Earthquakes

Liz LaRosa http://www.middleschoolscience.com 2009 for my 5th grade science class 2009

What is an earthquake?

• Used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip

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

earth.

Types of Faults

Strike-Slip Dip Slip

What causes earthquakes?• Tectonic plates move past each other causing

stress. Stress causes the rock to deform– Elastic deformation – rock stretches then reaches

a breaking point, releasing energy.

Elastic Rebound – deformed rock goes back to its original shape

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

Focus – point inside the Earth where an earthquake begins

Epicenter – point on Earth’s surface above focus

Primary Waves (P Waves)

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

• The first wave to arrive at an earthquake

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

Secondary Waves (S Waves)

• A type of seismic wave that moves the ground up and down or side to side

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

Comparing Seismic Waves

Surface Waves

• Move along the Earth’s surface• Produces motion in the upper crust

– Motion can be up and down– Motion can be around– Motion can be back and forth

• Travel more slowly than S and P waves• More destructive

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

How are Earthquakes Measured? Richter Scale

How are Earthquakes Measured? Mercalli Intensity Scale

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

Earthquake Waves & Earth’s Interior

Tsunamis

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

Formation of a tsunami

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

Tsunami Warning System

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

The Nazca Plate subducts beneath the South America plate at the Peru-Chile Trench offshore of western South America.   

At the latitude of the earthquake, the Nazca Plate moves to the east-northeast with respect to the South America Plate with a velocity of about 70 mm/y. 

Nazca Plate

South American Plate

Peru-Chile Trench

Magnitude 7.0 PERUMagnitude 7.0 PERUWednesday, September 25, 2013 at 16:42:42 UTC Wednesday, September 25, 2013 at 16:42:42 UTC

The record of the earthquake on the University of Portland seismometer (UPOR) is illustrated below. Portland is about 8355 km (~5192 miles, 75.27 degrees) from the location of this earthquake.

Traveling the same path as the P-wave, the S-waves (shear waves) travel at a slower velocity, arriving 21 minutes and 25 seconds (1285 seconds) after the earthquake.

The surface waves are the last to arrive approximately 39 minutes after the earthquake, having traveled 8355 km to Portland along the surface of the Earth.

P-waves are compressional waves that travel a curved path through the mantle. The P-waves arrived to the seismometer in Portland 11 minutes and 44 seconds (704 seconds) after the earthquake.

PP waves are compressional waves that bounce off the Earth’s surface halfway between the earthquake and the seismic station.

Magnitude 7.0 PERUMagnitude 7.0 PERUWednesday, September 25, 2013 at 16:42:42 UTC Wednesday, September 25, 2013 at 16:42:42 UTC

Modified Mercalli IntensityPerceived Shaking

Extreme

Violent

Severe

Very Strong

Strong

Moderate

Light

Weak

Not FeltUSGS Estimated shaking Intensity from M 7.0 Earthquake

The Modified Mercalli Intensity (MMI) scale depicts shaking severity. The area nearest the earthquake experienced strong shaking.

Image courtesy of the US Geological Survey

Shaking Intensity

Magnitude 7.0 PERUMagnitude 7.0 PERUWednesday, September 25, 2013 at 16:42:42 UTC Wednesday, September 25, 2013 at 16:42:42 UTC

This region experiences a large number of earthquakes.  Within 250 km of the epicenter, there have been 17 earthquakes of M6 and larger since 1973. The largest, a M8.4 earthquake of June 23, 2001, occurred along the plate boundary to the south.  It killed at least 74 people and destroyed over 17,000 homes. 

Earthquakes are shallow at the Peru-Chile Trench and increase to > 500 km depth towards the east as the Nazca Plate subducts deeper beneath the South American Plate.

Earthquake

Magnitude 7.0 PERUMagnitude 7.0 PERUWednesday, September 25, 2013 at 16:42:42 UTC Wednesday, September 25, 2013 at 16:42:42 UTC