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Laboratory measurements can determine the potential for geologic materials to generate unstable (seismic) slip, but a direct relation between sliding behavior in the laboratory and physical characteristics observable in the field is lacking, especially for the phyllosilicate-rich gouges that are widely observed in natural faults. We integrated laboratory friction experiments with surface topography microscopy and demonstrated a quantitative correlation between frictional slip behavior and fault surface morphology of centimeter-scale samples. Our results show that striated, smooth fault surfaces were formed in experiments that exhibited stable sliding, whereas potentially unstable sliding was associated with rougher, isotropic fault surfaces. We interpret that frictional stability and fault surface morphology are linked via the evolution of asperity contacts on localized slip surfaces. If fault surface roughness obeys a fractal relationship over a large range of length scales, then we infer that the morphological characteristics observed in the laboratory could indicate the earthquake nucleation potential on natural fault surfaces.

Slow slip events (SSEs) have been identified at subduction zones globally as an important link in the continuum between elastodynamic ruptures and stable creep. The northern Hikurangi margin is hom...

The generation of the 2011 Tohoku-oki earthquake has been modeled by many authors by considering a dynamic weakening mechanism such as thermal pressurization (TP). Because the effects of TP on afterslip have not been investigated, this study develops a 3D quasi-dynamic model of the earthquake cycle to investigate afterslip of the Tohoku-oki earthquake, considering TP and the geometry of the plate boundary. We employ several velocity-weakening (VW) patches for Mw 7 class events, and two large shallow VW patches. The frictional properties are set as velocity-strengthening (VS) outside the VW patches. The results show that, during megathrust earthquakes, fast slip propagates to the surrounding VS regions near the VW patches owing to weakening by TP. Following Mw 9 events, large afterslips occur in regions below the northern shallow rupture area in the off-Fukushima region close to the Japan Trench, which is consistent with observations. In the VS region near the VW patches, during the early afterslip period, frictional behavior exhibits less VS with increasing slip velocity due to pore pressure reduction. We also consider the frictional properties of the shallow plate boundary fault off Tohoku, which exhibits a transition from VW to VS from low to high slip velocities. The results show the occurrence of slow slip events (SSEs) at intervals of a few decades at the shallow plate boundary. During megathrust events, the VW property at low slip velocity promotes slip along the shallow SSE region more than the case with VS property throughout the entire velocity range.

Regarding the occurrence of seismicity on major plate-boundary fault zones, one leading hypothesis is that the processes of lithification is responsible transforming loose, unconsolidated sediment that does not host earthquake nucleation into the frictionally unstable rocks that inhabit the seismogenic zone. Previous laboratory studies comparing the frictional properties of intact rocks and powdered versions of the same rocks generally support this hypothesis. However, systematically quantifying frictional behavior as a function of lithification remains a challenge. Here, we simulate the lithification process in the laboratory by consolidating mixtures of halite and shale powders with halite-saturated brine, which we then desiccate. The desiccation allows precipitation of halite as cement, creating synthetic rocks. We quantify lithification by: (1) direct measurement of cohesion, and (2) measuring the porosity reduction of lithified samples compared to powders. We observe that powdered samples of each halite-shale proportion exhibit predominantly velocity-strengthening friction, whereas lithified samples exhibit a combination of velocity strengthening and significant velocity weakening when halite constitutes at least 30 wt% of the sample. Analysis of the individual rate-dependent friction parameters shows that the occurrence of velocity weakening is due to relatively low values of a for lithified samples. Larger velocity weakening is associated with cohesion of >∼1 MPa, and porosity reduction of >∼50 vol%. Microstructural images reveal that the shear surfaces for powders tend to exhibit small cracks not seen on the lithified sample shear surfaces. Our results suggest that lithification via cementation and porosity loss can facilitate slip instability, supporting the lithification hypothesis for seismogenic slip.

Calcareous‐pelagic input sediments are present at several subduction zones and deform differently to their siliciclastic counterparts. We investigate deformation in calcareous‐pelagic sediments drilled ~20 km seaward of the Hikurangi megathrust toe at Site U1520 during IODP Expeditions 372 and 375. Clusters of normal faults and subhorizontal stylolites in the sediments indicate both brittle faulting and viscous pressure solution operated at 150°C where frictional (possibly seismic) slip likely predominates. Plain Language Summary The type of sediments entering subduction zones will influence the way the plates in the subduction zone slide past one another. We looked at limestones in sediments drilled before they reach the subduction zone and found that because of the pressure they are under, they begin to crack and dissolve at very shallow depths. Most of the dissolution happens on thin layers where it concentrates clay by removing other, more soluble minerals. We compare how much vertical shortening we see in the sediments to a computer model. The model overestimates vertical shortening over the history of the sediment unless either high pressure fluids reduce the pressure felt by the sediments, or dissolution is governed by the largest sediment grains rather than their average size. Dissolving and cracking make the sediments weaker by concentrating soft materials such as clay. When these sediments enter the subduction zone, the two plates might slip past one another more easily on these weak regions, possibly during slow slip events.

Earthquakes occur by overcoming fault friction; therefore, quantifying fault resistance is central to earthquake physics. Values for both static and dynamic friction are required, and the latter is especially difficult to determine on natural faults. However, large earthquakes provide signals that can determine friction in situ. The Japan Trench Fast Drilling Project (JFAST), an Integrated Ocean Discovery Program expedition, determined stresses by collecting data directly from the fault 1–2 years after the 2011 Mw 9.1 Tohoku earthquake. Geological, rheological, and geophysical data record stress before, during, and after the earthquake. Together, the observations imply that the shear strength during the earthquake was substantially below that predicted by the traditional Byerlee's law. Locally the stress drop appears near total, and stress reversal is plausible. Most solutions to the energy balance require off-fault deformation to account for dissipation during rupture. These observations make extreme coseismic weakening the preferred model for fault behavior. ▪ Determining the friction during an earthquake is required to understand when and where earthquakes occur. ▪ Drilling into the Tohoku fault showed that friction during the earthquake was low. ▪ Dynamic friction during the earthquake was lower than static friction. ▪ Complete stress drop is possible, and stress reversal is plausible.

On major plate-boundary fault zones, it is generally observed that large-magnitude earthquakes tend to nucleate within a discrete depth range in the crust known as the seismogenic zone. This is generally explained by the contrast between frictionally stable, velocity strengthening sediments at shallow depths and lithified, velocity-weakening rocks at seismogenic (10’s of km) depth. Thus, it is hypothesized that diagenetic and low-grade metamorphic processes are responsible for the development of velocity-weakening frictional behavior in sediments that make up fault gouges. Previous laboratory studies comparing the frictional properties of intact rocks and powdered versions of the same rocks generally support this hypothesis, however controlling lithification in the laboratory and systematically quantifying frictional behavior as a function of lithification and remains a challenge.Here, we simulate the lithification process in the laboratory by using mixtures of halite and shale powders with halite-saturated brine, which we consolidate under 10 MPa normal stress and subsequently desiccate. The desiccation allows precipitation of halite as cement, creating synthetic rocks. We vary the proportion of salt to shale in our samples, which we use as a proxy for degree of lithification. We measure the frictional properties of our lithified samples, and equivalent powdered versions of these samples, with velocity-step tests in the range 10-7 – 3x10-5 m/s. We quantify lithification by two methods: (1) direct measurement of cohesion, and (2) measuring the porosity reduction of lithified samples compared to powders. Using these measurements, we systematically investigate the relationship between lithification and frictional slip behavior.We observe that powdered samples of every halite-shale proportion exhibits predominantly velocity-strengthening friction, whereas lithified samples exhibit a combination of velocity strengthening and significant velocity weakening when halite constitutes at least 30 wt% of the sample. Larger velocity weakening generally coincides with friction coefficients of > 0.62, cohesion of > ~1 MPa, and porosity reduction of > ~50 vol%. Although none of our lithified samples exhibit strictly velocity-weakening friction, this is consistent with the frictional behavior of pure halite under our experimental conditions. Scanning electron microscopy images do not show any clear characteristics attributable to velocity-weakening, but did reveal that the shear surfaces for powders tends to exhibit small cracks not seen in the lithified sample shear surfaces. These results suggest that lithification via cementation and porosity loss may facilitate slip instability, but that microstructural indicators are subtle.

Faults that are active at retrograde conditions tend to contain metastable fault rock assemblages that are prone to undergo fluid-consuming reactions. These reactions typically lead to growth of minerals that are viscously and frictionally weaker than the reactants. This is illustrated in the well-studied Outer Hebrides Fault Zone (OHFZ) of Scotland, and we add observations from the Kuckaus Mylonite Zone (KMZ), Namibia. In both locations, deformation is localised in anastomosing networks of phyllosilicates that developed during deformation of amphibolite and/or granulite assemblages at greenschist facies conditions. Microstructures of these phyllonites show generally well aligned phyllosilicates wrapping around fractured feldspars and quartz with features indicating dislocation creep.In the KMZ, further localization occurred in ultramylonites within the mylonite zone. These are characterised by a similar phyllosilicate proportion to surrounding mylonites, but lack interconnected phyllosilicate networks. Instead, they contain a very fine-grained assemblage of quartz, feldspar, and phyllosilicate, where both quartz and feldspar lack a CPO. We interpret this assemblage as having deformed through grain-size sensitive creep, at lower shear stress than the surrounding mylonite. It is possible that the ultramylonites developed by dismembering an earlier interconnected weak phase microstructure with increasing finite strain, as has been suggested experimentally by Cross and Skemer (2017).Whereas these exhumed fault zones deformed at greenschist facies conditions, continued activity would exhume similar fault rocks to shallower depth. We explored frictional properties and microstructure of greenschist facies fault rock at low temperature conditions by deforming chlorite-amphibole-epidote assemblages in single-direct shear at room temperature and 10 MPa normal stress under fluid saturated conditions. As inferred at greater depth, presence of chlorite weakens and promotes aseismic creep along these experimental faults. Presence of chlorite also correlates with the development of striations on fault surfaces. Lack of chlorite, on the other hand, leads to velocity-weakening behaviour and, in epidotite, a fault surface containing very fine grains that do not develop when ≥ 50 % chlorite is present. We suggest that chlorite supresses wear at contact asperities between stronger minerals, and therefore also supresses velocity-weakening behaviour.Overall, we see that growth of retrograde phyllosilicates lead to profound weakening, strain localisation, and frictional stabilisation of major shear zones, from greenschist facies to near-surface conditions. These processes and properties are, however, reliant on external fluids to allow hydration reactions in otherwise relatively dry host rocks. From scattered syn-deformational quartz veins, in the KMZ, such fluids appear to be of surface origin, whereas in the OHFZ, fluids were likely of a deeper, metamorphic or magmatic origin. Ready incorporation of such fluids into retrograde minerals would prevent substantial or widespread fluid overpressures from developing. These fluid sources are similar to present-day inferred fluid regimes in the Alpine and San Andreas Faults, respectively. We speculate that the variable slip behaviour seen on active retrograde faults relate to their degree of retrogression, and the development of time and strain-dependent microstructures with specific strengths and behaviours.

