Low Effective Pressure Research Articles

The Potential for Fracture Growth in Stepped Subglacial Topography as a Quarrying Mechanism

AbstractUnderstanding the rates and mechanisms of erosion by subglacial quarrying is a major unsolved problem in geomorphology. Stress enhancement due to load concentration on bedrock ledges between cavities is hypothesized to drive the growth of fractures. Prior work assumed the formation of vertically oriented tensile fractures at the downstream margins of cavities as the controlling process, but did not account for the evolution of the stress field as fractures lengthen, and in particular the dominance of the shearing mode at fracture tips. We used 2D finite element analysis and J‐integral methods to analyze stress intensity factors and fracture growth potentials at the tips of preexisting fractures in loaded bedrock steps, taking into account normal and shear components and measured rock strengths. By examining different step heights, step riser angles, rock types, prior fracture locations and orientations, and extents of ice‐rock contact zones, we identified some situations favorable for fracture growth, especially in brittle rock types. Typically, however, the growth direction will not be vertically downward but angled up‐glacier away from the step riser, a situation unfavorable for quarrying. Moreover, in many situations, the normal stress across fracture planes will be compressive. Non‐vertical step risers buttress the bedrock and also suppress fracture growth. In contrast, reducing the sizes of ice‐rock contact zones not only increases the loading magnitude, as previously recognized, but also increases intensification of tensile stress at the tips of fractures located just up‐glacier. Thus, larger cavities, and hence, fast sliding and low effective pressures, favor quarrying more strongly than previously recognized.

Water-Weakening and Time-Dependent Deformation of Organic-Rich Chalks

The Ghareb Formation is a shallowly buried porous chalk in southern Israel that is being considered as a host rock for a geologic nuclear waste repository. Setup and operation of a repository will induce significant mechanical, hydrological and chemical perturbations in the Ghareb. Developing a secure repository requires careful characterization of the rock behavior to different loads. To characterize hydromechanical behavior of the Ghareb, several short- and long-term deformation experiments were conducted. Hydrostatic loading tests were conducted both dry and water-saturated, using different setups to measure elastic properties, time-dependent behavior, and permeability. A set of triaxial tests were conducted to measure the elastic properties and rock strength under differential loading at dry and water-saturated conditions. The hydrostatic tests showed the Ghareb began to deform inelastically around 12–15 MPa, a relatively low effective pressure. Long-term permeability measurements demonstrated that permeability declined with increasing effective pressure and was permanently reduced by ~ 1 order of magnitude after unloading pressure. Triaxial tests showed that water saturation significantly degrades the rock properties of the Ghareb, indicating water-weakening is a significant risk during repository operation. Time-dependent deformation is observed during hold periods of both the hydrostatic and triaxial tests, with deformation being primarily visco-plastic. The rate of deformation and permeability loss is strongly controlled by the effective pressure as well. Additionally, during holds of both hydrostatic and triaxial tests, it is observed that when water-saturated, radial strain surpassed axial strain when above effective pressures of 13–20 MPa. Thus, deformation anisotropy may occur in situ during operations even if the stress conditions are hydrostatic when above this pressure range.

Effects of topography on dynamics and mass loss of lake-terminating glaciers in southern Patagonia

Abstract Calving glaciers are highly sensitive to bedrock geometry near their terminus. To understand the mechanisms controlling rapid calving glaciers’ mass loss, we measured the lake topography in front of four lake-terminating glaciers in the southern Patagonian icefield. Using remotely sensed surface elevation data, we calculated flotation height and surface slope and compared those with changes in ice-front position, surface speed and surface elevation. Rapid retreat accompanied by rapid flow acceleration and ice surface steepening was observed at Glaciar Upsala from 2008–2011, and at O'Higgins and Viedma glaciers from 2016–present. Surface lowering in the lower part of Glaciar Upsala reached 30 m a−1 and was 18 m a−1 and 12 m a−1 at O'Higgins and Viedma glaciers, respectively. Near- or super-buoyant conditions were observed prior to these events, leading to gradual flow acceleration due to low effective pressure and decoupling from the bed. The super-buoyant condition and gradual acceleration imply full-thickness buoyant calving, which causes the ice front to retreat from the shallow bedrock topography with substantial flow acceleration. We conclude that the buoyancy force plays an important role in the rapid mass loss of lake-terminating glaciers in southern Patagonia.

