dr. catherine petroff, university of washington - july 22, 2007 uw educational outreach – tsunami...

Sponsored by NOAA and USAID under the U.S. Indian Ocean Tsunami Warning System Program

Dr. Catherine Petroff, University of Washington - July 22, 2007UW Educational Outreach – Tsunami Science & Preparedness Program (Su 07)

Tsunami Basics



"Know your enemy and know yourself and you can fight a hundred battles without

disaster."

Sun TzuChinese general and author, b.500 BC



Some “Known” Large Tsunami Events:

Prehistoric (100 – 200 ka) - Hawaii Landslides1600 BC Santorini Explosion1491 Mahuika Crater impact, New Zealand (est Mw > 9.0) 1700 Cascadia (est Mw > 9.0) 1755 Lisbon, Portugal ( Mw = 8.5, 90,000 dead)1883 Krakatau (est 36,000 dead)1896 Sanriku, Japan (Ms = 7.0, est 22,000 dead)1960 Chile (Mw = 9.5, 500-2300 dead)1964 Alaska (Mw = 9.2, 122 dead)2004 Indonesia (Mw = 9.0, > 225,000 dead)

See http://www.msu.edu/~fujita/earthquake/bigquake.htmlFor historic earthquake data



1. Terminology

2. Waves in the Open Ocean

3. Exercise – Wave travel times

4. Tsunami Generation Mechanisms

Outline:



Terminology - :

Amplitude

Schematic of water wave



Terminology:

Deep Water Wave?

> 1/20?Wavelength

AmplitudeDepth

Deep water waves are usually driven by surface shear forces. The energy goes into the water from the surface (like with wind)



Wavelength

AmplitudeDepth

Terminology:

Shallow Water Wave?

< 1/20?

Shallow water waves have energy distributed thoughout the water depth. They “feel” the bottom of the ocean.



Other things that we often talk about with waves.

Long wave?

Small Amplitude Wave?

Answers to these and other questions about the waves we are studying lead us to decide on whether to use:

Linear shallow water wave theory

Non-linear shallow water wave theory

Finite amplitude wave theory

And other options…

?1Wavelength

Amplitude2



How do tsunamis differ from “day at the beach” waves?

Order of magnitude estimates:

Wind Waves Tsunamis

Wavelength 10-1000 m 100 km

Period 10 – 100 sec 10 min

Ocean Amplitude 1-100 m 1 m

Shore Amplitude 1-10 m 1-10 m *

Water motion More near surface Entire column

*except very near field events (100 m)



A word about energy

The energy carried by a wave must come from somewhere. For example in wind waves it is the wind that transfers energy to the water by pushing on the water surface.

In terms of mechanical energy there are two main components:

• Potential Energy: Raising a body of water up above its original level

And

• Kinetic Energy: The energy of the moving water

If we keep in mind that all waves take energy to create, then the issue of tsunamis becomes somewhat more straightforward…



A little exercise to amaze your friends:

Calculate the propagation time (travel time) of a tsunami using only a calculator and a map.

Recall: Tsunamis behave like shallow water waves. Their speed is governed by water depth.

In fact, a good first order estimate is:

c

metersindepths

mdepthgravity2

819.



Krakatau - 1883

Let’s estimate how long it took the wave to travel from Indonesia to Madagascar

http://www.ngdc.noaa.gov/seg/hazard/tsu_travel_time.shtml

http://www.timeanddate.com/worldclock/distance.html

http://www.pbs.org/wgbh/nova/teachers/activities/3208_tsunami.html



Now you try….

http://www.timeanddate.com/worldclock/distance.html

http://nctr.pmel.noaa.gov/education/science/docs/Alaska_travel_times.pdf

http://nctr.pmel.noaa.gov/education/science/docs/Alaska_Travel_times.xls



Causes of Tsunamis:

Any impulse that causes large scale displacement of the sea surface.

•Seismic Rupture•Landslide•Undersea Eruption or Explosion•Meteor Impact



More on energy

Some estimates put the seismic energy released during the Sumatra 2004 tsunami at 500 million tons of TNT – or about 33,000 times the energy released by the bomb at Hiroshima.

Where did all that energy go?

• Deforming a large part of the earth’s crust

• Seismic waves (earthquake)

• Tsunami waves - raising up (potential energy) and accelerating (kinetic energy) the water



Magnitude 9.0 OFF THE WEST COAST OF NORTHERN SUMATRA Sunday, December 26, 2004 at 00:58:53 UTC



Surface Projection of Slip Distribution

Courtesy ofChen Ji,Caltech



Projected Surface Displacements (by Chen Ji, Caltech)



Tsunami Generation:

Efficiency of tsunami generation by crustal deformation:

• Controlled by the speed at which the rupture propagates down the fault• Earthquakes with slow rupture velocities are the most efficient tsunami generators, “tsunami earthquakes”. •This capacity for tsunami generation is commonly characterized by a Tsunami Magnitude, Mt.



Recall: Basic Concept of Seismology

Seismic moment - measure of work done by earthquake.

Mo = m A D

where m = shear modulus of rock

A = rupture area

D = average displacement over rupture area

Moment magnitude

Mw = ------------ - 10.7log Mo

1.5



Tsunami Magnitude:

One formula for tsunami magnitude is:

Mt = log H2 + log X + 5.55

Mt: Tsunami magnitude

H2: Maximum crest-to-trough amplitude on tide gage record in meters

X: Distance from epicenter to station along the shortest oceanic path in km

(Ref. Abe, K., Phys. Earth Planet. Inter., 27, 194-205, 1981)



Tsunami Magnitude:

Some tsunami events with Mt = 9.0 and greater:

1837 Valdivia, Chile 9.31841 Kamchatka 9.01868 Arica, Chile 9.01877 Iquique, Chile 9.01946 Aleutians 9.31952 Kamchatka 9.01957 Aleutians 9.01960 Chile 9.41964 Alaska 9.12004 Sumatra, Indonesia 9.0

Ref: (Abe, K., J. Geophys. Res., 84, 1561-1568, 1979):



Landslide generation of tsunamis

So if we understand that a tsunami is generated by an impulse that causes large scale displacement of the sea surface then it is easy to see that landslides can also generate tsunamis.

Skagway, Alaska 1994



Skagway, Alaska 1994

Possible lanslide generated tsunami



Example of landslide source computation



What about Volcanoes?

So if we understand that a tsunami starts with a large scale displacement of the sea surface then it is easy to see that volcanoes can also generate tsunamis. The surface displacement can be caused by lava flows, sea floor motion or even the release of a large amount of gas.

http://www.volcanolive.com/tsunami10.html



And what about Meteors?

So if we understand that a tsunami is generated by an impulse that causes large scale displacement of the sea surface then it is easy to see that meteor/asteroid impacts can also generate tsunamis.



Phases of a Tsunami Event:

•Generation

•Propagation

•Shoaling

•Inundation

dr. catherine petroff, university of washington - july 22, 2007 uw educational outreach – tsunami...

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