Its a geometric issue associated with the very nature of a deep water gravity wave.
First from the derivation you realize that the trough and the crest are just parts of the same deformation of the surface.
Go hereto see derivation based on the PDE
Another analysis is found at
Basically the trough and the crest are part of the same beast...cant have one without the other...the trough leads because that is what builds the crest. tsunami
This is a water slosh that encompasses the full depth of the water. Now, on the figure, the arrow shows where we suddenly pull down on the crust. This is our big subduction earthquake. The water sloshes into the hole, and this starts to propagate over to Padang and Africa (negative slosh). It doesn't take long for the water to slam together, and send out a positive slosh. These slosh pulses involve the whole depth of the ocean, and travel quite slowly, as one would expect.
In one way, the people may be luckier with a big subduction earthquake, because the pull-down causes a leading negative slosh. Thus, when confronted with the ocean disappearing, it behooves one to run like hell to higher ground! Apparently, this might be too late for Padang; they have to start running as soon as they feel the shaking, because the run-up could be kilometers!
I hope this helps...
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