Stan Posted December 28, 2004 Share Posted December 28, 2004 Physics of Tsunamis The phenomenon we call a tsunami (soo-NAH-mee) is a series of waves of extremely long wave length and long period generated in a body of water by an impulsive disturbance that displaces the water. Tsunamis are primarily associated with earthquakes in oceanic and coastal regions. Landslides, volcanic eruptions, nuclear explosions, and even impacts of objects from outer space (such as meteorites, asteroids, and comets) can also generate tsunamis. As the tsunami crosses the deep ocean, its length from crest to crest may be a hundred miles or more, and its height from crest to trough will only be a few feet or less. They can not be felt aboard ships nor can they be seen from the air in the open ocean. In the deepest oceans, the waves will reach speeds exceeding 600 miles per hour (970 km/hr). When the tsunami enters the shoaling water of coastlines in its path, the velocity of its waves diminishes and the wave height increases. It is in these shallow waters that a large tsunami can crest to heights exceeding 100 feet (30 m) and strike with devastating force. The term tsunami was adopted for general use in 1963 by an international scientific conference. Tsunami is a Japanese word represented by two characters: "tsu" and "nami". The character "tsu" means harbor, while the character "nami" means wave. In the past, tsunamis were often referred to as "tidal waves" by many English speaking people. The term "tidal wave" is a misnomer. Tides are the result of gravitational influences of the moon, sun, and planets. Tsunamis are not caused by the tides and are unrelated to the tides; although a tsunami striking a coastal area is influenced by the tide level at the time of impact. Also in the past, the scientific community referred to tsunamis as "seismic sea waves". "Seismic" implies an earthquake-related mechanism of generation. Although tsunamis are usually generated by earthquakes, tsunamis are less commonly caused by landslides, infrequently by volcanic eruptions, and very rarely by a large meteorite impact in the ocean. Earthquakes generate tsunamis when the sea floor abruptly deforms and displaces the overlying water from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. The main factor which determines the initial size of a tsunami is the amount of vertical sea floor deformation. This is controlled by the earthquake's magnitude, depth, fault characteristics and coincident slumping of sediments or secondary faulting. Other features which influence the size of a tsunami along the coast are the shoreline and bathymetric configuration, the velocity of the sea floor deformation, the water depth near the earthquake source, and the efficiency which energy is transferred from the earth's crust to the water column. A tsunami can be generated by ANY disturbance that displaces a large water mass from its equilibrium position. Submarine landslides, which often occur during a large earthquake, can also create a tsunami. During a submarine landslide, the equilibrium sea-level is altered by sediment moving along the sea-floor. Gravitational forces then propagate the tsunami given the initial perturbation of the sea-level. Similarly, a violent marine volcanic eruption can create an impulsive force that displaces the water column and generates a tsunami. Above water (subarial) landslides and space born objects can disturb the water from above the surface. The falling debris displaces the water from its equilibrium position and produces a tsunami. Unlike ocean-wide tsunamis caused by some earthquakes, tsunamis generated by non-seismic mechanisms usually dissipate quickly and rarely affect coastlines far from the source area. Tsunamis are characterized as shallow-water waves. Shallow-water waves are different from wind-generated waves, the waves many of us have observed on a the beach. Wind-generated waves usually have period (time between two sucessional waves) of five to twenty seconds and a wavelength (distance between two sucessional waves) of about 100 to 200 meters (300 to 600 ft). A tsunami can have a period in the range of ten minutes to two hours and a wavelength in excess of 300 miles (500 km). It is because of their long wavelengths that tsunamis behave as shallow-water waves. A wave is characterized as a shallow-water wave when the ratio between the water depth and its wavelength gets very small. The speed of a shallow-water wave is equal to the square root of the product of the acceleration of gravity (32ft/sec/sec or 980cm/sec/sec) and the depth of the water. The rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami has a very large wave length, it will lose little energy as it propagates. Hence in very deep water, a tsunami will travel at high speeds and travel great transoceanic distances with limited energy loss. For example, when the ocean is 20,000 feet (6100 m) deep, unnoticed tsunami travel about 550 miles per hour (890 km/hr), the speed of a jet airplane. And they can move from one side of the Pacific Ocean to the other side in less than one day. As a tsunami leaves the deep water of the open sea and propagates into the more shallow waters near the coast, it undergoes a transformation. Since the speed of the tsunami is related to the water depth, as the depth of the water decreases, the speed of the tsunami diminishes. The change of total energy of the tsunami remains constant. Therefore, the speed of the tsunami decreases as it enters shallower water, and the height of the wave grows. Because of this "shoaling" effect, a tsunami that was imperceptible in deep water may grow to be several feet or more in height. When a tsunami finally reaches the shore, it may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore. Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approaches the shore. Tsunamis rarely become great, towering breaking waves. Sometimes the tsunami may break far offshore. Or it may form into a bore: a step-like wave with a steep breaking front. A bore can happen if the tsunami moves from deep water into a shallow bay or river. The water level on shore can rise many feet. In extreme cases, water level can rise to more than 50 feet (15 m) for tsunamis of distant origin and over 100 feet (30 m) for tsunami generated near the earthquake's epicenter. The first wave may not be the largest in the series of waves. One coastal area may see no damaging wave activity while in another area destructive waves can be large and violent. The flooding of an area can extend inland by 1000 feet (305 m) or more, covering large expanses of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat. Tsunamis may reach a maximum vertical height onshore above sea level, called a run-up height, of 30 meters (98 ft). A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 525 meter (1722 ft) wave. Since science cannot predict when earthquakes will occur, they cannot determine exactly when a tsunami will be generated. But, with the aid of historical records of tsunamis and numerical models, science can get an idea as to where they are most likely to be generated. Past tsunami height measurements and computer modeling help to forecast future tsunami impact and flooding limits at specific coastal areas. There is an average of two destructive tsunamis per year in the Pacific basin. Pacific wide tsunamis are a rare phenomenon, occurring every 10 - 12 years on the average SOURCE Quote If you receive benefit to being here please help out with expenses. https://www.paypal.me/clubadventist Administrator of a few websites like https://adventistdating.com Link to comment Share on other sites More sharing options...
