Description: Large magnitude (Mw ∼ ≥6) earthquakes in extensional settings are often associated with simultaneous rupture of multiple normal faults as a result of static and/or dynamic stress transfer. Here, we report details of the coseismic breaching of a previously unrecognized large-scale fault relay zone in central Greece, through three successive normal fault earthquakes of moderate magnitude (Mw 5.7–6.3) that occurred over a period of ∼10 days in March 2021. Specifically, joint analysis of InSAR, GNSS and seismological data, coupled with detailed field and digital fault mapping, reveals that the Tyrnavos Earthquake Sequence (TES) was accommodated at the northern end of a ∼100 km wide transfer structure, by faults largely unbroken during the Holocene. By contrast, the southern section of this relay zone appears to have accrued significant slip during Holocene. InSAR-derived displacements agree with the loci of eight subtle, previously undetected, faults that accommodated coseismic and/or syn-seismic normal fault slip during the TES. Kinematic modeling coupled with fault mapping suggests that all involved faults are interconnected at depth, with their conjugate fault-intersections acting largely as barriers to coseismic rupture propagation. We also find that the TES mainshocks were characterized by unusually high (〉6 MPa) stress-drop values that scale inversely with rupture length and earthquake magnitude. These findings, collectively suggest that the TES propagated north-westward to rupture increasingly stronger asperities at fault intersections, transferring slip between the tips of a well-established, but previously unrecognized, relay structure. Fault relay zones may be prone to high stress-drop earthquakes and associated elevated seismic hazard.

Description: Along the Northern Chilean active continental margin, the subducting Nazca plate is characterized by a rough sea floor topography that has been suggested to control the rupture behaviour of megathrust earthquakes. However, there is still debate of what structures exactly controlled the extent of the rupture of the Mw 8.12014 April 1st Iquique earthquake and why it only broke 1/3 of a large seismic gap that last ruptured completely in 1877. To better understand the seismotectonic segmentation of the northern Chilean convergent margin, we use datasets from different geophysical and geodetic studies in this area to produce a 3D model. We combine depth migrated images of the two northernmost multi-channel seismic reflection CINCA’95 (Crustal Investigations off- and onshore Nazca Plate/Central Andes) lines, bathymetry data, coseismic slip models, geodetic coupling, seismic b values, relocated seismic events and the morphology of the subduction interface from gravity modelling. The interface morphology shows a prominent surface relief that spacially correlates with the rupture process of the mainshock on April 1st and also for the largest aftershock on April 3rd. The main slip area exhibits a strong correlation with a large elongated topographic depression of the subducting slab. An elongated topographic high on the subducting plate to the south of that depression correlates with low pre-seismic locking and very likely acted as a barrier for rupture propagation for the main shock, as well as for the largest after shock. A subducted circular topographic high of 25 km in diameter located updip of the rupture area, possibly prevented coseismic slip to rupture all the way up to the trench axis. Thus, our observations support that subducting sea floor morphology plays an important role controlling rupture processes.

Description: This data set includes digital image correlation data from analog earthquakes experiments. The data consists of grids of surface strain and time series of surface displacement (horizontal and vertical) and strain. The data have been derived using a stereo camera setup and processed with LaVision Davis 10 software. Detailed descriptions of the experiments and results regarding the surface pattern of the strain can be found in Kosari et al. (in review), to which this data set is supplementary. We use an analog seismotectonic scale model approach (Rosenau et al., 2019 and 2017) to generate a catalog of analog megathrust earthquakes (Table 1). The presented experimental setup is modified from the 3D setup used in Rosenau et al. (2019) and Kosari et al. ( 2020). The subduction forearc model wedge is set up in a glass-sided box (1000 mm across strike, 800mm along strike, and 300 mm deep) with a dipping, elastic basal conveyor belt and a rigid backwall. An elastoplastic sand-rubber mixture (50 vol.% quartz sandG12: 50 vol.% EPDM rubber) is sieved into the setup representing a 240 km long forearc segment from the trench to the volcanic arc. The shallow part of the wedge includes a basal layer of sticky rice grains characterized by unstable stick-slip sliding representing the seismogenic zone. Stick-slip sliding in rice is governed by a rate-and-state dependent friction law similar to natural rocks. According to Coulomb wedge theory (Dahlen et al., 1984), two types of wedge configurations have been designed: a “compressional” configuration represents an interseismically compressional and coseismically stable wedge (compressional configuration), and a “critical” configuration, which is interseismically stable (close to critically compressional) and may reach a critical extensional state coseismically (critical configuration). In the compressional configuration, a flat-top (surface slope α=0) wedge overlies a single large rectangular in map view stick-slip patch (Width*Length=200*800 mm) over a 15-degree dipping basal thrust. In the critical configuration, the surface angle of the elastoplastic wedge varies from the coastal segment onshore (α=10) to the inner-wedge offshore (α=15) segments over a 5-degree dipping basal thrust. Slow continuous compression of the wedge by moving the basal conveyor belt at a speed velocity of 0.05 mm/s simulates plate convergence and results in the quasi-periodic nucleation of quasi-periodic stick-slip events (analog earthquakes) within the rice layer. The wedge responds elastically to these basal slip events, similar to crustal rebound during natural subduction megathrust earthquakes.

