Toe Of Wedge Research Articles

Experimental and numerical investigation of the effect of air injection on seawater intrusion mitigation

Previous studies have proposed compressed air injection to mitigate seawater intrusion, as a substitute for a hydraulic barrier created using freshwater injection. However, the current understanding of this strategy is based almost entirely on numerical simulation, and therefore, there is a need to investigate the complex processes accompanying this method within a physical setting. This study used sand tank experiments to explore the influence of air injection on the behavior of saltwater within a coastal confined aquifer, extending previous experiments by accounting for freshwater-saltwater density differences. Numerical modeling of the experiments assisted in interpreting experimental observations and allowed for field-scale conditions to be assessed. Numerical models were well matched to experimental observations, which showed a 34 % reduction in saltwater volume and a retreat of the saltwater toe length from 59 cm to 40 cm following air injection. A hypothetical field-scale confined aquifer model indicated that the largest seaward shift in the seawater wedge toe is obtained when the air-injection well is above the initial seawater wedge toe. However, maximal reductions in the seawater volume within the aquifer were obtained from injecting air within the original wedge rather than landward of the toe. This study provides the first physical evidence (through laboratory experiments) that injecting compressed air can effectively mitigate seawater intrusion. We considered a period of one year for field-scale applications, which show that the extent of desaturation likely needs to be mitigated from prolonged application of the method. Initial guidance is developed for the selection of air-injection locations, at least under simplified conditions (e.g. homogeneous, uniform aquifer, etc.).

Influence of tides on the effectiveness of artificial freshwater injection in mitigating seawater intrusion in an unconfined coastal aquifer

Managed aquifer recharge (MAR) using artificial freshwater recharge from infiltration basins or injection wells is an effective method to control seawater intrusion. The effect of oceanic tides, however, are usually not considered when dimensioning and operating freshwater injection installations. This study employed lab-scale physical experiments and groundwater numerical models to investigate salt transport and salinity distributions as driven by artificial freshwater injection under the influence of tides in an unconfined coastal aquifer. Results revealed that the effectiveness of artificial freshwater injection is significantly reduced under tidal conditions as compared to conditions without tides. The response of the upper saline plume (the tidal recirculation cell) to artificial freshwater injection in tidal conditions is nearly twice more rapid than that of the lower seawater wedge. The seaward displacement of the seawater wedge toe is higher for low seawater concentration isohalines (5%) than for higher concentration isohalines (50% to 95%). It was further found that an overshoot phenomenon (whereby the seawater toe first moves seaward before moving landward again) occurs and mainly affects the higher concentration isohalines. This further decreases the effectiveness of freshwater injection in controlling seawater intrusion, and the phenomenon is more pronounced with increased injection rates, increased distance between injection well and coastal line, increased tidal amplitude and decreased tidal periods. This study provides insights with direct implications for design of artificial freshwater injection wells to control seawater intrusion in tidally-influenced unconfined coastal aquifers.

Décollement strength and mechanical segmentation along the frontal wedge of the Hikurangi convergent margin (New Zealand): Insights from critical taper analysis

To better understand the mechanics of outer forearcs, which is influenced to a large degree by the frictional properties of the plate interface, is a pre-requisite to address topics like natural hazard such as large earthquakes and slumping. Previous research including geodetic and geophysical measurements as well as critical taper analyses (CTA) has revealed that the mechanics of the Hikurangi margin (HM) offshore New Zealand may vary along the margin. However, the relationship between the morphology of the submarine margin and effective décollement strength of the Pacific-Australian plate boundary is not fully understood. From areal gridded data of surface slope (α) and plate dip (β) we characterise the central HM as an intermediate accretionary wedge and the northern HM as a non-accretionary wedge sensu Lallemand et al. (1994). Our areal CTA, comprising the entire region of the submarine margin from the transition to strike-slip faulting in the south to subduction erosion in the northernmost part of HM reveals significant variation of the effective décollement strength of the Pacific-Australian plate boundary. Altogether the décollement strength shows significant regional variability along the HM (0.02 to 0.21), increasing to the toe of the accretionary wedge, and in regions with seamount subduction. Furthermore, mean values of the décollement strength are increasing from the central (0.05) to the northern HM (0.09). Also, our CTA suggests moderate to high fluid overpressure along the plate interface. Finally, out approach of an areal computation of the décollement strength provides a low-effort and on the same hand helpful tool to characterise convergent margin mechanics.

