It appears that operations of Orkuveita Reykjavíkur are creating the earthquake swarms that have been seen in Hengill volcanoo. But as part of operations in Hengill geothermal plant they pump down cold water into the bed rock under considerable pressure. This is according to Sigþrúður Ármannsdóttir a geologist at Icelandic Met Office. But Vísir.is quoted here on this earthquakes swarms in Hengill volcano.
This type of earthquake swarms are going to repeat them self for as long Orkuveita Reykjavíkur is pumping down cold water into the bed rock in Hengill volcano. As the water creates a pressure change in the surrounding rock.
The most intresting earthquakes that happened as because of this swarm where 3.8 km NE of Hveragerði. But the size of that earthquake was ML2.6 (automatic size). and it was considerable distance from Hengill volcano and appears to have been triggered by the earthquake swarm in Hengill volcano (unclear however and just my own observation).
News about this.
Orkuveitan framkallar jarðskjálfta í Henglinum (Vísir.is, Icelandic)