Not that I care (I didn't even care about the Tsunami) but it is actually reduced coastline. If you were to picture the island as a circle, a collapse of the coastline could only result in a circle with a smaller circumfernce. Unless somehow the collaps of the coastline left massive fjords like you see in Norway.
The ongoing lava flow, which caused the collapse, will fill in the gap and then some. This side of the island is a growing edge.