Slow slip events (SSEs) are recognized as an important component of plate boundary fault slip, and there is a need for laboratory friction data on natural samples to guide comparisons with natural SSEs. Here, we compile a comprehensive catalog of SSEs observed geodetically at the Hikurangi subduction zone offshore northern New Zealand, and compare it with results of laboratory friction experiments that produce laboratory SSEs under plate tectonic driving rates (5 cm/yr). We use samples from Ocean Drilling Program Site 1124 seaward of the Hikurangi subduction zone to represent the plate boundary that hosts shallow SSEs at Hikurangi. We find that laboratory SSEs exhibit a similar displacement record and range of stress drops as the natural SSEs. Results of velocity step tests, which can be used to evaluate frictional instability based on the critical stiffness criterion, indicate that the slow slip activity at Hikurangi is a form of stably-accelerating slip. Our laboratory SSEs provide an alternative method of quantifying (in)stability by direct measurement of the unloading stiffness during the stress drop. The observed dependence of laboratory SSE parameters on effective normal stress is consistent with critical stiffness theory; however, depth-increasing projections based on laboratory data do not match observations from natural SSEs. These differences are likely related to changing temperature and fault rock composition downdip but also complications related to scaling and/or limited sampling. Scientific drilling recently undertaken at the Hikurangi subduction zone should serve to improve and guide future studies of the role of frictional properties for the occurrence of SSEs.

This chapter describes the behaviour of faults in the laboratory. The chapter is organised from small scale to large scale experiments, introducing the reader to general and less general observations of faulting and friction, and showing how these observations are linked to faulting processes occurring in nature. The first section introduces cm-scale friction experiments on gouge materials including the concept of rate-and-state friction, i.e., how velocity affects friction in the quasi-static regime. The following section is devoted to dynamic friction, i.e., observations of friction at high velocity as well as observations of dynamic rupture. The third section discusses the evolution of discrete faults and fault zones in up to meter-scale physical analogue experiments, their dependence on material properties and their significance for the study of large-scale tectonic structures. Finally, the various microstructural features and their possible link to fault stability obtained in the quasi-static regime will be discussed.

In retrograde faults exhuming mafic rocks, shearing occurs in metamorphic and/or hydrothermally altered mineral assemblages whose frictional properties are not well known. Here, we present the results of laboratory shearing experiments on chlorite schist, epidotite, and hornblende‐dominated amphibolite and mixtures of these rocks and evaluate their frictional properties and microstructures. The experiments were conducted on powdered rock samples with starting grain size of

Geological records of past megathrust earthquakes on the Cascadia subduction zone give a valid reason to assume that another event will occur in the future. Both the magnitude and tsunamigenic potential of such an earthquake depend on the extent of seismic rupture and slip on the shallow megathrust, which are still under debate for Cascadia. The absence of shallow slow slip events at northern Cascadia has recently been interpreted to result from a megathrust that is locked and potentially seismogenic all the way to the trench. A crucial factor controlling both the nucleation and propagation of an earthquake is the frictional slip behavior of the material in the fault system, of which little is currently known for this setting. Here, we report on laboratory friction experiments on all major lithologies of Juan de Fuca plate sediments being input to the northern Cascadia subduction zone. The material was obtained from cores recovered during Integrated Ocean Drilling Program Expedition 301 at Site U1301 in the depth range of 65 to 260 meters below the seafloor. From tests on both intact and powdered samples under in-situ effective normal stress and room temperature, we observe a significant decrease in friction coefficient from 0.76 to 0.19 with increasing depth. A frictionally weak hemipelagic illite-rich unit located directly above the basement is expected to host the decollement upon subduction. This unit reveals predominantly velocity-strengthening frictional behavior at slip velocities of ∼ 10 − 9 to 10 − 4 m/s, and exhibits no slow slip events in the laboratory, indicating a frictionally stable material. Coulomb wedge modeling based on our friction measurements and interpretation of decollement stratigraphic location suggests significant overpressure on the decollement near the trench. Our data offer a possible explanation for the lack of shallow slow slip activity on the northern Cascadia plate-boundary fault, and raise the possibility that the megathrust is not locked near the trench.

