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The Hellenic Arc is an active seismogenic zone in the Mediterranean Sea that hosted at least two historical M≥8 earthquakes, which both caused destructive tsunamis. The low-angle geometry of its subduction interface could promote shallow slip amplification, enhancing seafloor displacement. Long-term seismic-probabilistic tsunami hazard assessment (S-PTHA, e.g., Scala et al., 2020) and early warning systems typically rely on kinematic models and Okada's analytical solution to compute static seafloor displacements. The static displacement is then used to source tsunami models. However, the complex interaction of earthquake dynamics and tsunami-genesis may not be fully captured.We recently demonstrated mechanically consistent dynamic rupture models in generic megathrust settings informed from long-term geodynamic modeling that can provide building blocks toward integrating physics-based dynamic rupture modeling in Probabilistic Tsunami Hazard Analysis (Wirp et al., 21). We here present a range of 3D multi-physics, high-resolution dynamic rupture subduction earthquake scenarios accounting for the complex slab geometry of the Hellenic Arc. We vary hypocenter locations, which leads to a wide range of rupture speeds, extent of shallow fault slip, and moment magnitudes. Our dynamic rupture models include highly resolved bathymetry and topography data and detailed knowledge of the tectonic structure of the Hellenic Arc (seismic velocity structure, stresses, and strengths). We use the slab geometry from the European Database of Seismogenic Faults (EDSF, Basili et al., 2013) to create a 3D dynamic rupture scenario that covers great parts of the Mediterranean Sea. The initial conditions in our models are constrained on the subduction zone scale (Ulrich et al., 2021) and specified for the Hellenic Arc region.Only part of the Hellenic Arc is fully seismically coupled (e.g., Laigle et al., 2004) and most of the convergence is assumed to occur as aseismic creep. We follow Ramos et al. (2021) and apply different friction parameters accounting for high or low coupling of the plate interface.Our modeling suggests that margin-wide rupture would yield an Mw 9.3 earthquake. More reasonable smaller magnitude earthquakes are obtained by increasing the along-arc complexity of the reference model. Different hypocenter locations result in remarkable differences in shallow fault slip penetrating into velocity-strengthening regions, which translate into strong variations of the final seafloor displacement across scenarios. In additional models with partially consolidated and totally unconsolidated sediments (Ulrich et al., 2021) we show that off-fault plastic yielding, which limits shallow fault slip, may drastically increase the seafloor uplift. Finally, we explore a novel 3D fully coupled earthquake-tsunami modeling approach (Lotto and Dunham, 2018; Krenz et al., 2021) by adding a water layer to the modeling domain. This enables simulating earthquake dynamics, acoustic waves, and the resulting tsunami simultaneously. The fully coupled model will capture the dynamics of the entire tsunami-genesis in a single simulation, overcoming typical approximations for standard earthquake-tsunami coupling workflows. We envision that mechanically consistent dynamic rupture models can provide building blocks toward combined, self-consistent, and physics-based Seismic and Tsunami Hazard Analysis.

With this contribution, we expand the discussion of effects that earthquakes induced by geo-energy projects can have on local communities, and that should probably be considered in future legislation or permitting processes. Inspired by consistent reports of felt and heard disturbances associated with the weeks-long stimulation of a 6-km-deep geothermal system in 2018 below the Otaniemi district of Espoo, Helsinki, we conduct numerical simulations of wave propagation in the solid earth and the atmosphere to assess the sensitivity of the ground shaking and audible noise patterns to various parameters. We explore the effects of three different local velocity models, realistic topography, variations of the source mechanism, and earthquake size on the loudness of the synthetic waves at frequencies up to 20 Hz, therefore reaching the lower limit of human sound sensitivity. We discuss the results of 18 elastic-acoustic coupled scenario simulations conducted on the Mahti high-performance computing infrastructure of the Finnish IT Center for Science CSC using the SeisSol wavefield solver. The computationally challenging simulations target the Otaniemi case study, i.e., we discretize a 12 km x 12 km x 15 km domain with a 2 km thick air layer over the solid earth domain. The earthquake point source is located at the 6.5 km deep location of the largest M1.8 event induced by the stimulation. In the target central area, we use a mesh with element lengths of about 14 m in the air and 97 m in the solid earth. Inside each element, we approximate the solution by a fifth-degree polynomial, by which we achieve a resolution of roughly 2.3 m in the air and 16 m in the earth. We develop an interactive visualization to facilitate instant access to the results governed by the different parameter combinations, where the synthetics are shown on top of a map of the Helsinki metropolitan region. This tool facilitates “what-if” analyses by quickly comparing the effects of fault orientation, source mechanism, and the velocity model. This supports effective communication of physics-based nuisance analysis to decision-makers and stakeholders, not only in environments such as the case study where there is little experience with natural earthquake phenomena. Together, these results resolve for the first time synthetic nuisance sound patterns at the 50 – 100 m scale in a densely populated capital region. The study highlights the mostly disregarded spatially variable audible effects that can negatively impact the public attitude towards geothermal stimulations, even if the ground shaking limits are safe, and it provides first estimates of the resources needed for comprehensive scenarios for future stimulation projects.

