Osaka area sits above the fault line that caused the Great Hanshin Earthquake.
Anytime it might happen again in the future. So if you're worried. Especially if you're travelling with your kids and family. Then better adjust your travel plans and avoid that place temporarily.
Not even say temporarily. If really worried about earthquakes then its best for them to totally avoid stepping into the Japan cities/prefectures (sorry I don't know which is the correct term) that are lying on top of and near the fault line.
There's always seismic activity going on because the plates are constantly moving, and Japan just happens to sit on the fault line on the joint of 4 different plates (Edit: I dug a little deeper and realised that my previous statement of Japan sitting on 2 plates were incorrect - Japan is on 4 different plates wut? Tokyo itself sits on the main Honshu Island which in turn sits on the intersection of 3 continental plates). I'm sure those who took Geography lessons last time in school would know what that means.
Anyway at the end of the day, earthquakes are unpredicatable - you'll never know when the next one will come. As said many times before in this thread, the next great Kanto quake (that will hit Tokyo) is way overdue already. When will it hit? No one knows. For those who are
really worried, then please don't choose Japan as your holiday destination.
Side (unrelated?) note: today Shinmoedake volcano in Kagoshima just erupted again (about 2hr ago?). The volcano has been quite active this 2018... last month got a big eruption, now another one.