Jökull


Jökull - 01.01.2015, Page 57

Jökull - 01.01.2015, Page 57
Seismicity beneath Þeistareykir, NE-Iceland wave reflected phase off the crust-mantle boundary at approximately 23 km depth. Figure 1a shows moment-tensor solutions for three deep events. Solution 1 (16–17 km depth b.s.l., seismograms in Figure 2) may represent a closing crack. However, if we include additional uncertainty in the source inversion, the solution pattern resembles a double-couple solution with one nodal plane closely aligned with the north-south direction. Solution 2 (15 km depth b.s.l.) seems to represent an opening crack with a significant non-double-couple component. The third solution (9 km depth b.s.l.), albeit constrained by limited azimuthal coverage, suggests a double-couple source mechanism with one nodal plane oriented ap- proximately in the NE-SW direction. We estimated the b-value within the area shown in Figure 1a using the formula by Aki (1965) and Bender (1983), which is based on the magnitude-frequency distribution of earthquakes. We included data from the IMO catalogue between 2009–2014. b-values be- low 1 (0.76±0.13) are estimated and representative for normal crust and volcanic areas. Similar values were found elsewhere along the Icelandic rift zone (e.g. Panzera et al., 2016). Here, magnitude binwidths of up to 0.5 were tested, the magnitude of complete- ness determined by the maximum curvature method and data prior to 2009 disregarded due to the network sparsity. The center of the shallow earthquake cluster is located at the west-northwestern part of the Bæjar- fjall mountain at 2–5 km depth b.s.l. and just out- side the limit of highly altered rocks mapped at the surface (Ármannsson, 2014). The shallow earthquake cluster correlates with seismicity reported by the SIL network (Hjaltadóttir and Vogfjörð, 2011) and tem- porary network (Vogfjörð, 2000). A relatively sharp base of the seismicity and spatial event clustering has also been found in Krafla and Askja beneath geother- mal fields and above velocity anomalies indicating the presence of melt (Schuler et al., 2015; Mitchell et al., 2013). We also interpret the shallow events beneath Þeistareykir to be linked to a shallow heat source, where earthquakes might be triggered by hot circulating fluids propagating along faults. For some shallow and well-constrained events, we performed source inversions to see whether they fit the double- couple (shear-fault) source model. In Figure 1b, we show three double-couple examples all with one nodal plane closely aligned in NE-SW direction following the fissure swarm. Results of the TEM/MT inversions by Karlsdóttir et al. (2012) do not show a low-resistivity anomaly directly beneath our shallow earthquake cluster at the west-northwestern part of Bæjarfjall, which would have supported our interpretation that a shallow heat source and hot geothermal fluids cause the increased seismicity there. The closest low-resistivity anomaly imaged lies beneath the southeastern part of Bæjar- fjall at 5–6 km b.s.l. The area of deep earthquakes is located outside the TEM/MT study area. CONCLUSIONS Microearthquakes located around Þeistareykir using a local seismic array reveal that the seismicity is con- centrated at 2–5 km depth b.s.l. beneath the northwest- ern end of Bæjarfjall mountain. Earthquakes south- east of Þeistareykir are scattered over a large area, at 8–20 km depth b.s.l. They are either linked with high strain rates or reduced normal friction caused by melt movement within the lower crust. ACKNOWLEDGEMENTS Seismometers of the Cambridge array were bor- rowed from the Natural Environment Research Coun- cil SEIS-UK under loan 891. The Icelandic Meteo- rological Office kindly provided additional data from stations DIM, GHA, GHS, KVO, MEL, REN, and SKI. Funding was provided to RSW by a grant from the Natural Environment Research Council. JS grate- fully acknowledges support by the Swiss National Science Foundation. We thank David Pugh for help- ing to calculate fault plane and moment tensor solu- tions. Generic Mapping Tools (Wessel et al., 2013) were used to make the figures and the Python frame- work ObsPy (Beyreuther et al, 2010) was used to ana- lyze our data. Constructive comments by two anony- mous reviewers helped improve the paper. JÖKULL No. 65, 2015 57
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