Here is another article about the same event (in german) that has an aerial view supposedly showing the distribution of the craters over one of the islands:

http://www.spektrum.de/news/7000-gasblasen-woelben-sich-in-s...

(The craters filled with water, hence the dark color. The picture doesn't say anything about the craters' depth)

I don't know about the source though, so I guess it should be taken with a grain of salt.