Slab Dip Research Articles

Slab Models Beneath Central Myanmar Revealed by a Joint Inversion of Regional and Teleseismic Traveltime Data

AbstractThe intermediate‐depth (50–180 km) seismicity beneath Myanmar provides direct evidence of the subducting Indian slab. However, the historic lack of regional seismic observations leads to previous low‐resolution models that show large variations in slab geometry beneath Myanmar. The depth extent and morphology of the slab are still poorly known. In this study, we conduct a joint inversion of regional and teleseismic P‐wave traveltimes from recently installed networks to image seismic velocity structures beneath central Myanmar by adopting an eikonal equation‐based traveltime tomography method. The observations contain a total of 6,069 regional first P‐wave arrivals and 29,787 teleseismic P‐wave differential traveltimes. We find a high P‐wave velocity anomaly beneath central Myanmar, which starts from ∼50 km depth and extends continuously to the mantle transition zone (MTZ) and is interpreted as the subducting Indian slab. Below 100 km depth, the dip angle of the slab in the south is ∼15° larger than that of the slab in the north, suggesting a possible slab tearing. Based on our tomographic results and previous studies, the slab in the north is inferred to have a deep stagnant segment lying above the 660‐km discontinuity in the MTZ, but whether it is connected with the shallow dipping slab cannot be confirmed. Meanwhile, the slab in the south may just stay in the upper mantle (above 410 km), but it may also have penetrated the 410‐km discontinuity. Taking into account all the scenarios, we propose four possible models of the Indian Plate subduction system beneath central Myanmar.

Controls of subducting slab dip and age on the extensional versus compressional deformation in the overriding plate

Oceanic plate subduction can exert extensional, compressional or both types of deformations simultaneously in overriding plates. In this study, we explore the factors controlling these two competing modes of deformation in response to contrasting stress fields. Two-dimensional computational fluid dynamics models are presented to show that the dip and age of subducting slabs are crucial parameters to modulate the extensional versus compressional deformations. Shallow-dipping (< 30°) and/or young (< 60 Myr) subducting slabs favor compressional tectonics to form an orogenic wedge (accretionary type), whereas steeply dipping (> 45°) and/or old (> 60 Myr) slabs facilitate extensional tectonics, expressed as a back-arc basin (non-accretionary type). We validate the parametric analysis by comparing our model results with the observed modes of overriding plate deformation in natural subduction zones.

The meeting place of backarc and foreland rifting: The example of the offshore western Sicily (Central Mediterranean)

Coeval rifting processes affecting both backarc, thrust belt and foreland occurred in offshore western Sicily (Central Mediterranean) as indicated by the analysis of a large seismic data set calibrated with well data, onshore data and dredges in a dedicated GIS software environment. Structural pattern of the investigated area, 3-D geological models and a Pliocene-Lower Pleistocene evolution of extensional sedimentary basins have been reconstructed. In detail, two zones characterized by basins with different directions of extension have been recognized: one trending NE-SW, located south of the Egadi Islands; another trending E-W, positioned north of the Egadi Islands. The first group of basins represents the continuation of those of the Africa foreland basin (Pelagian Block), as the second one characterizes the opening of the backarc (southern Tyrrhenian margin). Both basins affect the collisional chain (Maghrebian thrust belt). An eastward progression of extension, from lower Pliocene to lower Pleistocene, based on different onset of the basin fill, has been documented in the Southern Tyrrhenian margin. The study area represents the meeting place of backarc and foreland rifting that affected the late Miocene thrust belt. The recognized pattern of extensional deformation of the upper plate adds some constraint on the geodynamic evolution of the African and Eurasia plates and supports a model of westward dipping slab in the Central Mediterranean. It is hypothesized that the along-trench variation of the nature of the plates, due to paleo-margin configuration, has a profound impact on the upper crustal deformation during the convergence and thus the meeting place of backarc and foreland rifting could represent the surficial signature of the slab detachment since Pliocene times. • 3D models are constructed from geological and subsurface data • The study area represents the meeting place of backarc and foreland rifting affecting the late Miocene thrust belt. • A lower Pliocene-lower Pleistocene eastward progression of extension has been documented in the Southern Tyrrhenian margin. • The geological signature of slab detachment during Pliocene times is where backarc and foreland rifting meets.

