Thank you for sharing your concern. I appreciate this is a traumatic time for many people in Kyushu, however Japan has an excellent system for preventing and managing damage. Their infrastructure (water, roads) will be fixed quickly and to a good standard, so once that happens you can visit without much restrictions.
Kumamoto Airport has just reopened yesterday despite over 500 after shocks, so that is actually a very quick turn around. While I cannot predict earthquake activity, large earthquakes are rare and it is unlikely to be a recurring problem. I am basing this on the experience in Sendai in 2011, where after the initial earthquake, the airport and the transport infrastructure (shinkansen) was back up pretty quickly, despite the after shocks.
You should keep a watching brief on some of attractions, which may take longer to recover (e.g. Kumamoto Castle and Mt Aso). If they are closed during your visit, there are plenty of other attractions and experiences that make Kumamoto and Kyushu such a wonderful and relaxing place to visit.
The other prefectures (e.g. Fukuoka and Nagasaki) are largely unaffected.
I also suggest you register for the travel advisory services from your Government. The current advisories from Singapore and Australia advise caution but have said that these places are safe to travel.
Well, yes you should somehow be concerned.
If you plan to go to Kumamoto prefecture, it would be wiser to check if the place you want to visit and the place you plan to stay are open. Like Bonson said, forget about Kumamoto Castle. And if you are not going for humanitary reason, it might be wise just to avoid going to the most affected places until they have cleaned what was destroyed and relocated the people who lost their houses. If there is people without shelter and water, it does not sound like a place yo go as a tourist. Just wait at least a couple of weeks to let them fix their place.
But Kyushu is large, and if you are not too afraid of earthquakes (there is a lot of small ones right now), it should not be a problem, and there is a lof of things to see outside of Kumamoto