In tsunami, after the wave hit, why does the ocean recede,exposing sea bed?

its like when you slosh in the bathtub -- water goes to one end of the tub and then the other. it has to do with the physics of liquids and that makes my head hurt.

The ocean does that simply because it can... perhaps to baffle the minds of humans unable to seek answers for themselves... read a frickin' book!

The recede comes before the initial wave and possibly between the successive waves as there is typically more than one wave when a Tsunami hits. This occures because of the motion of the wave, which is actually somewhat complex but can be thought of as a cirular current as if it were a wheel rolling ashore.

The Tsunami is mostly created after the earthquake. The tectonic plates are moving up and down motion called compress ional. The water at the place where the tectonic plates moved will form a very huge heap of water for a few minutes only. During this time to compensate this huge heap of water the water in the shore will recede. After that only the huge water went up will be converted to a very long wave with very less height of about 300 to 600 mm only. But the Tsunami struck our coast on 26th Dec 2004 created the height of 250 c.m. When it hits the sea shore or moving over the shallow water will increase to enormous height. To know more about the behaviors of earthquake and Tsunami please visit my slide show at http://asia.pg.photos.yahoo.com/ph/quake

I could be wrong, but I think the ocean recedes BEFORE the wave hits. The water is being sucked out from the coastline TO the wave, which is building is size.

It's essentially the same as any other wave, just on a much larger scale. Before an ordinary ocean wave breaks on the beach the water in front of the wave recedes as the wave builds. Then the wave crest reaches critical height, breaks, and rushes up onto the shore. This process repeats with each wave crest in the series. In the case of a tsunami, the same process takes place, but greatly magnified. Much greater recession before the wave, much greater critical height, much greater invasion onto the land. Also, much greater wavelength, therefore greater time between successive crests. And, fortunately, due to the manner in which such waves are formed, successively smaller crests after the initial one.