The impact of sea-level rise on saltwater intrusion for barrier-island aquifers in North Carolina

Saltwater intrusion is an increasing problem for coastal aquifers due to rising sea levels, especially in areas of low hydraulic gradient, such as barrier-island aquifer systems. Large-scale studies are particularly useful for identifying trends in the effects of sea-level rise, especially for relatively understudied barrier-island aquifers. The barrier islands of coastal North Carolina, U.S.A., lie in a hot spot of sea level rise, where rates are projected to exceed global means throughout this century. Herein, we present a study of the effects of sea-level rise using four primary study sites, each with varying hydraulic properties, island width, and rates of sea-level rise. We use these sites to understand the viability of barrier-island aquifers in North Carolina, but also to understand in general the factors that are most important to the future viability of all barrier-island aquifers. We utilize a one-dimensional steady-state analytical model that allows the calculation of the position of the toe of the saltwater wedge at the base of the aquifer while limiting a rise in the water table. We determine the position of the toe for various sea-level projections up to the year 2100 and utilize this parameter as an assessment of the viability of the aquifer. Our findings suggest that higher island width and lower hydraulic conductivity are the most sensitive parameters and help to limit the effects of sea-level rise on aquifer viability. Aquifer risk maps calculated for the entire North Carolina coast, which equate risk with the position of the toe relative to island width, demonstrate that sea-level rise is projected to have increasing impact on aquifer viability over time, but that areas with narrower width and higher sea-level rise rates are the most vulnerable.

Seawater Intrusion in Extremely Heterogeneous Laboratory-Scale Aquifer: Steady-State Results

This work used experimental and numerical methods to investigate seawater intrusion (SWI) in a complex heterogeneous laboratory-scale aquifer. We started the analysis with a homogeneous isotropic aquifer as a reference case, then moved to heterogeneous layered aquifers. The study also investigated block-wise synthetic aquifers with different configurations. The seawater wedge toe length generally decreased under heterogeneous conditions, while the freshwater–saltwater dispersion/mixing zone generally increased when compared to the homogenous case. The saltwater–freshwater interface shows a distinct gradient change across boundaries at differing hydraulic conductivities. This was attributed to streamline refraction, which caused a reduction to the angle of intrusion when transitioning from high to low permeability zones and vice versa. The refraction also affected the mixing zone, where additional spreading was also observed when transitioning from high to low permeability zones and vice versa. When low permeability zones predominated the shoreline at the saline water boundary, this produced a shorter saline wedge in the horizontal direction, but it was more expanded vertically. This study provides insight into the general processes of SWI in heterogeneous aquifers and could be used as a basis for defining conceptual models of real-world systems. It highlights the capabilities of the image analysis to capture small perturbations.

The impact of aquifer stratification on saltwater intrusion characteristics. Comprehensive laboratory and numerical study

AbstractLaboratory experiments and numerical simulations were utilized in this study to assess the impact of aquifer stratification on saltwater intrusion. Three homogeneous and six layered aquifers were investigated. Image processing algorithms facilitated the precise calculation of saltwater wedge toe length, width of the mixing zone, and angle of intrusion. It was concluded that the length of intrusion in stratified aquifers is predominantly a function of permeability contrast, total aquifer transmissivity and the number of heterogeneous layers, being positively correlated to all three. When a lower permeability layer overlays or underlays more permeable zones its mixing zone widens, while it becomes thinner for the higher permeability strata. The change in the width of the mixing zone (WMZ) is positively correlated to permeability contrast, while it applies to all strata irrespectively of their relative vertical position in the aquifer. Variations in the applied hydraulic head causes the transient widening of WMZ. These peak WMZ values are larger during saltwater retreat and are negatively correlated to the layer's permeability and distance from the aquifer's bottom. Moreover, steeper angles of intrusion are observed in cases where low permeability layers overlay more permeable strata, and milder ones in the inverse aquifer setups. The presence of a low permeability upper layer results in the confinement of the saltwater wedge in the lower part of the stratified aquifer. This occurs until a critical hydraulic head difference is applied to the system. This hydraulic gradient value was found to be a function of layer width and permeability contrast alike.

