Showing papers on "Slab published in 2020"

Numerical investigation on load redistribution capacity of flat slab substructures to resist progressive collapse

Abstract: To study the load redistribution capacity of reinforced concrete (RC) flat slab structures subjected to a middle column loss scenario, high fidelity finite element (FE) models were built using commercial software LS-DYNA. The numerical models were validated by experimental results. It is found that the continuous surface cap model (CSCM) with an erosion criterion considering both the maximum principal and shear strain could effectively predict the punching shear failure at slab-column connections. The validated FE models were employed to investigate the effect of boundary conditions, amount of integrity reinforcement, and slab thickness on the load redistribution capacity of flat slab structures. Furthermore, multi-story RC flat slab substructures were built to capture the load redistribution behavior of different floors. Parametric studies indicate that ignoring the constraints from surrounding slabs may underestimate the load redistribution capacity of the flat slab substructures. Therefore, it is suggested that in future numerical or experimental studies, rigid horizontal constraints should be applied at the slab edge of the substructure to well represent the constraints from surrounding slabs. In addition, it is also found that the amount of integrity reinforcement would significantly affect the post-punching performance of flat slab structures. It is suggested that the minimum integrity reinforcement ratio should be 0.63%.

Detailed tectonic reconstructions of the Western Mediterranean region for the last 35 Ma, insights on driving mechanisms

Abstract: Slab retreat, slab tearing and interactions of slabs are first-order drivers of the deformation of the overriding lithosphere. An independent description of the tectonic evolution of the back-arc and peripheral regions is a pre-requisite to test the proposed conceptual, analogue and numerical models of these complex dynamics in 3-D. We propose here a new series of detailed kinematics and tectonic reconstructions from 35 Ma to the Present shedding light on the driving mechanisms of back-arc rifting in the Mediterranean where several back-arc basins all started to form in the Oligocene. The step-by-step backward reconstructions lead to an initial situation 35 Ma ago with two subduction zones with opposite direction, below the AlKaPeCa block (i.e. belonging to the Alboran, Kabylies, Peloritani, Calabrian internal zones). Extension directions are quite variable and extension rates in these basins are high compared to the Africa-Eurasia convergence velocity. The highest rates are found in the Western Mediterranean, the Liguro-Provencal, Alboran and Tyrrhenian basins. These reconstructions are based on shortening rates in the peripheral mountain belts, extension rates in the basins, paleomagnetic rotations, pressure-temperature-time paths of metamorphic complexes within the internal zones of orogens, and kinematics of the large bounding plates. Results allow visualizing the interactions between the Alps, Apennines, Pyrenean-Cantabrian belt, Betic Cordillera and Rif, as well as back-arc basins. These back-arc basins formed at the emplacement of mountain belts with superimposed volcanic arcs, thus with thick, hot and weak crusts explaining the formation of metamorphic core complexes and the exhumation of large portions of lower crustal domains during rifting. They emphasize the role of transfer faults zones accommodating differential rates of retreat above slab tears and their relations with magmatism. Several transfer zones are identified, separating four different kinematic domains, the largest one being the Catalan-Balearic-Sicily Transfer Zone. Their integration in the wider Mediterranean realm and a comparison of motion paths calculated in several kinematic frameworks with mantle fabric shows that fast slab retreat was the main driver of back-arc extension in this region and that large-scale convection was a subsidiary driver for the pre-8 Ma period, though it became dominant afterward. Slab retreat and back-arc extension was mostly NW-SE until ∼ 20 Ma and the docking of the AlKaPeCa continental blocks along the northern margin of Africa induced a slab detachment that propagated eastward and westward, thus inducing a change in the direction of extension from NW-SE to E-W. Fast slab retreat between 32 and 8 Ma and induced asthenospheric flow have prevented the transmission of the horizontal compression due to Africa-Eurasia convergence from Africa to Eurasia and favored instead upper-plate extension driven by slab retreat. Once slab retreat had slowed down in the Late Miocene, this N-S compression was felt and recorded again from the High Atlas to the Paris Basin.

