Subduction zones generate the largest and most devastating earthquakes and tsunamis on Earth as a result of seismic slip on the megathrust fault. In addition to being capable of generating magnitude 8+ earthquakes, megathrusts also accommodate plate convergence via aseismic creep processes including episodic slow slip events. Above the megathrust, a portion of the overall plate convergence is accommodated as finite permanent strain (shortening) via slip along upper plate faults, tectonic folding, and reduction of sediment porosity (compaction). The most seaward expression of tectonic shortening in a subduction zone is focused within the outer accretionary wedge, but can also extend seaward of the main frontal thrust into sediments of the trench. Quantifying the strain budget among these different processes is essential for a better understanding of the partitioning between permanent inelastic strain and elastic strain accumulation as part of the seismic cycle – and thus ultimately toward an improved picture of subduction zone behavior and tsunami hazard. In this study, we use exceptionally detailed seismic reflection depth imaging and P-wave velocities to characterize sediment compaction within the outer wedge and trench along a profile of the southern Hikurangi subduction margin. Complementing these data with new constraints on stratigraphy, lithology and sediment physical properties, we provide the first quantifications of tectonic shortening attributable to sediment compaction on the Hikurangi margin. Our results demonstrate a broad region of compaction that extends more than 15 km seaward of the outermost faults. Future work beyond this study will explore relationships between pore scale compaction, proto-thrust development and active creep near the trench, in an attempt to provide a holistic understanding of strain accumulation in the outer wedge and trench.

Earthquake nucleation is a fundamental problem in earthquake science and has practical implications for forecasting seismic hazards. Laboratory experiments performed on large, meter-scale fault systems offer unique insights into the nucleation process because the migration and expansion of the nucleation zone can be precisely detected, measured, and characterized using arrays of local strain and slip measurements. We report on a series of laboratory experiments conducted on a 1-m direct shear machine. We sheared layers of quartz gouge between roughened acrylic forcing blocks over a range of normal stresses between 3 and 12 MPa, generating a spectrum of slip modes, ranging from aseismic creep to fast-dynamic rupture. Co-seismic slip, peak slip velocity, and high-frequency acoustic energy content of laboratory earthquakes increases systematically with both cumulative fault slip and normal stress. Slower and smaller laboratory earthquake sequences have larger nucleation zones, creep more during their inter-seismic period, and are deficient in high-frequency energy compared to larger and faster rupture sequences. We find that the critical nucleation length scale, H*, scales inversely with cumulative fault slip and normal stress. A reduction in H* and an increase in event size can be explained by a decrease in the critical slip distance, D c, or an increase in the frictional rate parameter b-a and is likely driven by shear localization. Together, our results indicate that homogeneous, mature fault zones that have undergone more cumulative fault slip are expected to have smaller H* and can more easily host dynamic instabilities, relative to immature faults.

The Hikurangi margin has been an important global focus for subduction zone research for the last decade. International Ocean Discovery Program drilling and geophysical investigations have advanced our understanding of megathrust slip behavior. Along and across the margin, detailed imaging reveals that the megathrust structure varies spatially and evolves over time. Heterogeneous properties of the plate boundary zone and overriding plate are impacted by the evolving nature of regional tectonics and inherited overriding plate structure. Along-strike variability in thickness of subducting sediment and northward increasing influence of seamount subduction strongly influence megathrust lithologies, fluid pressure, and permeability structure. Together, these exert strong control on spatial variations in coupling, slow slip, and seismicity distribution. Thicker incoming sediment, combined with a compressional upper plate, influences deeper coupling at southern Hikurangi, where paleoseismic investigations reveal recurring great ( M w > 8.0) earthquakes. ▪ The Hikurangi Subduction Zone is marked by large-scale changes in the subducting Pacific Plate and the overlying plate, with varied tectonic stress, crustal thickness, and sediment cover. ▪ The roughness of the lower plate influences the variability in megathrust slip behavior, particularly where seamounts enhance subduction of fluid-rich sediments. ▪ Variations in sediment composition impact the strength of the subduction interface, with the southern Hikurangi Subduction Zone exhibiting a more uniform megathrust fault. ▪ Properties of the upper plate influence fluid pressures and contribute to the observed along-strike variations in Hikurangi plate coupling and slip behavior.