Measurements of Wave‐Induced Attenuation in Saturated Metapelite and the Band‐Limitation of Low‐Frequency Earthquakes

AbstractThe most common explanation for the depletion of high frequency waves that defines low‐frequency earthquakes (LFEs) and very low‐frequency earthquakes (VLFEs) is that fault rupture and slip are slower than typical earthquakes. However, it is difficult to rule out the possibility that the high frequency waves are produced during slip, but attenuated near the LFE source. One reason this hypothesis has been poorly tested is that there are no measurements of attenuation on the relevant rocks. We present the results of forced oscillation experiments that measure the frequency‐dependent attenuation of a chlorite‐rich metapelitic schist, a lithology found along the subduction plate boundary where LFEs and VLFEs have been documented. Experiments were run on dry and water‐saturated schist at effective pressures of 2–10 MPa and at frequencies of 2 × 10−5–30 Hz. We find that pore fluids and low effective pressure result in the attenuation of high frequencies. The frequency‐dependent attenuation is consistent with the concomitant operation of two wave‐induced fluid flow mechanisms, squirt flow, and patchy saturation. When the effects of these mechanisms are extrapolated to geologic conditions using rock physics models, our results predict that attenuation is capable of completely diminishing the frequencies depleted in LFEs and VLFEs. Therefore, LFEs and VLFEs may not necessarily record slow fault slip, but possibly the presence of high fluid pressure.

Persistent, extensive channelized drainage modeled beneath Thwaites Glacier, West Antarctica

Abstract. Subglacial hydrology is a leading control on basal friction and the dynamics of glaciers and ice sheets. At low discharge, subglacial water flows through high-pressure, sheet-like systems that lead to low effective pressures. However, at high discharge, subglacial water melts the overlying ice into localized channels that efficiently remove water from the bed, thereby increasing effective pressure and basal friction. Recent observations suggest channelized subglacial flow exists beneath Thwaites Glacier, yet it remains unclear if stable channelization is feasible in West Antarctica, where surface melting is nonexistent and water at the bed is limited. Here, we use the MPAS-Albany Land Ice model to run a suite of over 130 subglacial hydrology simulations of Thwaites Glacier across a wide range of physical parameter choices to assess the likelihood of channelization. We then narrow our range of viable simulations by comparing modeled water thicknesses to previously observed radar specularity content, which indicates flat, spatially extensive water bodies at the bed. In all of our data-compatible simulations, stable channels reliably form within 100–200 km of the grounding line and reach individual discharge rates of 35–110 m3 s−1 at the ice–ocean boundary. While only one to two channels typically form across the 200 km width of the glacier in our simulations, their high efficiency drains water across the entire lateral extent of the glacier. We posit the large catchment size of Thwaites Glacier, its funnel-like geometry, and high basal melt rates together accumulate enough water to form stable channels. No simulations resembled observed specularity content when channelization was disabled. Our results suggest channelized subglacial hydrology has two consequences for Thwaites Glacier dynamics: (i) amplifying submarine melting of the terminus and ice shelf while (ii) simultaneously raising effective pressure within 100 km of the grounding line and increasing basal friction. The distribution of effective pressure implied from our modeling differs from parameterizations typically used in large-scale ice sheet models, suggesting the development of more process-based parameterizations may be necessary.

