Faculty of Science, Charles University in Prague
Institute of Geophysics, The Czech Academy of Sciences
Relation of earthquakes and earthquake swarms in West Bohemia, Czech Republic and Reykjanes peninsula, Iceland Martin Labuta (1), Josef Horálek (2), Hana Čermáková (2), Jana Doubravová (2), Martin Hensch (3), and Kristín Vogfjörð (3) (1) Faculty of Science, Charles University in Prague, Czech Republic (
[email protected]), (2) Institute of Geophysics, Academy of Science, Prague, Czech Republic, (3) Icelandic Meteorological Office, Iceland
1. Introduction
4. The Reykjanes Peninsula
Earthquake swarms are characterized by series of earthquakes with several dominant shocks of similar strength. The origin of earthquake swarms is mostly a combination of tectonic stress and movement of hydrothermal fluids along the fault plane. There is a question about any similarities including b-values and the focal mechanism that could explain relations of origin of these swarms occurring in different tectonic settings - in the post-Alpine orogenic area of West Bohemia and on the active continental margin represented by the South Icelandic rift zone. This area of West Bohemia lies above intra-continental rift zone – Eger Rift and it was seismologically active in past years, including swarms in 2008, 2011, 2013 and 2014 with dominant shocks of magnitudes below ML 4.5. All these events are located in depths between 7 and 11 km and they create 8 km long focal zone with N-S orientation [1]. On the contrary, the region of Reykjanes peninsula is located above the Mid-Atlantic Ridge, which results in shallower depths between 2 to 9 km. Additionally, the earthquakes in this area are characteristic by typical swarm-like activity and by migrating along the rift in certain clusters, i.e. around Grindavík, Fagradalsfjall or Kleifarvatn.
Kleifarvatn area -> normal faulting ->
b-value 0.93
Fagradalsfjall area -> strike slip ->
b-value 0.86
Grindavík area ->
b-value 0.6
2. Methods For our study we used the Seismon software for picking events in combination with the NLLoc for locating. Firstly, we analysed data from the local seismic network REYKJANET from the last third of 2013 when a large swarm occurred there with a dominant main shock of magnitude ML 4.7 in the most South-Western part of peninsula. Secondly, we processed a swarm of maximum magnitude 4.5 recorded by the network WEBNET in WB from May 2014. From this data set we were able to define b-values using the GR law and calculate the moment seismic tensor for the focal mechanism.
3. West Bohemia Strike slip faulting with minority of events showing reverse and normal faulting. It could be caused by multiple faulting planes.
Time vs. Magnitude
b-value 0.97
5.Results and Discussion Time vs. Magnitude Firstly, we have discovered that the b-value for West Bohemia is higher (1.0) than the Icelandic one (0.8-0.9). The lowest b-value at Grindavík might be caused by non-recorded events. Secondly, accumulated stress is released during large timespan and migrates from east to west along the seismic zone in the Reykjanes peninsula. However, in WB stress is released in short series of larger earthquakes accompanied by aftershock activity.
References
European Geosciences Union General Assembly 2015
[1] Fischer, el al. (2014). Intra-continental earthquake swarms in West-Bohemia and Vogtland: A review. Tectonophysics [2] Geological map of Iceland based on data from The Icelandic Institute of Natural History, topographic map based on data from National Land Survey of Iceland, bathymetry based on data of International Bathymetric Chart of the Arctic Ocean [3] Geological map of West Bohemia based on data from Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Germany