Slow slip phenomena, including earthquake afterslip and discrete slow slip events (SSE), may have a major effect on megathrust earthquakes in subduction zones, however the nature of this effect is still incompletely understood. Here, we report that in friction experiments using samples from the plate boundary fault zone in the 2011 Tohoku-Oki earthquake, an increase in sliding velocity can induce a change from steady-state frictional strength or slip strengthening friction to slip-weakening frictional behavior. Specifically, significant slip weakening is caused by velocity steps with initial slip rates consistent with afterslip observed after the largest Tohoku earthquake foreshock on March 9th, 2011. This suggests that the foreshock afterslip, which is slightly faster than discrete SSEs, is favorable for fault weakening that may have contributed to the large coseismic slip during the mainshock on the shallow plate boundary fault during the Tohoku-Oki earthquake.

The morphology and geometry of the plate interface in a subduction zone is heterogeneous and influenced by lower-plate normal faulting, suggests an analysis of seismic data. These properties of subduction interfaces may influence how the largest earthquakes occur.

Slow slip events (SSEs) at the northern Hikurangi subduction margin, New Zealand, are among the best-documented shallow SSEs on Earth. International Ocean Discovery Program Expeditions 372 and 375 were undertaken to investigate the processes and in situ conditions that underlie subduction zone SSEs at the northern Hikurangi Trough. We accomplished this goal by (1) coring and geophysical logging at four sites, including penetration of an active thrust fault (the Pāpaku fault) near the deformation front, the upper plate above the SSE source region, and the incoming sedimentary succession in the Hikurangi Trough and atop the Tūranganui Knoll seamount; and (2) installing borehole observatories in the Pāpaku fault and in the upper plate overlying the slow slip source region. Logging-while-drilling (LWD) data for this project were acquired as part of Expedition 372, and coring, wireline logging, and observatory installations were conducted during Expedition 375. Northern Hikurangi subduction margin SSEs recur every 1–2 y and thus provide an ideal opportunity to monitor deformation and associated changes in chemical and physical properties throughout the slow slip cycle. In situ measurements and sampling of material from the sedimentary section and oceanic basement of the subducting plate reveal the rock properties, composition, lithology, and structural character of material that is transported downdip into the SSE source region. A recent seafloor geodetic experiment raises the possibility that SSEs at northern Hikurangi may propagate to the trench, indicating that the shallow thrust fault (the Pāpaku fault) targeted during Expeditions 372 and 375 may also lie in the SSE rupture area and host a portion of the slip in these events. Hence, sampling and logging at this location provides insights into the composition, physical properties, and architecture of a shallow fault that may host slow slip. Expeditions 372 and 375 were designed to address three fundamental scientific objectives: Characterize the state and composition of the incoming plate and shallow fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth and which may itself host shallow slow slip; Characterize material properties, thermal regime, and stress conditions in the upper plate directly above the SSE source region; and Install observatories in the Pāpaku fault near the deformation front and in the upper plate above the SSE source to measure temporal variations in deformation, temperature, and fluid flow. The observatories will monitor volumetric strain (via pore pressure as a proxy) and the evolution of physical, hydrological, and chemical properties throughout the SSE cycle. Together, the coring, logging, and observatory data will test a suite of hypotheses about the fundamental mechanics and behavior of SSEs and their relationship to great earthquakes along the subduction interface.

The central San Andreas Fault in California is known as a creeping fault, however recent studies have shown that it may be accumulating a slip deficit and thus its seismogenic potential should be seriously considered. We conducted laboratory friction experiments measuring time-dependent frictional strengthening (healing) on fault zone and wall rock samples recovered during drilling at the San Andreas Fault Observatory at Depth (SAFOD), located near the southern edge of the creeping section and in the direct vicinity of three repeating microearthquake clusters. We find that for hold times of up to 3000 s, frictional healing follows a log-linear dependence on hold time and that the healing rate is very low for a sample of the actively shearing fault core, consistent with previous results. However, considering longer hold times up to ∼350,000 s, the healing rate accelerates such that the data for all samples are better described by a power law relation. In general, samples having a higher content of phyllosilicate minerals exhibit low log-linear healing rates, and the notably clay-rich fault zone sample also exhibits strong power-law healing when longer hold times are included. Our data suggest that weak faults, such as the creeping section of the San Andreas Fault, can accumulate interseismic shear stress more rapidly than expected from previous friction data. Using the power-law dependence of frictional healing on hold time, calculations of recurrence interval and stress drop based on our data accurately match observations of discrete creep events and repeating M w = 2 earthquakes on the San Andreas Fault.