Aleutian island arc magma production rates and primary controlling factors

Island arc magmatism is considered a major mechanism of continental crustal growth; hence, the variation in island arc magma production rates and its influencing factors are of great importance. The along-arc strike variation in the island arc magma production rate is important for elucidating whether the subduction zone temperature is a controlling factor on arc magmatism. No back-arc spreading or ridge subduction is occurring in the intraoceanic subduction region of the Aleutian subduction zone, and the along-arc temperatures vary due to differences in the subducting plate age, convergence rate, and slab dip. Therefore, this is an ideal region to study the correlation between magma production rates and subduction zone temperatures. However, previous studies estimated the magma production rates at only a few locations due to the insufficient coverage of seismic data. Here, we employ the gravity inversion method based on density modelling and seismic data constraints to calculate the present-day arc crustal thickness and then obtain the magmatic thickness by removing the pre-existing crustal thickness from the present-day arc crustal thickness. Finally, the magma production rates along the main intraoceanic subduction region of the Aleutian Arc are mapped by estimating the island arc magmatic thickness and the most recent subduction inception dating result. In addition, the subduction zone temperatures and mantle melt amounts along five profiles across the Aleutian Arc are calculated. The magma production rate variation is highly correlated with the mantle melt amount. The slab temperature controls the extent of the slab dehydration, which is essential for mantle wedge melting, and the mantle wedge temperature controls the size of the hydrated mantle melting region and the melting fraction therein. Therefore, as the subduction zone temperature controls the mantle melt amount, we suggest that the subduction zone temperature is the first-order controlling factor on the magma production rate.

Orphaning Regimes: The Missing Link Between Flattened and Penetrating Slab Morphologies

Slab orphaning is a newly discovered phenomenological behaviour, where the slab tip breaks off at 660 km depth and is abandoned by its parent slab. Upon orphaning, subduction continues uninterrupted through the lateral motion of the parent slab above 660 km depth. In this work, we present a regime diagram for the range of conditions under which slabs can orphan at mid-mantle depths. We observe that a viscosity jump at 1000 km, separate from the endothermic phase change responsible for the 660 km seismic discontinuity, is necessary for orphaning, as is slab and mantle weakening between 660 and 1000 km. We show that orphan slabs, similar to other deep slab morphologies, can be the end result for a wide range of physical parameters governing slab dynamics: Slab orphaning persists across wide variations in slab dip, slab strength, Clapeyron slope and overriding plate nature. The diversity in orphan slab sizes and orphaning length scales is tied to the orphaning regime space, which describes a hitherto unexplored region between deflected and penetrating deep-subduction modes that can be found in the transition zone between upper and lower mantle. Orphaning provides a simple dynamic link between the well known deflection and penetration, and provides one possible way for subducting slabs to switch from direct penetration to deflection, littering the mantle with abandoned fragments.

Lithospheric Structure of Eastern Tibetan Plateau from Terrestrial and Satellite Gravity Data Modeling: Implication for Asthenospheric Underplating

