You could call it a bridle joint, too. It would help to see more of the bench and how it is put together.
Here is a free book that you can download or read online, with excellent drawings of woodworking joints. It is old, and it expects you to use hand tools, but the same woodwork joints are still in use today. You can easily find several joints that are similar to the one in your photo, including box joints and bridle joints:
Looking at a photo will tell very little about how out is constructed, a good craftsman would be capable of hiding anything extra such as dowels between the top and legs to retain the integrity of the design.
Wise men speak because they have something to say; fools because they have to say something -Plato
If it's Japanese, I'll bet my bottom dollar all of you are wrong. Some of the Japanese joinery is a compendium of very intricate and complex joints that will baffle the best of us. There are probably hidden joints under the surface that would lock this bench in place where no one will venture to guess how they did it.
Tony BRetired woodworker, among other things.
"Strive for excellence and settle for completion" Tony B