Towards a Correlation between Long-Term Seawater Intrusion Response and Water Level Fluctuations

Laboratory and numerical experiments were conducted to provide a quantitative steady-state analysis of the effect of incremental variations of water level on saltwater intrusion. The purpose was to seek mathematical correlations relating both the wedge toe length and the height along the coastline to the boundary head difference. The laboratory experiments were completed in a 2D sand tank where both freshwater and seawater levels were varied. The experiments were conducted for two bead sizes having different hydraulic conductivities. The numerical model SEAWAT was used to validate the results and then to perform sensitivity analysis. The experimental results show that at steady-state conditions, the logarithmic toe length could be expressed as a linear function of the boundary head difference. The linear relationship was recorded in both advancing and receding wedge phases. The linearity of the correlation was also well demonstrated with analytical solutions. Similar relationships were also derived in the scenarios where the sea level fluctuated while the freshwater boundary head was constant. The height of the saltwater wedge along the coastline was also found to be a linear function of the boundary head difference. The sensitivity analysis shows that the regression coefficients were sensitive to the hydraulic conductivity, the dispersivity, and the saltwater density, while the porosity and the rate of boundary head change induced negligible effects. The existence of a linear relationship between the logarithmic toe length and the boundary head difference was also well evidenced in a field-scale aquifer model for all the different hydrogeological aquifer conditions tested. This study is the first attempt in identifying the underlying correlation between the boundary water level variations and the main seawater intrusion (SWI) external metrics under controlled laboratory conditions, which is of great relevance from a water resources management perspective.

Numerical assessment of compressed air injection for mitigating seawater intrusion in a coastal unconfined aquifer

Subsurface fluids injection is a viable alternative for controlling seawater intrusion in coastal regions, and freshwater is usually served for valid fluids to generate pressure ridge towards the ocean. But other fluids injection, such as compressed air, which presents clean and readily available, is a more attractive solution for repulsing the intruded seawater where freshwater is insufficient. In this study, the performance of compressed air injection (i.e., air barriers) for mitigating seawater intrusion in a coastal unconfined aquifer was quantificationally assessed using a coupled water-air two-phase flow and saltwater transport model. As compressed air is introduced into the aquifer at the toe of saltwater wedge, the seaward hydraulic gradient adjacent to the coast is produced driven by the generated airflow, thereby causing salt-freshwater interface to retreat to the ocean. The magnitude of air injection rate, related to the operational cost of air barriers, does not increases significantly given the progressively decreasing change rate. Compared to the workability of air barriers in the confined aquifer under similar conditions, the reduction in intruded seawater at 365.0 d in the unconfined aquifer reaches only about 0.45 times that in the confined aquifer, which indicates the significance of the boundary condition of aquifer top. The sensitivity analysis of the overlying unsaturated zone reveals that with its permeability decreasing or its thickness increasing, the performance of air barriers is enhanced owing to the blocking effect of the overlying layer on the escape of generated airflow and tends progressively to be stable. Particularly, the efficacy of air barriers in the confined aquifer, including the reduction in intruded seawater and the air injection rate, also can be achieved in an unconfined aquifer with an overlaying sufficient-semipermeable layer.

Preventing Seawater Intrusion and Enhancing Safe Extraction Using Finite‐Length, Impermeable Subsurface Barriers: 3D Analysis

AbstractSubsurface physical barriers have been recognized as effective in mitigating seawater intrusion in coastal aquifers, although mainly 2D (cross‐sectional) barrier effects have been considered. In this study, impermeable barriers with finite shore‐parallel lengths are investigated through 3D numerical simulation, thereby extending previous analyses. Two scenarios are considered: (a) barrier‐only and (b) barrier‐well systems; and three available barrier types are analyzed and compared: (1) subsurface dam, (2) cutoff wall, and (3) fully penetrating barrier. Barrier location, length, and height are investigated, and barrier effectiveness is evaluated from seawater volumes, seawater wedge toe positions, and maximum safe pumping rates. In the barrier‐only system, a better performance in preventing seawater intrusion was achieved by cutoff walls rather than subsurface dams. Finite‐length subsurface dams may slightly enhance seawater extent along parts of the coastline that are beyond the dam's length. Cutoff walls performed best when located at relatively small distances from the coast in the barrier‐only system, whereas with a well at 450 m from the shoreline, the subsurface dam located at a critical distance from the sea (i.e., 300 m in the current study) performed optimally (from the tested cases) and was superior to cutoff walls in terms of the maximum safe pumping rate. A fully penetrating barrier outperformed cutoff walls and subsurface dams, as expected. Our investigation indicates that subsurface barrier design should consider the effect of the shore‐parallel length, because barrier benefits may otherwise be significantly overestimated.

