Honors Earthquakes Chp. 8

Earthquake: vibration of Earth produced by rapid release of energy, usually along a break in Earth’s crust

Focus: where earthquake originates (deep in Earth)

Epicenter: point directly above focus on Earths surface

Fault: fracture along which movement occurs, in the lithosphere

3 types of faults:1. Normal Fault: horizontal

tension, hanging wall drops down foot wall (foot wall become higher point)

2. Reverse fault: (Thrust): horizontal compression, hanging wall pushed up over foot wall (foot wall becomes lower point)

3. Strike-slip fault: horizontal shear

Hanging wall

Hanging wall

Footwall

Footwall

Cause of EarthquakesStress & Strain• Stress: force per unit

area causes fractures• Strain: deformation of

material due to stress

Lead to:• Elastic deformation:

material will return to normal if stress is decreased

• Elastic limit: point between elastic & ductile deformation

• Ductile Deformation: permanent deformation due to stress & strain, occurs after elastic limit is exceeded

• Failure: breakage, Earthquake

Elastic Rebound Hypothesis:

Aftershock: smaller earthquakes (could be hundreds) that follow a major earthquake, hinder rescue efforts

Foreshock: small earthquakes that happen before a major earthquake

Measuring Earthquakes• Seismographs: sensitive

instrument measures vibration

• Seismogram: (seismos = shake, gramma = what is written) written record produced by seismometer of all 3 waves

Seismic Waves1. Surface waves (L-

waves, complex waves): Rock moves up & down and side to side; side to side motion most destructive, most destructive wave overall, slowest moving wave (90% slower than S waves)

2. Primary waves (P-waves, longitudinal): squeezes & pulls rock in direction of traveling wave, fastest wave (1.7 X faster than S waves)

3. Secondary waves (S-waves, transverse): rocks move at right angle in relation to wave direction

Time-Travel Graph• S-wave: travel slower

than P-waves• P-waves: Travel

faster than S-waves

What is it used for?

Locating an Earthquake1. Determine P and S wave arrival time to seismographs

2. Calculate the difference between P & S wave arrival times

3. Determine epicentral distance

4. Minimum of three stations needed to determine epicenter

Seismic Belts• Areas of greatest seismic activity, narrow & follow

plate boundaries– 80% in Circum-Pacific Belt– 15% in Mediterranean-Asian Belt

Measuring EarthquakesIntensity: amount of

earthquake shaking at a given location (not quantitative)

Magnitude: amount of energy released from source of earthquake (quantitative)

Richter scale

• 1935– Charles Richter– Cal Tech (California

Institute of Technology)– Only useful for small

shallow earthquakes within 500 km of epicenter

– Used mostly by news reporters

Richter scale: based on amplitude of largest seismic wave generated, use logarithmic scale– Wave size increases by a

factor of 10• Magnitude 6 wave size is

10X lager than magnitude 5, and 100X larger than a magnitude 4

– Energy released increased by a factor of 32X

• Magnitude 6 energy is 32X greater than magnitude 5, and 1024X greater than a magnitude 4

Moment magnitude scale: used by most scientists, derived from amount of displacement

• Calculated using:– seismograph data size– average movement along fault – Area of surface breakage– Strength of rock

See table 1 page 227 in book

Modified Mercalli scale: measures intensity on I to XII scale, XII greatest damage earthquakes can cause

• The same earthquake can have different Mercalli scale ratings (depends on distance from epicenter) Why?

Depth & Focus related to earthquake magnitude• Deeper the focus smaller the vibrations

LithosphereHole thing

Crustdarkerarea

Destruction from EarthquakesDepends on:• Intensity (why intensity & not magnitude?)• Duration of vibration (material, magnitude)• Nature of material under structure or area

– Liquefaction: loosely compacted earth & saturated sediments • Liquefies during an earthquake causing settling

• Design of structure– Unreinforced masonry (brick) buildings most at

threat• Wood and steel have more flexibility• Generally why is wood better than steel?

Tsunamis: waves created by rapid displacement of water, with long wavelength off shore and short wavelength near shore (travel very fast off shore, slow near shore)

Tsunami warning systems are common in the Pacific Ocean & were not put in the Indian Ocean until after the 2002 Sumatra Tsunami

Amplitude increases in shallow Water only

Tsunamis are created by:• Earthquakes activity• volcanic activity• Landslides• Asteroids or meteor impacts

Tsunami damage is caused by huge surge that pushes inland long distances due to waves long wavelength

High ground is the safety zone during a tsunami

Before

After

Other dangers resulting from earthquakes:• Landslides• Ground subsidence• Fires

Predicting Earthquakes:Shore-range predictions: monitor & measure; uplift,

subsidence, strain in rock, water level, pressure in wells, radon gas emissions & electromagnetic properties in rock. Have not been successful

Long-range predictions: forecasts occurrence within 30 to 100+ years by studying historic records & seismic gaps, based on the idea earthquakes are cyclical, help in determination of building codes; Are predictions accurate? What is a seismic gap?

Earth’s Layers Defined by CompositionCrust: thin rock outermost layer

– Oceanic: 7 km, basalt & gabbro (density 3 g/cm3)– Continental: 8-75km (average 40km), granitic

(grandiorite), less dense than oceanic (density 2.7 g/cm3)

Mantle: 82% of Earth’s volume, dominant rock type peridotite, extends to depth of 2890 km, density 3.4 g/cm3

Core: center sphere composed of iron-nickel greatest concentrations of metal), extreme pressure density 13 g/cm3

Earth’s Layers Defined by Physical Properties

Lithosphere: outer most layer crust and upper mantle, cool ridged shell, 100 km in thickness

Asthenosphere: small amount of melting due to temp & pressure, weak layer because near melting point

Upper mantle: lower lithosphere & asthenosphere

Lower mantle: more ridged layer, rocks very hot capable of gradual flow

Outer Core: liquid layer, flow generates Earth’s magnetic field, 2260 km thick

Inner Core: solid due to pressure, 1220 km thick

Discovering Earth's layersMohorovicic’s evidence of

1909: velocity of seismic waves increased abruptly below 50km (the Moho)

Moho: boundary between crust and mantle

More evidence:

P waves can travel through a liquid (but they lose velocity)

S waves can not travel through liquid

Shadow zones:

P wave: due to refraction in core, 105 degrees to 140 degrees

S wave: due to outer core (cannot travel through liquid), waves stop at 105 degree mark

103 degreesOuter core low viscosity

AsthenosphereHigher viscosity

Outer core Lower viscosity

Inner coresolid

Discovering Earth’s Composition• Drilling (direct observation)

• Seismic data– P & S wave shadow zones

• Nuclear test– 1960 allowed for measurement of inner core

Kola Peninsula Deepest Well Cool Facts:

Began to drill in 1970– 5 years to drill 7000 m (1975)– 9 years to drill the next 5000 m! (1984)– Stuck at 12,000 m in 1989– Reached 12,262 m in 1994– At the bottom of the well…

• Rocks 2.9 billion years old

• Temperature 190°C ~ 2X the temp of boiling water

Local Faults: