A rule of thumb, in cases like this, is to ask who benefits from this. The cable connects Europe and Asia, and so would affect all or most Eurasian countries to a greater or lesser extent. Further, as traffic is redirected through intact cables, there could be indirect effects across the world. Finally, the perpetrator must be capable of cutting not one but four metre-thick cables at the bottom of the sea.
Once you consider these points, there is only one possible suspect.
Godzilla.