Stan Posted December 28, 2004 Author Share Posted December 28, 2004 Some have calculated or miscalculated this last one to have been over 2000 miles per hour. I suspect they may not have taken into consideration a time zone change or something like that. Even still, having these monsters travel faster than a passenger jet is incredible, and they are all underwater so you can not see them coming. Imagine a 30 foot wave hitting a beach at 550+ miles per hour. No wonder it did the surprise damage it did. Quote If you receive benefit to being here please help out with expenses. https://www.paypal.me/clubadventist Administrator of a few websites like https://adventistdating.com Link to comment Share on other sites More sharing options...
Stan Posted December 28, 2004 Author Share Posted December 28, 2004 further internet research shows they move a lot slower... Quote If you receive benefit to being here please help out with expenses. https://www.paypal.me/clubadventist Administrator of a few websites like https://adventistdating.com Link to comment Share on other sites More sharing options...
Amelia Posted December 28, 2004 Share Posted December 28, 2004 []http://www.geophys.washington.edu/tsunami/images/tsunamihead.gif[/] http://www.geophys.washington.edu/tsunami/intro.html []http://walrus.wr.usgs.gov/tsunami/images/home_title.gif[/] http://walrus.wr.usgs.gov/tsunami/ Quote <p><span style="color:#0000FF;"><span style="font-weight:bold;"><span style="font-style:italic;">"Do not use harmful words, but only helpful words, the kind that build up and provide what is needed, so that what you say will do good to those who hear you."</span></span> Eph 4:29</span><br><br><img src="http://banners.wunderground.com/weathersticker/gizmotimetemp_both/US/OR/Fairview.gif" alt="Fairview.gif"> Fairview Or</p> Link to comment Share on other sites More sharing options...
Amelia Posted December 28, 2004 Share Posted December 28, 2004 Something interesting on our local news today. The 9+ earthquake that caused the tsunami, registered on our Mt. Hood and North Sister seismographs. Plus on Washingtons Mt. Rainier seismographs. There are signs like these []http://www.kgw.com/news-local/stories/M_IMAGE.100c6c60ffc.93.88.fa.7c.4f07e616.jpg[/] dot our coastline. Quote <p><span style="color:#0000FF;"><span style="font-weight:bold;"><span style="font-style:italic;">"Do not use harmful words, but only helpful words, the kind that build up and provide what is needed, so that what you say will do good to those who hear you."</span></span> Eph 4:29</span><br><br><img src="http://banners.wunderground.com/weathersticker/gizmotimetemp_both/US/OR/Fairview.gif" alt="Fairview.gif"> Fairview Or</p> Link to comment Share on other sites More sharing options...
Moderators Gregory Matthews Posted December 28, 2004 Moderators Share Posted December 28, 2004 Just a point of interest, according to my rough estimate, the speed of sound is jsut under 75 miles per hour. So, they travel much faster. Yes, earthquakes travel through (some of the waves) the center of the earth to the other side. Other waves travel on the surface/under the surface. So, they will register all over the world. Quote Gregory Link to comment Share on other sites More sharing options...
Moderators Nan Posted December 28, 2004 Moderators Share Posted December 28, 2004 </font><blockquote><font class="small">Quote:</font><hr /> the speed of sound is jsut under 75 miles per hour <hr /></blockquote><font class="post"> ????? I thought the speed of sound was roughly 1000 feet per second - more like 750 mph? I presume a typo above. Quote Link to comment Share on other sites More sharing options...
