A sign of a tsunami is that the water rushes away from the shore, then comes back to higher levels. It seems that waves should be both + and – polarized and that some tsunamis should go in the opposite direction. That is the first indication of them would be that the water begins rising. However, other than situations very close to the source, it seems that the wave always begins with the water drawing away from the coast.
For example, the wikipedia article on tsunamis states that:
In the 2004 Indian Ocean tsunami
drawback was not reported on the
African coast or any other eastern
coasts it reached. This was because
the wave moved downwards on the
eastern side of the fault line and
upwards on the western side. The
western pulse hit coastal Africa and
other western areas.
The above is widely repeated. However, when you search the scientific literature, you find that this is not the case:
Proc. IASPEI General Assembly 2009, Cape Town, South Africa., Hermann M. Fritza, Jose C. Borrerob, "Somalia Field Survey after the December 2004 Indian Ocean Tsunami":
The Italian-speaking vice council,
Mahad X. Said, standing at the
waterfront outside the mosque upon the
arrival of the tsunami (Figure 10a),
gave a very detailed description of
the initial wave sequence. At first, a
100-m drawback was noticed, followed
by a first wave flooding the beach.
Next, the water withdrew again by 900
m before the second wave partially
flooded the town. Finally, the water
withdrew again by 1,300 m offshore
before the third and most powerful
wave washed through the town. These
drawbacks correspond to 0.5-m, 4-m,
and 6-m depths. The detailed
eyewitness account of the numerous
drawbacks is founded on the locations
of the offshore pillars.
So is there a physical reason why tsunamis, perhaps over longer distances, tend to be oriented so that the first effect is a withdrawal of the water?
Best Answer
The positive or negative elevation of the leading wave is related to what happened to the ocean floor during the earthquake. If the seabed was raised, a crest should lead the tsunami; if it was lowered then a trough leads.
In the 2004 Sumatra tsunami, a piece of seabed of about 1000 km x 100 km went down a few meters. Another strip, located to the west of the first and with roughly the same size, went up. Because of this, the wave moving east (towards Thailand) was led by a trough. The one moving west, which reached Sri Lanka, India and Africa, was led by a crest.
This is seen in the simulation of that tsunami shown below (by Kenji Satake, of the Active Fault Research Center in Tsukuba, Japan). In the red areas the water surface is higher than normal, and in the blue ones it is lower.
A similar animation, with some data added, is at http://es.ucsc.edu/~ward/indo.mov