Patterns of strain accumulation and release offshore in subduction zones are directly linked to the potential for shallow coseismic slip and tsunamigenesis, but these patterns remain elusive. In this work, we analyze formation pore pressure records from three offshore borehole observatories at the Nankai subduction zone, Honshu, Japan, to capture detailed slip-time histories of two slow slip events (SSEs) along the outermost reaches of the plate boundary. Slip initiates ~30 kilometers landward of the trench; migrates seaward at 1 to 2 kilometers per day to within a few kilometers of, and possibly breaching, the trench; and coincides with the onset and migration of tremor and/or very-low-frequency earthquakes. The SSE source region lies in a zone of high pore fluid pressure and low stress, which provides clear observational evidence linking these factors to shallow slow earthquakes.

Heterogeneity in geometry, stress, and material properties is widely invoked to explain the observed spectrum of slow earthquake phenomena. However, the effects of length scale of heterogeneity on macroscopic fault sliding behavior remain underexplored. We investigate this question for subduction megathrusts, via linear stability analysis and quasi‐dynamic simulations of slip on a dipping fault characterized by rate‐and‐state friction. Frictional heterogeneity is imposed through alternating velocity‐strengthening and velocity‐weakening (VW) patches, over length scales spanning from those representative of basement relief (several km) to the entrainment of contrasting lithologies (100s of m). The resulting fault behavior is controlled by: (a) the average frictional properties of the fault, and (b) the size of VW blocks relative to a critical length scale. Reasonable ranges of these properties yield sliding behaviors spanning from stable sliding, to slow and seismic slip events that are confined within VW blocks or propagate along the entire fault.

The physical properties of subduction inputs profoundly influence megathrust slip behavior. Seismic data reveal extensive polygonal fault systems (PFSs) in the input sequences of the Hikurangi Margin and Nankai Trough. The mechanical and hydrological effects of these incoming PFSs on subduction zones are potentially substantial. Here, we investigate their effects following transport into the accretionary wedge by integrating discrete-element modeling with three-dimensional seismic interpretation. We find that the typical dips of the incoming PFSs overlap with modeled dips prone to reactivation and confirm that subducting PFSs can be reactivated and gradually evolve into major thrust faults. Comparisons with electromagnetic data indicate that PFSs may provide conduits for fluid leakage along the plate interface, coincide with disrupted strata and decreased shear stress, and enhance geometric and stress heterogeneity along the megathrust. These suggest that PFSs may play a previously unrecognized role in contributing to shallow slow earthquake phenomena in subduction zones.

JAMSTEC have been monitoring changes in underground fluid pressure, or "pore pressure," from boreholes near the site of the 1944 Tonankai earthquake in southwestern Japan. These changes are linked to Slow Slip Events (SSEs), which occur on the boundary between the Eurasian plate and the subducting Philippine Sea plate beneath the Nankai Trough. By connecting their borehole observatory (LTBMS) to a seafloor monitoring network (DONET), they now collect real-time pore pressure data, allowing them to update their SSE catalog.This updated catalog revealed something unusual: the SSE in February 2012 lasted significantly longer than similar events. Researchers studied pore pressure and seafloor pressure data to understand why. We found that the February SSE moved more slowly and lasted longer because of two key factors: internal and external forces.Internally, the SSE occurred in a region where little stress had built up on the fault, causing it to slip more slowly, consistent with frictional behavior on faults. Externally, we found that changes in seafloor pressure, driven by shifts in the Kuroshio Current (a major ocean current), coincided with the end of the February SSE. This suggests that the Kuroshio Current's meander may influence the duration of SSEs.Our study highlights that SSEs are not only shaped by fault interactions but also by environmental factors like ocean currents and atmospheric pressure. Understanding these influences is key to better predicting such events. These findings are based on a paper accepted by Tectonophysics (https://doi.org/10.1016/j.tecto.2024.230439), and we plans to share additional insights and recent practical analysis in our presentation.

Whether Earth materials exhibit frictional creep or catastrophic failure is a crucial but unresolved problem in predicting landslide and earthquake hazards. Here, we show that field-scale observations of sliding velocity and pore water pressure at two creeping landslides are explained by velocity-strengthening friction, in close agreement with laboratory measurements on similar materials. This suggests that the rate-strengthening friction commonly measured in clay-rich materials may govern episodic slow slip in landslides, in addition to tectonic faults. Further, our results show more generally that transient slow slip can arise in velocity-strengthening materials from modulation of effective normal stress through pore pressure fluctuations. This challenges the idea that episodic slow slip requires a narrow range of transitional frictional properties near the stability threshold, or pore pressure feedbacks operating on initially unstable frictional slip.