Interseismic recovery of fault strength (healing) following earthquake failure is a fundamental requirement of the seismic cycle and likely plays a key role in determining the stability and slip behavior of tectonic faults. We report on laboratory measurements of time- and slip-dependent frictional strengthening for natural and synthetic gouges to evaluate the role of mineralogy in frictional strengthening. We performed slide-hold-slide (SHS) shearing experiments on nine natural fault gouges and eight synthetic gouges at conditions of 20 MPa normal stress, 100% relative humidity (RH), large shear strain (~15), and room temperature. Phyllosilicate-rich rocks show the lowest rates of frictional strengthening. Samples rich in quartz and feldspar exhibit intermediate rates of frictional strengthening, and calcite-rich gouges show the largest values. Our results show that (1) the rates of frictional strengthening and creep relaxation scale with frictional strength, (2) phyllosilicate-rich fault gouges have low strength and healing characteristics that promote stable, aseismic creep, (3) most natural fault gouges exhibit intermediate rates of frictional strengthening, consistent with a broad range of fault slip behaviors, and (4) calcite-rich fault rocks show the highest rates of frictional strengthening, low values of dilation upon reshear, and high frictional strengths, all of which would promote seismogenic behavior.

The Nankai Trough is an exceptionally well-studied convergent margin known to host damaging megathrust earthquakes as well as various forms of slow fault slip. Outcrop studies of exhumed analogues for the modern subduction thrust, as well as seismic reflection images of the active subduction zone, suggest that the basaltic basement participates in plate-boundary shearing at depths of a few km and greater. We obtained altered basaltic upper basement from the modern Nankai Trough seaward of the trench, recovered by drilling on IODP Expedition 333. We performed laboratory friction experiments on bare surfaces and gouge powders of this material, in addition to and in combination with other materials for comparison. The altered basalt exhibits predominantly velocity-strengthening frictional behavior, indicating a tendency for stable slip. For bare surface experiments, few instances of velocity-weakening friction occur and are restricted to slip rates of

Previous work suggests that in subduction zones, the onset of large earthquake nucleation at depths > ~ 5–10 km is likely driven by a combination of factors associated with the process of lithification. At these depths, lithification processes affect the entire fault system by modifying the mechanical properties of both the plate boundary fault zone and the wall-rock. To test the hypothesis that lithification of subduction zone sediments produces rocks capable of earthquake nucleation via diagenesis and low-grade metamorphism, we conducted friction experiments on fossil subduction zone sediments recovered from exposures in the Shimanto Belt in SW Japan. These meta-sediments represent accreted and subducted material which has experienced maximum temperatures of 125 to 225 °C, which are representative of seismogenic depths along the active Nankai subduction megathrust in the foreland of the Shimanto Belt. We find that intact Shimanto rock samples, which preserve the influence of diagenetic and metamorphic processes, exhibit the potential for unstable slip under in-situ pressure conditions. Powdered versions of the same samples tested under the same conditions exhibit only velocity-strengthening friction, thus demonstrating that destroying the lithification state also removes the potential for unstable slip. Using advanced porosity loss to quantify the lithification process, we demonstrate that increased velocity weakening correlates with increasingly advanced lithification. In combination with documented frictionally stable behavior of subduction zone sediments from shallower depths, our results provide evidence that the sediment lithification hypothesis can explain the depth-dependent onset of large earthquake nucleation along subduction zone megathrusts.

Factor-of-safety analyses of submarine slope failure depend critically on the shear strength of the slope material, which is often evaluated with residual strength values and for normally consolidated sediments. Here, we report on direct measurements of both shear strength and cohesion for a quartz–clay mixture over a wide range of overconsolidation ratios (OCRs). For normally consolidated sediment at low stresses, cohesion is the dominant source of shear strength compared to friction. Significant increases in peak shear strength occur for OCR > 4, and the primary source of this strength increase is due to increased cohesion, rather than friction. The proportion of added shear strength due to cohesion depends log-linearly on the OCR. We show that at shallow depths where OCR values can be high, overconsolidated clays can be stronger than pure or nearly pure quartz sediments, which are cohesionless under near-surface conditions. Our data also suggest that areas which have experienced significant unroofing due to previous mass movements are less likely to experience subsequent failure at shallow depths due to increased peak strength, and if failure occurs it is expected to be deeper where the OCR is lower. In seismically active areas, this is one potential explanation for the general observation of lower slope failure recurrence compared to rates expected from triggering due to local earthquakes.

Narrow, highly-comminuted shear localization features in faults, known as principal slip zones (PSZs), are commonly associated with large-offset seismogenic faults. In this study, laboratory friction experiments were performed using shale and slate gouges where deformation was encouraged to localize at the gouge–wall-rock boundary. The slate gouges develop a black, narrow PSZ composed of densely packed submicron particles that appear sintered while the spectator gouge remains largely undeformed. These PSZs form at subseismic slip velocities of ∼10 –5 m/s and with a calculated temperature rise of ∼3 °C. Instances of velocity-weakening friction, which is necessary for unstable fault slip, are only observed for slate samples with a PSZ; shale gouges, however, do not develop a PSZ and exhibit only velocity-strengthening frictional behavior. The development of a PSZ may therefore be a prerequisite for future earthquake slip to occur, rather than unequivocal evidence of past earthquake slip.