Abstract The lithosphere of the eastern Tibetan plateau is underlain by a low-velocity zone at shallow depths which is interpreted as asthenospheric material in the upper-most mantle in various seismic tomography studies. The driving mechanism for the presence of asthenospheric material in the upper-most mantle is not well understood, and the spatial extent of the asthenospheric material is not well delineated. We use 2.5D gravity models to assess what drove the asthenospheric flow upwards in the past and determine the lateral extent of the asthenospheric material in the upper-most mantle. The models also allow us to determine the Indian slab configuration below the Tibetan plateau. The gravity models show that lithospheric thickness increases from ~120 km in the central and eastern parts of the plateau to ~150 km in the west, indicating that the lithosphere in the central and eastern parts of the plateau may have been delaminated. The ~30 km shallower Lithosphere-Asthenosphere Boundary in the central and eastern Tibetan plateau may indicate that asthenospheric flow could have been induced in the past by a combination of lithospheric delamination and a slab break-off event of the Greater Indian slab. The spatial extent of the asthenospheric material in the upper-most mantle beneath the Tibetan plateau is ~15,000 km2 (N−S length=500 km and thickness=30 km) between 85°E and 88°E, which could even extend east of 92°E. The Indian slab is dipping more steeply in the east. The slab dip along the Indian plate increases from ~10° in the west to ~18° in the central (~87°E) and ~25° in the eastern part (~91°E) of the plateau, indicating that the style of lithospheric deformation changes from underthrusting to slab roll-back from west to east.

Calcite twinning fabrics along the Middle America trench, Costa Rica and the Motagua sinistral fault, Honduras and Jamaica: Tectonic implications for the Caribbean plate

The Cretaceous oceanic crust of the Caribbean plate has a long and complex tectonic history as an indentor plate bounded by transcurrent faults (N and S) and subduction margins (W [east slab dip] and E [west slab dip]) with no strain data. Miocene forearc sediments and accreted ophiolites are exposed along the central Pacific coast of Costa Rica and preserve a variety of folding and faulting styles. These clastic sediments, deposited in the forearc as the Middle America trench evolved, contain calcite cements and are cross cut by calcite veins. Analysis of the mechanical twins in these two calcite groups (8 samples, n = 267 twins) at Punta Dominical and the Nicoya Peninsula ophiolite preserve a consistent sub-horizontal shortening strain that is oriented parallel to the current trench axis, as are most of the accretion-related fold axes. Differential stress magnitudes for the calcite twinning strain average −450 bars, and there is little evidence of a twinning strain overprint (low NEVs). Obduction along the Middle America trench involves, in part, a component of trench-parallel horizontal shortening.The Motagua fault originates in the Middle America trench and strikes east as the northern boundary of the Caribbean plate as a sinistral margin. The Roatan Islands are a series of islands composed of south-dipping rocks along the southern side of the Motagua fault. Recent, deformed clastic sediments are drapped over gneisses, schists (40Ar-39Ar biotite age of 14 Ma), conodont-free marbles and ophiolitic fragments of presumed Cretaceous age. Twinning studies (n = 6 samples, n = 425 twins) of these marbles preserve a N–S, horizontal shortening strain, normal to the sinistral plate boundary. Differential stress magnitudes for the calcite twinning strain average −36 MPa. Only in a combined sample (n = 189 grains) is a strain overprint (high NEVs; n = 47) present, which records horizontal shortening strain at 132°, 5°. Jamaica is also along the eastern Motagua fault (Cayman trough) and twinning strain studies in Eocene limestones and younger calcite veins (n = 4 samples; n = 136 twins) preserve a horizontal shortening strain normal to the sinistral plate boundary.Twinning strain results are compared to regional GPS, focal mechanism and SKS anisotropy results around the Caribbean plate, especially along the Middle America trench on the western margin of Costa Rica with some new insights into subduction dynamics.

Cold Plumes Initiated by Rayleigh‐Taylor Instabilities in Subduction Zones, and Their Characteristic Volcanic Distributions: The Role of Slab Dip

