# Tsunami receeding water question

1. Jan 5, 2005

### cbrauner

1. What causes water to leave the shore preceeding the tsunami wave peak?

2. Is the cause of that receeding water different from when regular waves approach the shore?

2. Jan 5, 2005

3. Jan 5, 2005

### cbrauner

preceding trough

The two responses regarding the "outflow" of water preceding the tsunami wave do not support each other. I need a better explanation of the physics.

Furthermore, how does a single upthrust of a tectonic plate generate a series of waves?

4. Jan 5, 2005

### Staff: Mentor

The same way dropping a stone in a lake creates a series of waves - it starts an oscillation that continues until damped. Water is pushed away, then rebounds (actually sending a column of water up), then that water falls away, etc., etc., etc.

HERE is a simulation of a diver entering a pool - notice the series of waves. Pause the video and look at the pattern - notice the small initial wave followed by a deep depression and then the largest wave is second - just like with the tsunami (and like I said in the other thread). HERE is a water-drop splash. You can't see the waves very well because its edge-on, but you can see the rebound and oscillation of the initial drop: its amost as if the initial drop of water is shot back into the air. From the first frame of THIS animation from NOAA, you can see that the biggest trough also preceded the biggest wave (the dark blue line) - obviously its a much more complicated event than a single splash, though. It also looks like what traveled west was a different shape - probably due to the sea-foor shape.

Also, an earthquake isn't a single event: they generally last 30-60 seconds, which also causes a series of waves.

Last edited: Jan 5, 2005
5. Jan 6, 2005

### cbrauner

Thank you

Thank you Russ.
We just learned of this website and were figuring out how it works.

This more detailed explanation and links helped to expand and clairfy your first explanation in the other thread. We didn't realize that there were smaller "tsunami" waves that came before the large one or that they could cause such a big trough before the "big" wave first hit land.

6. Jan 7, 2005

### Orion1

A Tsunami is a wave which damps off as its amplitude increases. The increase in amplitude is due to a decrease in waveform velocity due to a shallowing ocean depth and conservation of wave energy.

Tsunami celerity velocity:
$$v_c = \sqrt{gd}$$
$$g$$ - gravitational acceleration constant.
$$d$$ - ocean depth

A tsunami 'wave recession' is a 'leading wave' of negative amplitude. The negative amplitude height being equal to the height difference between its maximum recession ocean level and its 'normal' ocean level.

Reference:
http://www.prh.noaa.gov/itic/library/pubs/glossary/tsu_glossary_images/bigimage1_pg14.jpg
http://www.scubageek.com/geek/articles/wwwceler.html

Tsunami Photos:
http://img93.exs.cx/img93/6783/tsunami29bh.jpg
http://img93.exs.cx/img93/4707/tsunami39ge.jpg
http://img93.exs.cx/img93/6052/tsunami45sw.jpg
http://img93.exs.cx/img93/4176/tsunami53jm.jpg
http://img84.exs.cx/img84/7036/tsunami61kb.jpg
http://img84.exs.cx/img84/8669/tsunami72fb.jpg
http://www.pmel.noaa.gov/tsunami/Oceans99/Images/pic_1_2.jpg
http://www.buffalonews.com/graphics/2004/12/27/actualsize/1227waves.jpg

Last edited: Jan 7, 2005