Determining the static Young’s modulus and Poisson’s ratio, and compressive strength of the friable Erin Formation rocks using P-wave velocity

It is important to understand the mechanical properties (elastic properties and strength) of rocks as these properties are used to model the rock's deformation in civil and underground engineering projects. Determining these properties for friable rocks are very difficult and scarce due to the rock's poor cementation making it challenging to retrieve samples from outcrop and boreholes, and difficult to prepare specimens for laboratory testing. To improve on the lack of mechanical properties of friable rocks, this study aims to measure the mechanical properties and P- and S-wave velocities and determine if empirical relationships exist between them. Experiments were carried out at effective pressures up to 130 MPa under dry and saturated conditions on friable sandstone and thin-bed shale lithofacies from the Erin Formation. The static Young's modulus of the rocks is less than 8 MPa and the unconfined strength ranged from 1.0 to 2.2 MPa, which indicate that the rock is very weak. The confined compressive strength is relatively high due to large strain accumulation (average of 20% strain), which resulted in significant strain hardening. At low effective pressures, velocity measurements were highly attenuated, resulting in limited S-wave velocity measurements. Empirical relationships established between the mechanical properties and P-wave velocities were predominantly weak to strong. Moderate and strong linear relationships were established between the unconfined compressive strength and static elastic properties and between the tensile strength and static Poisson's ratio. The results of this study could be carefully applied to velocity data that are readily available from well logs to estimate the mechanical properties of friable rocks.

Combustion Modes Induced by Oil-Droplet Gas-Phase Pre-ignition in the Chamber under Different Environmental Conditions

ABSTRACT The combustion of combustible gaseous mixture with/without oil droplet is studied based on a rapid compression machine under conditions comparable with the state at the end of compression in SI-engine cylinder. The ignition and burning processes of gaseous mixtures with oil droplet and the pressure traces for same initial state cases with and without oil droplets are obtained experimentally. The combustion of the gaseous mixture without oil droplet is in an integrated homogeneous auto-ignition mode at lower effective pressures or temperature, while it is in an integrated sequential auto-ignition and detonation mode at higher effective pressures and temperature, which is controlled by the ignition delay of gaseous mixtures. For gaseous mixture with oil droplet, the gas-phase pre-ignition can be induced, while the combustion modes are affected significantly by negative temperature coefficient character of the ignition delay. The combustion of gaseous mixture is in the end-gas homogeneous auto-ignition mode with no knock when its effective temperature is in the pre-region of NTC influence region, while it is in the end-gas sequential auto-ignition and detonation mode with knock or super knock when its effective temperature is in the NTC region. The peak pressure is close to that of constant-volume combustion for the integrated (end-gas) homogeneous auto-ignition mode. The peak pressure is close to that of detonation for the integrated (end-gas) sequential auto-ignition and detonation mode under lower effective pressure conditions. However, it is much larger than that of detonation for the end-gas sequential auto-ignition and detonation mode under higher effective pressure conditions.

Compactive Deformation of Sandstone Under Crustal Pressure and Temperature Conditions

AbstractThe transition from macroscopically brittle to macroscopically ductile deformation in porous sandstones is known to be pressure dependent, with compactive, ductile behavior occurring only once significant effective pressures have been reached. Within the crust, such effective pressures are associated with burial depths in the range 0.5–6 km, where the temperature is likely 35°C–200°C. To test the importance of such elevated temperature on the strength and deformability of sandstone, a series of constant strain rate, triaxial deformation experiments were performed on three different water saturated sandstones at either ambient temperature or 150°C. For each sandstone, an effective pressure range was used which spanned both the brittle and ductile deformation regimes, up to a maximum of 120 MPa. In the brittle regime, we observed a temperature‐dependent lowering of the yield stress of between 8% and 17%. Within the ductile regime, we observed an even greater reduction in the yield stress of between 9% and 37%. A further notable observation is that the transition from dilatant, brittle behavior to compactive, ductile behavior tends to occur at a lower effective pressure at elevated temperature. The weakening observed at elevated temperature can be explained by a reduction in fracture toughness, which is shown mathematically to cause greater weakening in the ductile regime than in the brittle regime. The apparent reduction in fracture toughness at elevated temperature is potentially driven by a combination of a reduction in surface energy and, to a minor extent, an increase in subcritical crack growth rate.

Study on mechanical characteristics and damage mechanism of the Longmaxi Formation shale in southern Sichuan Basin, China

During the development of shale gas reservoirs, the mechanical parameters of shale rocks are greatly significant to guide reservoir drilling design, reservoir stimulation, scheme design, and wellbore stability evaluation. Compared with conventional gas reservoirs, the mechanical properties of shale gas reservoir are more complex and lack of the comprehensive understanding. In this study, the uniaxial and triaxial compression experiments of the Longmaxi shale in South China were conducted to understand the mechanical characteristics and damage mechanism of shale rocks. Combined with the characteristics of shale microstructure, the effects of the mechanical properties such as mineral composition, deformation, stress–strain, and fracture damage characteristics were analyzed and discussed. The results show that the stress–strain curves of shale are two main types of elastic deformation and elastic–plastic deformation. There is a significant difference in effective pressure of yield platform in different shale post-peak deformation. The effective pressure of rigid shale is 60 MPa while that of other shales is 30 MPa. With the increase of effective pressure, the increase of peak compressive strength decreases gradually, and the effect of effective pressure on compressive strength decreases gradually with the transition from the brittleness to the ductility. The higher the effective pressure of the brittleness to the ductility is, the greater the change of compressive strength with effective pressure is, and the smaller the compressive strength under low effective pressure is, the smaller the variation with pressure is. Shale rocks with relatively high brittleness under different effective pressures are mainly characterized by brittle split fracture dominated by tensile fracture, while other shales with low brittleness are dominated by single shear fracture plane.

Influence of alteration on the mechanical behaviour and failure mode of andesite: implications for shallow seismicity and volcano monitoring

Volcanoes often host hydrothermal systems that alter the host rock. To understand the influence of alteration on mechanical behaviour of edifice-forming rock, we performed a series of triaxial deformation experiments on variably altered andesite from Mt. Ruapehu (New Zealand) under constant effective pressure. Under the imposed conditions, andesite with intermediate argillic alteration deforms in a brittle manner forming fractures. By contrast, andesite with advanced argillic alteration deforms in a ductile manner, with sample failure driven by distributed cataclastic pore collapse. We consider this the result of an increase in porosity and clay content with increasing alteration. Ancillary experiments highlight that the brittle-ductile transition occurs at lower effective pressure (i.e. at shallower depths) in andesites with advanced argillic alteration relative to unaltered andesites of comparable porosity. We conclude that advanced argillic alteration can create an anomalous shallow ductile zone, which has important implications for fluid flow and pre-eruptive seismicity.

Reaction fronts, permeability and fluid pressure development during dehydration reactions

Fluids released by prograde metamorphism are often invoked to explain a range of crustal processes from earthquake triggering to metasomatism. These fluids can be either trapped and overpressured or released and channelized depending on the interplay between permeability, reaction rate and compaction. Experimental data are presented, measuring permeability, porosity and microstructural evolution throughout the dehydration of gypsum to form bassanite. Reaction fronts, regions over which the reaction largely occurs, are used as a framework to explain the results. Experiments were conducted under hydrostatic conditions at a constant temperature of 115 °C at two effective pressures of 60 MPa and 110 MPa and three pore-fluid pressures of 20, 40 and 60 MPa. At high effective pressure, creep of the gypsum solid framework results in low porosity and permeability, producing high pore-fluid pressure build-up that slows the reaction rate. A clearly defined narrow reaction front migrates along the sample and the average permeability remains low until the front sweeps across the entire sample. Conversely, at low effective pressure the reaction front is wide producing a permeable, drained network. Average permeability is enhanced significantly after only a small fraction of the reaction has completed, by the interconnection of open pores. This study shows that the width of reaction fronts and hence the permeability development is strongly controlled by compaction. The reaction front velocity is broadly dependent on permeability and the reaction driving force. A simple quantitative model for these relationships is developed.

Access to CPAP treatment in patients with moderate to severe sleep apnea in a Latin American City.