AbstractDehydration melting in subduction zones often produces cold plumes, initiated by Rayleigh‐Taylor instabilities in the buoyant partially molten zones lying above the dipping subducting slabs. We use scaled laboratory experiments to demonstrate how the slab dip (α) can control the evolution of such plumes. For α &gt; 0°, Rayleigh‐Taylor instabilities evolve as two orthogonal waves, one trench perpendicular with wavelength λL and the other one trench parallel with wavelength λT (λT &gt; λL). We show that two competing processes, (1) λL‐controlled updip advection of partially molten materials and (2) λT/λL interference, determine the modes of plume growth. The λT/λL interference gives rise to an areal distribution of plumes (Mode 1), whereas advection leads to a linear distribution of plumes (Mode 2) at the upper fringe of the partially molten layer. The λT wave instabilities do not grow when α exceeds a threshold value (α* = 30°). For α &gt; α*, λL‐driven advection takes the control to produce exclusively Mode 2 plumes. We performed a series of 2‐D and 3‐D computational fluid dynamics simulations to test the criticality of slab dip in switching the Mode 1 to Mode 2 transition at α*. We discuss the effects of viscosity ratio (R) and the density contrast (Δρ) between the source layers and ambient mantle, source layer thickness (Ts), and slab velocity (Us) on the development of cold plumes. Finally, we discuss the areal versus linear distributions of volcanoes from natural subduction zones as possible examples of Mode 1 versus Mode 2 plume products.

How Sediment Thickness Influences Subduction Dynamics and Seismicity

AbstractIt has long been recognized that sediments subducting along the megathrust influence the occurrence of giant (Mw ≥ 8.5) megathrust earthquakes. However, the limited observation span and the concurrent influence of multiple parameters on megathrust behavior prevent us from understanding how sediments affect earthquake size and frequency. Here, we address these limitations by using two‐dimensional, visco‐elasto‐plastic, seismo‐thermo‐mechanical numerical models to isolate how sediment thickness affects subduction geometry and seismicity. Our results show that increasing sediment thickness on the incoming plate results in a decrease of the slab dip, as the trench retreats due to the seaward growth of the sedimentary wedge that also unbends the slab. This decrease in megathrust dip results in a wider seismogenic zone, so that the maximum magnitude of megathrust earthquakes increases. Concurrently, the recurrence time of characteristic events increases and partial ruptures are introduced. The maximum magnitude estimated for subduction segments with the thickest sediment input (Makran, West‐Aegean, and Calabria) is distinctly higher than the instrumentally recorded magnitude. These segments may thus experience larger than as of yet observed earthquakes, albeit infrequently. Increasing sediment thickness also decreases megathrust normal stresses, as the seismogenic zone is more shallow and overlain by a lighter forearc structure. Thicker incoming plate sediments also favor more splay fault activity, whereas we observe more outer rise events for low sediment thickness. Finally, we demonstrate that modeling long‐term subduction dynamics and sediment subduction is crucial for understanding and quantifying megathrust seismicity and seismic potential of subduction zones.

Subduction Dynamics and Mantle Pressure: 2. Towards a Global Understanding of Slab Dip and Upper Mantle Circulation

AbstractWe investigate the relationship between the global distribution of deep slab dips (at 250‐ to 300‐km depth) and pressure and circulation in the upper mantle. Using an analytic method to compute dynamic pressure in a 3‐D global upper mantle domain, and a force balance between slab dip, slab buoyancy, and pressure, we model dips for all major subduction zones. Overall, our models suggest that global‐scale mantle flow, as dictated by the shapes and velocities of Earth's plates and slabs, plays a fundamental role in creating the global pattern of slab dips. The dip trends of the South American and western Pacific subduction zones are controlled, in our models, by spatial variations in the dynamic pressure associated with flow. Our best fitting models produce global root mean square dip misfits of less than 10° for asthenospheric viscosities of 2.5–4.0 × 1020 Pas. This result is only obtained with a large flux of asthenosphere from upper to lower mantle at subduction boundaries, occurring on the overriding plate side of slabs, without which dips are significantly steeper than observed. This effect cannot be resolved by processes that affect only certain subduction systems and requires flux of asthenosphere into the lower mantle at subduction systems globally (or an alternative mechanism that produces more negative pressures on the overriding plate side of slabs). Upper mantle pressure fields that fit global slab dips yield negative dynamic pressure on the upper plate side of slabs, positive pressure on the subducting plate side, and an east‐to‐west pressure increase beneath the Pacific Plate.