Seismic velocity and reflectivity analysis of concentrated gas hydrate deposits on the southern Hikurangi Margin (New Zealand)

In the southern Hikurangi subduction margin, widespread gas hydrate accumulations are inferred based on the presence of bottom simulating reflections and recovered gas hydrate samples, mainly associated with thrust ridges. We present a detailed analysis of high- and medium-resolution seismic reflection data across Glendhu and Honeycomb ridges, two elongated four-way closure systems at the toe of the deformation wedge. High-amplitude reflections within the gas hydrate stability zone, coincident with high seismic velocities, suggest the presence of highly concentrated gas hydrate accumulations in the core regions of the anticlinal ridges. A novel method involving combined seismic velocity and reflectivity analysis and rock physics modelling is used to estimate hydrate saturations in localised areas. The effective medium model consistently predicts gas hydrate saturations of ~30% of the pore space at Glendhu Ridge and >60% at Honeycomb Ridge, whereas the empirical three-phases weighted equation likely underestimates the amount of gas hydrate present. We note that our estimates are dependent on the vertical resolution of the seismic data (5–14 m), and that the existence of thin layers hosting gas hydrate at higher concentrations is likely based on observations made elsewhere in similar depositional environments. A comparison between the two ridges provides insights into the evolution of thrust related anticlines at the toe of the accretionary wedge. We propose that the main driving mechanism for concentrated hydrate accumulation in the study area is along-strata gas migration. The vertical extent of these accumulations is a function of the steepness of the strata crossing the base of gas hydrate stability, and of the volume of sediments from which fluid flows into each structure. According to our interpretation, older structures situated further landward ofthe deformation front are more likely to host more extensive concentrated hydrate deposits than younger ridges situated at the deformation front and characterised by more gentle folding. The method introduced in this work is useful to retrieve quantitative estimates of gas hydrate saturations based on multi-channel seismic data.

Assessing the Effectiveness of Using Recharge Wells for Controlling the Saltwater Intrusion in Unconfined Coastal Aquifers with Sloping Beds: Numerical Study

Groundwater systems are considered major freshwater sources for many coastal aquifers worldwide. Seawater intrusion (SWI) inland into freshwater coastal aquifers is a common environmental problem that causes deterioration of the groundwater quality. This research investigates the effectiveness of using an injection through a well to mitigate the SWI in sloping beds of unconfined coastal aquifers. The interface was simulated using SEAWAT code. The repulsion ratios due to the length of the SWI wedge (RL) and the area of the saltwater wedge (RA) were computed. A sensitivity analysis was conducted to recognize the change in the confining layer bed slope (horizontal, positive, and negative) and hydraulic parameters of the value of the SWI repulsion ratio. Injection at the toe itself achieved higher repulsion ratios. RL and RA declined if the injection point was located remotely and higher than the toe of the seawater wedge. Installation at the toe achieved a higher RL in positive sloping followed by horizontal and negative slopes. Moreover, the highest value of RA could be reached by injecting at the toe itself with a horizontal bed aquifer, followed by negative and positive slopes. The recharge well is confirmed as one of the most effective applications for the mitigation of SWI in sloping bed aquifers. The Akrotiri case study shows that the proposed recharging water method has a significant impact on controlling SWI and declines in both SWI wedge length and area.

