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MIDWEEK ESSAY: Physics of the Tsunami - and Understanding Why It Causes Devastation

Introduction

On Boxing Day December 26, 2004, the world awoke to a horrific news: a tsunami had 'boxed" out whole towns and villages in and around islands in the Indian Ocean/Pacific Ocean of South-East Asia (from Indonesia to India to Thailand), and extended its grievous reach to the Horn of Africa (Somalia).  The death toll is still rising - and at last count as many as 150,000 human beings - not to talk of fauna and flora - are missing and/or presumed dead.

How could this devastation have occurred?

Well, let us look at some back-of-the-envelope arithmetics of the physics of the Tsunami.

  Tsu-nami (Japanese for 'harbor-wave")

Some speeds to consider

In normal stride, an adult walks roughly at the speed of 5 kilometers per hour (km/h) or 3.1 miles per hour (mph). A person who runs 100 meters in 9 seconds runs at 40 km/h - or roughly 25 mph [1 mph = 1.60934 km/h;  1 km/h =0.621371 mph.].   Above 55 mph (or 88 km/h), you will get a speeding ticket driving in most highways in the US.  Commercial jet airplanes typically fly at 540 mph (870 km/h) - roughly  ten times the typical car speeding limit  The speed of sound at sea level is roughly 760 mph or 1,223 km/h  [= Mach 1] and the speed of light is just about less than 1,000 times the speed of sound - 1,079,252.849 km/h to be exact.

The Tsunami and Channel Flow

Think of a long rectangular container filled with water, tilted such that water can flow out  from one end.   For a tsunami here, the water is ocean water; the bottom of the container is the ocean floor  - and the sudden tilt of the container is caused by an earthquake. In the case in question, it was a megathrust earthquake of moment magnitude 9.0 on the Richter scale.

And that water flows onto coastal land and beyond (as far as 1 km or 0.6 miles inland)  - into where human beings live.

The Speed of the Tsunami

So what was the speed of the Tsunami that hit parts of South-East Asia on December 26, 2004 ?

It depends on where in the ocean the quake which started the channel flow of water began, causing a shallow-water wave.  Multiply the ocean depth (in meters) by roughly 10 (actually 9.8 meter per second squared), square root the product, multiply it by 3.6 - and you get roughly its speed in km/h [ see Table 1 below.] 

We are told that the SE-Asia tsunami moved up at speeds up to 800 km/h - which means that it must have come from depths of about

5,000 m (5 km). [Average depth of the Pacific Ocean is about 4,188 m, and its deepest Mariana Trench is about 11,000 m.]

Of Tornadoes and Hurricanes

The speed of the water when it got to land is much slower - but you get the idea that it could be sufficiently fast to sweep objects in its path away.  Even equally devastating is the HEIGHT of the water when it hits land, drowning life.

I have also given in Table 2 the equivalent wind speeds and categories for  the more familiar depressions, storms,  tornadoes and hurricanes.  In August/September 2004, the mid-Atlantic/Southeast states (particularly the state of Florida) witnessed five major land-falling hurricanes:  Hurricane Charley ( around August 13) was 145 mph [233 km/h;  Level 4] and Gaston (at 75 mph, a Level 1 Hurricane in August; reclassified as a hurricane after the season),   and the rest in September were  Frances (around September 5; 105 mph; 169 km/h;  Level 2), Ivan (around September 16; at 200 km/h, a Level 3 Hurricane) while Hurricane Jeanne was about 75 mph (120 km/h a Level 1 Hurricane; around September 26).  Only Gaston missed Florida.

Noting that the density of sea water is about 1,030 kg / cubic meter while that of dry air is 1.2 kg per cubic meter at room temperature - and with kinetic energy directly proportional to mass and to the square of the moving medium's speed - one can see that water of the same speed as air packs about 1000 times more energy ! [Doubling the speed (for example) quadruples the kinetic energy.]

Lord have mercy !

So Please Donate

Now that you have a rough idea of how devastating a tsunami is - and why it wrought what it did in South-East Asia and the Horn of Africa, please donate to your favorite charity: 

Bibliography

http://news.bbc.co.uk/1/hi/world/asia-pacific/4136289.stm

Taken from 'Animated Guide: The Tsunami disaster"

http://en.wikipedia.org/wiki/Density_of_air

Density of Air

http://hypertextbook.com/facts/2002/EdwardLaValley.shtml

Density of Sea Water

http://www.geophys.washington.edu/tsunami/general/physics/meaning.html

What Does 'Tsunami" mean ?

http://www.news-journalonline.com/NewsJournalOnline/News/Neighbors/DailyJournal/03AreaDJ02123004.htm

Table 1:  Relationship of Tsunami Water Speeds to Ocean Depth

Source: http://mbgnet.mobot.org/salt/oceans/data.htm

   Table 2:  Classifying Depressions, Storms, Hurricanes and Tornadoes

*The term hurricane is used for Northern Hemisphere tropical cyclones east of the International

Dateline to the Greenwich Meridian. The term typhoon is used for Pacific tropical cyclones north

of the Equator west of the International Dateline.

Sources:

http://hypertextbook.com/facts/StephanieStern.shtml

http://www.infoplease.com/ipa/A0193170.html

http://www.ucar.edu/educ_outreach/webweather/hurricane2.html

http://www.hawaii.navy.mil/hurricane/hurr_info.htm

http://www.aoml.noaa.gov/general/lib/laescae.html

http://www.nhc.noaa.gov/aboutgloss.shtml