Moderators Gregory Matthews Posted December 28, 2004 Moderators Share Posted December 28, 2004 The speed of sound is aprox. 1,100 feet per second. P. S. About an hour, or so, ago I left a dental chair after having three (3) root canals which due to problems were not completed. I am between working off the effects of the dental pain killers, and deciding whether/when to take the pills that will make me more comfortable. At this point in time, powers of ten are probable not my primary focus. <img src="/ubbthreads/images/graemlins/Nixe_nixe02b.gif" alt="" /> That episode was two solid hours of drilling, poking, and more that ended up incomplete. You are quite correct. I worked the multiplicatiion in my head, and the division with a pencil and paper. While doing the multiplication I dropped a power of ten. My sixth sense told me that I was off by a power of ten, but I was not able to figure out where I had dropped it until you pointed it out. Thanks. Quote Gregory Link to comment Share on other sites More sharing options...
Moderators Bravus Posted December 28, 2004 Moderators Share Posted December 28, 2004 Heh, the good old 'does your answer make sense' test I drum into my physics students: most racing cars are going siginificantly faster than 75 mph, but we don't get too many sonic booms at the track. <img src="/ubbthreads/images/graemlins/wink.gif" alt="" /> But yes, the waves are travelling at several hundred kph (which is a few less hundred mph). Remember that in the deep ocean the water itself isn't moving along at this speed, just the energy pulse. It's only as the water gets shallow that water starts to move in the direction of the wave, and it would come ashore considerably slower than those speeds. But still, if you saw it coming, it would be way too late to outrun or outdrive it. But all this science is whistling in the dark against a human tragedy that looks set to have a death toll fifty times that of September 11. It's a sobering reminder that there's no such thing as perfect safety and security in this world. Quote Truth is important Link to comment Share on other sites More sharing options...
Guest Posted December 28, 2004 Share Posted December 28, 2004 Ok...like...I usually ask the class after we go over a theme... What is the personal application to this?? This is a no brainer as far as I am concerned... I will continue to make waves. Quote Link to comment Share on other sites More sharing options...
Moderators Nan Posted December 28, 2004 Moderators Share Posted December 28, 2004 </font><blockquote><font class="small">Quote:</font><hr /> But all this science is whistling in the dark against a human tragedy that looks set to have a death toll fifty times that of September 11. It's a sobering reminder that there's no such thing as perfect safety and security in this world. <hr /></blockquote><font class="post"> Gregory's explanation is accepted - with sympathy. Thank you for the basic physics Bravus ! There are so many aspects to this tragedy - and not one of us knows the time of our own departing. (that is the personal aspect for me JimBob). Quote Link to comment Share on other sites More sharing options...
bevin Posted December 28, 2004 Share Posted December 28, 2004 The speed of sound depends on the media. In air, it depends on the pressure and temperature. http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe.html The speed of sound in air at normal sea level conditions is around 340 meters/sec = 750 miles per hour But the events being described here happen in water and rock. The speed in sea water is around 1500 meters/sec or about 4000 miles per hour http://freespace.virgin.net/sd.richards/speed.html In rock... http://hypertextbook.com/facts/2001/PamelaSpiegel.shtml /Bevin Quote Link to comment Share on other sites More sharing options...
bevin Posted December 28, 2004 Share Posted December 28, 2004 Quote: set to have a death toll fifty times that of September 11 3,000 * 50 = 150,000 So far the biggest number I have heard is around 50,000 /Bevin Quote Link to comment Share on other sites More sharing options...
Moderators Bravus Posted December 28, 2004 Moderators Share Posted December 28, 2004 Ack, now it's my turn to mess up the math! I was using the number, reported in Australian newspapers, of a projected 60,000, so I should have said 20 times. I won't edit the post above now that I've been corrected in the thread. The number is appalling either way. Quote Truth is important Link to comment Share on other sites More sharing options...
Stan Posted December 28, 2004 Author Share Posted December 28, 2004 ok the breaking the sound barrier is around 550 or so MPH.. I understood fully what I meant <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> Quote If you receive benefit to being here please help out with expenses. https://www.paypal.me/clubadventist Administrator of a few websites like https://adventistdating.com Link to comment Share on other sites More sharing options...
Peter Posted December 29, 2004 Share Posted December 29, 2004 Read about this volcanic eruption that caused 100 foot tsunami's in 1883 and killed over 36000 in the same area but on the other side of the island the 9.0 quake took place. http://volcano.und.nodak.edu/vwdocs/frequent_questions/grp7/asia/question879.html Quote Link to comment Share on other sites More sharing options...