Subduction Duration and Slab Dip

Abstract: The dip angles of slabs are among the clearest characteristics of subduction zones, but the factors that control them remain obscure. Here slab dip angles and subduction parameters, including subduction duration, the nature of the overriding plate, slab age and convergence rate, are determined for 153 transects along subduction zones for the present‐day. We present a comprehensive tabulation of subduction duration based on isotopic ages of arc initiation and stratigraphic, structural, plate tectonic and seismic indicators of subduction initiation. We present two ages for subduction zones, a long‐term age and a reinitiation age. Using cross correlation and multivariate regression, we find that: (1) subduction duration is the primary parameter controlling slab dips with slabs tending to have shallower dips at subduction zones that have been in existence longer; (2) the long‐term age of subduction duration better explains variation of shallow dip than reinitiation age; (3) overriding plate nature could influence shallow dip angle, where slabs below continents tend to have shallower dips; (4) slab age contributes to slab dip, with younger slabs having steeper shallow dips; and (5) the relations between slab dip and subduction parameters are depth‐dependent, where the ability of subduction duration and overriding plate nature to explain observed variation decreases with depth. The analysis emphasizes the importance of subduction history and the long‐term regional state of a subduction zone in determining slab dip and are consistent with mechanical models of subduction.

Geochemical Evolution of Arc and Slab Following Subduction Initiation: a Record from the Bonin Islands, Japan

Abstract: Volcanism following the initiation of subduction is vital to our understanding of this specific magma-generation environment. This setting is represented by the first development of the Izu–Bonin–Mariana arc system as subduction commenced along the Western Pacific margin in the Eocene. A new collection of volcanic rocks recovered from the islands and exposed crustal sections of the Bonin Ridge spans the first 10 Myr of arc evolution. An elemental and radiogenic isotope dataset from this material is presented in conjuction with new 40Ar/39Ar ages and a stratigraphic framework developed by a detailed mapping campaign through the volcanic sections of the Bonin Islands. The dating results reveal that both the locus and type of magmatism systematically changed with time in response to the progressive sinking of the slab until the establishment of steady-state subduction at around 7–8 Ma. Following initial mid-ocean ridge basalt (MORB)-like spreading-related basalt magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite or high-Mg andesite, then finally tholeiitic or calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominates the slab input to the shallow source of high-Si boninites at 49 Ma, but high-precision Pb isotope data show that this sediment varies in composition along the subducting plate. At around 45 Ma, volcanism switched to low-Si boninite and the pelagic sediment signature was almost entirely replaced by volcanic or volcaniclastic material originating from a HIMU ocean island source. These low-Si boninites are isotopically consistent with a slab component comprising variable proportions of HIMU volcaniclastic rocks and Pacific MORB. In turn, this signature was replaced by a Pacific MORB-dominated flux in the post 45 Ma tholeiite and calcalkaline volcanic rocks. Notably, each change in slab-derived flux coincided with a change in the magma type. Fluctuations in the slab-derived geochemical signature were superimposed on a change in the mantle wedge source from highly depleted harzburgite to a depleted MORB-type mantle-type source. In turn, this may correspond to the increasing depth of the leading edge of the slab through this 5 Myr period.

Aqueous fluids are effective oxidizing agents of the mantle in subduction zones

Abstract: Aqueous fluids produced by dehydration of the downgoing slab facilitate chemical exchange in subduction zones, but the efficiency of fluid-mediated redox transfer as a mechanism to deliver oxidized material from the slab to the sub-arc mantle remains hotly debated. Here we report the first direct measurements of the oxidation state of experimentally produced slab fluids using in situ redox sensors. Our experiments show that the dehydration of natural antigorite serpentinite at shallow subduction zone conditions (1 GPa, 800 °C) produces moderately oxidizing fluids (QFM + 2) with elevated concentrations of Na, K, Ca, and Mg. The composition and redox of the experimental fluids are then used to parameterize a thermodynamic reactive transport model to investigate the interaction of slab fluid with the sub-arc mantle from 1–4 GPa and 700–900 °C. Recently determined equation of state parameters for aqueous fluids at high pressures now enables thermodynamic modeling of aqueous fluid–rock interactions at conditions relevant to deep subduction zones for the first time. Our thermodynamic modeling demonstrates that aqueous fluid can efficiently oxidize Fe in mantle minerals via the reduction of H+ to H2 in the fluid. We estimate that < 1–3 kg of serpentinite-derived fluid at 850–900 °C is required to increase the Fe3+/ΣFe in 1 kg of sub-arc mantle from MORB-like values (0.15) to those of primitive arc basalts (0.2–0.3). We calculate that a slab fluid flux of 1.4 × 109–1.4 × 1014 kg year−1 is required to oxidize sufficient sub-arc mantle to produce the average annual flux of magmas at arcs, which overlaps with the estimated range of H2O flux in subduction zones.