Experimental and numerical assessment of saltwater recession in coastal aquifers by constructing check dams

Artificial freshwater recharge has been considered as a feasible and effective procedure to mitigate seawater intrusion in coastal regions. The efficiency of freshwater infiltration through a check dam reservoir on saltwater recession (SWR) is investigated using two physical models. The results demonstrate the apparent tendency of recharge freshwater to move horizontally toward the boundaries rather than flowing downward to influence saltwater wedge toe. Thereby, it would affect the saltwater wedge tip instead of its toe due to the new establishment of a positive hydraulic gradient from a dam reservoir to the boundaries. Moreover, numerical dispersive simulations have been carried out on a large-scale aquifer to find the optimum location of the dam as well as the aquifer characteristics impacts on SWR efficiency. The results show that the best location to construct a check dam is immediately above the saltwater wedge toe. It is found that when saltwater head declines, the steeper hydraulic gradient between boundaries is established and the efficiency of recharge performance will be improved. Moreover, the reduction of hydraulic conductivity in vertical direction improves SWR, while higher hydraulic conductivity in the homogeneous cases only accelerates the infiltration rate but has no meaningful effect in the long term. The considered recharge method also works better in scenarios with higher dispersivity. However, the construction of check dams on floodways might be a practical and low-cost solution but it can be concluded that as the dominant direction of the recharged freshwater is toward boundaries, it cannot promptly retreat saltwater around toe position.

Investigation of transient sea level rise impacts on water quality of unconfined shallow coastal aquifers

This study analyzes the impacts of 1 m gradual and instantaneous sea level rise combined with pumping activity on seawater wedge toe location in a shallow coastal aquifer located in the southern shores of the Caspian Sea. The gradual sea level rise scenario investigates the transition variation of seawater wedge toe due to linear and parabolic between the years 2015 and 2100. Moreover, the maximum value of saltwater intrusion due to variations in hydraulic conductivity and aquifer’s recharge rates is analyzed and compared with flux-controlled and head-controlled boundary condition systems. A tree model is also applied to develop a linear regression between the sea level rise and aquifer parameters derived from different real coastal aquifers in the literature. The results indicate that 1 m gradual sea level rise in the study area by the end of the year 2100 considering all 1320 pumping wells leads to a 6.5% depletion of freshwater volume. The results also show that the peak saltwater intrusion rate during nonlinear gradual sea level rise rises with increasing the aquifer depth and horizontal hydraulic conductivity and the time to reach the saltwater intrusion peak rate considering the current recharge rate of an aquifer is about 48 m. The maximum saltwater intrusion lengths estimated by the tree model are 48 m and 10.42 m for head-controlled and flux-controlled boundary conditions, respectively, indicating that the saltwater intrusion in the aquifer due to instantaneous seal level rise is closer to head-controlled boundary conditions.

Seawater intrusion and retreat in tidally-affected unconfined aquifers: Laboratory experiments and numerical simulations

Based on combined laboratory experiments and numerical simulations, this paper examined seawater intrusion (SWI) and seawater retreat (SWR) processes caused by abrupt inland watertable changes in a laboratory-scale unconfined aquifer (length of 7.7 m and thickness of 1.0 m) subjected to a synthetic sinusoidal tide. The results showed that the salinity distribution was relatively stable and that SWI and SWR processes were almost temporally symmetrical given relatively large horizontal hydraulic gradients (0.0269, 0.0209 and 0.0149) between the inland watertable and the mean sea level. However, the salt distribution changed significantly in response to the inland watertable variations when the horizontal hydraulic gradient was relatively small (0.0030). The speed of the SWI and SWR response to the inland watertable variations was temporally asymmetric, e.g., SWR was quicker than SWI by a factor of 9 with respect to the observed saltwater wedge toe locations. As a relatively thick mixing zone (transition between freshwater and saltwater zones) was induced by the tide, simulated saltwater wedge toe locations, as indicated by the 5%, 50% and 95% isohalines, changed inconsistently. Different hysteresis behaviors were found in the relationship between the SW toe locations and total salt mass stored in the aquifer. Sensitivity analyses demonstrated that the response of both SWI and SWR to the inland watertable variations could be prolonged by a decreased tidal amplitude or decreased tidal period.