Description: This data set includes data derived from high-speed surface displacement observations from analog earthquake experiments. The data consists of surface displacement of the experiment upper plate and slab, slip distribution, and grids of Coulomb Failure Stress (CFS). The surface displacement observations have been captured using a highspeed CMOS (Complementary Metal Oxide Semiconductor) camera (Phantom VEO 640L camera, 12 bit) and processed with LaVision Davis 10 software. Description of the experiments and results regarding the surface displacement observation, Slip distribution, and CFS can be found in Kosari et al. (2022), to which this data set is supplementary. We use an analog seismotectonic scale model approach (Rosenau et al., 2019 and 2017) to generate a catalog of analog megathrust earthquakes. The presented experimental setup is modified from the 3D setup used in Rosenau et al. (2019) and Kosari et al. ( 2020 and 2022). The subduction forearc model wedge is set up in a glass-sided box (1000 mm across strike, 800mm along strike, and 300 mm deep) with a dipping, elastic basal conveyor belt, and a rigid backwall. An elastoplastic sand-rubber mixture (50 vol.% quartz sandG12: 50 vol.% EPDM rubber) is sieved into the setup representing a 240 km long forearc segment from the trench to the volcanic arc. The shallow part of the wedge includes a basal layer of sticky rice grains characterized by unstable stick-slip sliding representing the seismogenic zone. The Stick-slip sliding in rice is governed by a rate-and-state dependent friction law similar to natural rocks. A flat-top (surface slope α=0) wedge overlies rectangular stick-slip patch/es over a 15-degree dipping basal thrust. Two different seismic configurations of the shallow part of the wedge base (the megathrust) represent the depth extent of the seismogenic zone in nature. In the first configuration (homogeneous configuration), a single large rectangular stick-slip patch (Width*Length=200*800 mm) is implemented as the main slip patch (MSP). In the second case (heterogeneous configuration), two square-shaped MSPs (200*200mm) have been emplaced, acting as two medium-size seismogenic asperities surrounded by a salt matrix hosting frequent small events. Slow continuous compression of the wedge by moving the basal conveyor belt at a speed velocity of 0.05 mm/s simulates plate convergence and results in the quasi-periodic nucleation of quasi-periodic stick-slip events (analog earthquakes) within the sticky-rice layer. The wedge responds elastically to these basal slip events, similar to crustal rebound during natural subduction megathrust earthquakes.

Description: Understanding the along-strike seismogenic behavior of megathrusts is crucial to anticipating seismic hazards in subduction zones. However, if and how spatiotemporal frictional heterogeneity (high and low kinematic coupling) at depth feeds back into the upper-plate deformation pattern and how the upper-plate elastic signals and permanent records may correlate have yet to be fully understood. Hence, we mimic subduction megathrust seismic cycles using an analog seismotectonic model of an elastoplastic wedge overlying a frictionally heterogeneous megathrust. Coseismically, the zone above the down-dip limit of the aseismic and seismogenic patches undergoes extension and contraction, respectively, while the strain state shows a switch in polarity from coseismic to interseismic. The down-dip limit of the creeping zone produces permanent along-strike extension or contraction, depending on the frictional barrier strength. Our experiments show that the frictional locking heterogeneity generates more segmented along-strike strain patterns elastically (short term) than permanently (long term). Moreover, our results suggest that along-strike upper-plate strain patterns could serve as a proxy for interpreting persistent lateral variations of seismogenic behavior in subduction megathrusts.

