Chapter 15

EARTHQUAKES!

BFRB Pages 215-226

Earthquake causes

•An earthquake is the shaking of the Earth’s crust caused by a release of energy

•The movement of the Earth’s plates causes most earthquakes

Elastic rebound theory

• As Earth’s plates move past one another, friction causes the plates to get “stuck”

• Stress and pressure builds up and causes the plate to become deformed (bend) – (the plates are solid, but elastic - they can

change shape) • Eventually, the pressure overcomes the

friction and the plates slip past one another

• This sudden plate movement is the earthquake. The greater the plate movement, the larger the earthquake.

• The plates are in a new location, but they go back to their original shape (elastic rebound).

Earthquake Depth

• Earthquake depth depends on the type of plate boundary where the movement occurs

• Diverging boundaries (mid-Atlantic Ridge), and transform (sliding) boundaries (San Andreas Fault) tend to have earthquakes which are closer to the surface (shallow focus)

• Subduction boundaries tend to have very deep earthquakes as the subducting plate plunges deeper into the mantle (deep focus)

Subducting plate

Shallow focus

Intermediate focus

Deep focus

• The point where the actual movement of the plates takes place, and where the energy is released from is called the focus

• The point on the Earth’s surface that is directly above the focus is called the epicenter

• When an earthquake occurs, energy waves are released and move outward from the focus

Types of Faults(We’ll discuss these more when we do

Geologic History)• NORMAL - one side of a fault slips down

relative to another • REVERSE (& Thrust) - one side of a fault

is driven up and over the other • STRIKE-SLIP – occur where plates meet

evenly and slip past each other horizontally. (The angle at which a fault cuts through the earth is referred to as the strike, so a strike-slip fault happens when plates slip along the strike). – most common fault type in California

FAULT ANIMATIONS

Seismometers/ Seismographs

• Earthquakes generate seismic waves which can be detected with a sensitive instrument called a seismograph aka seismometer

• The actual readout of a seismometer is called a seismogram

3 Types of Waves

P-WAVE

S-WAVE

L-WAVE

P-Waves•COMPRESSIONAL WAVES (push)•cause the rocks in the crust to

be squeezed and stretched • the first waves received by a

seismograph station because they move the fastest

•These waves can move through solids, liquids, and gases

S-Waves•SHEAR WAVES •Cause the rocks to move side to side, causing a ripping (shearing) motion

•Slower than p-waves•Can only move through solids ****S waves = Side to Side, Slow, Solids

P & S Wave Animations

L-Waves•LONGITUDINAL WAVES•aka SURFACE WAVES•when P & S-waves reach

the surface, they cause wave-like ripples on the surface of Earth like when you throw a rock in a pond

Wave Speed•Determined by the

characteristics of the material that the earthquake’s wave is moving through!

•The more solid the material, the faster the waves can move through it (but remember, P-waves will always move faster than S-waves)

Remember when we were studying the layers of the Earth and you asked me, “ If we’ve never been there, how do we know that these

layers exist?

Well, here’s how!!!!

Movement of Waves Inside the Earth

• The velocity of the P-waves and S-waves increase as they travel deeper into the Earth

• The boundary between the crust and the mantle is a thin layer of denser rocks. Because of this increase in density, the P & S-waves suddenly increase in velocity.

• This thin, dense layer is called the Mohorovicic discontinuity or Moho

…Wave Movement…• When the waves reach the outer core, the

P-waves slow down and the S-waves stop. • S-waves can’t travel through liquids, and

P-waves move fastest through solids. • **The change in velocity of the P-Waves

and ceasing of the S-waves is evidence that the outer core is liquid!!!**

• After passing through the outer core, the P-waves increase in velocity as they pass through the solid inner core, then decrease as they go through the other side of the liquid outer core, then increase in velocity again when they go through the rest of the solid Earth.

• Seismograph stations on the opposite side of the earth from the epicenter only receive P-waves (the liquid outer core block the S-waves).

• Some seismograph stations do not receive any waves – neither P nor S.

• This is because the P-waves refract (bend) as they travel through the liquid outer core, and the S-waves don’t make it through at all.

• This area where no waves reach is called the SHADOW ZONE

• The location of the shadow zone, of course, depends on the location of the epicenter

…Wave Movement…

Locating an Epicenter

• The location of an earthquake epicenter can be found by using the measurement

of seismic waves from 3 seismic stations and a P-wave/ S-wave Time Travel Graph (ESRT’s page 11)

• Epicenters are found by drawing circles around each seismograph station that are the exact distance from the epicenter that was determined using the Time Travel Graph.

• Where all three circles intersect is the location of the epicenter.

Things to Remember

• P-waves always arrive before S-waves • The farther a seismograph is from the

epicenter of the earthquake, the greater the time difference between the arrival of the P-waves and S-waves

• Knowing the distance from the epicenter of one or two seismographs will only give you a rough area of where the earthquake occurred. To find the exact position of the epicenter, you must have three seismograph station recordings

• Count ‘em…THREE• Did I mention that the number was 3 – no

more, no less….

Earthquake Strength and Damage

•Seismometers can also be used to determine the magnitude (strength) of an earthquake

•The Richter Scale is used to measure the amount of energy released by an earthquake

Richter Scale• Rating scale from 1 to 10• Each increase on the Richter

Scale is an increase of 10 times in the strength of the earthquake–Ex. an earthquake measuring a 5 is 10 times stronger than a 4, and 100 times stronger than a 3

• Each increase is also 32 times greater in energy

Damage & Prediction

• Essentially, the seismic waves that cause the ground to shake cause earthquake damage– The shaking, depending on severity,

can cause building foundations to fail, gas and water lines to rupture, electrical disruptions, avalanches, etc

• Earthquake prediction is important to determine the areas where damage is most likely to occur

Partial collapse of the 'Million Dollar Bridge' into the Copper River in Alaska following the

1964 earthquake. (9.2 magnitude)

LIQUIFACTION –

The ground literally flows like a liquid due to the pressure that the earthquake waves put on the water that surrounds the particles. The water exerts enough pressure on the particles that they no longer are held together by the friction between them – the water in between the particles enables them to flow easily and no longer support the structure that is built on it.

This was a communications building in Spitak during the 1988 Armenian

earthquake.

Elementary School torn apart by a landslide that occurred during the 1964 Alaska earthquake. Notice the large crack in the foreground that also was caused by

the earthquake. (9.2 magnitude)

Land Subsidence of 2 meters was caused by the Edgecumbe, New Zealand earthquake.

(6.3 magnitude) This fault is now known as the Edgecumbe

Fault

Epicenter of the Manhattan earthquake on

January 17, 2001 is plotted on an aerial photograph

Emergency Preparedness

– Scientists use EQ data to identify places where earthquakes are likely to occur

– Government authorities then use this information to write emergency preparedness plans.

– These plans are for both before AND after an earthquake strikes.

– From you’ve seen in the previous slides and what you’ve learned about earthquakes, complete the information on the following slides.

Emergency Preparedness

– What can authorities do BEFORE an EQ strikes to mitigate (reduce) the damage and deaths that an EQ may cause?

Emergency Preparedness

– What can citizens do BEFORE an EQ strikes to mitigate (reduce) the damage and deaths that an EQ may cause?

Emergency Preparedness

– What can authorities plan for AFTER an EQ strikes to mitigate (reduce) the damage and deaths that an EQ may cause?

Emergency Preparedness

– What can citizens plan for AFTER an EQ strikes to mitigate (reduce) the damage and deaths that an EQ may cause?