Well there is this, "The map of the sea floor, made by the ship’s multibeam sonar, indicates that as much as 5 cubic kilometers of magma erupted onto the sea floor."

I think 5 km^3 is a large eruption by any standards. Looking at lists of modern ones it's a index 6 event which happen every few decades on land. From the article it sounds like if it weren't close to Mayotte it wouldn't have been picked up seismically.