Seismic Waves

Seismic WavesSeismic Waves

CH 8.2CH 8.2

NCSCoS 2.04NCSCoS 2.04

Warm-upWarm-up How are earthquakes and plate tectonics How are earthquakes and plate tectonics

related?related? What do you know about waves?What do you know about waves?

What do you remember about the major What do you remember about the major tsunami in the Indian Ocean on tsunami in the Indian Ocean on December 26, 2004?December 26, 2004?

ObjectivesObjectives

Compare and contrast the three types of Compare and contrast the three types of seismic waves.seismic waves.

Locate earthquake epicenters using Locate earthquake epicenters using seismograms and a travel-time graph.seismograms and a travel-time graph.

Sumatra Tsunami Sumatra Tsunami December 26, 2004December 26, 2004

““Earthquake triggers deadly tsunami.Earthquake triggers deadly tsunami.”” ““The world's most powerful earthquake in more than 40 years struck deep The world's most powerful earthquake in more than 40 years struck deep

under the Indian Ocean on December 26, triggering massive tsunamis under the Indian Ocean on December 26, triggering massive tsunamis that obliterated cities, seaside communities and holiday resorts, killing that obliterated cities, seaside communities and holiday resorts, killing tens of thousands of people in a dozen countries.tens of thousands of people in a dozen countries.””

-CNN News-CNN News

http://www.time.com/time/photoessays/asia_earthquake/index.html

http://www.cnn.com/SPECIALS/2004/tsunami.disaster/

NOVA interactive siteNOVA interactive sitehttp://www.pbs.org/wgbh/nova/tsunami/anat-flash.htmlhttp://www.pbs.org/wgbh/nova/tsunami/anat-flash.html

Seismic Waves radiate from focus Seismic Waves radiate from focus and transmit energy.and transmit energy.

Body waves Body waves travel travel through through earth.earth.

Surface Surface waves travel waves travel on surface.on surface.

SURFACE WAVES

BODY WAVES

Three Types of Seismic WavesThree Types of Seismic Waves http://www.gmi.edu/~drussell/Demos/waves/wavemotion.htmlhttp://www.gmi.edu/~drussell/Demos/waves/wavemotion.html

1. 1. P-wavesP-waves = primary waves = primary waves Body waveBody wave push-pull = push-pull = compression waves Particles move parallel to wave direction.

Fastest Travels through solids and liquids

2. S-waves = secondary waves2. S-waves = secondary waves body wavesbody waves side-to-side = transverse wavesside-to-side = transverse waves Particles move perpendicular to wave Particles move perpendicular to wave

direction.direction.

intermediate speedintermediate speed cannot travel through liquidscannot travel through liquids

3. Surface waves3. Surface waves Particles at surface move in a circular Particles at surface move in a circular

motion.motion.

SlowestSlowest Most destructiveMost destructive

SeismogramSeismogram

P-wave S-wave

Surface

Review/Preview:Review/Preview:

Sketch and label a seismogram with s-p Sketch and label a seismogram with s-p lag time.lag time.

What does lag time tell you?What does lag time tell you?

Seismograph – instrument used to Seismograph – instrument used to

record seismic waves record seismic waves

http://www.thetech.org/exhibits/online/quakes/seismo/

Heavy weight keeps pen still due to inertia.

Drum moves beneath pen.

Travel Time Graph

• S-P lag time –time between P and S wave arrivals

•Greater S-P lag time = greater distance from epicenter

P wave

S wave

S-P lag time

TriangulationTriangulation

Use at least three Use at least three stations to locate stations to locate epicenter. epicenter.

Epicenter is where all Epicenter is where all three lines meet.three lines meet.

Check For Understanding

Which wave is an s-wave?

How does material move and change in a p-wave?

Which wave is faster?

What is the What is the arrival time of arrival time of the p-wave?the p-wave?

The s-wave?The s-wave? What is the What is the

distance to distance to the epicenter the epicenter if the S-P lag if the S-P lag time is 5 time is 5 minutes?minutes?

Which station is the Which station is the closest to the closest to the epicenter?epicenter?

Where is the Where is the epicenter?epicenter?

Why canWhy can’’t you just t you just use two stations?use two stations?

B

AC

Review/Review/PreviePreviewwMake your own

sheet this week.

Write many observations of this diagram. (also on p. 236)

3) Earthquake destruction

important factors:Intensity & duration of shaking Intensity & duration of shaking Soil type (soft? hard rock?)Soil type (soft? hard rock?)Building designBuilding design

other effects:

LiquifactionLiquifactionTsunamisTsunamis



other effects:


Bay Area, CA1989


important factors:Intensity & duration of shaking Soil type (soft? hard rock?)Building design

other effects:

LiquifactionTsunamis



other effects:


Intensity of shaking decreases with distance from epicenter (MM Intensity scale).

M6.7, 1994 Northridge, California earthquake

Intensity of shaking decreases with distance from epicenter (Peak horizontal acceleration).


Intensity of shaking decreases with distance from epicenter (star; USGS Shake Map).


Richter ScaleRichter Scale

Based on amplitude of the largest waveBased on amplitude of the largest wave Logarithmic scaleLogarithmic scale

Ex) 3.0 is 10x greater than 2.0Ex) 3.0 is 10x greater than 2.0

Moment MagnitudeMoment Magnitude

Scientists use this nowScientists use this now Based on several factors (more Based on several factors (more

complicated)complicated) But tries to measure displacement & energy But tries to measure displacement & energy

that is releasedthat is released >6 = significant>6 = significant >7 = major>7 = major >8 = great>8 = great

2) Earthquake Intensity and magnitude

Mercalli intensity scale

Magnitude

Intensity of shaking & damage at a specific location

A measure of the energy released in an earthquake

Depends on distance to earthquake& strength of earthquake

Depends on size of fault that breaks

1) Earthquake “belts” Fault Types

Documents

Seismic Waves