Early Cretaceous greywacke from Colville Knolls, New Zealand

ABSTRACTBlocks of greywacke sandstone were dredged from the Colville Knolls, on the margin of the Zealandia continent, in 1989. Re-examination shows them to be quartz-poor feldspathic litharenites similar to those recorded from the basement, rather than sedimentary cover, of the North Island. U-Pb dating of detrital zircons reveals significant age populations at 107, 115, 122, and 245 Ma. These compositions and ages best match those of Waipapa Composite Terrane greywacke basement on nearby Great Barrier Island. As such, an earlier Waipapa correlation is preferred, but Colville Knolls detrital petrography and zircon ages also resemble those from some Early Cretaceous Torlesse Composite Terrane greywackes. Irrespective of terrane correlation, Colville Knolls is a submarine outcrop of Cretaceous greywacke deposited at the toe of the Gondwana Mesozoic accretionary wedge, and likely represents the outer, northeastern margin of Zealandia continental crust.

An alternative approach to control saltwater intrusion in coastal aquifers using a freshwater surface recharge canal

Aquifers are a major source of freshwater in many parts of the world. Saltwater intrusion from the sea or saline lakes into freshwater aquifers degrades the potable quality of these resources. Various methods have been introduced to mitigate saltwater intrusion, such as recharge wells and physical subsurface barriers. This paper presents an alternative approach to control saltwater intrusion in coastal aquifers using a surface water recharge canal. In this paper, the effectiveness of a recharge canal at mitigating saltwater intrusion is evaluated numerically using SEAWAT. The results indicate that the recharge canal leads to a reduction in the extent of the saltwater intrusion. Under a fixed hydraulic gradient, the extent of this reduction is dependent on the location of the recharge canal relative to the saltwater source. As the hydraulic gradient increases, with the optimum location of the recharge canal approaches the saltwater source location. The results also indicate that more effective saltwater repulsion is achieved when the recharge canal is located near the toe of the saltwater wedge. The results of a field scale case study indicate that a recharge canal with relatively small dimensions could have a significant effect on reduction in the extent of the saltwater intrusion.

A giant late Precambrian chert-bearing olistostrome discovered in the Bohemian Massif: A record of Ocean Plate Stratigraphy (OPS) disrupted by mass-wasting along an outer trench slope

An intriguing example of chert–graywacke olistostrome is exceptionally well preserved within the late Neoproterozoic to early Cambrian Blovice accretionary wedge, Bohemian Massif. The olistostrome exhibits a block-in-matrix fabric defined by chert blocks isolated within the graywacke matrix. The major and trace element composition indicates two distinct types of cherts that formed either in a hydrothermal pelagic or hemipelagic environment supplied with a distal terrigenous material. The former is documented by elevated contents of Fe, Co, Zn, Ni, and Ti whereas the latter by high Al2O3 contents, relatively lower LaN/CeN ratios, and higher Eu/Eu* and Ce/Ce* values. Based on these geochemical data integrated with field observations and detrital zircon U–Pb ages of the host graywackes (determined using laser ablation ICP-MS), a new model for the origin of chert–graywacke association is proposed. The cherts are interpreted as representing pelagic and hemipelagic members of the Ocean Plate Stratigraphy (OPS) that formed in a sedimentary basin, carried on top of a subducting plate towards the trench. While moving over the outer swell (rise), the chert basin was intensely fractured and disrupted into large blocks or slabs. Subsequent motion of the plate brought the blocks onto an outer trench slope where they became gravitationally unstable to slide down and mix in the trench with distal, ca. 580–570 Ma turbidites derived from the overriding plate. Finally, this chert–graywacke olistostrome was covered by younger, ca. 560–547 Ma trench-fill turbidites (devoid of chert blocks) and accreted to the accretionary wedge toe, deformed, buried, and exhumed back to the wedge surface. We propose that such an olistostrome composed of pelagic/hemipelagic chert blocks and terrigenous, arc-derived graywacke matrix represents a rarely documented case of submarine, outer trench slope mass-wasting deposits and may be considered a new type of subduction-related mélanges. We coin the term outer-trench-slope mélange.

