Description: Currently, it is unknown how seismic and aseismic slip influences the recurrence and magnitude of earthquakes. Modern seismic hazard assessment is therefore based on statistics combined with numerical simulations of fault slip and stress transfer. To improve the underlying statistical models we conduct low velocity shear experiments with glass micro‐beads as fault gouge analogue at confining stresses of 5–20 kPa. As a result, we show that characteristic slip events emerge, ranging from fast and large slip to small scale oscillating creep and stable sliding. In particular, we observe small scale slip events that occur immediately before large scale slip events for a specific set of experiments. Similar to natural faults we find a separation of scales by several orders of magnitude for slow events and fast events. Enhanced creep and transient dilatational events pinpoint that the granular analogue is close to failure. From slide‐hold‐slide tests, we find that the rate‐and‐state properties are in the same range as estimates for natural faults and fault rocks. The fault shows velocity weakening characteristics with a reduction of frictional strength between 0.8% and 1.3% per e‐fold increase in sliding velocity. Furthermore, the slip modes that are observed in the normal shear experiments are in good agreement with analytical solutions. Our findings highlight the influence of micromechanical processes on macroscopic fault behavior. The comprehensive data set associated with this study can act as a benchmark for numerical simulations and improve the understanding of observations of natural faults.

Description: Plain Language Summary: Earthquakes occur when two continental plates slide past each other. The motion is concentrated at the interface of the two plates which is called a fault. In many cases the fault is filled with granular material, called gouge, that supports the pressure between the plates. Therefore, the properties of this gouge determine how fast and how large an earthquake can be. It also has an influence on the time between earthquakes. In our study, we examine a simplified version of a fault gouge in a simple small‐scale model. Instead of rock material we use glass beads and measure how different conditions affect the motion of the model. We find that our model reproduces features of fault gouge because it shows similar behavior. When there is no motion our model fault becomes stronger with a rate equal to fault gouge. Also, the type of strengthening is analogous to fault gouge. During slip, the glass beads become weaker as the slip velocity increases in a similar manner as in natural faults. These results improve the understanding of computer simulations and natural observations.

Description: Key Points: Slip modes in granular gouge are akin to natural fault slip. Glass beads are a suitable granular analogue for fault gouge and show rate‐and‐state dependent friction. Enhanced creep and small scale events are signals for imminent failure and indicate fault criticality.

Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659

Description: 亥姆霍兹联合会致力, Helmholtz‐Zentrum Potsdam ‐ Deutsches GeoForschungsZentrum GFZ (GFZ) http://dx.doi.org/10.13039/501100010956

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: The 250 km long profile 3B/MVE (East) was recorded in 1990 as part of the joint seismic reflection venture DEKORP 1990-3/MVE (Muenchberg-Vogtland-Erzgebirge) between the two former German Republics shortly before their unification. The aim of DEKORP 1990-3/MVE was to explore the structure of the crust from the Rhenish Shield through the Bohemian Massif to the Ore Mountains. The entire profile consists of DEKORP 3A, DEKORP 3B/MVE (West) and its prolongation to the east DEKORP 3B/MVE (East). Its total length amounts to about 600 km. 24 short seismic cross lines and associated 3D blocks with single fold coverage were also recorded. The seismic survey of 3B/MVE (East) was conducted to investigate the deep crustal structure of the Saxothuringian Zone of the Central European Variscian Belt along the northern margin of the Bohemian Massif with high-fold near-vertical incidence vibroseis acquisition. The main objectives were to image the deep structures of the Muenchberg Gneiss Complex, to concern the volume of Variscan granites by combining seismic and gravity data as well as to determine the origin and nature of the deep regional NW-trending fault systems. Details of the experiment, preliminary results and interpretations were published by DEKORP Research Group (B) et al. (1994) and Förste, Lück & Schulze (1994). The Technical Report of DEKORP 3B/MVE (East) gives complete information about acquisition and processing parameters. The European Variscides, extending from the French Central Massif to the East European Platform, originated during the collision between Gondwana and Baltica in the Late Palaeozoic. Due to involvement of various crustal blocks in the orogenesis, the mountain belt is subdivided into distinct zones. The external fold-and-thrust belts of the Rhenohercynian and Saxothuringian as well as the predominantly crystalline body of the Moldanubian dominate the central European segment of the Variscides. Polyphase tectonic deformation, magmatism and metamorphic processes led to a complex interlinking between the units. The 3B/MVE (East) line runs in SW-NE direction along the southern margin of the Saxothuringian belt from the Franconian Line, the southwestern boundary fault zone of the Bohemian Massif, to the Lausitz Massif. It traverses the allochthonous Muenchberg Gneiss Complex, the Cambro-Ordovician South Vogtland Syncline Zone, the Eibenstock-Karlovy Vary Granite Complex as well as the Ore Mountains crystalline blocks, the most significant Bouguer gravity low in Central Europe. Besides, the line intersects several NW-fault systems such as the Floeha Zone fault system and the Mid Saxon fault (DEKORP Research Group (B) et al., 1994). The line 3B/MVE (East) is complemented by eight short cross lines. To the west the profile is extended by DEKORP 3B/MVE (West). In the Muenchberg Gneiss Complex the 3B/MVE (East) profile is crossed by DEKORP 4N, which runs parallel to the western border of the Bohemian Massif near the KTB drilling site. Farther to the east the line has an intersection with DEKORP 9501 (GRANU).

