The article indicates that you need register in advance. My impression is that it may be difficult to register and work as a volunteer unless you understand Japanese but, if you're willing and able, you can search for "volunteer" at kumamoto.kirkmasden.com (be sure to sort by "Newest first") to find some posts I made about how to register. -- Kirk

https://japantoday.com/category/national/over-1-200-disaster-relief-volunteers-enter-quake-hit-kumamoto