Subduction Dynamics and Mantle Pressure: 1. An Analytical Framework Relating Subduction Geometry, Plate Motion, and Asthenospheric Pressure

AbstractRecent work suggests that slab dip may be controlled by differences in dynamic pressure above and below the slab at mid‐asthenospheric depth, motivating exploration of how plate and slab motion are linked to dynamic pressure in the asthenosphere. This paper, the first of two companion papers, presents an analytical method for determining dynamic pressure and velocity in a thin asthenospheric channel, with linear viscosity, consistent with prescribed plate and slab kinematics. Slabs are explicitly incorporated as vertical barriers to asthenospheric flow. Dynamic pressure is expressed as sums of functions with two functional forms, Pedge and Pwall; the former generates discontinuities in velocity (but not pressure) at non‐slab boundaries, and the latter generates discontinuities in pressure (but not velocity) at boundaries with slabs. Application to simple rectangular plate systems demonstrates that asthenospheric velocities within about one trench length of slabs are insensitive to processes on the opposing side of the slab, that dynamic pressure scales linearly with trench length for a given plate aspect ratio and scales approximately linearly with the shortest plate dimension. Although dynamic pressure is quasisymmetrical across convergent plate boundaries that lack slabs, the presence of slabs creates discontinuity and asymmetry in dynamic pressure across subduction zones. When flux of asthenosphere into the lower mantle occurs adjacent to subducting slabs, dynamic pressure is decreased on the side of the slab where down‐flux occurs. This analytical method, applied to systems with multiple arbitrarily shaped plate boundaries, yields excellent results as benchmarked against 3‐D numerical solutions.

Uncovering the physical controls of deep subduction zone slow slip using supervised classification of subducting plate features

Summary Deep slow slip events (SSEs) at subduction zones have significantly contributed to refining our understanding of the megathrust earthquake cycle at the brittle-ductile transition. However, the specific combination of factors that determine their occurrence has not yet been fully explored. Here we evaluate the contribution of several of these characteristics using globally mapped geophysical data that are used as proxies for physical properties of the subducting plate. This is performed by classifying 25 km-wide, trench-parallel segments into binary classes based on the observation (or lack thereof) of deep, short- or long-term SSEs. The five characteristics explored here include subducting plate age, sediment thickness, relative plate velocity, slab dip, and plate surface roughness. We use these characteristics to train six Machine Learning models based on different learning algorithms: Gaussian Naïve Bayes, Logistic Regression, Linear Discriminant Analysis, Random Forest, Support Vector Machine, and K-Nearest Neighbour. Short-term SSE models show that subducting plate age, relative velocity, and sediment thickness have the strongest predictive power with the first two characteristics negatively correlating and sediment thickness positively correlating with SSE occurrence, respectively. These results are consistent with a conceptual model where slow slip is controlled by conditions favoring the enduring release (and possible storage) of fluids near the source region. However, the relationship between these features and elevated pore fluid pressures is not established here and further evidence is needed to validate this hypothesis. We then use a final model constructed as a weighted average of the best performing models to make predictions on the probability of SSE occurrence, with predicted short-term SSE occurrence in South America, the Aleutians, Sumatra, Vanuatu and Solomon, as well as long-term SSE occurrence in the Aleutians, Izu-Bonin, Kuril-Kamchatka, Mariana, and Tonga-Kermadec. Overall, long-term SSE models do not perform as well as the short-term SSE models which may indicate that long-term SSEs are controlled by a different and/or extended set of physical characteristics than the short-term SSEs.