Amelia Posted December 29, 2004 Share Posted December 29, 2004 Whatever the speed is, it took the first wave just under 2 hours to reach Sri Lanka from the epicenter. <img src="/ubbthreads/images/graemlins/shocked.gif" alt="" /> Quote <p><span style="color:#0000FF;"><span style="font-weight:bold;"><span style="font-style:italic;">"Do not use harmful words, but only helpful words, the kind that build up and provide what is needed, so that what you say will do good to those who hear you."</span></span> Eph 4:29</span><br><br><img src="http://banners.wunderground.com/weathersticker/gizmotimetemp_both/US/OR/Fairview.gif" alt="Fairview.gif"> Fairview Or</p> Link to comment Share on other sites More sharing options...
Peter Posted December 29, 2004 Share Posted December 29, 2004 Sri Lanka is 1000 miles from the epicenter so that means the wave was traveling at 500 mph which is what tsunami's travel. Here is the math. The deeper the water the faster the wave travels. http://www.pmel.noaa.gov/tsunami/Faq/x006_speed Quote Link to comment Share on other sites More sharing options...
Amelia Posted December 29, 2004 Share Posted December 29, 2004 Wouldnt tsunamis travel at different speeds according to the strength of the earthquake? Quote <p><span style="color:#0000FF;"><span style="font-weight:bold;"><span style="font-style:italic;">"Do not use harmful words, but only helpful words, the kind that build up and provide what is needed, so that what you say will do good to those who hear you."</span></span> Eph 4:29</span><br><br><img src="http://banners.wunderground.com/weathersticker/gizmotimetemp_both/US/OR/Fairview.gif" alt="Fairview.gif"> Fairview Or</p> Link to comment Share on other sites More sharing options...
Moderators Nan Posted December 29, 2004 Moderators Share Posted December 29, 2004 I think the speed is more related to the medium thru which it is travelling than the strength of the originating earthquake. Quote Link to comment Share on other sites More sharing options...
bevin Posted December 30, 2004 Share Posted December 30, 2004 Quote: 3,000 * 50 = 150,000 So far the biggest number I have heard is around 50,000 Sadly - at this point 150,000 is certainly a reachable number. /Bevin Quote Link to comment Share on other sites More sharing options...
Moderators Nan Posted December 30, 2004 Moderators Share Posted December 30, 2004 They are quoting 120 000 this morning. Aceh in Indonesia has suffered unbelievably, looks like the water was 60 feet high - from the air whole towns devastated and no survivors visible.... Quote Link to comment Share on other sites More sharing options...
Neil D Posted December 30, 2004 Share Posted December 30, 2004 I guess I just don't understand how a wall of water is displaced in the ocean [in a relatively small area] and that force is equally spread or intensivfied over areas many times the size of the origional force....I thought force was displaced over area with time.... Quote Democracy is a device that ensures we shall be governed no better than we deserve. George Bernard Shaw Link to comment Share on other sites More sharing options...
Moderators Nan Posted December 30, 2004 Moderators Share Posted December 30, 2004 The force would dissipate as it spread over a wider area wouldnt it? The problem arises when the force/shock wave reaches shallow water. It does not do damage in the deeper ocean - there were some Australians out snorkelling some hundreds of metres off shore and the wave just went under them - so others exploring an underwater cave were not hurt either. I am not sure in all this how far out the fishing boats had to be, to be safe. The only ones I have seen or heard about were washed up or missing. Bravus where are you? Quote Link to comment Share on other sites More sharing options...
Moderators Bravus Posted December 31, 2004 Moderators Share Posted December 31, 2004 As I understand it, the energy (better to talk about that than force in this context) would be decreasing linearly as the wave got further from its centre: it's spreading out in a circle, and the circumference of a circle is just over 6 (2 pi) times the radius, so it's a linear inverse relationship. The same amount of energy is having to spread itself out over more and more distance. (The inverse square law we see for forces like gravity is because they're spreading out in a sphere, and the surface area of a sphere is related to the square of the radius.) But apparently the earthquake involved a fall or rise of the sea floor of 30 metres (about 100 feet) over quite a large area, so the energy imparted by that event is almost unimaginable. It did decrease with distance, but it still retained plenty of energy by the time it made landfall all around the region. Apparently in deep water a tsunami can go under you and you barely notice - it's only a metre or two high. When it gets to shallow water, it's the same process as surf, only dramatically magnified. You've seen the way fairly gentle swells turn into breakers, and the up and down movement of the water converts into horizontal movement as the wave gets into the shallows. The same happens with a tsunami, but with a 60-100 foot (20-35 m, roughly) wave, 'shallows' is a pretty relative term, and I imagine boats and things quite far out were effected. It would depend on whether the shore sloped away gently or dropped off dramatically: if the latter, the divers could have been safe quite close to shore (though why you'd snorkle in very deep water is beyond me). Quote Truth is important Link to comment Share on other sites More sharing options...
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