It has long been known that rock fabric plays a key role in dictating rock strength and rheology throughout Earth's crust; however the processes and conditions under which rock fabric impacts brittle failure and frictional strength are still under investigation. Here, we report on laboratory experiments designed to analyze the effect of foliation orientation on the mechanical behavior and associated microstructures of simulated fault rock sheared at constant normal stress of 50MPa. Intact samples of Pennsylvania slate were sheared with a range of initial fabric orientations from 0 to 165° with respect to the imposed shear direction. Foliation orientations of 0-30° produce initial failure at the lowest shear stress; while samples oriented at higher angles are less favorable resulting in higher peak failure strength. In all samples, post-peak development of through-going deformation zones in the R1 Riedel orientation results in a residual strength that is lower than that observed for powdered gouge from the same material. As shear strain increases, all samples approach a residual apparent friction, defined by the ratio of shear strength to normal stress, of ~0.4. However, the final deformation microstructure depends strongly on the orientation of the pre-existing foliation. When fabric is oriented at low angles to the shear direction (

One poorly understood feature of the subduction inputs to the Nankai Trough subduction zone (SW Japan) is a stratigraphic interval with an anomalously high porosity zone (HPZ), which is up to 240 m thick and located within the clay- and volcanic ash-rich Shikoku Basin facies. To investigate the origin of their peculiar physical properties, we integrated logging-while-drilling (LWD) data, shipboard density measurements, and visual descriptions of core samples recovered from four drill sites of the Ocean Drilling Program and Integrated Ocean Drilling Program. We combined those observations with scanning electron microscopy (SEM) and laboratory consolidation tests on both HPZ samples and artificial mixtures of ash (glass shards) + smectite and vesicular pumice + smectite. LWD data indicate that the HPZ mudstones have a large proportion of dispersed volcanic ash (~ 20–30%). The consolidation tests show that the rate of porosity loss with increasing effective stress (consolidation behavior) is consistent among HPZ specimens and matches artificial mixtures containing up to 60% volcanic material. However, absolute values of porosity remain higher for HPZ samples compared to artificial mixtures, so processes in addition to the mechanical effects of volcanic clasts must be contributing. We suggest that hydration and partial dissolution turns clusters of volcanic glass into aggregates with distinct microfabric. SEM images confirm the presence of strengthened grain-to-grain contacts, which probably inhibit the collapse of the intra-aggregate pore space. The aggregates behave like grains, so that cohesive strength of the bulk sediment and rate of porosity loss remain nearly unchanged during burial. The two-step diagenetic process of dissolution and precipitation depends critically on a threshold abundance of fine-grained dispersed volcanic ash/pumice. Older units in the Shikoku Basin with only traces of dispersed ash show no such effects. HPZs should be expected in other subduction zones with similar compositional and diagenetic prerequisites.

The 2011 Tohoku-Oki earthquake demonstrated that the shallowest reaches of plate boundary subduction megathrusts can host substantial coseismic slip that generates large and destructive tsunamis, contrary to the common assumption that the frictional properties of unconsolidated clay-rich sediments at depths less than ∼ 5 km should inhibit rupture. We report on laboratory shearing experiments at low sliding velocities ( 1 mm / s ) using borehole samples recovered during IODP Expedition 343 (JFAST), spanning the plate-boundary decollement within the region of large coseismic slip during the Tohoku earthquake. We show that at sub-seismic slip rates the fault is weak (sliding friction μ s = 0.2 – 0.26 ), in contrast to the much stronger wall rocks ( μ s > ∼ 0.5 ). The fault is weak due to elevated smectite clay content and is frictionally similar to a pelagic clay layer of similar composition. The higher cohesion of intact wall rock samples coupled with their higher amorphous silica content suggests that the wall rock is stronger due to diagenetic cementation and low clay content. Our measurements also show that the strongly developed in-situ fabric in the fault zone does not contribute to its frictional weakness, but does lead to a near-cohesionless fault zone, which may facilitate rupture propagation by reducing shear strength and surface energy at the tip of the rupture front. We suggest that the shallow rupture and large coseismic slip during the 2011 Tohoku earthquake was facilitated by a weak and cohesionless fault combined with strong wall rocks that drive localized deformation within a narrow zone.

The very large slip on the shallow portion of the subduction interface during the 2011 Tohoku-oki earthquake (M w 9.0) caused a huge tsunami along the northeast coast of Honshu, Japan. In order to elucidate the mechanics of such tsunamigenic slip, the Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project, JFAST), was carried out one year after the earthquake and succeeded in recovering rocks constituting the active plate boundary fault. Our mineralogical analyses using X-ray diffraction reveal that the shallow portion of the fault zone that caused the earthquake is significantly enriched in smectite compared to the surrounding sediments, which may be intimately linked to the tsunamigenic shallow faulting. For comparison, we also analyzed mineralogical features of incoming sediments just prior to subduction, recovered on the outer rise of the Japan Trench (Site 436, Deep Sea Drilling Project Leg 56), and found a characteristic smectite-rich horizon in the uppermost ∼5 m of the pelagic clay layer. This horizon should be mechanically weak and will become the future plate boundary fault, as observed in the JFAST cores. The smectite-rich deposits are broadly distributed in the northwestern Pacific Ocean, and may therefore potentially enhance conditions for large shallow slip during earthquakes that occur over a broad area of the Japan Trench plate boundary, which would result in large tsunamis for this region.