The effects of the thermal state of overriding continental plate on subduction dynamics: Two-dimensional thermal-mechanical modeling

The dynamic process of ocean-continent subduction depends on not only the properties of the subducting oceanic plate, but also the characteristics and state of the overriding continental plate. Numerical models conducted to date have mostly focused on the oceanic lithosphere in this regard; research on the properties of overriding continental lithosphere remains relatively limited, especially the influence of its thermal state on subduction dynamics. Here we explored the performance of continental lithosphere with different thermal states during the subduction process using two-dimensional thermal-mechanical modeling and systematically investigated the effects of the thermal state of overriding continental plate, the age of subducting oceanic plate, and relative convergence rate on subduction dynamics. Modeling results show that: (1) When the geothermal gradient of continental crust is low (between 10 and 15 °C km−1), the oceanic plate first subducts at a low angle. As subduction continues, the slab dip gradually increases and the slab begins to retreat rapidly driven by its negative buoyancy, opening an ocean basin ranging from 600 to 1100 km in width. This leads to the decoupling between the overriding continental plate and oceanic plate. As the trench retreat continues, the horizontal deviatoric stress inside the overriding continental crust alternates between being positive and negative in a local area. Thinning of the overriding lithosphere mainly occurs at the region adjacent to the subduction zone, where the surface experiences significant subsidence. (2) When the geothermal gradient of continental crust is higher (greater than 15 °C km−1), oceanic plate retreat causes the overriding continental plate to be strongly stretched. In this case, the trench retreat distance decreases and the width of the ocean basin also reduces by between 100 and 1000 km. The horizontal deviatoric stress inside the whole overriding continental crust first manifests as compression and then changes into extension, which causes the surface to first uplift and then slowly subside. (3) Increasing the age of oceanic lithosphere accelerates trench retreat and promotes overriding plate thinning. (4) An advancing overriding continental plate slows down trench retreat. In cases where the geothermal gradient of continental crust is greater than 17.5 °C km−1, the hot continental crust experiences gravitational collapse and is overthrusted onto oceanic lithosphere, resulting in slow trench retreat. We analyzed the subduction process of the western Paleo-Pacific Plate in the Early Cretaceous based on our modeling results and discussed its possible control on the tectonic evolution of the rift basins in east Asia. We suggest that the development of a wide rift basin system on the Amurian Superterrane in the Early Cretaceous was likely related to slow trench retreat and the collapse of the hot crust, and the formation of a series of passive rift basins in the North China Craton was likely caused by the relatively cold thermal state of the lithosphere and the rapid retreat of the Paleo-Pacific Plate.

Detailed Structure of the Subducted Nazca Slab into the Lower Mantle Derived From Continent‐Scale Teleseismic P Wave Tomography

AbstractNazca subduction beneath South America is one of our best modern examples of long‐lived ocean‐continent subduction on the planet, serving as a foundation for our understanding of subduction processes. Within that framework, persistent heterogeneities at a range of scales in both the South America and Nazca plates is difficult to reconcile without detailed knowledge of the subducted Nazca slab structure. Here we use teleseismic travel time residuals from &gt;1,000 broadband and short‐period seismic stations across South America in a single tomographic inversion to produce the highest‐resolution contiguous P wave tomography model of the subducting slab and surrounding mantle beneath South America to date. Our model reveals a continuous trench‐parallel fast seismic velocity anomaly across the majority of South America that is consistent with the subducting Nazca slab. The imaged anomaly indicates a number of robust features of the subducted slab, including variable slab dip, extensive lower mantle penetration, slab stagnation in the lower mantle, and variable slab amplitude, that are incorporated into a new, comprehensive model of the geometry of the Nazca slab surface to ~1,100 km depth. Lower mantle slab penetration along the entire margin suggests that lower mantle slab anchoring is insufficient to explain along strike upper plate variability while slab stagnation in the lower mantle indicates that the 1,000 km discontinuity is dominant beneath South America.

Subduction Duration and Slab Dip

AbstractThe 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 is consistent with mechanical models of subduction.

