Earthquakes Waves & Seismograms Lecture prepared by Mr. B.

EarthquakesWaves &

SeismogramsLecture prepared by Mr. B

Topics

•Elastic rebound theory

•Seismic waves

•Seismograph and seismogram

•Finding distance to an earthquake

•Locating an earthquake

•Distribution of earthquakes

•Depth of earthquakes

•Earthquake intensity

•Earthquake magnitude

Earth In Cross

Section

Under the

Mantle, it’s like a

Lava Lamp

Turkey, 1999

Can Earthquakes be Predicted?

Earthquake Precursors

1. changes in elevation or tilting of land surface2. fluctuations in groundwater levels3. magnetic field4. electrical resistance of the ground5. Release of gases

Can Earthquakes be Predicted?Earthquake Prediction Programs

– include laboratory and field studies of rocks before, during, and after earthquakes

– monitor activity along major faults

– produce risk assessments

Elastic Rebound TheoryRocks bend under

stress while storing elastic energy. When the strain in the rocks exceeds their strength, breaking willoccur along the fault.Stored elastic energy is released as the earthquake. Rocks“snap back”, or rebound to their original condition.

Spread of the Seismic Waves

Types of Earthquake (Seismic) Waves

• Body Waves– P-Waves (primary waves)– S-Waves (secondary waves)

• Surface Waves- L – Love Waves- R – Raleigh Waves (surface, vertical)

P-Waves

P Wave Animation

S-Waves

S Wave Animation

Body Waves: P and S waves

• Body waves– P or primary waves

• fastest waves• travel through solids,

liquids, or gases• compressional wave,

material movement is in the same direction as wave movement

– S or secondary waves• slower than P waves• travel through solids

only• shear waves - move

material perpendicular to wave movement

Surface Waves: R and L waves

• Surface Waves

– Travel just below or along the ground’s surface

– Slower than body waves; rolling and side-to-side movement

– Especially damaging to buildings

The Poor House

Seismograph(Horizontal)

Seismograph (Vertical)

Distribution of Quakes

A direct result of Continental Drift

Earthquake Risk

The Continental Plates

Profile of Subduction Zone

Tsunami

Tsunami

Seismogram

How is an Earthquake’s Epicenter Located? Seismic wave behavior

– 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?

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

Locating the Earthquake

How is an Earthquake’s Epicenter Located?

• Three seismograph stations are needed to locate the epicenter of an earthquake

• A circle where the radius equals the distance to the epicenter is drawn

• The intersection of the circles locates the epicenter

How are the Size and Strength of an Earthquake Measured?

• Modified Mercalli Intensity Map

– 1994 Northridge, CA earthquake, magnitude 6.7

• Intensity

– subjective measure of the kind of damage done and people’s reactions to it

– isoseismal lines identify areas of equal intensity

Mercalli Scale of Earthquake Intensity

• Advantages:– No high-tech instruments are required.

• Disadvantages:– Damage depends on geologic materials

and type of structures in area– Damage varies with distance from

epicenter– Subjective - different people may view

damage and effects very differently

The Goofy Mercalli Scale

I. People do not feel any Earth movement.

II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring

IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.

XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

How are the Size and Strength of an Earthquake Measured?

• Magnitude

– Richter scale measures total amount of energy released by an earthquake; independent of intensity

– Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

Richter Scale Math• Increase by 1 whole number means a 10X

increase in the Magnitude of the quake

• For every increase on the Richter Scale, the amount of energy released increases 30X

Compare a 5.0 to a 7.0 quake

7.0 has 10 X 10 = 100 times greater magnitude

7.0 has 30 X 30 = 900 times more energy!

Compared to a 1.0 quake, a 7.0 has:

10 X 10 X 10 X 10 X 10 X 10 = 1 000 000 greater strength

And 30 X 30 X 30 X 30 X 30 X 30 = 729 000 000 more energy

Earthquake Magnitude aand Worldwide

OccurrenceMagnitude Number/YearLess than 2.0600,0002.0-2.9 300,0003.0-3.9 49,0004.0-4.9 6,2005.0-5.9 8006.0-6.9 2667.0-7.9 18Greater than 8,0 1

Terminology Used in the Study of Earthquakes

Earthquake intensityEarthquake

magnitudeRichter ScaleMercalli ScaleElastic reboundFaultFocus

P-waveSeismic waveSeismogramSeismographS-waveTsunami