Description: An earthquake-induced stress drop on a megathrust instigates different responses on the upper plate and slab. We mimic homogenous and heterogeneous megathrust interfaces at the laboratory scale to monitor the strain relaxation on two elastically bi-material plates by establishing analog velocity weakening and neutral materials. A sequential elastic rebound follows the coseismic shear-stress drop in our elastoplastic-frictional models: a fast rebound of the upper plate and the delayed and smaller rebound on the elastic belt (model slab). A combination of the rebound of the slab and the rapid relaxation (i.e., elastic restoration) of the upper plate after an elastic overshooting may accelerate the relocking of the megathrust. This acceleration triggers/antedates the failure of a nearby asperity and enhances the early slip reversal in the rupture area. Hence, the trench-normal landward displacement in the upper plate may reach a significant amount of the entire interseismic slip reversal and speeds up the stress build-up on the upper plate backthrust that emerges self-consistently at the downdip end of the seismogenic zones. Moreover, the backthrust switches its kinematic mode from a normal to reverse mechanism during the coseismic and postseismic stages, reflecting the sense of shear on the interface.

Description: Upper-plate normal faults along forearcs often accumulate slip during 〉Mw 6 earthquakes. Such normal faults traverse the forearc of the Hellenic Subduction System (HSS) in Greece and are the focus of this study. Here, we use detailed field-mapping and analysis of high-resolution Digital Elevation Models (DEMs) to study 42 active normal faults on the islands of Kythira and Antikythira in the Aegean Sea. Onshore fault kinematic data are complemented by seabed bathymetry mapping of ten offshore faults that extend along the Kythira-Antikythira Strait (KAS). We find that normal faults in the KAS have lengths of ∼1–58 km and scarps ranging in height from 1.5 m to 2.8 km, accommodating, during the Quaternary, trench-orthogonal (NE-SW) extension of ∼2.46 ± 1.53 mm/a. Twenty-eight of these faults have ruptured since the Last Glacial Maximum, with their postglacial (16 ± 2 ka) displacement rates (0.19–1.25 mm/a) exceeding their Quaternary (≤0.7–3 Ma) rates (0.03–0.37 mm/a) by more than one order of magnitude. Rate variability, which is more pronounced on short (〈8 km) faults, is thought to arise due to temporally clustered paleoearthquakes on individual KAS faults. When displacement accumulation is considered across the entire onshore fault network, rate variability between the two time-intervals examined decreases significantly (2.79 ± 0.41 vs 1.29 ± 0.99 mm/a), a feature that suggests that earthquake clustering in the KAS may occur over ≤16 ka timescales.

Description: Cataclasites are a characteristic rock type found in drill cores from active faults as well as in exposed fossil subduction faults. Here, cataclasites are commonly associated with evidence for pervasive pressure solution and abundant hydro­ fracturing. They host the principal slip of regular earthquakes and the family of so­called slow earthquakes (episodic slip and tremor, low to very low frequency earthquakes, etc.). Slip velocities associated with the formation of the different types of cataclasites and conditions controlling slip are poorly constrained both from direct observations in nature as well as from experimental research. In this study, we explore exposed sections of subduction faults and their dominant microstructures. We use recently proposed constitutive laws to estimate deformation rates, and we compare predicted rates with instrumental observations from subduction zones. By identifying the maximum strain rates using fault scaling relations to constrain the fault core thickness, we find that the instrumental shear strain rates identified for the family of “slow earthquakes” features range from 10−3s−1 to 10−5s−1. These values agree with estimated rates for stress corrosion creep or brittle creep possibly controlling cataclastic deformation rates near the failure threshold. Typically, pore­fluid pressures are suggested to be high in subduction zones triggering brittle deformation and fault slip. However, seismic slip events causing local dilatancy may reduce fluid pressures promoting pressure­solution creep (yielding rates of 〈10−8 to 10−12s−1) during the interseismic period in agreement with dominant fabrics in plate interface zones. Our observations suggest that cataclasis is controlled by stress corrosion creep and driven by fluid pressure fluctuations at near­lithostatic effective pressure and shear stresses close to failure. We posit that cataclastic flow is the dominant physical mechanism governing transient creep episodes such as slow slip events (SSEs), accelerating preparatory slip before seismic events, and early afterslip in the seismogenic zone.