Observations and modeling of flat subduction and its geological effects

Flat subduction refers to low-angle (<10°) or sub-horizontal subduction of oceanic slabs. Flat subduction is only recognized in ~10% of present-day subduction zones, but its impact on the behavior of the overriding plate is particularly strong. For example, flat subduction zones are typically associated with stronger earthquakes. The deformation caused by typical flat subduction will transfer from the trench to the overriding continental interior and form a broad magma belt. The formation mechanism of flat subduction has been linked to the relative buoyancy of subducted oceanic plateaus, overthrusting of the overriding plate, hydrodynamic suction, and trench retreat. However, these mechanisms remain debated. This paper systematically analyzes and summarizes previous studies on flat subduction, and outlines the possible geological effects of flat subduction, such as intracontinental orogeny and magmatism. Using examples from numerical modeling, we discuss the possible formation mechanisms. The most important factors that control the formation of flat subduction are associated with overthrusting of the overriding plate and the arrival of an oceanic plateau at the subduction zone. In addition, trench retreat is necessary to enable flat subduction. Hydrodynamic suction contributes to the reduction of the slab dip angle, but is insufficient to form flat subduction. Future numerical modeling of flat subduction should carry out three-dimensional high-resolution thermo-mechanical simulation, considering the influence of crustal eclogitization (negative buoyancy) and mantle serpentinization (positive buoyancy) of oceanic lithosphere, in combination with geological and geophysical data.

Larger tsunamis from megathrust earthquakes where slab dip is reduced

A subset of megathrust earthquakes produce anomalously large tsunamis for their magnitude. All of these recorded ‘tsunami earthquakes’ in the past 50 years had extensional aftershocks in the upper plate. These include the two largest and most destructive earthquakes of that period, the 2004 Sumatra–Andaman and the 2011 Tohoku events. Evidence from the region of Tohoku indicates that normal fault slip in the upper plate during the earthquake may have contributed to the tsunami size. Here we present a numerical model that shows how a reduction of the dip of a subducting slab, on a timescale of millions of years, can result in an extensional fault failure above a megathrust earthquake on timescales of seconds to months. Slab dip reduction bends the upper plate so that the shallow part fails in extension when a megathrust rupture relieves compressional stress. This results in a distribution of extensional aftershocks comparable to that seen above the Tohoku megathrust. Volcanic arc migration and uplift data for Tohoku and several other tsunami earthquakes is consistent with slab dip reduction. The collection of more such data might identify other areas of tsunami hazard related to slab dip reduction. Tsunami generation by megathrust earthquakes is enhanced by extensional faulting in the upper plate when the subducting slab shallows, according to numerical modelling and observations from the Sumatra–Andaman and Tohoku earthquake–tsunami events.

Coupled Evolution of Deformation, Pore Fluid Pressure, and Fluid Flow in Shallow Subduction Forearcs

AbstractDeformation and fluid flow in subduction zone forearcs are dynamically coupled, but our quantitative understanding of their coupling is incomplete. In this work, we investigate the hydrological and mechanical coupling in shallow forearcs, using a Lagrangian‐Eulerian finite element model that incorporates constitutive and transport properties of sediments and faults constrained by laboratory and field measurements. Wide‐ranging observations show that sediment thickness and composition, plate convergence rate, basement strength and roughness, and subducting slab dip angle vary between subduction zones. We therefore systematically study their effects on forearc stress and pore fluid pressure states, consolidation and dewatering patterns, and margin morphology. Our models, with the incorporation of a simple description of permeability enhancement along fault damage zones, yield a range of fault permeability (10−13–10−17 m2) consistent with previous estimates and describe the important role of upper plate splay faults in causing heterogeneous dewatering and consolidation patterns and in modulating effective normal stress on the plate interface. Spatial variations in tectonic loading and sediment consolidation can also be caused by subducting basement roughness such as a horst‐and‐graben structure. For typically observed relief and spacing, our models predict locally enhanced porosity reduction by up to 50% at the downdip edge of the horsts and anomalously high sediment porosity above the geometrical highs. At the margin scale, our results demonstrate that sediment permeability and thickness are dominant controls on fluid overpressure, sediment compaction, and megathrust strength. Rough and frictionally strong megathrusts produce similar effects in driving high wedge tapers.