The Alpine Fault is a major plate-boundary fault zone that poses a major seismic hazard in southern New Zealand. The initial stage of the Deep Fault Drilling Project has provided sample material from the major lithological constituents of the Alpine Fault from two pilot boreholes. We use laboratory shearing experiments to show that the friction coefficient µ of fault-related rocks and their precursors varies between 0.38 and 0.80 depending on the lithology, presence of pore fluid, effective normal stress, and temperature. Under conditions appropriate for several kilometers depth on the Alpine Fault (100 MPa, 160 °C, fluid-saturated), a gouge sample located very near to the principal slip zone exhibits µ = 0.67, which is high compared with other major fault zones targeted by scientific drilling, and suggests the capacity for large shear stresses at depth. A consistent observation is that every major lithological unit tested exhibits positive and negative values of friction velocity dependence. Critical nucleation patch lengths estimated using representative values of the friction velocity-dependent parameter a−b and the critical slip distance D c , combined with previously documented elastic properties of the wall rock, may be as low as ~3 m. This small value, consistent with a seismic moment M o = ~4 × 1010 for an M w = ~1 earthquake, suggests that events of this size or larger are expected to occur as ordinary earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3–7 km.

Experimental shearing at plate rate speeds reveals slow slip events and earthquake potential on weak, shallow faults., The near-surface areas of major faults commonly contain weak, phyllosilicate minerals, which, based on laboratory friction measurements, are assumed to creep stably. However, it is now known that shallow faults can experience tens of meters of earthquake slip and also host slow and transient slip events. Laboratory experiments are generally performed at least two orders of magnitude faster than plate tectonic speeds, which are the natural driving conditions for major faults; the absence of experimental data for natural driving rates represents a critical knowledge gap. We use laboratory friction experiments on natural fault zone samples at driving rates of centimeters per year to demonstrate that there is abundant evidence of unstable slip behavior that was not previously predicted. Specifically, weak clay-rich fault samples generate slow slip events (SSEs) and have frictional properties favorable for earthquake rupture. Our work explains growing field observations of shallow SSE and surface-breaking earthquake slip, and predicts that such phenomena should be more widely expected.

[1] The mechanical behavior of the plate boundary fault zone is of paramount importance in subduction zones, because it controls megathrust earthquake nucleation and propagation as well as the structural style of the forearc. In the Nankai area along the NanTroSEIZE (Kumano) drilling transect offshore SW Japan, a heterogeneous sedimentary sequence overlying the oceanic crust enters the subduction zone. In order to predict how variations in lithology, and thus mechanical properties, affect the formation and evolution of the plate boundary fault, we conducted laboratory tests measuring the shear strengths of sediments approaching the trench covering each major lithological sedimentary unit. We observe that shear strength increases nonlinearly with depth, such that the (apparent) coefficient of friction decreases. In combination with a critical taper analysis, the results imply that the plate boundary position is located on the main frontal thrust. Further landward, the plate boundary is expected to step down into progressively lower stratigraphic units, assisted by moderately elevated pore pressures. As seismogenic depths are approached, the decollement may further step down to lower volcaniclastic or pelagic strata but this requires specific overpressure conditions. High-taper angle and elevated strengths in the toe region may be local features restricted to the Kumano transect.

Slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake. Laboratory simulations of slow slip in rock samples taken from the Nankai subduction zone, Japan, reveal similar characteristics to fast earthquakes, implying that some slow slip events could be prematurely arrested earthquakes.

The NE portion of the Gela Basin (Strait of Sicily) shows evidence of multiple mass wasting events of predominantly translational character. In this context, recent investigations implicate volcanoclastic layers as key stratigraphic surfaces acting as preferential planes of failure. We present an integrated analysis of a representative sedimentary transition from overlying homogeneous background sedimentation of silty clay to a volcanoclastic layer. A high-resolution CT scan and three drained direct-shear laboratory experiments from a 20 cm whole-round section (~28.2 mbsf) allow the delineation of the role of this volcanoclastic layer in the framework of slope stability and failure initiation. The mechanical results indicate a general strengthening of the material with increased volcanoclastic content. Tendency for failure is expected to be highest within the silty clay due to relatively lower shear strength and strain-weakening behaviour, which promotes progressive sediment failure. In contrast with recent findings, this suggests that volcanoclastic sediment would not act as a weak layer. However, the volcanoclastic layer exhibits significant mesoporosity (i.e., fracturing) and may therefore host large volumes of fluid. Temporarily undrained conditions, for example during seismic activity, could transiently elevate fluid pressures and thus reduce the material shear strength below that of the surrounding silty clay. Such a weak layer may preferentially form along the interface of fractured volcanoclastic material and relatively impermeable silty clay, where differences in material strengths are lowest.