Glacio-eustatic cycles lead to changes in sedimentation on all types of continental margins. There is, however, a paucity of sedimentation rate data over eustatic sea-level cycles in active subduction zones. During International Ocean Discovery Program Expedition 375, coring of the upper similar to 110 m of the northern Hikurangi Trough Site U1520 recovered a turbidite-dominated succession deposited during the last similar to 45 kyrs (Marine Isotope Stages (MIS) 1-3). We present an age model integrating radiocarbon dates, tephrochronology, and delta O-18 stratigraphy, to evaluate the bed recurrence interval (RI) and sediment accumulation rate (SAR). Our analyses indicate mean bed RI varies from similar to 322 yrs in MIS1, similar to 49 yrs in MIS2, and similar to 231 yrs in MIS3. Large (6-fold) and abrupt variations in SAR are recorded across MIS transitions, with rates of up to similar to 10 m/kyr occurring during the Last Glacial Maximum (LGM), and <1 m/kyr during MIS1 and 3. The pronounced variability in SAR, with extremely high rates during the LGM, even for a subduction zone, are the result of changes in regional sediment supply associated with climate-driven changes in terrestrial catchment erosion, and critical thresholds of eustatic sea-level change altering the degree of sediment bypassing the continental shelf and slope via submarine canyon systems.

The Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) and the Long Term Borehole Monitoring System (LTBMS), installed above the source region of the 1944 Tonankai earthquake, revealed that crustal deformation is driven by slow slip events (SSEs) in the shallower extension of megathrust earthquakes. However, there are unresolved questions about (A) the duration of the SSE in February 2012, which was longer than expected for SSEs with similar magnitudes, and (B) the relationship of the spatial distribution of fault slip between the SSEs in February and December 2012 under the condition of drilling disturbance. To clarify these questions, we re-analyzed the pore/seafloor pressure data associated with the SSEs. Our refined fault models show that the SSE in February had a significantly slower propagation speed and a longer duration than others, while the SSE in December was comparable to others. We interpret that the difference in duration and propagation speed is related to external and internal stress perturbations, respectively. Using the ocean modeling JCOPE (Japan Coastal Ocean Predictability Experiment), we identified that the decrease and subsequent increase in seafloor pressure due to the passage of the Kuroshio meander coincided with the latter part of the longer duration of the SSE in February and its termination, respectively. This suggests that the Kuroshio meander might affect the duration of SSEs. Our refined fault model also indicates that the amount of shear stress accumulation was small before the occurrence of the SSE in February, which triggered the slow propagation of aseismic slip based on a rate- and state-dependent friction law. These results imply that we need to consider the variety of SSEs from the viewpoint of stress perturbation due to not only interaction between fault segments but also external forces from oceanographic phenomena.

ABSTRACT: We study the influence of an upper-plate fault on the stress state of accreting sediments under large-scale deformation. We develop drained evolutionary geomechanical models using the Finite Element program Elfen. We simulate sediments as porous-elastoplastic material, and we model the fault as a pre-existing contact surface with a varying frictional strength that is lower than the intact sediment. The weaker fault results in a decrease in sediment differential stress near and especially seaward of the fault. A significant section of the wedge is affected by this stress variation. In contrast, the stress ratio is that of Coulomb failure further away from the fault. We also show that the maximum principal stress the sediments can support decreases with decreasing fault strength. This study offers a significant improvement over previous models of continuum wedge sediments that predict Coulomb failure throughout the wedge. Our results improve our understanding of near-fault stress state, hence improving our understanding of seismic hazards in subduction zones and providing practical insights for reservoir quality and the design of safe and economic well trajectories. 1 INTRODUCTION Accretionary wedges are geological structures that develop at convergent plate boundaries, particularly at subduction zones. They form as a result of offscraping sediments from the subducting oceanic plate, which then accumulates on the leading edge of the overriding plate (e.g., Moore et al., 2011; Buiter et al., 2016; Gao et al., 2018). Understanding the state of stress in accretionary wedges is crucial for the study of earthquake mechanics in subduction zones (e.g., Brodsky et al., 2017; Huffman and Saffer, 2016; Suppe, 2007). It provides insights for earthquake occurrence, large or slow-slip events (e.g., Kodaira et al., 2012; Tobin et al., 2022; Liu and Rice, 2007) and tsunami potential (e.g., Dean et al., 2010; Riedel et al., 2016). In addition, estimates of mean and differential stress, as well as porosity evolution can help improve the assessment of reservoir quality and the design of safe and economic well trajectories (e.g., Morley et al., 2011). The accumulation of sediments in an accretionary wedge resembles a bulldozer gathering snow as it moves. Hence, wedge sediments are often assumed to be at compressional failure (e.g., Davis et al., 1983; Dahlen et al., 1984; Flemings and Saffer, 2018). Several field observations of faulting, folding and lateral compression as seen in seismic images and drilling measurements (e.g., Flemings and Saffer, 2018; Henry et al., 2003; Moore et al., 1990; Westbrook et al., 1988) provide evidence that the accretionary wedge is at a state of failure in compression.