Geometry of the Deep Calabrian Subduction (Central Mediterranean Sea) From Wide‐Angle Seismic Data and 3‐D Gravity Modeling

AbstractThe Calabrian subduction zone is one of the narrowest arcs on Earth and a key area to understand the geodynamic evolution of the Mediterranean and other marginal seas. Here in the Ionian Sea, the African plate subducts beneath Eurasia. Imaging the boundary between the downgoing slab and the upper plate along the Calabrian subduction zone is important for assessing the potential of the subduction zone to generate megathrust earthquakes and was the main objective of this study. Here we present and analyze the results from a 380‐km‐long, wide‐angle seismic profile spanning the complete subduction zone, from the deep Ionian Basin and the accretionary wedge to NE Sicily, with additional constraints offered by 3‐D gravity modeling and the analysis of earthquake hypocenters. The velocity model for the wide‐angle seismic profile images thin oceanic crust throughout the basin. The Calabrian backstop extends underneath the accretionary wedge to about 100 km SE of the coast. The seismic model was extended in depth using earthquake hypocenters. The combined results indicate that the slab dip increases abruptly from 2–3° to 60–70° over a distance of ≤50 km underneath the Calabrian backstop. This abrupt steepening is likely related to the rollback geodynamic evolution of the narrow Calabrian slab, which shows great similarity to the shallow and deep geometry of the Gibraltar slab.

Coseismic coastal subsidence associated with unusually wide rupture of prehistoric earthquakes on the Kamchatka subduction zone: A record in buried erosional scarps and tsunami deposits

The prograding strand plain of Avachinsky Bay, Kamchatka, Russia, along the highly active Kamchatka subduction zone, exhibits geological evidence--buried erosional scarps--for coseismic subsidence only three times in the last four millennia, the last event about 1200 years ago. This same coast has a historical record (since A.D. 1737) of five subduction-zone earthquakes with large tsunami runup (>5 m), the last of which was the 1952 Mw 9 Kamchatka earthquake, and a geological record of more than 30 large tsunamis in the last 4000 years. This rarity of buried scarps relative to large earthquakes contrasts with the Cascadia strand plain in SW Washington State, where most or all large events are represented by buried scarps. A strong factor in the amplitude and sign of coseismic deformation is distance from the seaward edge of a subduction zone (the trench); the Avachinsky Bay coastline is 180–200 km from the trench, with ∼25° slab dip, requiring unusually wide ruptures to generate significant coseismic subsidence. This coastal zone is undergoing net subsidence approximately equivalent to the total of the three coseismic subsidence events, generating a sequence of beach ridges that increase in elevation seaward.Each of the three unusual (coseismic subsidence) events comprises a) an earthquake whose deformation field caused b) onshore coseismic subsidence, thus local sea-level rise and c) sufficient deformation offshore to produce a large tsunami; a,b,c followed by d) a period of coastal erosion and shoreline retreat, leaving e) an erosional beach scarp that was f) subsequently buried once progradation resumed. We identified, dated and correlated the scarps and tsunami deposits from these events with several field methods, including trenching, tephrostratigraphy and ground penetrating radar. The scarps were correlated over an alongshore distance of 50–70 km. The most recent event (event 1) occurred ∼800 cal AD (1100–1250 14С years BP), event 2–600 cal BC (2400–2450 14С years BP), and event 3–1700 cal BC (3300–3500 14С years BP). We developed methods for quantifying subsidence, coastal erosion and tsunami size for each of these events. All three retain evidence of ∼0.4–1.2 m of coseismic subsidence; coastal erosion in the case of event 1 averaged more than 100 m; all three “event” tsunamis were amongst the largest in the last 4000 years.