Generally, you can assume a burst effect is an echo burst if a separate soft damage cap/'attenuation value' is mentioned. IE for something to have its own soft damage cap, it must be its own attack/hit.
Unfortunately, Marduk's doesn't have an attenuation value listed. You'll have to go the description of her first ability in the after awakening section. The last sentence in that description will be something like 'also, if Marduk herself is in a fortified state, a burst attack will also occur'. Then when you hover over that line, you should see the japanese text popup. Somewhere in there will mention 'burst ni (pursuit) (attack) mo hasseisuru'. Pursuit attacks tend to be separate attacks/hits.