Earthquake rupture dynamic models capture the variability of slip in space and time while accounting for the structural complexity which is characteristic for subduction zones. The use of a geodynamic subduction and seismic cycling (SC) model to initialize dynamic rupture (DR) ensures that initial conditions are self-consistent and reflect long-term behavior. We extend the 2D geodynamical subduction and SC model of van Zelst et al. (2019) and use it as input for realistic 3-dimensional DR megathrust earthquake models. We follow the subduction to tsunami run-up linking approach described in Madden et al. (2020), including a complex subduction setup along with their resulting tsunamis. The distinct variation of shear traction and friction coefficients with depth lead to realistic average rupture speeds and dynamic stress drop as well as efficient tsunami generation. We here analyze a total of 14 subduction-initialized 3D dynamic rupture-tsunami scenarios. By varying the hypocentral location along arc and depth, we generate 12 distinct unilateral and bilateral earthquakes with depth-variable slip distribution and directivity, bimaterial, and geometrical effects in the dynamic slip evolutions. While depth variations of the hypocenters barely influence the tsunami behavior, lateral varying nucleation locations lead to a shift in the on-fault slip which causes time delays of the wave arrival at the coast. The fault geometry of our DR model that arises during the SC model is non-planar and includes large-scale roughness. These features (topographic highs) trigger supershear rupture propagation in up-dip or down-dip direction, depending on the hypocentral depth.In two additional scenarios, we analyze variations in the energy budget of the DR scenarios. In the SC model, an incompressible medium is assumed (ν=0.5) which is valid only for small changes in pressure and temperature. Unlike in the DR model where the material is compressible and a different Poisson’s ratio (ν=0.25) has to be assigned. Poisson’s ratios between 0.1 and 0.4 stand for various compressible materials. To achieve a lower shear strength of all material on and off the megathrust fault and to facilitate slip, we increase the Poisson ratio in the DR model to ν=0.3 which is consistent with basaltic rocks. As a result, larger fault slip is concentrated at shallower depths and generates higher vertical seafloor displacement and earthquake moment magnitude respectively. Even though the tsunami amplitudes are much higher, the same dynamic behavior as in the twelve hypocenter-variable models can be observed. Lastly, we increase fracture energy by changing the critical slip distance in the linear slip-weakening frictional parameterization. This generates a tsunami earthquake (Kanamori, 1972) characterized by low rupture velocity (on average half the amount of s-wave speed) and low peak slip rate, but at the same time large shallow fault slip and moment magnitude. The shallow fault slip of this event causes the highest vertical seafloor uplift compared to all other simulations. This leads to the highest tsunami amplitude and inundation area while the wavefront hits the coast delayed compared to the other scenarios.

From interpreting data to scenario modeling of subduction events, numerical modeling has been crucial for studying tsunami generation by earthquakes. Seafloor instruments in the source region featu...

3D imaging reveals striking along-trench structural variations of subduction zones world-wide (e.g., Han et al, JGR 2018). Subduction zones include basins, sediments, splay and back-thrusting faults that evolve over a large time span due to tectonic processes, and may crucially affect earthquake dynamics and tsunami genesis. Such features should be taken into account for realistic hazard assessment. Numerical modeling bridges time scales of millions of years of subduction evolution to seconds governing dynamic earthquake rupture, as well as spatial scales of hundreds of kilometers of megathrust geometry to meters of an earthquake rupture front.Recently, an innovative framework linking long-term geodynamic subduction and seismic cycle models to dynamic rupture models of the earthquake process and seismic wave propagation at coseismic timescales was presented (van Zelst et al., JGR 2019). This workflow was extended in a simple test case to link the 2D seismic cycle model to a three-dimensional earthquake rupture mode, which was then linked to a tsunami model (Madden et al., EarthArxiv, doi:10.31223/osf.io/rzvn2). Here, we couple a 2D seismic cycle model to 3D earthquake and tsunami models and assess the geophysical aspects of this coupling. We extract all 2D material properties, stresses and the strength of the megathrust, and its geometry, from the seismic cycling model at a time step right before a typical megathrust event to use as initial conditions for the 3D dynamic rupture models. We explore the effects of along-arc variations of megathrust curvature, sediment content, and closeness to failure of the wedge on earthquake dynamics by studying the effects on slip, rupture velocity, stress drop and seafloor deformation.In a next step, the dynamic seafloor displacements are linked to tsunami simulations that use depth-integrated (hydrostatic) shallow water equations. This approach efficiently models wave propagations and large-scale horizontal flows. We also present novel, fully coupled 3D dynamic rupture-tsunami simulations (Krenz et al., AGU19; Abrahams et al., AGU19; Lotto and Dunham et al., 2015, Computational Geosciences) which solve simultaneously for the solid earth and ocean response, taking gravity into account via a modified free surface boundary condition.Earthquake rupture modeling and the fully-coupled tsunami modeling utilize SeisSol (www.seissol.org), a flagship code of the ChEESE project (www.cheese-coe.eu). SeisSol is an open source software package using unstructured tetrahedral meshes that are optimally suited for the complex geometries of subduction zones. The here presented links between geodynamic subduction and seismic cycling model with earthquake dynamics and tsunami models better account for the complexity of subduction zones and help evaluate the effects of along arc heterogeneities on earthquake and tsunami behavior and advance physics-based assessments of earthquake-tsunami hazards.