Per- and polyfluoroalkyl substances (PFAS) can represent a significant human health risk if present in aquifers used as a drinking water source. Accurate assessment of PFAS exposure risks requires an improved understanding of field-scale PFAS transport in groundwater. Activities at a former firefighter training site in University Park, Pennsylvania introduced perfluorooctanesulfonic acid (PFOS) to the underlying dolomite aquifer. Groundwater sampling from 2015 to 2018 delineated a PFOS plume with two concentration maxima located approximately 20 and approximately 220 m downgradient of the training site, separated by a zone of lower concentrations. We use a combination of analytical and numerical models, informed by independent measurements of aquifer porosity, hydraulic conductivity, and organic carbon content, to interpret the field observations. Our analysis demonstrates that preferential retention and transport resulting from simple heterogeneity in bedrock sorption, as caused by organic carbon (OC) content variability, provides a plausible explanation for plume separation. Dissolved PFOS partitions strongly to organic solids (high K-oc), so even a small OC (<1 wt%) significantly retards PFOS transport, whereas zones with little to no OC allow for transport rates that approximate those of a conservative solute. Our work highlights an important consideration for modeling the groundwater transport of PFOS, and other compounds with high K-oc. In aquifers with discrete layers of varying OC, models using a uniform site-average OC will underestimate transport distances, thereby misrepresenting exposure risks for downgradient communities.

Frictional healing is fundamental to the seismic cycle and plays a role in the energy balance, dynamics, and recurrence interval of earthquakes and slow slip events. Although the healing behavior of quartz has been studied extensively, the role of clay content is less understood. We tested synthetic mixtures of quartz and smectite (10%–100% smectite) in a double‐direct shear configuration to measure frictional healing. We show that the magnitude and rate of healing decreases systematically with higher clay content (from 0.008/decade at 10% smectite to 0.002/decade at 100% smectite). Healing scales with both the magnitude of stress relaxation during holds and layer‐normal compaction of the gouge. We suggest this reflects the alignment of clay minerals, leading to saturation of the real area of contact that limits restrengthening during holds. The low healing rates of clay‐rich faults—together with rate‐neutral to rate‐strengthening friction—should promote frequent, small failures or stable sliding.

Subduction megathrusts exhibit a spectrum of slip modes, including catastrophic earthquakes. Although the mechanical and frictional properties of materials sampled from subduction zones have been studied extensively, few datasets have been collected for compositions and at pressure and temperature conditions representative of those in situ. The Nankai subduction zone in southwest Japan is a well-studied margin, and abundant data provide an opportunity to advance our understanding of fault and earthquake physics. Here, we use samples exhumed in the Shimanto and Sanbagawa Belts on Shikoku Island of southwest Japan that represent analogs for materials along the present-day megathrust at depths of similar to 5->25 km, and we shear these at their peak in situ pressure-temperature (P-T) conditions. Effective normal stresses range from 28 to 192 MPa, and temperatures from 105 degrees C to 470 degrees C. We used pore fluid pressures of 45-240 MPa, corresponding to fluid overpressure ratios lambda of 0.65 and 0.90. Slip velocities of 0.1-100 mu m/s were used, in order to focus on the nucleation of instability and earthquakes. We found predominantly velocity-strengthening (inherently stable) behavior under all conditions for lambda = 0.65. For lambda = 0.90, velocity-weakening behavior was observed at 350 degrees C, with velocity-strengthening behavior at lower and higher temperatures. The rate/state frictional stability parameter (a-b) increases with slip velocity at temperatures up to similar to 200 degrees C and remains constant or decreases with slip velocity at higher temperatures. Overall, our results demonstrate the potentially important roles of both temperature and slip velocity in controlling the distribution of stress and frictional rheology along subduction thrusts.