Introduction:The most effective treatment for moderate to severe obstructive sleep apnea (OSA) is continuous positive airway pressure (CPAP) but adherence may be a limiting factor. Most compliance studies often only include patients under CPAP treatment, neglecting the importance of access to treatment. The aim of this study was to evaluate CPAP access and compliance in OSA patients, after CPAP indication and titration.Methods:We included moderate to severe OSA patients, diagnosed by in-lab polysomnography (PSG), with CPAP indication and effective pressure titration. Between 12 to 18 months after treatment was indicated a telephone questionnaire was administered including questions about access to CPAP, reasons of no access, reported adherence and symptoms improvement.Results: A total of 213 patients responded to the survey (171 males, mean age 53.4±13.5 and BMI 34.02±8.8 kg/m2). Almost a third of the patients (28.2%) did not initiate CPAP treatment. Out of 213, 153 patients (71.8%) started treatment with CPAP and 120 (56.3%) reported still being under treatment a year after indication, additionally, 85.8% reported that they were using it =4hs/night. Those who accessed to CPAP were on average, older age, had full coverage of treatment by their medical insurance, required lower effective pressure and experienced more severe sleepiness compared to those individuals who did not accessed to CPAP.Discussion:A significant proportion of OSA patients with CPAP indication did not initiate and/or eventually abandoned CPAP. Approximately only 50% of the patients were still under treatment, with acceptable self-reported adherence rate and clinical response, one year after the initial treatment indication. Additional measures are necessary to increase access to CPAP and improve long-term compliance.

Shear Modulus Dispersion in Cracked and Fluid‐Saturated Quartzites: Experimental Observations and Modeling

AbstractThe effect of pore fluids on acoustic wave dispersion in rocks with low aspect ratio crack porosity is important for the interpretation of laboratory and field observations in hard rock mineral exploration environments. Here we make laboratory measurements of shear modulus dispersion at frequencies 0.01–1 Hz and at 1 MHz with different saturating fluids (dry, argon, and water saturated) in two thermally cracked quartzite samples with ~2% total porosity. Measurements are made across a range of effective pressures (10–150 MPa), with the resulting very low permeabilities of the samples varying from 1–300 × 10−21 m2. Moduli across the 0.01–1 Hz band were typically independent of frequency. The shear moduli measured at sub‐Hz frequencies are unaffected by fluid saturation, as expected for the saturated isobaric (Gassmann) regime. In marked contrast, water saturation of the cracked rocks results in very large increases in the shear moduli measured at 1 MHz and low effective pressures, indicative of saturated isolated conditions. Thus, at an effective pressure of 20 MPa, the shear moduli for the two water‐saturated quartzites increase by 74% and 98% from 1 Hz to 1 MHz. The contrast in elastic moduli between dry and water‐saturated conditions is well represented by the theoretical model developed by Walsh and others. The observed dispersion highlights the need for care in seismological application of results obtained at MHz frequencies from laboratory ultrasonic measurements.

Physical mechanism of ice/structure interaction

ABSTRACTTo obtain the effect of velocity and structural natural frequency (structural stiffness) on ice failure, an extended dynamic Van der Pol-based single degree-of-freedom ice/structure interaction model is developed. Three basic modes of response were reproduced: intermittent crushing, frequency lock-in and continuous crushing. Further analysis on the physical mechanism of ice/structure interaction is presented on the basis of feedback mechanism and energy mechanism, respectively. Internal effect and external effect from ice and structure were both explained in the feedback branch. Based on reproduced results, energy exchanges at different configurations are computed from the energy conservation using the first law of thermodynamics. A general conclusion on the predominant type of vibration when the ice velocity increases during the interaction process is forced, self-excited and forced in each of the three modes of responses. Ice force variations also show that there is more impulse energy during the lock-in range. Moreover, ice-induced vibration demonstrates an analogy of friction-induced self-excited vibration. Finally, the similarity between strain-stress curve and Stribeck curve shows that static and kinetic friction force variations are attributed to ice force characteristic, and can be used to explain the lower effective pressure magnitude during continuous crushing than the peak pressure during intermittent crushing.