ExaHyPE (“An Exascale Hyperbolic PDE Engine”) is a software engine for solving systems of first-order hyperbolic partial differential equations (PDEs). Hyperbolic PDEs are typically derived from the conservation laws of physics and are useful in a wide range of application areas. Applications powered by ExaHyPE can be run on a student’s laptop, but are also able to exploit thousands of processor cores on state-of-the-art supercomputers. The engine is able to dynamically increase the accuracy of the simulation using adaptive mesh refinement where required. Due to the robustness and shock capturing abilities of ExaHyPE’s numerical methods, users of the engine can simulate linear and non-linear hyperbolic PDEs with very high accuracy. Users can tailor the engine to their particular PDE by specifying evolved quantities, fluxes, and source terms. A complete simulation code for a new hyperbolic PDE can often be realised within a few hours — a task that, traditionally, can take weeks, months, often years for researchers starting from scratch. In this paper, we showcase ExaHyPE’s workflow and capabilities through real-world scenarios from our two main application areas: seismology and astrophysics.

Tsunami generation by offshore earthquakes is a problem of scientific interest and practical relevance, and one that requires numerical modeling for data interpretation and hazard assessment. Most numerical models utilize two-step methods with one-way coupling between separate earthquake and tsunami models, based on approximations that might limit the applicability and accuracy of the resulting solution. In particular, standard methods focus exclusively on tsunami wave modeling, neglecting larger amplitude ocean acoustic and seismic waves that are superimposed on tsunami waves in the source region. In this study, we compare four earthquake-tsunami modeling methods. We identify dimensionless parameters to quantitatively approximate dominant wave modes in the earthquake-tsunami source region, highlighting how the method assumptions affect the results and discuss which methods are appropriate for various applications such as interpretation of data from offshore instruments in the source region. Most methods couple a 3D solid Earth model, which provides the seismic wavefield or at least the static elastic displacements, with a 2D depth-averaged shallow water tsunami model. Assuming the ocean is incompressible and tsunami propagation is negligible over the earthquake duration leads to the instantaneous source method, which equates the static earthquake seafloor uplift with the initial tsunami sea surface height. For longer duration earthquakes, it is appropriate to follow the time-dependent source method, which uses time-dependent earthquake seafloor velocity as a forcing term in the tsunami mass balance. Neither method captures ocean acoustic or seismic waves, motivating more advanced methods that capture the full wavefield. The superposition method of Saito et al. (2019) solves the 3D elastic and acoustic equations to model the seismic wavefield and response of a compressible ocean without gravity. Then, changes in sea surface height from the zero-gravity solution are used as a forcing term in a separate tsunami simulation, typically run with a shallow water solver. A superposition of the earthquake and tsunami solutions provides an approximation to the complete wavefield. This method is algorithmically a two-step method. The complete wavefield is captured in the fully-coupled method, which utilizes a coupled solid Earth and compressible ocean model with gravity (Lotto & Dunham, 2015). The fully-coupled method, recently incorporated into the 3D open-source code SeisSol, simultaneously solves earthquake rupture, seismic waves, and ocean response (including gravity). We show that the superposition method emerges as an approximation to the fully-coupled method subject to often well-justified assumptions. Furthermore, using the fully-coupled method, we examine how the source spectrum and ocean depth influence the expression of oceanic Rayleigh waves. Understanding the range of validity of each method, as well as its computational expense, facilitates the selection of modeling methods for the accurate assessment of earthquake and tsunami hazards and the interpretation of data from offshore instruments.