Very interesting investigation made by GeoLurking from Volcanocafe
[link to volcanocafe.wordpress.com]

"We can find dates of 871, 932, 1480, 1783, 1862. Calculating the repose intervals, there is an average of 247.75 years between events… but that is only four intervals and is statistically dubious at best. (it means that there is a lot of room for error)

However, it does also indicate that about 4.6 meters of crust expansion had occurred when the events happened. (low – 1.15 meters, high 10.3 meters), provided that my 18.8 mm/y. rate is valid that many years into the past… and it’s based on data from 2004 to the present. (pretty short sample period)

Anyway, that’s my take on it.

Right now we are at 2.8 meters of expansion since Trollagigar, 4.3 meters since Laki."