Plate motion on shallow subduction megathrusts is accommodated by a spectrum of tectonic slip modes. However, the frictional properties and conditions that sustain these diverse slip behaviors remain enigmatic. Frictional healing is one such property, which describes the degree of fault restrengthening between earthquakes. We show that the frictional healing rate of materials entrained along the megathrust at the northern Hikurangi margin, which hosts well-characterized recurring shallow slow slip events (SSEs), is nearly zero (<0.0001 per decade). These low healing rates provide a mechanism for the low stress drops (<50 kilopascals) and short recurrence times (1 to 2 years) characteristic of shallow SSEs at Hikurangi and other subduction margins. We suggest that near-zero frictional healing rates, associated with weak phyllosilicates that are common in subduction zones, may promote frequent, small-stress-drop, slow ruptures near the trench.

We study stress, pressure, and rock properties in evolving accretionary wedges using analytical formulations and geomechanical models. The evolution of the stress state from that imposed by uniaxial burial seaward of the trench to Coulomb failure within the wedge generates overpressure and drives compaction above the décollement. Changes in both mean and shear stress generate overpressure and shear‐induced pressures play a particularly important role in the trench area. In the transition zone between uniaxial burial and Coulomb failure, shear‐induced overpressures increase more than overburden and are higher than footwall pressures. This rapid increase in overpressure reduces the effective normal stress and weakens the plate interface along a zone that onsets ahead of the trench and persists well into the subduction zone. It also drives dewatering at the trench, which enables compaction of the hanging‐wall sediments and a porosity offset at the décollement. Within the accretionary wedge, sediments are at Coulomb failure and the pore pressure response is proportional to changes in mean stress. Low permeability and high convergence rates promote overpressure generation in the wedge, which limits sediment strength. Our results may provide a hydromechanical explanation for a wide range of observed behaviors, including the development of protothrust zones, widespread occurrence of shallow slow earthquake phenomena, and the propagation of large shallow coseismic slip.

Perfluoroalkyl acids (PFAAs), a group of synthetic compounds associated with adverse human health impacts, are commonly found in effluent discharged from wastewater treatment facilities. When that effluent is used for irrigation, the fate of PFAAs depends strongly on vadose zone solute retention properties and loading history. The relative importance of PFAA retention factors under natural conditions remains uncertain, and the historical record of effluent PFAA concentrations is limited. Using soil cores collected from the Penn State Living Filter (irrigated with treated wastewater effluent for nearly 60 years), we evaluated PFAA transport under near-natural conditions, and estimated historical PFAA concentrations in the irrigated effluent. Total perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) masses stored in soils in 2014 were more than 450 times greater than the masses applied during the 2020 effluent irrigation. Equilibrium piston-flow transport models reproduced the observed PFOS and PFOA profiles, allowing us to estimate historical effluent PFOS and PFOA concentrations: 70-170 ng L-1 and 1000-1300 ng L-1, respectively. Estimated concentrations were comparable to concentrations measured in other wastewater effluents in the 1990s and 2000s, indicating that when interpreted with transport modeling, wastewater-irrigated soils function as integrated records of historical PFAA loading. Simulated PFOS breakthrough to groundwater occurred 50 years after the start of wastewater irrigation, while simulated PFOA breakthrough occurred after only 10 years of irrigation. Thus, while wastewater irrigation of soils facilitates retention and reduces effluent PFAA loading to surface waters, the resulting increased PFAA storage in soils potentially creates long-term sources of PFAAs to groundwater.

Many geoscientific problems require us to exploit synergies of experimental and numerical approaches, which in turn lead to questions regarding the significance of experimental details for validation of numerical codes. We report results of an interlaboratory comparison regarding experimental determination of mechanical and hydraulic properties of samples from five rock types, three sandstone varieties with porosities ranging from 5% to 20%, a marble, and a granite. The objective of this study was to build confidence in the participating laboratories’ testing approaches and to establish tractable standards for several physical properties of rocks. We addressed the issue of sample-to-sample variability by investigating the variability of basic physical properties of samples of a particular rock type and by performing repeat tests. Compressive strength of the different rock types spans an order of magnitude and shows close agreement between the laboratories. However, differences among stress–strain relations indicate that the external measurement of axial displacement and the determination of system stiffness require special attention, apparently more so than the external load measurement. Furthermore, post-failure behavior seems to exhibit some machine-dependence. The different methods used for the determination of hydraulic permeability, covering six orders of magnitude for the sample suite, yield differences in absolute values and pressure dependence for some rocks but not for others. The origin of the differences in permeability, in no case exceeding an order of magnitude, correlate with the compressive strength and potentially reflect a convolution of end plug–sample interaction, sample-to-sample variability, heterogeneity on sample scale, and/or anisotropy, the last two aspects are notably not accounted for by the applied evaluation procedures. Our study provides an extensive data set apt for “benchmarking” considerations, be it regarding new laboratory equipment or numerical modeling approaches.