If God were good, why would He let what happened to Japan happen? I think this comes from a larger issue of, "If God, why evil?" This specific situation falls under "natural evils" or "natural disasters" for obvious reasons that which are understood. I want to answer this question delicately due to the emotions that are going about this, but I want to answer this Biblically, because I believe the Bible is a book that though it cuts through the heart...it also brings the gauze to heal the wounds as well. I have three reasons that I can find in Scripture for this.
Reason 1 comes from Genesis 3:17-19, "And to Adam he said, 'Because you have listened to the voice of your wife and have eaten of the tree of which I commanded you, "You shall not eat of it," cursed is the ground because of you; in pain you shall eat of it all the days of your life; thorns and thistles it shall bring forth for you; and you shall eat the plants of the field. By the sweat of your face you shall eat bread, till you return to the ground, for out of it you were taken; for you are dust, and to dust you shall return.'" If you notice the phrase, cursed is the ground because of you, what this is saying is that all of nature, because of the sin of man, is cursed. Paul mentions this again in Romans 8:20-21 "For the creation was subjected to futility, not willingly, but because of him who subjected it, in hope that the creation itself will be set free from its bondage to corruption and obtain the freedom of the glory of the children of God." So, at the Fall, creation was subject to a curse, and Tsunamis, Earthquakes, Volcanic Eruptions, Floods, Tornadoes, Hurricanes...etc. happen because of this. Even though, God is still sovereign over it, our sin caused it, so blame must be shifted from God...to us.
Reason 2 is one that we all do not like to hear, but we must hear it. Often times, even with our friends, we must tell them something that is the truth and we know it is good for them but it may be hard to tell them because they won't like it at first...yea, this is kind of a reason like that, so please, bear with me. Reason two is judgment. Romans 3:23 says, "for all have sinned and fall short of the glory of God," and since the word, "all" is not limited in this passage except for, "all" humans...then we must consider this and follow it out. If the Japanese people are humans, then they are under judgment as well because by this passage, they are sinners as well. I told you that you would not like this, but do hear me out, please. We all are sinners, all in desperate need of Jesus Christ to save us, and because we are sinners, we are subject and deserving of God's wrath. It stinks sometimes, but just because we may or may not like it, does not discount it from being reality. I think a more important question that follows is, “Why didn’t God allow this to happen to us?” We need to remember Reason 2; we all are sinners, Japan, China, Iraq, Zimbabwe, Uganda, and America. You and me, we all are sinners, so the real question is not, “Why would God allow this to happen to Japan?” but rather, “Why did God not allow this to happen to us since we are deserving of it as well?”
Reason 3 that I have found is, it is because of natural disasters, Christians can rise up, be a Christian by giving support financially, giving time to go over and help rebuild and care for people's emotional needs, giving supplies to organizations to send over to help with relief, and in this, spread the Gospel. Letting things like this point people to Jesus, so that when we lose everything we have, we can cling to Jesus. This may seem selfish to some people, understandably so. Do remember, if Jesus Christ is what Christians view as the most valuable thing they have, then giving the most valuable thing they have to someone is the most selfless act a Christian can do, so it is all about perspective. We can be a Christian in word and deed, which is what James says in chapter 2, and which is what Paul says in Colossians 3:17, but moreover , what James, Paul and most importantly, Jesus Christ actually did. They all lived a life doing deeds that glorified God and that people took notice of, and they preached Gospel.