Earthquake swarms in West Bohemia-Vogtland and South-West Iceland: are they of similar nature?

Earthquake swarms occurred worldwide in diverse geological units, however, their origin is still unclear. West Bohemia-Vogtland represents one of the most active intraplate earthquake-swarm areas in Europe, South-West Iceland is characterized by intense interplate earthquake swarms. Both these areas exhibit high activity of crustal fluids.

We investigated earthquake swarms from W-Bohemia and from different areas in SW-Iceland: the Hengill volcanic complex, Ölfus transition zone (the edge of the SISZ), and Reykjanes Peninsula, from the perspective of their magnitude-time development, seismic moment release with time, the magnitude-frequency distribution and distribution of the inter-event times, and the space and time distribution of the foci. The aim was to determine the swarm characteristics that are dependent or vice-versa independent on the tectonic environment, and also the characteristics which should help us to distinguish more precisely earthquake swarms from mainshock-aftershock sequences.

We found that the frequency-magnitude (b-values) and inter-event-time distributions are similar for both areas, while total seismic moment release and its rate are much larger for the SW Icelandic activities compared to the W-Bohemia ones. One dominant short-term swarm phase with one or a few dominant events in which significant part of M_0tot released, is typical of the SW Icelandic swarms, whereas the W-Bohemia swarms are characterised by stepwise seismic moment release, which is manifested by several swarm phases. MFDs of the SW-Iceland swarms indicate significantly lower a-value (number of M_L > 0 evens), particularly of those on the Reykjanes Peninsula, compared to W-Bohemia swarms; it is due to the fact that considerable amount of M_0tot released in quasi-mainshocks and the rest in aftershocks; lower a-value was also found for the W-Bohemian mainshock-aftershock sequence in 2014. The W-Bohemian swarms took place in a bounded focal zone consisting of several fault segments but the SW-Icelandic swarms correspond well to tectonic structures along the Mid Atlantic Ridge. We conclude that most of the W-Bohemia earthquake swarms were series of subswarms with one or more embedded mainshock-aftershock sequences, while the SW-Icelandic swarms (particularly those on the Reykjanes Peninsula appear to be a transition between earthquake swarm and mainshock-aftershock sequence. The W-Bohemia and SW-Iceland focal zones are characterized by complex system of short, differently oriented faults/fault segments; interestingly, the W-Bohemia and some SW-Icelandic focal zones exhibit coexistence of faults susceptible to earthquake swarms and differently oriented faults predisposed to common earthquakes (mainshock-aftershocks).