Local Earthquake Tomography in the Tjornes Fracture Zone (North Iceland)

Abstract

The Tjörnes Fracture Zone (TFZ) is a transform zone that connects the Kolbeinsey Ridge (KR) to the Northern Volcanic Zone (NVZ) of Iceland. The TFZ has two main sub-parallel structures approximately oriented SE-NW: the Húsavík Flatey Fault (HFF) and the Grímsey Oblique Rift (GOR). Additionally, the KR continues to the south as the Eyjafjarðaráll Rift (ER) on the western border of the TFZ. Together these parts of the TFZ demark the Tjörnes Microplate (TM) (see Figure 1). The HFF, the GOR and the ER encompass most of the seismicity in the region (Figure 2). The seismicity of the area is characterized by the SIL earthquake catalog of the Icelandic Meteorological Office (IMO) (Bödvarsson et al., 1999;Rögnvaldsson et al., 1998).The HFF is a WNW-striking, right-lateral strike-slip fault that extends from the Þeistareykir fissure swarm in the NVZ to the southern end of the ER. The eastern part of the HFF is a set of subparallel faults located on land on the Tjörnes Peninsula. The central and western parts are located offshore (Figure 1). The ER is a pull-apart basin characterized by a semi-symmetric pattern of normal faulting on a north-striking axis Abstract Local earthquake tomography has been carried out in the Tjörnes Fracture Zone. This transform region connects the Mid-Atlantic Ridge to the Northern Volcanic Zone in Iceland in a mostly offshore area. The challenge to record seismic information in this area was the motivation for the North ICeland Experiment (NICE). Fourteen ocean-bottom seismometers and eleven on-land stations were installed in the project and operated simultaneously with the permanent Icelandic seismic network (SIL) during summer 2004. Data from the experiment were used to estimate P-and S-wave crustal velocities. Also, the gravity anomaly was derived for comparison with the tomographic results. Upper-crustal velocities are found to be relatively low in the offshore region. In particular, low velocities are mapped along the Húsavík-Flatey Fault, where a more confined negative gravity anomaly and a sedimentary basin are found. Low velocities are also mapped along the Grímsey Oblique Rift and in a zone connecting these two main lineaments. The northern half of the aseismic Grímsey Shoal appears as a fast anomaly. Furthermore, localized high-velocity anomalies are found beneath northern Tröllaskagi and Flateyjarskagi Peninsulas, where bedrock dates from Upper and Middle Miocene (10-15 Ma). Regions of low Vp/Vs ratios are mapped at depth along the main lineaments. Low velocities along the lineaments are interpreted as due to fracturing extending into the middle crust, while high velocities in the upper crust beneath Tertiary formations are associated with relic volcanoes. Low Vp/Vs variations along the lineaments are interpreted as due to the presence of supercritical fluids.

Physics-based dynamic rupture models, fault interaction and ground motion simulations for the segmented Húsavík-Flatey Fault Zone, Northern Iceland

“…We first construct a highly segmented model of the HFFZ (Figure 1b, Model-A), consisting of 55 partially intersecting, non-planar vertical faults, each also intersecting with the complex bathy-topography of the free surface. We integrate data from highresolution bathymetry interpretation, o shore seismic reflection campaigns in Northern Iceland (Magnúsdóttir & Brandsdóttir, 2011;Magnúsdóttir et al, 2015;Hjartardóttir et al, 2016) and relocated seismicity (Abril et al, 2018(Abril et al, , 2019 (Figure 1a). We assume vertical faults which is supported by the depth distribution of the recently relocated HFFZlocal seismicity from 1993 to 2019 considering the lateral variability of the crustal structure (Abril et al, 2018(Abril et al, , 2019(Abril et al, , 2021.…”

“…We integrate data from highresolution bathymetry interpretation, o shore seismic reflection campaigns in Northern Iceland (Magnúsdóttir & Brandsdóttir, 2011;Magnúsdóttir et al, 2015;Hjartardóttir et al, 2016) and relocated seismicity (Abril et al, 2018(Abril et al, , 2019 (Figure 1a). We assume vertical faults which is supported by the depth distribution of the recently relocated HFFZlocal seismicity from 1993 to 2019 considering the lateral variability of the crustal structure (Abril et al, 2018(Abril et al, , 2019(Abril et al, , 2021. Seismicity, initially located based on the recorded data by the permanent Icelandic -SIL network, has been relocated with a 3D tomographic velocity model of the study region.…”

“…We do not account for but discuss the e ect of additional local variations of seismogenic depth. We embed all fault systems in the same 3D velocity model that was used for seismicity relocation (Abril et al, 2019(Abril et al, , 2021 and use attenuation factors Q s = 50V s (V s in km/s) and Q p = 2Q s , following the empirical relations in Olsen et al (2009).…”

“…In this study, we develop a suite of 3D spontaneous dynamic earthquake rupture scenarios, based on varying levels of fault geometrical complexity and segmentation and varying hypocenter locations. We account for regional 3D subsurface structure, bathymetry (Abril et al, 2018(Abril et al, , 2019 and red stars mark the locations of historic large earthquakes (Stefansson et al, 2008). The red lines show the complex segmented fault traces of the Húsavík-Flatey fault zone (HFFZ) (Halldórsson, 2019) (Magnúsdóttir & Brandsdóttir, 2011;Magnúsdóttir et al, 2015;Hjartardóttir et al, 2016).…”

“…(a) Map of the Tjörnes Fracture Zone (TFZ). White dots show relocated earthquakes from 1993 to 2019(Abril et al, 2018(Abril et al, , 2019 and red stars mark the locations of historic…”