Seismicity of the Hengill area, SW Iceland: Details revealed by catalog relocation and collapsing

Inflation-Deflation Episodes in the Hengill and Hrómundartindur Volcanic Complexes, SW Iceland

“…This depth coincides broadly with the brittleductile boundary of the Hengill area, estimated using maximum earthquake depth, approximately 6-7 km below the surface, thinning towards the western end of the Reykjanes Peninsula and thickening through the eastern part of the SISZ (Foulger, 1995;Li et al, 2019). This 6 year uplift episode was associated with new surface fractures (Clifton et al, 2002) and intense seismic activity (with earthquakes reaching M L > 5) in the whole area of Hengill (Sigmundsson et al, 1997;Feigl et al, 2000;Li et al, 2019;Parameswaran et al, 2020;Blanck et al, 2020). A seismic tomography study of the 1993-1999 uplifting volume from Tryggvason et al (2002), shows a lower ratio of P-to S-wave velocities in its estimated source location, which is interpreted as the presence of supercritical fluids at these depths rather than a large partially melted magmatic body.…”

“…A simple point source of pressure within an elastic halfspace model (Mogi, 1958) was used by Feigl et al (2000) to constrain a source at 7 ± 1 km depth with a volume increase of 3.9 × 10 6 m 3 per year, which was located approximately 3 km north of the town of Hveragerði (Figure 1). This depth coincides broadly with the brittleductile boundary of the Hengill area, estimated using maximum earthquake depth, approximately 6-7 km below the surface, thinning towards the western end of the Reykjanes Peninsula and thickening through the eastern part of the SISZ (Foulger, 1995;Li et al, 2019). This 6 year uplift episode was associated with new surface fractures (Clifton et al, 2002) and intense seismic activity (with earthquakes reaching M L > 5) in the whole area of Hengill (Sigmundsson et al, 1997;Feigl et al, 2000;Li et al, 2019;Parameswaran et al, 2020;Blanck et al, 2020).…”

“…A seismic tomography study of the 1993-1999 uplifting volume from Tryggvason et al (2002), shows a lower ratio of P-to S-wave velocities in its estimated source location, which is interpreted as the presence of supercritical fluids at these depths rather than a large partially melted magmatic body. Recent seismic and resistivity studies seem to agree with the findings of the aforementioned seismic tomography (Jousset et al, 2011;Gasperikova et al, 2015;Li et al, 2019). The integration of resistivity models based on electromagnetic data and seismic tomography highlights a layer with low electric resistivity anomaly and high Vp/Vs at 2.5-4 km depth, which possibly extends deeper (Jousset et al, 2011;Árnason et al, 2010).…”

“…The optimum depth associated with the pennyshaped crack model is ∼11 km, deeper than the point source model (∼6-7 km). This estimated depth is shallower than the inferred Mohorovičić discontinuity (∼14-15 km; Weir et al, 2001) but deeper than the inferred brittle-ductile boundary in the area (Foulger, 1995;Li et al, 2019), indicating a possible lower-crustal deformation source.…”

“…The 2017-2018 source of inflation and the 2006-2017 contracting source in eastern Hengill (Results and Table 2; Juncu et al, 2017), appear to be in proximity (∼3 km NNW) of the point source estimated for the uplift episode in 1993-1999, at a depth of approximately 5-7 km (Figure 4; Sigmundsson et al, 1997;Feigl et al, 2000). This depth range is near or within the inferred brittle-ductile transition zone of the area (Foulger, 1995;Li et al, 2019). The inferred 2017-2018 inflating and 2006-2017 contracting sources are located under Ölkelduháls, between the Hengill, Hrómundartindur and Grensdalur volcanic systems (Figure 4 and Figure 7).…”