Flexural behavior of basalt fiber–reinforced concrete slab strips with BFRP bars : experimental testing and numerical simulation

Abstract: This study investigated the flexural behavior of a new one-way concrete slab system reinforced longitudinally with basalt fiber–reinforced polymer (BFRP) bars and cast with basalt fiber–rei...

Control of subduction zone age and size on flat slab subduction

Abstract: Flat slab subduction is an enigmatic style of subduction where the slab attains a horizontal orientation for up to several hundred kilometers below the base of the overriding plate. It has been linked to the subduction of buoyant aseismic ridges or plateaus, but the spatial correlation is problematic, as there are subducting aseismic ridges and plateaus that do not produce a flat slab, most notably in the Western Pacific, and there are flat slabs without an aseismic ridge or plateau. In this paper an alternative hypothesis is investigated which poses that flat slab subduction is associated with subduction zones that are both old (active for a long time) and wide (large trench-parallel extent). A global subduction zone compilation is presented showing that flat slabs preferentially occur at old (> ~80-100 Myr) and wide (≥ ~6000 km) subduction zones. This is explained by the tendency for wide subduction zones to decrease their dip angle in the uppermost mantle with progressive time, especially in the center. A set of numerical subduction models confirms this behavior, showing that only the central parts of wide slabs progressively reduce their slab dip, such that slab flattening, and ultimately flat slab subduction, can occur. The models further show that a progressive decrease in slab dip angle for wide slabs leads to increased vertical deviatoric tensional stresses at the top surface of the slab (mantle wedge suction), facilitating flat slab subduction, while narrow slabs retain steep dip angles and low vertical deviatoric tensional stresses. The results provide a potential explanation why present-day flat slabs only occur in the Eastern Pacific, as only here subduction zones were old and wide enough to initiate flat slab subduction, and why Laramide flat slab subduction and South China flat slab subduction were possible in the geological past.

The Slab Puzzle of the Alpine-Mediterranean Region: Insights From a New, High-Resolution, Shear Wave Velocity Model of the Upper Mantle

Abstract: Mediterranean tectonics since the Lower Cretaceous has been characterized by a multiphase subduction and collision history with temporally and spatially variable, small ‐ scale plate con fi gurations. A new shear wave velocity model of the Mediterranean upper mantle (MeRE2020), constrained by a very large set of over 200,000 broadband (8 – 350 s), interstation, Rayleigh wave, phase velocity curves, illuminates the complex structure and fragmentation of the subducting slabs. Phase velocity maps computed using these measurements were inverted for depth ‐ dependent, shear wave velocities using a stochastic particle ‐ swarm ‐ optimization (PSO) algorithm. The resulting three ‐ dimensional (3 ‐ D) model makes possible an inventory of slab segments across the Mediterranean. Fourteen slab segments of 200 – 800 km length along ‐ strike are identi fi ed. We distinguish three categories of subducted slabs: attached slabs reaching down to the bottom of the model; shallow slabs of shorter length in downdip direction, terminating shallower than 300 km depth; and detached slab segments. The location of slab segments are consistent with and validated by the intermediate ‐ depth seismicity, where it is present. The new high ‐ resolution tomography demonstrates the intricate relationships between slab fragmentation and the evolution of the relatively small and highly curved subduction zones and collisional orogens characteristic of the Mediterranean realm.

