Abstract Subduction is substantially multiscale process where the stresses are built by long‐term tectonic motions, modified by sudden jerky deformations during earthquakes, and then restored by following multiple relaxation processes. Here we develop a cross‐scale thermomechanical model aimed to simulate the subduction process from 1 min to million years' time scale. The model employs elasticity, nonlinear transient viscous rheology, and rate‐and‐state friction. It generates spontaneous earthquake sequences and by using an adaptive time step algorithm, recreates the deformation process as observed naturally during the seismic cycle and multiple seismic cycles. The model predicts that viscosity in the mantle wedge drops by more than three orders of magnitude during the great earthquake with a magnitude above 9. As a result, the surface velocities just an hour or day after the earthquake are controlled by viscoelastic relaxation in the several hundred km of mantle landward of the trench and not by the afterslip localized at the fault as is currently believed. Our model replicates centuries‐long seismic cycles exhibited by the greatest earthquakes and is consistent with the postseismic surface displacements recorded after the Great Tohoku Earthquake. We demonstrate that there is no contradiction between extremely low mechanical coupling at the subduction megathrust in South Chile inferred from long‐term geodynamic models and appearance of the largest earthquakes, like the Great Chile 1960 Earthquake.
Abstract The magnitude of earthquakes on continental normal faults rarely exceeds 7.0 Mw. However, because of their vicinity to large population centers they can be highly destructive. Long recurrence time, relatively small deformations, and limited observations hinder our understanding of the deformation patterns and mechanisms controlling the magnitude of events. Here, this problem is addressed with 2D thermomechanical modeling of normal fault seismic cycles. The 2020 Samos, Greece Mw7.0 earthquake is used as an example as it is one of the largest and most studied continental normal fault earthquakes. The modeling approach employs visco‐elasto‐plastic rheology, compressibility, free surface, and a rate‐and‐state friction law for the fault. Modeling of the Samos earthquake suggests the pore fluid pressure ratio on the fault ranges from 0 to 0.7. The model demonstrates that most of the deformation during interseismic and coseismic periods, besides on the fault, occurs in the hanging wall and footwall below the seismogenic part of the fault. The largest vertical surface displacement during the earthquake is the subsidence of the hanging wall in the vicinity of the fault, while the uplift of the footwall and remote part of the hanging wall is significantly smaller. Modeling of the seismic cycles on normal faults with different setups shows the dependency of the magnitude on the thermal profile and dipping angle of the fault; low heat flow and low dipping angle are favorable conditions for the largest events, while steep normal faults in the areas of high heat flow tend to have the smallest magnitudes.
Abstract Giant earthquakes with magnitudes above 8.5 occur only in subduction zones. Despite the developments made in observing large subduction zone earthquakes with geophysical instruments, the factors controlling the maximum size of these earthquakes are still poorly understood. Previous studies have suggested the importance of slab shape, roughness of the plate interface contact, state of the strain in the upper plate, thickness of sediments filling the trenches, and subduction rate. Here, we present 2‐D cross‐scale numerical models of seismic cycles for subduction zones with various geometries, subduction channel friction configurations, and subduction rates. We found that low‐angle subduction and thick sediments in the subduction channel are the necessary conditions for generating giant earthquakes, while the subduction rate has a negligible effect. We suggest that these key parameters determine the maximum magnitude of a subduction earthquake by controlling the seismogenic zone width and smoothness of the subduction interface. This interpretation supports previous studies that are based upon observations and scaling laws. Our modeling results also suggest that low static friction in the sediment‐filled subduction channel results in neutral or moderate compressive deformation in the overriding plate for low‐angle subduction zones hosting giant earthquakes. These modeling results agree well with observations for the largest earthquakes. Based on our models we predict maximum magnitudes of subduction earthquakes worldwide, demonstrating the fit to magnitudes of all giant earthquakes of the 20th and 21st centuries and good agreement with the predictions based on statistical analyses of observations.
