Seismic Waves CH 8.2 NCSCoS 2.04

Warm-up  How are earthquakes and plate tectonics related?  What do you know about waves?  What do you remember about the major tsunami in the Indian Ocean on December 26, 2004?

Objectives  Compare and contrast the three types of seismic waves.  Locate earthquake epicenters using seismograms and a travel-time graph.

Sumatra Tsunami December 26, 2004 “Earthquake triggers deadly tsunami.” “Earthquake triggers deadly tsunami.” “The world's most powerful earthquake in more than 40 years struck deep under the Indian Ocean on December 26, triggering massive tsunamis that obliterated cities, seaside communities and holiday resorts, killing tens of thousands of people in a dozen countries.” “The world's most powerful earthquake in more than 40 years struck deep under the Indian Ocean on December 26, triggering massive tsunamis that obliterated cities, seaside communities and holiday resorts, killing tens of thousands of people in a dozen countries.” -CNN News NOVA interactive site

Seismic Waves radiate from focus and transmit energy.

 Body waves travel through earth.  Surface waves travel on surface. SURFACE WAVES BODY WAVES

Three Types of Seismic Waves Three Types of Seismic Waves P-waves = primary waves  Body wave  push-pull =  push-pull = compression waves   Particles move parallel to wave direction.   Fastest   Travels through solids and liquids

2. S-waves = secondary waves  body waves  side-to-side = transverse waves  Particles move perpendicular to wave direction.  intermediate speed  cannot travel through liquids

3. Surface waves  Particles at surface move in a circular motion.  Slowest  Most destructive

Seismogram P-wave S-wave Surface

Review/Preview:  Sketch and label a seismogram with s-p lag time.  What does lag time tell you?

Seismograph – instrument used to record seismic waves  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

Triangulation  Use at least three stations to locate epicenter.  Epicenter is where all 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 arrival time of the p-wave?  The s-wave?  What is the distance to the epicenter if the S-P lag time is 5 minutes?

 Which station is the closest to the epicenter?  Where is the epicenter?  Why can’t you just use two stations? B A C

Review/ Preview Make your own sheet this week. Write many observations of this diagram. (also on p. 236)

3) Earthquake destruction l important factors: Intensity & duration of shaking Soil type (soft? hard rock?) Building design l other effects: LiquifactionTsunamis

3) Earthquake destruction l important factors: Intensity & duration of shaking Soil type (soft? hard rock?) Building design l other effects: LiquifactionTsunamis Bay Area, CA 1989

3) Earthquake destruction l important factors: Intensity & duration of shaking Soil type (soft? hard rock?) Building design l other effects: Liquifaction Tsunamis

3) Earthquake destruction l important factors: Intensity & duration of shaking Soil type (soft? hard rock?) Building design l other effects: LiquifactionTsunamis

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). M6.7, 1994 Northridge, California earthquake

Intensity of shaking decreases with distance from epicenter (star; USGS Shake Map). M6.7, 1994 Northridge, California earthquake

Richter Scale  Based on amplitude of the largest wave  Logarithmic scale  Ex) 3.0 is 10x greater than 2.0

Moment Magnitude  Scientists use this now  Based on several factors (more complicated)  But tries to measure displacement & energy that is released  >6 = significant  >7 = major  >8 = great

2) Earthquake Intensity and magnitude l Mercalli intensity scale l 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” l Fault Types