Earthquakes

Earthquakes

Natural vibrations of the ground caused by movement at faults

Faults: Fracture in the Earth’s crust Faults form when rocks break

deep within the Earth’s surface due to stress

In conclusion…

Earth’s rocks deform when stress is exerted on them. If the stress exceeds a certain limit, the rocks fracture to form faults. Movement along faults causes most earthquakes.

Earthquake Waves

Seismic Waves – The vibrations of the ground during

an earthquake

3 Types of Seismic wavesPrimary waves (aka P waves)

Push/pull rocks same way waves are traveling

Travels through Earth’s interior

Secondary waves (S waves) Move perpendicular to the

direction of the waves Travel through interior of Earth

Surface waves Move in 2 directions

Like the ocean and like a snake at the same time

Travel on the surface of Earth

Some more vocabulary

Focus Point of failure of the rocks Where the earthquake startsUsually several kms below the

surface

EpicenterThe point on Earth’s surface directly

above the focus

Locating Earthquakes

There are more than 1 million earthquakes each year…

However, 90% of them are not felt by people.

The exact location of the epicenter and the time of occurrence are initially unknown.

For any distance from the epicenter, the P waves always arrive first.

The further the recording instrument is from the epicenter, the greater the difference in arrival times of the 1st P wave and the 1st S wave.

This information is used to determine the exact location of the epicenter.

Gathering data….

5 10 15 20 25

P wave

S wave

Need data from several locations: At least 3 locations

A travel-time graph is used to determine the distance from a recording station to the epicenter Based on travel time of the P and S

waves The time between when the P wave

arrives and the S wave arrives is called the S-P lag

Epicenter Distance (km)

S-P

lag (m

in)

Using the info…

Once you figure out the distance of the epicenter to the recording instrument you plot a CIRCLE on a map with the radius being the distance discovered.

Where the 3 intersect is the epicenter

Measuring Earthquakes

There are several different scales used to measure earthquakes.Measure size of seismic wavesMeasurements of the fault

Size, amount of movement, type of rock occurred in

Measure amount of damage done on surface of Earth

CD-ROM video clips

Slides on CD-ROM Types of seismic waves

369-371

Locating an epicenter 375-387

Factors Influencing Intensity

Size of seismic waves produced

Depth of the focusDistance from the epicenter

of the quake

Using Earthquakes for Info

Most of the knowledge of Earth’s interior comes from the study of seismic waves.These waves change speed and

direction when they encounter different materials.

Scientists track where they are detected and at what strength.

Understanding Stress

Fractures form when the stress exceeds the strength of the rocks

3 Kinds of Stress: a forceCompression – push motionTension – pull motionShear – twist motion

Strain – deformation in material due to stress.

Stre

ss

Strain

Elastic Deformation – not permanent

Ductile Deformation- permanent damage

Can Vary Based on Temperature!

Failure