Dynamic earthquake rupture scenarios and physics-based seismic hazard assessment for the segmented Húsavík–Flatey fault zone, North Iceland

Abstract:

The Húsavík–Flatey fault (HFF) network is one of the seismically most active zones in Iceland. This complex fault system crosses from off-shore to on-shore directly underneath the town of Húsavík and consists of multiple right-lateral strike slip segments distributed across ~100 km. The HFF hosted several historical earthquakes with M>6. The moment it accumulated since last major earthquakes (two M6.5 events in 1872) may result in an earthquake of magnitude 6.8 to 7 (Metzger & Jonsson, 2014), posing a high seismic risk to Húsavík’s community, flourishing tourism and heavy industry. However, seismic hazard assessment (SHA) remains challenging.

In this study, we use the open-source software package SeisSol (www.seissol.org) for 3-D spontaneous dynamic rupture scenarios across the HFF system, accounting for newly inferred fault geometries (Einarsson et al., 2019), 3-D subsurface structure (Abril et al., 2020), bathymetry and topography of the area, primary stress orientations and the stress shape ratio constrained by the inversion of earthquake focal mechanisms (Ziegler et al., 2016). The earthquake scenarios include viscoelastic attenuation, the possibility of off-fault plastic yielding (Wollherr et al., 2019) and fault roughness.

We here explore rupture scenarios of different magnitudes and varying hypocenter locations, and analyze the corresponding ground motions towards physics-based hazard maps. Using the epicenters of historical large events on the HFF, we reproduce comparable magnitude events controlled by spontaneous fault interaction in terms of dynamic and static stress transfer and rupture jumping across the complex fault network. In our models, localized supershear rupture is often observed due to fault orientation changes and segment gaps. Our models also reveal highly heterogeneous ground shaking intensity, in particular, intense localization of shaking in the vicinity of fault geometric complexities, such as fault bends.

Observationally constrained dynamic rupture scenarios and associated ground shaking can contribute to the SHA in the HFF region, specifically in a probabilistic sense. Hundreds of dynamic simulations will run to build the rupture and ground shaking database of mechanically plausible events, with the linked rupture probability estimation to perform PSHA.