Chapter 5: EARTHQUAKES &EARTH’S INTERIOR

Chapter 5: EARTHQUAKES &EARTH’S INTERIOR

Earthquakes & earthquake hazards• Earthquake

– Sudden release of energy • Seismology

– Scientific study of earthquakes & seismic waves

• Seismic waves– Shock waves traveling out

in all directions from earthquake’s source

Earthquake Hazards & Predictions• Primary hazard –

ground motion– Collapsing buildings,

bridges & other structures

– Aftershocks

Earthquake Hazards &Predictions• Secondary hazards

– Landslides– Fires– Ground liquefaction– Tsunamis

Earthquake Hazards &Predictions

Earthquake Hazards &Predictions• Recent Tsunami

– 2004 Sumatra– Magnitude 9.1– ~ 30 ft– 225,000

casualties

– 2011 Japan– Magnitude 9.0– ~25 ft.– 15,000

casualties

Earthquake prediction• Short-term

prediction & early warning– Precursor

phenomena– Foreshocks

• Long-term forecasting

• Paleoseismology– Study of prehistoric

earthquakes

Earthquake Readiness• Mercalli Intensity Scale

– Based on observations & damage to structures

– Subjective

The Science of Seismology• Seismograph

– Instrument to detect & measure vibrations of Earth’s surface

• Seismogram– Record made by a seismograph

Seismic waves• Body wave

– Travels through Earth’s interior• Surface wave

– Travels along Earth’s surface• Focus

– Where rupture occurs & earthquake’s energy is first released

• Epicenter– Point on Earth’s surface

directly above earthquake’s focus

Locating earthquakes• P (Primary) or Compressional waves:

– Alternating pulses of compression & expansion– Pass through all states of

matter• S (Secondary) or Shear

waves:– Side to side or up &down

movement– Pass only through solids– Travel slower than P waves

Locating earthquakes• Surface or L (Love)

wave:– Pass around Earth, not

through it– Cause most ground

shaking & damage– Slower than P & S

waves

Locating earthquakes• Finding the epicenter

– S – P lag time– Triangulation (3 points needed)

Measuring Earthquakes• Richter Magnitude

Scale– Magnitude vs. Intensity– Based on amplitudes of

seismic waves shown on seismograph

– Logarithmic scale• 10 fold increase in

amplitude = 1 unit increase

• 32-fold increase in energy = 1 unit increase

Measuring Earthquakes• Moment Magnitude Scale

– Based on rupture size, rock properties, & amount of displacement along fault surface

– Used for large earthquakes

Studying Earth’s Interior• Seismic

discontinuity– Boundary inside

Earth where seismic wave velocities change abruptly

Studying Earth’s Interior• 3 things can happen to seismic waves if they meet

a boundary:– Refraction: waves bend as they pass from one material to

another– Reflection: some or all of wave energy bounces back – Absorption: some or all of wave energy is blocked

How geologists look into Earth’s interior• Direct observation

– Drilling– Xenoliths

How geologists look into Earth’s interior• Indirect observation

– Magnetism– Astronomical observations

• Density– Meteorites

A Multilayered Planet• Crust

– Outermost compositional layer of solid Earth, part of lithosphere• Lithosphere = brittle

& solid– Thickness varies

• 8 km (oceanic) • 45 km (continental)

– Moho boundary

A Multilayered Planet• Mantle

– Middle compositional layer of Earth, between core & crust

– Asthenosphere: mantle where rock is near melting • Plastic & weak

– Mesosphere– Mantle-core

boundary

• Core– Innermost layer

• Where magnetic field is generated & most geothermal energy resides

– Outer core (liquid) – Inner core

(solid)

A Multilayered Planet