A vibration of the Earth produced by a rapid release of energy

Often occur along faults – breaks in the Earths crust and mantle (plate boundaries)

What is an earthquake?

1. Focus – Point within the Earth where the earthquake starts◦ Energy moves in all directions from this point

2. Epicenter – Point on the Earth’s surface directly above the focus◦ Most intense movement during the earthquake

2 important points

The Elastic Rebound Hypothesis

◦Stress builds along an existing fault

◦Deformation of crustal rock – bent features of the rocks caused by increasing stress – elastic energy

◦Slippage (earthquake) – energy is stored in the rock overcomes frictional forces keeping crust in place

◦Energy released – rock returns to original shape

Causes of Earthquakes

At spreading ridges, or similar extensional boundaries, earthquakes are shallow, and tend to be smaller than magnitude 8.

Where are Earthquakes

At compressional boundaries, earthquakes are found from the very near surface to several hundred kilometers depth, ◦ since the coldness of the subducting plate permits

brittle failure down to as much as 700 km.

Compressional boundaries host Earth's largest quakes, with some events on subduction zones in Alaska and Chile having exceeded magnitude 9.

At transforms, earthquakes are shallow, as deep as 25 km; mechanisms indicate strike-slip motion. ◦ Tend to have earthquakes smaller than magnitude 8.5.

The San Andreas fault in California is an example of a transform, separating the Pacific from the North American plate.

At transforms the plates mostly slide past each other laterally, producing less sinking or lifting of the ground than extensional or compressional environments. ◦ The yellow dots below locate earthquakes along strands of this fault system in the San Francisco Bay area.

Measured by a seismograph (instrument)

Seismographs produce seismograms – the written record of the movement

Earthquake Waves

Surface Waves – earthquake waves that travel along the Earth’s outer layer◦Up and down, side to side, twisting

motion◦Most destructive of the waves◦Slowest wave

3 Types of Earthquake Waves

P waves: (Body waves) Compression waves Alternately expand

and compress material they pass through

Travel the fastest of all three waves

Travels parallel to its movement

3 Types of Earthquake Waves

S Waves (Body waves)• Transverse wave• Travel slower than P

waves• Travels at a right

angle to the direction of its movement

• Will not travel through liquids and gases

3 Types of Earthquake Waves

Factors contributing to damage Duration Intensity Building Design – reinforced/flexible

buildings best Materials built on – hard, dense material

the best Liquefaction - when loose sediments are

saturated with water, during an earthquake, shaking creates a liquid like material not able to support structure – structure sinks

Richter scale: This scale was based on the ground motion measured◦ there is an upper limit on the highest measurable

magnitude, and all large earthquakes will tend to have a local magnitude of around 7.

◦ the magnitude becomes unreliable at a distance of more than about 600 km (370 mi) from the epicenter

Moment Magnitude Scale (MW or M) is used to measure the size of earthqukes in terms of the energy released◦ Rigidity of Earth x Amount of Slip x Area that

slipped

Measuring Earthquake Strength

Tsunamis – sea waves

caused by an earthquake that displaces the ocean floor vertically◦ Convergent and strike-slip boundaries

water is pushed upwards and toward the land

speeds of 500/9000 km/hr low waves in open water – as wave

nears shore water builds upwards

Other Dangers

Fires - from broken gas pipes and falling power lines

Landslides – rock and soil slide downhill from shaking

Predicting Earthquakes

No predicting devices Only measurements – stress along

faults, water level in wells, gas emissions from fractures

Seismic gap – time between earthquakes

Foreshocks-small quakes preceeding big one◦ Could be hours, days, even years before

Aftershocks-small quakes following big one

The Earth’s Interior

Knowledge of Earth’s interior comes from Earthquake waves

By measuring the speed of a wave - able to determine the composition of the Earth

P waves travel through liquids and solids

S waves travel only through solids

As P waves travel they bend as they enter new material - bending shows change of material

S waves not traveling through the outer core also shows change in material

Conclusion – Earth is made up of different states of material