Description: Several decades of field, geophysical, analogue, and numerical modeling investigations have enabled documentation of the wide range of tectonic transport processes in accretionary wedges, which constitute some of the most dynamic plate boundary environments on Earth. Active convergent margins can exhibit basal accretion (via underplating) leading to the formation of variably thick duplex structures or tectonic erosion, the latter known to lead to the consumption of the previously accreted material and eventually the forearc continental crust. We herein review natural examples of actively underplating systems (with a focus on circum-Pacific settings) as well as field examples highlighting internal wedge dynamics recorded by fossil accretionary systems. Duplex formation in deep paleo–accretionary systems is known to leave in the rock record (1) diagnostic macro- and microscopic deformation patterns as well as (2) large-scale geochronological characteristics such as the downstepping of deformation and metamorphic ages. Zircon detrital ages have also proved to be a powerful approach to deciphering tectonic transport in ancient active margins. Yet, fundamental questions remain in order to understand the interplay of forces at the origin of mass transfer and crustal recycling in deep accretionary systems. We address these questions by presenting a suite of two-dimensional thermo-mechanical experiments that enable unravelling the mass-flow pathways and the long-term distribution of stresses along and above the subduction interface as well as investigating the importance of parameters such as fluids and slab roughness. These results suggest the dynamical instability of fluid-bearing accretionary systems causes either an episodic or a periodic character of subduction erosion and accretion processes as well as their topographic expression. The instability can be partly deciphered through metamorphic and strain records, thus explaining the relative scarcity of paleo–accretionary systems worldwide despite the tremendous amounts of material buried by the subduction process over time scales of tens or hundreds of millions of years. We finally stress that the understanding of the physical processes at the origin of underplating processes as well as the forearc topographic response paves the way for refining our vision of long-term plate-interface coupling as well as the rheological behavior of the seismogenic zone in active subduction settings.

Description: The behavior of the shallow portion of the subduction zone, which generates the largest earthquakes and devastating tsunamis, is still insufficiently constrained. Monitoring only a fraction of a single megathrust earthquake cycle and the offshore location of the source of these earthquakes are the foremost reasons for the insufficient understanding. The frictional-elastoplastic interaction between the megathrust interface and its overlying wedge causes variable surface strain signals such that the wedge strain patterns may reveal the mechanical state of the interface. To contribute to this understanding, we employ Seismotectonic Scale Modeling and simplify elastoplastic megathrust subduction to generate hundreds of analog seismic cycles at a laboratory scale and monitor the surface strain signals over the model's forearc over high to low temporal resolutions. We establish two compressional and critical wedge configurations to explore the mechanical and kinematic interaction between the shallow wedge and the interface. Our results demonstrate that this interaction can partition the wedge into different segments such that the anelastic extensional segment overlays the seismogenic zone at depth. Moreover, the different segments of the wedge may switch their state from compression/extension to extension/compression domains. We highlight that a more segmented upper plate represents megathrust subduction that generates more characteristic and periodic events. Additionally, the strain time series reveals that the strain state may remain quasi-stable over a few seismic cycles in the coastal zone and then switch to the opposite mode. These observations are crucial for evaluating earthquake-related morphotectonic markers and short-term interseismic time-series of the coastal regions.

