Earthquakes

What is an earthquake?

• Defined as movements of the ground that are caused by a sudden release of energy when rocks along a fault move.– Sudden slip on a fault

– Resulting ground shaking

– Radiated seismic energy caused by the slip

• Caused by volcanic or magmatic activity, • Caused by other sudden stress changes in the earth.

What causes earthquakes?• Tectonic plates move past each other

causing stress. Rocks near tectonic plate boundaries experience a great amount of stress.– Elastic deformation – rock stretches then

reaches a breaking point, releasing energy.

Plate motion simulation

Elastic Rebound – sudden return of elastically deformed rock to its

undeformed shape

http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM

Focus – point inside the Earth where the first motion of an earthquake

beginsEpicenter – point on Earth’s surface above

focus

Seismic Waves

• Body Waves – waves that travel through the body of a medium– P Waves– S Waves

• Surface Waves – waves that travel along the surface of a body rather than through the middle– Love Waves– Rayleigh Waves

Primary Waves (P Waves)

• A type of seismic wave that compresses and expands the ground.

• The first wave to arrive at an earthquake.• Particles of rock move in a back-and-forth direction

that is parallel to the direction in which the waves are traveling.

• Can move through solids, liquids and gases.

http://daphne.meccahosting.com/~a0000e89/insideearth2.htm

Secondary Waves (S Waves)• Second fastest wave.• A type of seismic wave that moves the ground up and down or side to

side.• Can only travel through solid material.

http://daphne.meccahosting.com/~a0000e89/insideearth2.htm

Comparing Seismic Waves

Surface WavesThe surface waves are the slowest of the

three earthquake wave types.

Two basic types of surface waves

• Love Waves

• Rayleigh Waves

1. L-waves or love waves.Complex motion. Up-and-down and side-to-side. Slowest. Causes damage to structures during an earthquake.

2. Rayleigh waves involve rolling motions. A surface particle moves in a circle or ellipse in the direction of propagation.

Using Seismic Waves to Study Earth's Interior

Seismic Waves travel through the entire Earth

Both S and P waves travel throughout the body of the earth, and can be picked up by seismograph - machines that record earthquakes (ground vibrations) - anywhere in the world.

Seismic waves as “x-rays” to look inside the earth

• P-Waves travel through solid and liquid

• However, it turns out that S waves cannot travel through the core, and only P waves are recorded in some places:

S-Waves travel only through solids

Seismic waves as “x-rays” to look inside the earth

• Seismic waves travel faster through denser material.

• Because of this, the path traveled by a seismic wave is bent towards the surface.

• Shadow zones are areas where there are no direct seismic waves from a particular earthquake.

Using Seismic Waves to Study Earth's Interior

Earthquakes and Tectonic Environments

• Convergent Oceanic Environments– Plates move toward each other and collide– Denser plate subducts– Top plate scrapes across subducting plate– Occurs between two oceanic plates or one oceanic plate and one continental

plate

Fault zones form at plate boundaries because intense stress occurs there when the plates separate, collide, subduct, or slide past each other.

Earthquakes and Tectonic Environments

• Divergent Oceanic Environments– Plates move away from each other– Oceanic lithosphere is pulling away from both sides of each

ridge– Spreading motion causes earthquakes

Earthquakes and Tectonic Environments

• Continental Environments– Continental plates converge, diverge, or move horizontally in

opposite directions– Rock experiences stress – mountains form or earthquakes occur

Seismograph

Seismograph Video

• A seismograph is an instrument that records ground vibrations from earthquakes. Mechanical versions work by way of a large mass, freely suspended.

• In the example on the left, a rotating drum records a red line on a sheet of paper. If the earth moves (in this case from left to right) the whole machine will vibrate too.

• However, the large mass tends to stay still, so the drum shakes beneath the pen, recording a squiggle!

How do scientists calculate how far a location is from the epicenter of an earthquake?

• Scientists calculate the difference between arrival times of the P waves and S waves

• The further away an earthquake is, the greater the time between the arrival of the P waves and the S waves

Typical Seismogramtracing of earthquake motion

http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt

Earthquake Strength

The intensity or strength of an earthquake is measured by seismologist in two main ways:

1.The Richter Scale • measures the amount of energy that an

earthquake releases as ground motion • Each number of magnitude is 10x stronger

than the number below it.

How are Earthquakes Measured? Richter Scale

The Moment Magnitude ScaleGeologists use this scale today

It’s a rating system that estimates the total energy released by an earthquake based on the size of the fault area that moves, how far fault blocks move, and the rigidity of rocks.

Can be used for any kind of earthquakes, near or far

Some news reports may mention the Richter scale, but the magnitude number they quote is almost always the moment magnitude for that earthquake

Modified Mercalli Scale

• Measures the intensity of an earthquake • Ranges from I to XII • Based on common earthquake

occurrences such as "noticeable by people" "damage to buildings" chimneys collapse" "fissures open in the ground”.

The Modified Mercalli ScaleDeveloped in the twentieth century to rate earthquakes according to their intensity

The intensity of an earthquake is the strength of ground motion in a given place

Is not a precise measurement

But, the 12 steps explain the damage given to people, land surface, and buildings

The same earthquake could have different Mercalli ratings because of the different amount of damage in different spots

XII is total destruction

•The Mercalli scale uses Roman numerals to rank earthquakes by how much damage they cause

How are Earthquakes Measured? Mercalli Intensity Scale

Click Link for Interactive Demo http://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf

How Earthquakes Cause Damage

The severe shaking provided by seismic waves can damage (building may sway or collapse) or destroy buildings and bridges, topple utility poles, and damage gas and water mains

With their side to side, up and down movement, S waves can damage or destroy buildings, bridges, and fracture gas mains.

Tsunami – giant ocean wave

http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM

Formation of a tsunamiwaves get bigger as they move toward

shore

http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt

Tsunami Warning System

http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt

Tsumani’s Attack

Causes:•Undersea earthquakes•Landslides•Volcanic eruptions

Earthquake Safety

• Be prepared before an earthquake occurs

• Know evacuation procedure

• Stay calm

• Protect yourself

• Check for fire and other hazards

Earthquake damage in Anchorage on March 27, 1964

Earthquake Warning and Forecasts

• Scientists study past earthquakes to predict future earthquakes

• Still no reliable way to predict exactly when or where an earthquake will occur

• Seismic Gaps – an area along a fault where relatively few earthquakes have recently but where strong earthquakes occurred in the past

San Francisco are built on sandy soil or fill. Many homes built on this type of soil were badly damaged during the 1989 Loma Prieta earthquake.

Earthquake Warning and Forecasts

• Seismic Gaps – an area along a fault where relatively few earthquakes have occurred recently but where strong earthquakes occurred in the past

• Foreshocks – small earthquakes that can potentially precede an earthquake by a few seconds or a few weeks

• Changes in Rocks– cracks filled with water– natural gas seepage