NOTESNOTES

What are Earthquakes?

• A vibration of Earth’s crust caused by a sudden release of energy

• Caused by faulting or breaking of rocks

• Aftershocks – continued re-adjustment of Earth’s crust

• Foreshocks – small, often precede major e-quakes

• Usually occur at plate boundaries

Explains how energy is released during an earthquake

Rocks deform or bend

Rocks rupture when pressure accumulates in rocks on either side of a fault and build to a level which exceeds the rocks' strength.

Finally, rocks rebound and return to their original shape when the accumulated pressure is released.

Elastic Rebound Theory

Rocks rebound to original un-deformed shape

Deformation

Rupture and release of energy

Fence

Original position

Fault

Elastic Rebound Theory

Where do earthquakes happen?

• The point within Earth’s crust where faulting begins is the focus. This is where energy is released.

• The point directly above the focus on the surface is the epicenter.

Where do earthquakes Where do earthquakes occur?occur?

• 85% around the Pacific belt

• 15% around the Mediterranean-Asiatic belt

• 5% along divergent boundaries and plate interiors

900,000 occur each year 31,000 are strong enough to be felt

Seismology – study of earthquakesSeismogram – the record of an earthquakeSeismograph – instrument used to record

earthquakes (creates seismograms)

Studying EarthquakesStudying Earthquakes

SeismogramSeismogram

Seismic Waves

Most of the damage and the shaking people feel during an earthquake is from the seismic waves.

Two types of seismic waves:

Body waves travel through Earth

Surface waves travel along or just below the surface

Body WavesBody Waves

• P (Primary) waves– fastest waves– travel through solids,

liquids, or gases– compressional wave,

material movement is in the same direction as wave movement

• S (Secondary) waves– slower than P waves– travel through solids

only– shear waves - move

material perpendicular to wave movement

Undisturbed material

Focus

Surface

Primary wave (P-wave)

Com

pres

sion

Com

pres

sion

Com

pres

sion

Exp

ansi

on

Exp

ansi

on

Undisturbedmaterial

Direction of wave movement

Secondary wave (S-wave)

Wavelength

How do seismic wave help us How do seismic wave help us infer Earth’s internal infer Earth’s internal

structure?structure?

Surface Waves: R and L waves

Surface Waves•Slower than body waves

•R (Rayleigh) – rolling motion

•L (Love) – side-to-side motion

•Especially damaging to buildings

Love wave (L-wave)

Undisturbed material

Rayleigh wave (R-wave)

Rayleigh wave Love wave

How is an Earthquake’s Epicenter Located?How is an Earthquake’s Epicenter Located?

• P waves arrive first, then S waves, then L and R

• Average speeds for all these waves is known

• After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter

How is an Earthquake’s Epicenter Located?How is an Earthquake’s Epicenter Located?

• Time-distance graph showing the average travel times for P- and S-waves.

• The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

Which seismic station is closer to the epicenter?

A

B

How is an Earthquake’s Epicenter Located?How is an Earthquake’s Epicenter Located?

Triangulation•3 seismograph stations are needed to locate the epicenter•A circle where the radius equals the distance to the epicenter is drawn•The intersection of the circles locates the epicenter

Measuring the Size & Measuring the Size & Strength Strength

of an Earthquake of an Earthquake

Intensity is a qualitative measurementMercalli Scale

Scale of I – XIIMeasures damage/sensations

Mercalli Scale - IntensityMercalli Scale - Intensity

• Modified Mercalli Intensity Map

– 1994 Northridge, CA earthquake, magnitude 6.7

Measuring the Size & Measuring the Size & Strength Strength

of an Earthquake of an Earthquake

Magnitude is a quantitative measurementRichter ScaleMeasures total energy releasedAmplitude of largest waveExponential scale (30-fold energy increase for

each unit)6.5 releases 30x more energy than 5.56.5 releases 900x more energy than 4.5 (30 x 30)

Strongest recorded: • 9.5 (Chile 1960)• 9.2 (Alaska 1964)• 9.1 (Indian Ocean 2004)• 9.0 (Japan 2011)

The Destructive Effects of The Destructive Effects of EarthquakesEarthquakes

Ground ShakingThe most destructive of all earthquake hazards

is ground shaking.

An area's geology, earthquake magnitude, the type of building construction, and duration of shaking determine the amount of damage caused.

Fig. 8.13, p. 202, Fig. 8.15, p. 203

The Destructive Effects of The Destructive Effects of EarthquakesEarthquakes

Liquefaction occurs when unconsolidated material is saturated with water and it loses its cohesive strength

Fig. 8.14, p. 203

LiquifactionLiquifaction

The Destructive Effects of The Destructive Effects of Earthquakes Earthquakes

Fire occurs when gas and water lines break

Geo-inSight 4. and 7. , p. 205

The Destructive Effects of EarthquakesThe Destructive Effects of EarthquakesTsunami: seismic sea wave caused by vertical

displacement along a fault or an underwater landslide

BEFORE

AFTER Fig. 8.16, p. 207

• 2004, a magnitude 9.0 earthquake offshore from Sumatra generated the deadliest tsunami in history

Fig. 8-16b, p. 207

Stepped Art

Fig. 8.17, p. 208

The Destructive Effects of EarthquakesThe Destructive Effects of EarthquakesGround Failure – landslides and rock slides are

responsible for huge amounts of damage and many deaths.

Destructive Effects of Destructive Effects of EarthquakesEarthquakes

Ground failure can result in building / road collapse

Geo-inSight 5. and 6.; p. 205

Earthquake PredictionEarthquake Prediction Seismic risk maps help geologists in determining the

likelihood and potential severity of future earthquakes based on the intensity of past earthquakes.

Fig. 8.18, p. 209

Earthquake PredictionEarthquake Prediction

Earthquake Precursors – short-term and long-term changes within the Earth prior to an earthquake that assist in prediction.

Seismic gaps – locked portions of the fault where pressure is building

Surface elevation changes and tilting

Ground water table fluctuations Anomalous animal behavior

Fig. 8.19, p. 210