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ABSTRACT CyberShake is a high-performance computing workflow for kinematic fault-rupture and earthquake ground-motion simulation developed by the Statewide California Earthquake Center to facilitate physics-based probabilistic seismic hazard assessment (PSHA). CyberShake exploits seismic reciprocity for wave propagation by computing strain green tensors along fault planes, which in turn are convolved with rupture models to generate surface seismograms. Combined with a faultwide hypocentral variation of each simulated rupture, this procedure allows for generating ground-motion synthetics that account for realistic source variability. This study validates the platform’s kinematic modeling of physics-based seismic wave propagation simulations in Southwest Iceland as the first step toward migrating CyberShake from its original study region in California. Specifically, we have implemented CyberShake workflows to model 2103 fault ruptures and simulate the corresponding two horizontal components of ground-motion velocity on a 5 km grid of 625 stations in Southwest Iceland. A 500-yr-long earthquake rupture forecast consisting of 223 hypothetical finite-fault sources of Mw 5–7 was generated using a physics-based model of the bookshelf fault system of the Southwest Iceland transform zone. For each station, every reciprocal simulation uses 0–1 Hz Gaussian point sources polarized along two horizontal grid directions. Comparison of the results in the form of rotation-invariant synthetic pseudoacceleration spectral response values at 3, 4, and 5 s periods are in good agreement with the Icelandic strong motion data set and a suite of empirical Bayesian ground-motion prediction equations (GMPEs). The vast majority of the physics-based simulations fall within one standard deviation of the mean GMPE predictions, previously estimated for the area. At large magnitudes for which no data exist in Iceland, the synthetic data set may play an important role in constraining GMPEs for future applications. Our results comprise the first step toward comprehensive and physics-based PSHA for Southwest Iceland. 

Abstract. Tsunamigenic earthquakes pose considerable risks, both economically and socially, yet earthquake and tsunami hazard assessments are typically conducted separately. Earthquakes associated with unexpected tsunamis, such as the 2018 Mw  7.5 strike-slip Sulawesi earthquake, emphasize the need to study the tsunami potential of active submarine faults in different tectonic settings. Here, we investigate physics-based scenarios combining simulations of 3D earthquake dynamic rupture and seismic wave propagation with tsunami generation and propagation. We present time-dependent modeling of one-way linked and 3D fully coupled earthquakes and tsunamis for the ∼ 100 km long Húsavík–Flatey Fault Zone (HFFZ) in North Iceland. Our analysis shows that the HFFZ has the potential to generate sizable tsunamis. The six dynamic rupture models sourcing our tsunami scenarios vary regarding hypocenter location, spatiotemporal evolution, fault slip, and fault structure complexity but coincide with historical earthquake magnitudes. Earthquake dynamic rupture scenarios on a less segmented fault system, particularly with a hypocenter location in the eastern part of the fault system, have a larger potential for local tsunami generation. Here, dynamically evolving large shallow fault slip (∼ 8 m), near-surface rake rotation (± 20∘), and significant coseismic vertical displacements of the local bathymetry (± 1 m) facilitate strike-slip faulting tsunami generation. We model tsunami crest to trough differences (total wave heights) of up to ∼ 0.9 m near the town Ólafsfjörður. In contrast, none of our scenarios endanger the town of Akureyri, which is shielded by multiple reflections within the narrow Eyjafjörður bay and by Hrísey island. We compare the modeled one-way linked tsunami waveforms with simulation results using a 3D fully coupled approach. We find good agreement in the tsunami arrival times and location of maximum tsunami heights. While seismic waves result in transient motions of the sea surface and affect the ocean response, they do not appear to contribute to tsunami generation. However, complex source effects arise in the fully coupled simulations, such as tsunami dispersion effects and the complex superposition of seismic and acoustic waves within the shallow continental shelf of North Iceland. We find that the vertical velocity amplitudes of near-source acoustic waves are unexpectedly high – larger than those corresponding to the actual tsunami – which may serve as a rapid indicator of surface dynamic rupture. Our results have important implications for understanding the tsunamigenic potential of strike-slip fault systems worldwide and the coseismic acoustic wave excitation during tsunami generation and may help to inform future tsunami early warning systems. 

Abstract The Húsavík‐Flatey Fault Zone (HFFZ) is the largest strike‐slip fault in Iceland and poses a high seismic risk to coastal communities. To investigate physics‐based constraints on earthquake hazards, we construct three fault system models of varying geometric complexity and model 79 3‐D multi‐fault dynamic rupture scenarios in the HFFZ. By assuming a simple regional prestress and varying hypocenter locations, we analyze the rupture dynamics, fault interactions, and the associated ground motions up to 2.5 Hz. All models account for regional seismotectonics, topo‐bathymetry, 3‐D subsurface velocity, viscoelastic attenuation, and off‐fault plasticity, and we explore the effect of fault roughness. The rupture scenarios obey earthquake scaling relations and predict magnitudes comparable to those of historical events. We show how fault system geometry and segmentation, hypocenter location, and prestress can affect the potential for rupture cascading, leading to varying slip distributions across different portions of the fault system. Our earthquake scenarios yield spatially heterogeneous near‐field ground motions modulated by geometric complexities, topography, and rupture directivity, particularly in the near‐field. The average ground motion attenuation characteristics of dynamic rupture scenarios of comparable magnitudes and mean stress drop are independent of variations in source complexity, magnitude‐consistent and in good agreement with the latest regional empirical ground motion models. However, physics‐based ground motion variability changes considerably with fault‐distance and increases for unilateral compared to bilateral ruptures. Systematic variations in physics‐based near‐fault ground motions provide important insights into the mechanics and potential earthquake hazard of large strike‐slip fault systems, such as the HFFZ.