Description: This data set is digital image correlation data, including surface displacement and strain data from laboratory subduction megathrust earthquake cycles. The data consists of grids of surface strain (elastic and permanent), trench-normal surface displacement, vorticity and divergence maps over analog seismic cycles, and time series of surface displacement. The data have been derived using a stereo camera setup and processed with LaVision Davis 10 software. Detailed descriptions of the experiments and results regarding the surface pattern of the strain can be found in Kosari et al. (2023), to which this data set is supplementary. We use three configurations to mimic the along-strike heterogeneous spatiotemporal distribution of frictional locking (Rosenau et al., 2019; Kosari et al., 2022b). A central patch separates two stick-slip zones as an aseismic barrier in all configurations. The frictional properties of the central patch vary as a velocity-strengthening (VS configuration), a velocity-neutral (VN), and a velocity-weakening (VW configuration). The VW zone generates smaller slip events with a higher frequency (i.e., recurrence interval) than the stick-slip zones. Four frictionally different materials have been emplaced on the interface: The sticky-rice as velocity-weakening material (a-blt;0) resulting in stick-slip cycles simulating earthquake cycles, fine-grained sugar and rubber-sand mixture as velocity-strengthening (a-bgt;0) and velocity-neutral (a-b=0) material, and fine-grained salt as velocity-weakening material (a-blt;0) (Kosari et al., 2023).

Description: The Kaikōura Earthquake uplifted Kaikōura Peninsula by ≤∼1 m. Uplift in 2016 mainly resulted from slip on an offshore thrust fault (OSTF), modelled to splay from the plate-interface, and was further influenced by slip on two newly identified faults (Armers Beach Fault, ABF; Te Taumanu Fault, TTF) mapped onshore from differential lidar (D-lidar). Forward dislocation modelling indicates that 2016 peninsula uplift can be reproduced by mean slip of ∼2.3 m on the OSTF and 0.25–0.5 m on the ABF and TTF. The variable co-seismic uplift recorded during the 2016 earthquake differs from the near-uniform (1.2 ± 0.2°) northwest tilting of MIS5c (96 ± 5 ka) and MIS5e (123 ± 5 ka) marine terraces; these ages are constrained by Optically Stimulated Luminescence (OSL) dating and correlation to sea-level curves. Tilting of Late Quaternary marine terraces can be primarily reproduced by slip rates of ∼0.8–2.7 mm/yr on the OSTF and 0.3–0.6 mm/yr on the ABF. Slip on the TTF is not required to produce tilting of the marine terraces, suggesting that it may have ruptured less frequently than the OSTF and ABF in the Late Quaternary. The OSTF links 2016 ruptures north and south of Kaikōura, with the earthquake rupturing an interconnected network of faults.

Description: The present dataset is a comprehensive earthquake catalogue for the Northern Chile subduction zone forearc covering the period 2007-2021, determined from IPOC seismic station data (GFZ and CNRS-INSU 2006; https://doi.org/10.14470/pk615318) plus some auxiliary stations (IPOC = Integrated Plate Boundary Observatory Chile; http://www.ipoc-network.org). The method of automatized earthquake catalogue retrieval, the different relocation steps as well as the different earthquake class labels, and the structures outlined by the seismicity are described in detail in Sippl et al. (2023). The catalogue builds on the one from Sippl et al. (2018; https://doi.org/10.5880/GFZ.4.1.2018.001), but uses a slightly deviating parameter set and a new event category. The columns of the data files are: year, month, day, hour, minute, second, latitude [dec. degrees], longitude [dec. degrees], depth [km], magnitude [ML], identifier The identifier term provides a first-order spatial classification of the seismicity, an explanation is given in Sippl et al. (2023).

Description: Slow-slip events (SSEs), although widely recorded in various convergent margins globally, only recently have been reported in the Eastern Mediterranean, with one of them triggering the 2018 ~M7 Zakynthos Earthquake along the western Hellenic Subduction System (HSS). Here, we explore the distribution, size and duration of SSEs along the HSS and assess their importance in subduction-related strain accumulation and release. To achieve this, we analyse geodetic timeseries from a dense network of permanent GNSS stations on Western Peloponnese, Crete and surrounding islands that collectively span a time-period of ~10 years. We use greedy linear regression techniques to estimate displacement trajectory models for each station and thus we identify transient displacement signals, associated with aseismic processes at depth. To further constrain the spatial extent and size of the SSEs we invert the GNSS transient displacements for variable distributed slip at depth and we, therefore, discuss likely scenarios of aseismic and seismic strain distribution (and partitioning) within the HSS’s complex plate-interface zone.