We present 3-D spontaneous dynamic rupture earthquake scenarios for the Húsavík–Flatey Fault Zone (HFFZ) in Northern Iceland. We construct three fault system models consisting of up to 55 segments of varying geometric complexity. By varying hypocenter locations, we analyze rupture dynamics, fault interactions and their associated ground motions and observational uncertainties in 79 scenarios. We use regional observations to constrain 3-D subsurface velocities and viscoelastic attenuation as well as fault stress and strength. Our models account for topo-bathymetry, off-fault plasticity and we explore the effect of fault roughness. Our spontaneous dynamic rupture scenarios can match historic magnitudes. We show that the fault system segmentation and geometry, hypocenter locations, initial stress conditions and fault roughness have strong effects on multi-fault rupture dynamics across the HFFZ. Breaking of different portions of the same fault system leads to varying rupture dynamics, slip distributions and magnitudes. All dynamic rupture scenarios yield highly heterogeneous near-field ground motions. We observe amplification from rupture directivity, geometric complexities, and amplification and shielding due to topography. We recover a magnitude-consistent attenuation relationship in good agreement with new regional empirical ground motion models. Physics-based ground motion variability changes with distance and increases for unilateral vs. bilateral rupture. Our study illustrates important ingredients for fully physics-based, regional earthquake scenarios, their respective importance for rupture dynamics and ground motion modeling and how they can be observationally constrained and verified. We entail that dynamic rupture scenarios can be useful for non-ergodic probabilistic seismic hazard assessment, specifically in data-limited regions.

Local earthquake tomography has been carried out in the Tjornes Fracture Zone. This transform region connects the Mid-Atlantic Ridge with 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 Bouguer 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 Husavik-Flatey Fault, where a more confined negative gravity anomaly and a sedimentary basin are found. Low velocities are also mapped along the Grimsey Oblique Rift and in a zone connecting these two main lineaments north of Skjalfandi Bay. The northern half of the aseismic Grimsey Shoal appears as a fast anomaly. Furthermore, localized high-velocity anomalies are found beneath northern Trolaskagi and Flateyjarskagi Peninsulas, where bedrock dates from Upper and Middle Miocene (10-15 Ma). Regions of low Vp/Vs ratio 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 fast upper-crustal velocities beneath Tertiary formations are associated with relic volcanoes. Low Vp/Vs ratios along the lineaments are interpreted as due to the presence of supercritical fluids.