Kirk here with a little more information about last night's earthquake. Though it was big, there was relatively little damage, thankfully. It seems that a few people were hurt but that there were no fatalities.
The following article (in English) has a summary of what happened:
https://english.kyodonews.net/news/2022/01/8ffee7f69134-urgent-m64-quake-hits-southwestern-japan-no-fear-of-tsunami.html
The end of this article talks about how damage from an earthquake in the Nankai Trough could far exceed damage from recent earthquakes, including the 2016 Kumamoto quake. But, it doesn't explain what the Nankai Trough is or how it relates to what we felt last night. That made me curious so I did a little searching and found some information. Here are two points:
1. You can learn about where the Nankai Trough is and why it is so feared here:
https://www.nippon.com/en/news/fnn20190524001/a-disaster-to-dwarf-311-the-predicted-nankai-quake.html
2. Last night's quake was in the area of the Nankai Trough but the moving and shaking was caused by a deeper plate, not the one that people are most worried about. I learned this from the following article in Japanese:
https://english.kyodonews.net/news/2022/01/8ffee7f69134-urgent-m64-quake-hits-southwestern-japan-no-fear-of-tsunami.html
A Disaster to Dwarf 3/11? The Predicted Nankai Quake
Experts believe there is a 70% to 80% probability of a severe Nankai Trough earthquake within 30 years. More than 70 years have passed since the previous such quake in this region, which sees a major shake every 100 to 150 years.