Load Transfer Method for Energy Piles in a Group with Pile–Soil–Slab–Pile Interaction

Abstract: The behavior of an energy pile in a group is different from that of an isolated energy pile due to thermal and mechanical interactions. This study aims to extend the load transfer method to...

Distinct slab interfaces imaged within the mantle transition zone

Abstract: Oceanic lithosphere descends into Earth’s mantle at subduction zones and drives material exchange between Earth’s surface and its deep interior. The subduction process creates chemical and thermal heterogeneities in the mantle, with the strongest gradients located at the interfaces between subducted slabs and the surrounding mantle. Seismic imaging of slab interfaces is key to understanding slab compositional layering, deep-water cycling and melting, yet the existence of slab interfaces below 200 km remains unconfirmed. Here, we observe two sharp and slightly dipping seismic discontinuities within the mantle transition zone beneath the western Pacific subduction zone that coincide spatially with the upper and lower bounds of the high-velocity slab. Based on a multi-frequency receiver function waveform modelling, we found the upper discontinuity to be consistent with the Mohorovicic discontinuity of the subducted oceanic lithosphere in the mantle transition zone. The lower discontinuity could be caused by partial melting of sub-slab asthenosphere under hydrous conditions in the seaward portion of the slab. Our observations show distinct slab–mantle boundaries at depths between 410 and 660 km, deeper than previously observed, suggesting a compositionally layered slab and high water contents beneath the slab. Two seismic discontinuities in the mantle transition zone beneath the western Pacific represent subducted slab interfaces that could be the slab Moho and partially molten sub-slab asthenosphere, according to an analysis of seismic data.

Slab break-offs in the Alpine subduction zone

Abstract: After the onset of plate collision in the Alps, at 32–34 Ma, the deep structure of the orogen is inferred to have changed dramatically: European plate break-offs in various places of the Alpine arc, as well as a possible reversal of subduction polarity in the eastern Alps have been proposed. We review different high-resolution tomographic studies of the upper mantle and combine shear- and body-wave models to assess the most reliable geometries of the slabs. Several hypotheses for the tectonic evolution are presented and tested against the tomographic model interpretations and constraints from geologic and geodetic observations. We favor the interpretation of a recent European slab break-off under the western Alps. In the eastern Alps, we review three published scenarios for the subduction structure and propose a fourth one to reconcile the results from tomography and geology. We suggest that the fast slab anomalies are mainly due to European subduction; Adriatic subduction plays no or only a minor role along the Tauern window sections, possibly increasing towards the Dinarides. The apparent northward dip of the slab under the eastern Alps may be caused by imaging a combination of Adriatic slab, from the Dinaric subduction system, and a deeper lying European one, as well as by an overturned, retreating European slab.

The 2018 Mw 6.8 Zakynthos, Greece, Earthquake: Dominant Strike‐Slip Faulting near Subducting Slab