Description: The 187 km long line 4N was recorded in 1985 as part of the DEKORP project, the German continental seismic reflection program, and served as a basis for a network of six seismic reflection lines KTB 8501 – 8506, which were performed to investigate the planned target area for the Continental Deep Drilling Program (KTB) in the Upper Palatinate. The aim of the survey 4N was to explore the crustal structure of the central Mid-European Variscides down to the Moho and the uppermost mantle with high-fold near-vertical incidence vibroseis acquisition and, in particular, to scan the suture between the Moldanubian Zone and the northward adjacent Saxothuringian Zone. Details of the experiment, first results and interpretations were published by DEKORP Research Group (1987, 1988). The Technical Report of line 4N gives complete information about acquisition and processing parameters. The European Variscides, extending from the French Central Massif to the East European Platform, originated during the collision between Gondwana and Baltica in the Late Palaeozoic. Due to involvement of various crustal blocks in the orogenesis, the mountain belt is subdivided into distinct zones. The external fold-and-thrust belts of the Rhenohercynian and Saxothuringian as well as the predominantly crystalline body of the Moldanubian dominate the central European segment of the Variscides. Polyphase tectonic deformation, magmatism and metamorphic processes led to a complex interlinking between the units. The Saxothuringian represents the infill of a Cambro-Ordovician basin. The Moldanubian contains blocks of pre-Variscan crust and their Palaezoic cover. During the Variscan orogeny the Moldanubian crust was thrust towards the NW over the Saxothuringian foreland. Both units were welded together by a low-pressure metamorphism accompanied by polyphase deformation (DEKORP Research Group, 1987, 1988). The SE-NW striking line 4N runs along the western border of the Bohemian Massif perpendicular to the main tectonic trend (SW-NE). The profile starts in the Bavarian Forest and runs across the Upper Palatinate Forest. Shortly before the NE-trending Erbendorf Line, which separates the Moldanubian unit from the Saxothuringian unit, the profile runs through the area of the KTB drill site. In the Saxothuringian DEKORP 4N runs through the Fichtel Mountains, the Muenchberg Gneiss Complex and ends in the Franconian Forest. In the Bavarian Forest the line 4N traverses DEKORP 4Q nearly perpendicularly. Farther northwest the profile crosses KTB 8501 – 8503, which were arranged parallel to strike of the orogenic belt, as well as the DEKORP 3-D survey ISO 1989 around the KTB drill hole. In the Muenchberg Gneiss Complex the 4N profile is intersected by DEKORP 3B/MVE (East), which runs along the southern margin of the Saxothuringian belt in a SW-NE direction.