Here is a first draft of an updated summary of responses so far. I have grouped them together into similar categories.
Please feel free to suggest improvements and additions. I am certain it needs a lot of work but its a start.
Thank you to everbody who has contributed so far.
The "rational responses" do not cover all of the excellent range of contributions so my apologies in advance.
1. Answers that seek to change the question.
For example...
Blaming humans for damaging the earth
References to "the fall"
Slippery-slope arguments such as, "if god prevents a tsunami where does he stop?"
Any answer that appeals to free-will.
Rational Response
The question of this thread is very specific. It only concerns "natural evil". In theology this term relates to suffering that is not caused by human actions. Earthquakes and tsunamis have been happening for millions of years before humans appeared. They are a result of plate tectonics. They are not caused by anything humans have done.
If god chose to do so he could refuse to stop suffering caused by human actions while preventing the negative consequences of how he chose to make the world. Either directly or indirectly, the tsunami was entirely an act of god.
If god had calmed the wave before it had even reached the surface no human would ever have known about it and no free will would have been effected in any way.
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2. Answers that call into question god's ability to prevent the tsunami
For example...
There may have been unforeseen consequences of stopping the disaster
God is not all-powerful
Rational Response
The god of theism is the omnipotent creator. It would have been trivially easy for him to calm the wave and prevent any other negative consequences.
The question is not a problem for worshippers of a lesser god.
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3. Answers that seek to shift the blame
For example...
People should not have lived so near the sea
People should have known how to read the signs
Humans do bad things too
Satan did it
Rational Response...
This is morally equivalent to throwing rocks at a crowd of people and blaming them for not ducking. It portrays god as vindictive and unloving.
The people who drowned were not living on top of a fault-line or beside a live volcano. The tsunami impacted on thousands of miles of coastline around the Pacific Rim. Most of the victims had no opportunity to take evasive action.
Measuring the morality of god against that of human tyrants is setting the bar low for the god of christian theism.
The concept of god's nemesis doesn't even appear in the bible until post-exile when the Jews came into contact with the dualistic Zoroastrian religion. It is a strange pagan notion that makes god look pathetically weak.
Theism does not teach that god handed over his entire creation to a diabolical enemy with no restrictions. The devil can do nothing without the knowledge and permission of god. If Satan caused the tsunami he did so with the full knowledge and permission of god.
This answer is the moral equivalent of letting a wild lion loose in a village and then blaming the animal for the deaths.
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4. Answers that seek to find some benefit in the disaster
For example...
Suffering makes us stronger
Suffering teaches us compassion
It afforded the opportunity for people to offer help
Rational Response...
These are Ivory Tower responses that takes no account of the reality of human suffering. A quarter of a million people learned nothing from being drowned. Their hopes and dreams perished in a moment. Hundreds of thousands of survivors were left bereaved and without homes, jobs or the necessities of life.
Human efforts to clear up god's mess does not excuse his passivity. It is also astonishing hubris that diminishes the lives of a quarter of million people into a commodity to be used for the benefit of producing better christians.
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5. Answers that try to reject the question
For example...
Who are we to judge god?
God can do whatever he chooses
We just need to trust that whatever god does is for the best.
Rational Response...
This answer requires that we unhitch the word "love" from any meaningful definition. We may think we know what love means but god demonstrates that we have not the slightest idea. Love could just as easily mean the capricious annihilation of a quarter of a million innocent people. It destroys our ability to make moral judgements. "Good" is whatever pleases god from moment to moment. Mass destruction is just as morally good as altruism and self-sacrifice.
If god is love, everything he does must be motivated by love, even when he judges. Love is not a hat he can take off for a while and replace with one labelled "vengeance".
Ethics become a matter of divine fiat and the value of human life is trivialised. This defence reduces god to a celestial Pol Pot who may choose on a whim to eradicate our lives in the manner of the killing fields of Cambodia.
If neglecting to stop a wave that drowns a quarter of a million people doesn't give us pause to reconsider the wisdom of blindly trusting of god what would??
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6. Answers that Retreat into temporary Deism
For example...
This is not god's time to intervene
God has promised to end suffering at a future time
Rational Response
According to christian theism, god is intimately involved in human affairs. Not just in giving strength to cope with whatever happens in life, but in actually changing events for the benefit of those who ask in faith.
"If you remain in me and my words remain in you, ask whatever you wish, and it will be done for you." - John 15:7
"Dear friend, I pray that you may enjoy good health and that all may go well with you , even as your soul is getting along well." - 3 John :2
If all christians comply with Jesus' words to petition god for things, and imitate John by praying that god bless and prosper others, then god is active every second of every day responding to millions of requests.
If god answers even one of those prayers it destroys the argument that god was not in the intervention business on 26th December 2004
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7. Answers that trivialise the reality of human suffering
For example..
Suffering will be unimportant compared to eternal rewards
Rational Response
This is ethically repugnant. Suffering is not reducible to arithmetic. This life really matters. Any philosophy that minimises the importance of physical human life is dangerous. It is the same mentality that leads to religious extremism and flies aeorplanes into tall buildings.
It is an extreme example of "the end justifies the means" defence, so beloved of tyrants.
Like other theodicies it is dehumanising by reducing humans to pawns in god's game.
Imagine that scientists developed a pill that would eradicate all unwelcome memories and create a feeling of bliss. How would you judge a scientist who imposed the most horrific suffering on millions of people, as unwilling subjects of his experiment, but who gave all of them one of the magic pills when it was over?