The series of new earthquakes is exactly the right length for an M6.4.
You can see that the earthquakes (including the March one) are lined up along the monster valley that contains Zagreb. I've written many times about monster valleys in the heart of a compressive mountainous regime. They are the locations for huge 'blind thrust' earthquakes. These are earthquakes that turn everything in the valley to dust. (Look up blind thrust, on my search, on the right)
This is the exact situation the Italian seismologists found themselves in. When there was a smaller earthquake in the mountains, the newspapers asked if it meant anything for a large earthquakes. The seismologists came up with the usual '1 in 100' odds. A politician translated it to 'No Chance!'. A day later a big one came. The seismologists went to jail.
Because of the geology I give it a 1 in 20 of an M8 in the next year. Does this mean everybody should live in tents in the winter? No, maybe the odds are really 1 in 100. I don't want to go to jail.
But, we were looking at Croatia for a post-covid trip. Nope. I'll go somewhere perfectly safe, like Indonesia or Greece. :)
The truth is that you can't find any spectacular scenery without an earthquake risk.
ps. the return time for a monster earthquake should be 3-500 years. You can look it up. These earthquakes are so dangerous because that is way beyond living memory, and everybody puts up stone-rubble houses in the last 100 years or so of the time left.