3. Osaka, Japan
Metro area population: 19,341,976
Uemachi Fault Group consisting of Butsunenjiyama Fault, Uemachi Fault, and Nagai Fault runs beneath Osaka, the second largest population center in Japan. Ueamachi runs in North-South direction and is some 40 kilometers long. The fault zone is well researched with numerous boreholes that house sensors which could hopefully provide enough warning in case of the activation.