Abstract: Cite this article as Sokos, E., F. Gallovič, C. P. Evangelidis, A. Serpetsidaki, V. Plicka, J. Kostelecký, and J. Zahradník (2020). The 2018 Mw 6.8 Zakynthos, Greece, Earthquake: Dominant Strike-Slip Faulting near Subducting Slab, Seismol. Res. Lett. 91, 721–732, doi: 10.1785/0220190169. Supplemental Material With different styles of faulting, the eastern Ionian Sea is an ideal natural laboratory to investigate interactions between adjacent faults during strong earthquakes. The 2018 Mw 6.8 Zakynthos earthquake, well recorded by broadband and strong-motion networks, provides an opportunity to resolve such faulting complexity. Here, we focus on waveform inversion and backprojection of strong-motion data, partly checked by coseismic Global Navigation Satellite System data. We show that the region is under subhorizontal southwest–northeast compression, enabling mixed thrust faulting and strike-slip (SS) faulting. The 2018 mainshock consisted of two fault segments: a lowdip thrust, and a dominant, moderate-dip, right-lateral SS, both in the crust. Slip vectors, oriented to southwest, are consistent with platemotion. The sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. The 2018 event, and anMw 6.6 event of 1997, occurred near three localized swarms of 2016 and 2017. Future numerical models of the slab deformation and ocean-bottom seismometer observations may illuminate possible relations among earthquakes, swarms, and fluid paths in the region. Introduction Multiple faults acting during an earthquake have been generally well known, but on global scale less observations have been available for near-simultaneous ruptures of different faulting mechanisms, specifically in subduction zones. For example, Lay et al. (2013) reported a doublet of twoMw ∼ 7 events below Japan trench, where a thrust faulting (TF) along the subduction interface was followed after 14 s by a shallower normal faulting in the overriding plate. A rare evidence of a thrust event on a plate interface, which triggered a normal-faulting event in the overriding plate was provided for an Mw ∼ 7 earthquake in the Chile subduction zone (Hicks and Rietbrock, 2015). Particularly challenging in terms of strain partitioning are the regions where subduction terminates, and plate motions continue along transform faults. A good example is the 2016 Mw 7.8 earthquake in New Zealand, in which a dominant strike-slip (SS) faulting occurred in the upper plate and possibly triggered minor slip on the underlying subduction thrust (Mouslopoulou et al., 2019; Ulrich et al., 2019). Resolving fault complexity for strong earthquakes (Mw 6.0–6.9) in the shallowest parts of the subduction termination zones is even more challenging, and this article focuses on such a task in western Greece. The major ongoing convergent tectonic process in western Greece is subduction, imaged by seismic tomography and other structural studies (Spakman et al., 1993; Laigle et al., 2004; Suckale et al., 2009; Sachpazi et al., 2016; Halpaap et al., 2019). The active overriding of the Aegean plate over the subducting African plate, derived from Global Navigation Satellite System (GNSS) data, is oriented toward southwest, approximately perpendicular to the trench, being consistent with slip vectors of many earthquakes (Kiratzi and Louvari, 2003; Hollenstein et al., 2006; Shaw and Jackson, 2010). Zakynthos (or Zante) Island is situated at a subduction-termination zone, a part of the Ionian Islands–Akarnania Block (IAB) (Pérouse et al., 2017). At the northwest edge, this block is separated from Apulian–Ionian microplate by the right-lateral Cephalonia transform fault. The southwest boundary of IAB is the Hellenic subduction backstop front. The northeast boundary of IAB is a mixture of SS and extensional structures (e.g., the Corinth Gulf). The southeast boundary of IAB has not been well known until the 2008Mw 6.3 Movri Mountain earthquake 1. Department of Geology, Seismological Laboratory, University of Patras, Patras, Greece; 2. Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic; 3. National Observatory of Athens, Institute of Geodynamics, Athens, Greece; 4. Faculty of Mining and Geology, University of Ostrava, Ostrava, Czech Republic *Corresponding author: esokos@upatras.gr © Seismological Society of America Volume 91 • Number 2A • March 2020 • www.srl-online.org Seismological Research Letters 721 Downloaded from http://pubs.geoscienceworld.org/ssa/srl/article-pdf/91/2A/721/4956466/srl-2019169.1.pdf by Charles University user on 21 December 2020 (Gallovič et al., 2009; Konstantinou et al., 2009; Serpetsidaki et al., 2014), which proved activity of a blind right-lateral transform fault crossing the western Peloponnese and possibly continuing further toward the southwest into the Ionian Sea. The interior of IAB, south of Zakynthos, is a zone of “diffuse deformation” (Pérouse et al., 2017). Although being continuously seismically active, and being obviously related to shallow subduction process, its faulting style has not been understood yet. In this respect, the recent 2018 Mw 6.8 event plays an important role for seismotectonic