in the same direction the wave is moving

Secondary Waves (S) – move through the Earth causing particles to move at right angles to the direction of the wave.

Wave Facts Surface waves (S) are the slowest waves, primary (P Waves) are

the fastest. P Waves can only travel through solids. P waves travel 1.7 times faster than S waves

Surface waves cause the most destruction during an earthquake.

Measuring Earthquakes Seismologists use seismographs to measure

earthquakes. The more energy that an earthquake releases, the

greater its magnitude. One of the largest earthquakes occurred in Haiti

very recently and killed over 200,000 people

Finding the Epicenter- Distance Remember P Waves travel faster

than S waves The greater the distance between

the P wave and S wave the farther the source

1) find the time interval between the first P wave and first S wave using the seismogram

2) use the graph to find distance from the seismogram

Finding the Epicenter- Direction Can use 3 or more

seismograms to do this Create a circle using

direction found on graph Where circles intersect

is the exact location of the epicenter

Richter Earthquakes are ranked on the Richter Scale – measures amount of energy released 1-10 For each increase of 1.0,

amplitude of the largest surface wave is 10 times greater

About 32 times as much energy is released

Tsunami Means “Harbor Wave” If an earthquake happens underwater, it can create

a huge wave. This wave travels to the surface After it reaches the surface, it can spread out for

thousands of kilometers in every direction As the wave approaches the shore, it can form a

towering crest that can reach 30m high.

Earthquake Safety Be prepared – know if an

area has had earthquakes in the past or if it is near a pate boundary.

Make your home earthquake safe

Take heavy objects down from high shelves

Hot water heaters and gas appliances should be securely in place

During earthquakes, stay away from windows or anything that could fall on you.

Living on a Fault Seismic Safe structures

Resistant to vibrations Buildings with bases of alternating rubber and steel can

withstand quakes of up to 8.3 on the Richter scale without major damage

Buildings and highways can be made so they will bend but not break

Seismic safe structures are more expensive to build than regular structures.

More Earthquake DamageAnchorage, Alaska 1964 – Magnitude 9.2

Oakland, CA 1989

Northridge, CA 1994 – 62 killed

Former hospital in Mexico City 1985 – Magnitude 8.1 with 7.5 aftershock 18 hours

later

Turkey 1999