Insight Into Frontal Seismogenic Zone in the Mentawai Locked Region From Seismic Full Waveform Inversion of Ultralong Offset Streamer Data

AbstractSumatra subduction zone is one of the most seismically active zones on Earth. After having produced three Mw > 8.4 earthquakes and several Mw > 7.5 earthquakes, including the Mw = 7.8 2010 tsunami earthquake, the northern Mentawai segment is still locked and is capable of generating a great earthquake, possibly a disastrous tsunami. We analyzed ultralong offset seismic reflection data from this locked zone to characterize the nature of the accretionary prism and the plate interface using a combination of traveltime tomography, full waveform inversion, and prestack depth migration. In order to enhance the refractions, we downward extrapolate the streamer data to the seafloor, allowing the refraction arrivals to be observed from near‐zero offset up to far offset, and use a traveltime tomography to determine the background velocity in the upper sediments. Starting from these velocities, we perform a multiscale elastic full waveform inversion to determine the detailed P wave velocity structure of the subsurface. Based on this velocity, we perform a prestack depth migration to obtain seismic image in the depth domain and compute the porosity of the sediments to determine fluid content along faults. Our results show a low‐velocity subduction channel with high porosity at the plate interface that connects the likely active frontal thrusts at the toe of accretionary wedge, suggesting that the frontal section of the prism is seismogenic. We have also observed a low‐velocity layer in the middle of wedge separating old sediments below from new sediments above, defining the roots of bivergent thrust faults up to the seafloor, which can be interpreted as a psudo‐décollement.

Formation of the frontal thrust zone of accretionary wedges

The combination of recent megathrust earthquakes that have ruptured the frontal portions of accretionary prisms and historical evidence for large wedge-front, fold-and-thrust belt events has sparked interest in the development of wedge-front fault systems. Here we explore the formation of these faults using two-dimensional finite-difference models of spontaneous thrust fault formation and wedge accretion. A reference model with uniform material strength parameters predicts wedge-front thrusts to nucleate near the surface of the sedimentary layer and propagate down towards the basal décollement. The formation of a new frontal thrust is preceded by incipient faulting over a zone several kilometers wide forward of the wedge toe, resembling the protothrust zone (PTZ) imaged in seismic reflection data from the front of submarine accretionary prisms. Similar behavior, with a new frontal thrust forming from top to bottom, occurs if the fraction (λ) of pore-fluid pressure relative to the total (dynamic) pressure is uniform and <0.8 throughout the sediment section, or if λ increases with depth and is <0.7 at the base of the décollement. For greater values of λ, models predict more frontal thrusts to form from the bottom to top. These results are reasonably well explained by a simple elastic stress analysis. Models that simulate cohesive brittle strength that is negligible in the shallow sediments and increases with depth also predict new thrust faults to form from top to bottom. Models in which viscous flow at shallow depths represents creep of sediments predict bottom-up thrust nucleation when viscous stresses due to plate convergence remain lower than the brittle yield stress in the top ∼15–50% of the sediment section. Top-down nucleation is otherwise favored when viscous stresses exceed the brittle yield strength throughout most of the sediments. Our study illuminates key controls on the mechanical and temporal links between the PTZ, the forward propagation of slip along the underlying décollement, and the formation of a new frontal thrust.

Modeling the Geometry of Plate Boundary and Seismic Structure in the Southern Ryukyu Trench Subduction Zone, Japan, Using Amphibious Seismic Observations

AbstractHere we present the new model, the geometry of the subducted Philippine Sea Plate interface beneath the southern Ryukyu Trench subduction zone, estimated from seismic tomography and focal mechanism estimation by using passive and active data from a temporary amphibious seismic network and permanent land stations. Using relocated low‐angle thrust‐type earthquakes, repeating earthquakes, and structural information, we constrained the geometry of plate boundary from the trench axis to a 60 km depth with uncertainties of less than 5 km. The estimated plate geometry model exhibited large variation, including a pronounced convex structure that may be evidence of a subducted seamount in the eastern portion of study area, whereas the western part appeared smooth. We also found that the active earthquake region near the plate boundary, defined by the distance from our plate geometry model, was clearly separated from the area dominated by short‐term slow‐slip events (SSEs). The oceanic crust just beneath the SSE‐dominant region, the western part of the study area, showed high Vp/Vs ratios (&gt;1.8), whereas the eastern side showed moderate or low Vp/Vs (&lt;1.75). We interpreted this as an indication that high fluid pressures near the surface of the slab are contributing to the SSE activities. Within the toe of the mantle wedge, P and S wave velocities (&lt;7.5 and &lt;4.2 km/s, respectively) lower than those observed through normal mantle peridotite might suggest that some portions of the mantle may be at least 40% serpentinized.