interpretations in western Greece. Lessons learned here may apply also for other subduction-termination zones (Mouslopoulou et al., 2019), where the deformation partitioning between the slab and upper plate may take a variety of forms. Knowledge of the crustal structure of the studied region has been significantly improved by a mixed onshore and offshore (ocean-bottom) temporary seismic network (Papoulia et al., 2014). The latter study illuminated the spatial variation of the Moho depth, and provided a layered velocity model consistently used throughout this article. A weakly northeastdipping (∼5°) seismic reflector has been detected at the depth of ∼10–15 km, supposedly mapping the top of the subducting plate in the area west and southwest of Zakynthos (Clément et al., 2000; Laigle et al., 2004), situated ∼10 km above Moho (Halpaap et al., 2019). Lacking more detailed information, in the following parts of this article we use the wellmapped Moho depth of Papoulia et al. (2014) and plot the slab top 10 km above the Moho. It provides an approximate (schematic) location of the slab top, at the depths 10–20 km, close to the above-cited reflector depths. In the instrumental era, the region near Zakynthos experienced three Mw 6–7 earthquakes, roughly every 20 yr (1959, 1976, 1997, and 2018), consistently with a high seismic coupling (Laigle et al., 2002; Chousianitis et al., 2015). No historicalM > 7 event has been documented. On 25 October 2018, at 22:54 UTC, an Mw 6.8 earthquake occurred southwest of Zakynthos. It caused limited damage on the island and no casualties (Institute of Engineering Seismology and Earthquake Engineering [ITSAK], 2018). The event was observed globally, and its broad characteristics were soon outlined as follows. The Global Centroid Moment Tensor (Global CMT) project suggested a centroid depth of 12 km, scalar moment M0 2:3 × 1019 N · m, and strike/dip/rake angles of 13°/24°/165°, an oblique-TF mechanism. The Global CMT solution comprised a significant non-double-couple (non-DC) component, namely a compressional compensated linear vector dipole, CLVD −44%. The European data centers reported centroid depths <20 km, with focal mechanisms ranging from the mixed SS and thrust type to an almost pure SS, often with a notable non-DC component. For example, the National Observatory of Athens (NOA) published the moment tensor (MT) with CLVD of −61%. Interestingly, an Mw 4.8 foreshock was a pure low-dip thrusting mechanism (strike/dip/rake ∼300°=10°=100°), similar to four major Mw 5+ aftershocks; however, smaller aftershocks were of both types, thrust and SS. Our preliminary analysis, reported to European Mediterranean Seismological Centre two weeks after the event, speculated about a segmented fault (Zahradník et al., 2018). The goal of this article is to improve understanding of the complex faulting style taking place near Zakynthos, complementing our previous earthquake studies of the Ionian Sea islands of Lefkada and Cephalonia (Sokos et al., 2015, 2016). To this goal, we analyze source process of the 2018 mainshock and aftershocks using regional broadband, accelerometric, and GNSS data, considering also the 2011–2018 seismicity of the region. We interpret the mainshock in terms of a segmented source model, possibly related to trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. Source Modeling Point-source models of mainshock and aftershocks The mainshock nucleated ∼45 km southwest of Zakynthos, close to a local bathymetric low (the sea depth of 4 km; see Fig. 1). Exact hypocenter position is unknown because a small foreshock of an unknown position and magnitude preceded the mainshock by a few seconds, thus complicating the arrivaltime picking. For the same reason, the first-motion polarities are problematic. We made a probabilistic location (Lomax et al., 2001), see Text S1, and Figures S1 and S2 (available in the supplemental material to this article), pointing to a shallow depth, and hereafter we use the epicenter (latitude/longitude 37.27°/20.43°) corresponding to the arbitrarily fixed source depth of 5 km, with origin time of 22:54:47.5 UTC. None of the following modeling methodologies relies on the particular hypocenter position. A significant Mw 4.8 foreshock occurred 32 min before the mainshock (Fig. S1). The mainshock was followed by a standard exponentially decaying aftershock sequence that we first located with Hypoinverse (Klein, 2002), and then relocated with hypoDD code (Waldhauser, 2001). Their median formal errors are ∼1 km, and a few hundred meters, respectively. The sequence included one Mw 5.1 event 15 min after mainshock, and four other events of Mw 5+ in the first week. After ∼80 days, anot

On the existence of translating solutions of mean curvature flow in slab regions

Abstract: We prove, in all dimensions n≥2, that there exists a convex translator lying in a slab of width πsec𝜃 in ℝn+1 (and in no smaller slab) if and only if 𝜃∈[0,π2]. We also obtain convexity and regularity results for translators which admit appropriate symmetries and study the asymptotics and reflection symmetry of translators lying in slab regions.