EARTHQUAKES

What is an Earthquake?What is an Earthquake?

• Sudden movement in the earth’s crust which releases energy

• Movement occurs along areas of weakness in the earth’s crust

• Areas of weakness are generally cracks in the rock

Crack in the earth’s crust along which movement occurs

Place where rock begins to separateduring an earthquake. Generally underground

Place on earth’s surface directly abovefocus

SEISMIC WAVES

• When earthquakes occur, waves of energy seismic waves travel outward from the earthquake focus.

• 3 types of seismic waves are produced AT THE SAME TIME but each behaves differently within earth.

P WAVES

• Primary waves or compression waves vibrate parallel to the direction of movement. (push-pull)

• Travel faster than any other wave (6-8 km./s)

• Travel through solids, liquids, and gases

P Waves

• Longitudinal:– Particles move

parallel to the direction wave travels

S WAVES• Shear wave or secondary waves vibrate

back and forth perpendicular to the direction the wave is moving (side-side)

• Slower than P waves (4-5 kms./s) • TRAVEL THROUGH SOLIDS ONLY

S Waves• Transverse waves

– Particles move perpendicular to direction of wave motion

SURFACE OR LONG WAVES• Vibrations travel along earth’s surface in a

circular motion at relatively slow speeds (2 kms/s) like waves in a pond

Do more Do more damage damage because they because they produce produce more ground more ground movementmovement

Seismic WavesSeismic Waves are either: are either:

Body waves: travel through the earth– P-waves– S-waves

or Surface waves (Long waves): – Form when p and s waves reach the surface– Move slowly– Cause the most damage

How are earthquakes detected?How are earthquakes detected?

• Seismograph: instrument which detects motion

seismogram: recording of the earthquake

seismograph

Seismogram

P WAVES TRAVEL FASTER THAN S WAVES

AS DISTANCE FROM EPICENTER INCREASES -THE GREATER THE TIME INTERVAL BETWEEN P AND S WAVES

VELOCITY OF SEISMIC WAVES

• Velocity depends on the material they are passing through

a.Increase density and pressure – greater the velocity

b.Waves are refracted or bent as waves pass through material with different densities

EARTHQUAKE SHADOW ZONES

• At an angle of 103° (distance 11,000 kms) from the epicenter both P and S waves disappear

• P waves can again be detected at l42° (16,000 kms.)

NO S WAVES EVER APPEAR AGAIN

• THIS BAND OF 39° IN WHICH NO WAVE ARE OBSERVED IS CALLED THE EARTHQUAKE’S SHADOW ZONE

EARTHQUAKE SHADOW ZONES

P WAVES DISAPPEAR FROM 105º -140º

S WAVES DISAPPEAR FROM 105º NEVER TO APPEAR AGAIN

ANALYSIS OF SEISMIC WAVES HAVE RESULTED IN THE INFERENCE ABOUT EARTH’S INTERIOR

S WAVES CANNOT PASS THROUGH THE LIQUID OUTER CORE

How many…

• In a day? 80 800 8000 (choose one)• In a day? 80 800 8000

• That’s about one every ten seconds.• But we don’t feel all of them, do we?

No, most are smaller earthquakes.

How is the strength of an How is the strength of an earthquake measured?earthquake measured?

• Mercalli scale – Based upon reports of people in effected area

• Richter Scale:– Based upon size of waves on seismogram– Logarithmic scale (ex. An earthquake of 5

has 10 times as much movement as a 4)

Measuring Earthquakes (1)Measuring Earthquakes (1)

Mercalli Index• Measure of intensity

based on descriptions of structural damage and what is felt

• Scale of I to XII• Often dependent on

the proximity to EQ focus– One EQ will have many

Mercalli scores

ExamplesII. A few people might notice

movement if they are at rest and/or on the upper floors of tall buildings.

VI. Everyone feels movement. People have trouble walking. Objects fall from shelves.

X. Most buildings and their foundations are destroyed.

XII. Almost everything is destroyed. Objects are thrown into the air.

Measuring Earthquakes (2)Measuring Earthquakes (2)Richter Magnitude• Measures the energy

released from an earthquake

• 1 – 10 scale• Every EQ has a

unique magnitude, but effects will vary due to distance, ground conditions, etc.

Examples• Less than 3.5 – Generally not felt,

but recorded.

• 6.1-6.9 – Can be destructive in areas up to about 100 kilometers across where people live.

• 8 or greater – Great earthquake. Can cause serious damage in areas several hundred kilometers across.

Descriptor Magnitude Annual average

Great 8 or higher 11

Major 7–7.9 172

Strong 6–6.9 1342

Moderate 5–5.9 1,3192

Light 4–4.9 c. 13,000

Minor 3–3.9 c. 130,000

Very minor 2–2.9 c. 1,300,000

Frequency of Earthquakes WorldwideFrequency of Earthquakes Worldwide

National Earthquake Information Center, U.S. Geological Survey.

Indian Ocean – 2004 12 26 9.0Japan - 2011 03 11 9.0

Body Wave ReviewP Waves

•Are Primary waves•They arrive first•They are the fastest•They are compressional waves•The travel through anything

S Waves•Are Secondary waves•They arrive second•They are Shear waves•They travel through Solids only