Abstract This paper presents the time and space evolution of crustal deformation and their respective sedimentary infill of the 600 km wide, asymmetric conjugate rifted margin of the Santos–Benguela basins. Based on a geoseismic transect obtained with interpretation of long-offset seismic reflection and tied by wells, we interpret six main synrift unconformities, corresponding to different deformation phases processed from the Valanganian to Early Albian. Confined by these unconformities, sedimentary growths with progressively young relative ages towards the boundary with the oceanic crust are interpreted as evidence of oceanward rift migration. The combination of this information with crustal structure derived from long-offset seismic reflection illuminating the deep crust of the Santos–Benguela conjugate margins, resulted in a complete view of sedimentary infill, internal compartments, and crustal structure. These data were used to guide a dynamic model of rifting resulting in a simulated lithospheric section. We show that the margin architecture can be explained by the combination of an early, protracted phase of distributed deformation, followed by basinward rift migration. Distributed deformation lasted from the Valanginian to Early Aptian (135–117 Ma), initiating with isolated lakes that later coalesced into a wide basin-scale lake (>450 km). From the Mid Aptian to Early Albian (117–110 Ma), rift migration formed the main structural compartments and unconformities, as well as the distal hinge zones we observe today in the seismic lines. During this time, the inner proximal margins were left behind to thermally subside, whereas outer proximal and distal margins were tectonically active. Coexistence of these two processes explains the enigmatic simultaneous formation of proximal sag-like geometries, with late synrift accumulation of a salt layer up to 3 km thick, with tectonically active faults in the distal margin, promoting crestal block uplift that could explain the deposition of Late Aptian, shallow water, pre-salt carbonate rocks.
<p>Rifts and rifted margins result from interaction of several physical processes, which produce a range of crustal structures, subsidence histories, and sedimentary architectures. Study of these processes in academia and industry includes kinematic modelling (i.e. cross-section restoration, backstripping) combined with simple thermomechanical models and dynamic modelling. In kinematic models, the thinning of the lower crust and mantle is kinematically imposed in the form of pure shear, which contradicts natural non-linear viscous behavior. Although, kinematic modelling can provide a crustal thinning profile, heatflow estimates, subsidence rates etc., imposed extension of the lower crust and mantle might strongly impact the result. On the other hand, a dynamic approach allows to model the whole range of possible physical processes, but it cannot be used to model particular extension histories.<br>Here, we show a new modelling technique, namely KineDyn, to combine the advantages of the above-mentioned approaches into a single modelling framework. Our method employs full non-linear visco-elasto-plastic rheology, surface process of erosion and sediment transport, decompression melting of the mantle, and serpentinization of mantle rocks. Faults are introduced as weak planes in the upper crust, in order to simulate faulting during the model run. In our approach, faults are initially controlled by prescribed initial locations, offsets and timings, while the rest of the model is resolved in a fully dynamic mode. Since fault planes are much weaker than the surrounding upper crust, extension of the model naturally leads to slip on the faults. We demonstrate that faults modelled this way reproduce a natural behavior, including rotation due to flexure and unloading of the fault plane. <br>In order to reconstruct the evolution of an existing rift or rifted margin we model extension of the lithosphere with controlled faulting. To do this we use the interpreted spatio-temporal evolution of the faulting from a seismic profile to guide the evolution of the dynamic model. After a trial-and-error process, where we correct the faults&#8217; locations, the thicknesses of layers, surface process&#8217;s parameters, initial thermal gradient etc., we obtain the model that best fits the observations. Thus, KineDyn gives, in effect, the same results as existing section restoration techniques (i.e. the potential history of faulting) and forward modeling techniques (i.e. the likely history of sedimentation, thinning, heat flow and subsidence), while simultaneously taking into account non-linear interactions between processes occurring during rifting. <br>In this work we show the methodology, examples, tests and benchmarks of the technique. Finally, we present applications of KineDyn for the following rifts and rifted margins: Malawi Rift, East African Rift System, hyper-extended West Iberia Margin, and ultra-wide Santos-Benguela Rifted Margin.</p>
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