Description: Geodetic, seismological, gravimetric, and geomorphic proxies have widely been used to understand the behavior of the shallow portion of subduction megathrusts and answer questions related to seismic asperities: Where are they located, and how large are they? How close are they to failure, and how strong are they coupled? Our current knowledge of the kinematics and dynamics of megathrust earthquakes is limited due to their offshore location, and that our observations only cover a fraction of one megathrust earthquake cycle. The frictional-elastoplastic interaction between the interface and its overriding wedge causes variable surface strain signals such that the wedge strain pattern may reveal the mechanical state of the interface. We here contribute to this discussion using observations and interpretations of controlled analog megathrust experiments highlighting the variability of deformation signals in subduction zones. To examine the interaction, we investigate seismotectonic scale models representing a seismically heterogenous interface and capture the model’s surface displacements by employing a “laboratory-geodetic” method with high spatio-temporal resolution. Our experiments generate physically self‐consistent, analog megathrust earthquake ruptures over multiple seismic cycles at laboratory scale to study the interplay between short-term elastic and long-term permanent deformation. Our results demonstrate that frictional-elastoplastic interaction partitions the upper plate into a trench-parallel and -perpendicular strain domain, experiencing opposite strain (contraction vs. extension) during the co- and interseismic phase of the seismic cycle. Moreover, the pattern differs in the off- and onshore segments of the upper plate. This implies that the seismic potential of the shallow (offshore) portion of the megathrust may be underrepresented if only onshore observations are included in the estimate. However, our models suggest that, in the case of strong frictional contrast (velocity weakening vs. strengthening) on the interface, the short-term, onshore strain pattern (dominated by elastic deformation) may suffice to map the frictional heterogeneity of the shallow interface along strike. Finally, the frictional heterogeneity of the shallow interface is well reflected by the permanent surface strain observed offshore and partially in the strain observed at the coastal region. The observed along-trench segmentation predicted by our models is reasonably compatible with short-term, elastic geodetic observations and permanent geomorphic features in nature.

Description: Large earthquakes impose differential stresses in the crust and upper mantle that are transiently relaxed during the postseismic phase mostly due to afterslip on the fault interface, viscoelastic relaxation in the lower crust and upper mantle, and poroelastic rebound in the upper crust. During the last years, the wealth of geophysical and geodetic observations, as well as great effort in forward and inverse modelling have allowed a better comprehension of the role of these mechanisms during the postseismic period. However, it is still an open question to what extent postseismic processes contribute to the surface deformation signal, especially during the early postseismic period. In this study, we use GNSS and InSAR observations collected in the first 48 days following the 2010 Maule earthquake in Chile along with a model approach that integrates afterslip, poroelasticity, and temperature-controlled power-law (non-linear viscosity) rheology. The afterslip distribution is obtained from a geodetic data inversion after removing the poro-viscoelastic component by forward modelling to the geodetic data. We find that our model approach explains the geodetic cumulative signal 14% better than a pure elastic model inverting for afterslip. This improvement is mainly produced by the better fit to the geodetic signal at the volcanic and back-arc regions due to the inclusion of non-linear viscoelastic processes, which can explain 〉 60% of the observed surface displacements in these regions. We also show that poroelastic processes play a significant role locally, specifically near the region where the coseismic slip was largest. Here, poroelastic processes explain most of the cumulative observed GNSS uplift signal and produce surface landward patterns that affect the horizontal GNSS component by up to 15% in the opposite direction. If poroelastic processes are ignored, our results reveal that the resulting afterslip amplitude is both amplified and suppressed by up to 40% in regions of ~50 x 50 km2. Our findings have implications for the calculation of the postseismic slip budget, and therefore the seismic hazard assessment of future earthquakes.

Description: Megathrust earthquakes impose changes of differential stress and pore pressure in the lithosphere-asthenosphere system that are transiently relaxed during the postseismic period primarily due to afterslip, viscoelastic and poroelastic processes. Especially during the early postseismic phase, however, the relative contribution of these processes to the observed surface deformation is unclear. To investigate this, we use geodetic data collected in the first 48 days following the 2010 Maule earthquake and a poro-viscoelastic forward model combined with an afterslip inversion. This model approach fits the geodetic data 14% better than a pure elastic model. Particularly near the region of maximum coseismic slip, the predicted surface poroelastic uplift pattern explains well the observations. If poroelasticity is neglected, the spatial afterslip distribution is locally altered by up to ±40%. Moreover, we find that shallow crustal aftershocks mostly occur in regions of increased postseismic pore-pressure changes, indicating that both processes might be mechanically coupled.