At subduction zones, the permeability of major fault zones influences pore pressure generation, controls fluid flow pathways and rates, and affects fault slip behavior and mechanical strength by mediating effective normal stress. Therefore, there is a need for detailed and systematic permeability measurements of natural materials from fault systems, particularly measurements that allow direct comparison between the permeability of sheared and unsheared samples from the same host rock or sediment. We conducted laboratory experiments to compare the permeability of sheared and uniaxially consolidated (unsheared) marine sediments sampled during IODP Expedition 316 and ODP Leg 190 to the Nankai Trough offshore Japan. These samples were retrieved from: (1) The decollement zone and incoming trench fill offshore Shikoku Island (the Muroto transect); (2) Slope sediments sampled offshore SW Honshu (the Kumano transect) ~ 25 km landward of the trench, including material overriden by a major out-of-sequence thrust fault, termed the “megasplay”; and (3) A region of diffuse thrust faulting near the toe of the accretionary prism along the Kumano transect. Our results show that shearing reduces fault-normal permeability by up to 1 order of magnitude, and this reduction is largest for shallow (

We investigate the mechanics of slope failures on the Nankai accretionary complex offshore Japan in the vicinity of a major out-of-sequence thrust fault (termed the “megasplay”). Incorporating laboratory-measured shear strength of slope sediments sampled during Integrated Ocean Drilling Project (IODP) Expeditions 315 and 316 with local seafloor slope angles from bathymetric data and constraints on in-situ effective stress conditions from drilling, we find that slopes in the study area are stable and submarine landslides are not expected to occur under static conditions. In order to assess the possibility of slope failure triggered by coseismic rupture of the megasplay fault, we use empirical relations for strong ground motion attenuation from earthquakes with Mw 6–9. We find that the slope sediments should be stable based on computations from one model, developed from a catalog of worldwide subduction zone earthquakes ( Youngs et al., 1997 ). However, using a different model developed primarily from a catalog of crustal earthquakes in Japan ( Kanno et al., 2006 ), we find that slopes should be unstable for earthquakes 8 ≤ Mw ≤ 9, and possibly unstable for events with 6 ≤ Mw

A fundamental problem in fault mechanics is whether slip instability associated with earthquake nucleation depends on absolute fault strength. We present laboratory experimental evidence for a systematic relationship between frictional strength and friction rate dependence, one of the key parameters controlling stability, for a wide range of constituent minerals relevant to natural faults. All of the frictionally weak gouges (coefficient of sliding friction, μ < 0.5) are composed of phyllosilicate minerals and exhibit increased friction with slip velocity, known as velocity-strengthening behavior, which suppresses frictional instability. In contrast, fault gouges with higher frictional strength exhibit both velocity-weakening and velocity-strengthening frictional behavior. These materials are dominantly quartzofeldspathic in composition, but in some cases include certain phyllosilicate-rich gouges with high friction coefficients. We also find that frictional velocity dependence evolves systematically with shear strain, such that a critical shear strain is required to allow slip instability. As applied to tectonic faults, our results suggest that seismic behavior and the mode of fault slip may evolve predictably as a function of accumulated offset.

Sediment hydraulic properties and consolidation behavior are key parameters that affect pore pressure generation, fluid migration, deformation, and slip behavior and mechanical strength of subduction zone megathrusts. We report on a set of constant rate of strain consolidation experiments and isostatic deformation and permeability experiments on whole-round core samples obtained during Integrated Ocean Drilling Program (IODP) Expedition 343 as part of the Japan Trench Fast Drilling Project (JFAST). The samples were collected from the frontal prism at 697.18 meters below seafloor (mbsf) and the underthrust sediment section at 831.45 mbsf at IODP Site C0019, 6.0 km landward of the Japan Trench. The permeability of sediment from the frontal prism decreases from 7.3 × 10 –18 to 3.5 × 10 –19 m 2 as effective mean stress increases from 14.6 to 62.3 MPa under a uniaxial loading path and from 5.4 × 10 –18 to 2.9 × 10 –19 m 2 over effective stresses from 3.6 to 54.6 MPa under an isostatic loading path. Porosity decreases from 46% to 27% as effective mean stress increases from 0.73 to 62.3 MPa and 46% to 29% as stress increases from 3.6 to 64.6 MPa for uniaxial and isostatic loading of the prism sediment, respectively. Permeability of the underthrust sediment sample decreases from 4.0 × 10 –18 to 3.2 × 10 –19 m 2 as stress increases from 3.6 to 86.6 MPa under isostatic conditions. Porosity decreases from 49% to 37% over the stress range from 0.73 to 69.6 MPa and 48% to 36% from 3.6 to 86.6 MPa for uniaxial and isostatic loading paths, respectively. For both samples, permeability exhibits a log-linear decrease with decreasing porosity. In situ permeability for the prism and underthrust sediment samples are estimated from our laboratory-defined permeability-porosity relationships. The in situ porosity values for the prism sediment samples are 44% and 45% for uniaxial and isostatic conditions, respectively. The corresponding in situ permeability values are 4.3 × 10 –18 and 4.9 × 10 –18 m 2 . In situ porosity for the underthrust sample is 47% under both uniaxial and isostatic conditions. The permeability value for the underthrust sediment sample is 4.9 × 10 –18 m 2 under isostatic loading conditions. The coefficient of consolidation (Cv) decreases with increasing effective axial stress to near-constant values of 4.2 × 10 –7 and 9.0 × 10 –6 m 2 /s, and the compression index (Cc) is 0.70 and 0.57 for the prism and underthrust sediment samples, respectively. Preconsolidation stresses (Pc′) for the prism and underthrust samples, defined using the Casagrande method, are 17.0