Physical Conditions of Fast Glacier Flow: 1. Measurements From Boreholes Drilled to the Bed of Store Glacier, West Greenland

Marine‐terminating outlet glaciers of the Greenland Ice Sheet make significant contributions to global sea level rise, yet the conditions that facilitate their fast flow remain poorly constrained owing to a paucity of data. We drilled and instrumented seven boreholes on Store Glacier, Greenland, to monitor subglacial water pressure, temperature, electrical conductivity, and turbidity along with englacial ice temperature and deformation. These observations were supplemented by surface velocity and meteorological measurements to gain insight into the conditions and mechanisms of fast glacier flow. Located 30 km from the calving front, each borehole drained rapidly on attaining ∼600 m depth indicating a direct connection with an active subglacial hydrological system. Persistently high subglacial water pressures indicate low effective pressure (180–280 kPa), with small‐amplitude variations correlated with notable peaks in surface velocity driven by the diurnal melt cycle and longer periods of melt and rainfall. The englacial deformation profile determined from borehole tilt measurements indicates that 63–71% of total ice motion occurred at the bed, with the remaining 29–37% predominantly attributed to enhanced deformation in the lowermost 50–100 m of the ice column. We interpret this lowermost 100 m to be formed of warmer, pre‐Holocene ice overlying a thin (0–8 m) layer of temperate basal ice. Our observations are consistent with a spatially extensive and persistently inefficient subglacial drainage system that we hypothesize comprises drainage both at the ice‐sediment interface and through subglacial sediments. This configuration has similarities to that interpreted beneath dynamically analogous Antarctic ice streams, Alaskan tidewater glaciers, and glaciers in surge.

Microscopic investigation of near-field spray characteristics of 2-methylfuran, ethanol and isooctane under flash boiling conditions

Atomization of fuel sprays is a key factor in the control of combustion quality in direct-injection engines. In the present work, the near-field spray patterns of 2-methylfuran (MF), ethanol (ETH) and isooctane (ISO) under non-flash boiling and flash boiling conditions were investigated using an ultra-highspeed imaging technique. Fuel was injected from a single-hole solenoid injector into an optically accessible constant volume chamber at the injection pressure of 40 MPa. Various conditions were tested, ranging from non-flash boiling conditions (ambient) to flare-flash boiling conditions with fuel temperatures of 20 °C and 80 °C and different back pressures. High-speed imaging was performed using a long-distance microscope coupled with an ultra-highspeed camera (1 million fps). Results showed that under flash boiling conditions, near-nozzle spray patterns changed significantly and clear radial expansion was observed due to bubble formation and explosion. Among the three fuels, MF showed the most intense flash boiling behavior due to it having the highest vapor pressure. The effects of different non-dimensional numbers were also considered and it was found that saturation ratio and cavitation number were the two main governing factors for the near-nozzle spray behaviors. During the end of the injection process, the low effective pressure led to poorly atomized spray with a compact liquid column and large ligaments; this could result in poor air/fuel mixing and thus higher HC and particle emissions. Significant improvements were observed at Rs = 0.2 where flash boiling greatly promoted the spray atomization, even with low fuel velocity.

Pressure and Fluid Effect on Frequency‐Dependent Elastic Moduli in Fully Saturated Tight Sandstone

AbstractWe developed a system to explore the effects of pressure and fluid viscosity on the dispersion and attenuation of fully saturated tight sandstones, especially at seismic frequencies. Calibration of the new system revealed that the system can operate reliably at frequencies of [2–200, 106] Hz. Tight sandstone with a “crack–pore” microstructure was tested under nitrogen gas (dry), brine, and glycerin saturation. A frequency‐dependent effect was not found for the dry case. However, apparent dispersion and attenuation for the undrained/unrelaxed transition was clearly observed for sample under brine or glycerin saturation, the magnitude of which was largely suppressed by increasing effective pressure. The measurement results illustrated that increasing the fluid viscosity or the effective pressure will shift the dispersion curve to the lower frequency range. A simple squirt‐flow model with dual‐porosity scheme was used to compare with the measurement results. Although the estimated values deviated slightly from the data, the trend fitted the saturated data relatively well, especially at low effective pressures. Therefore, considering the crack–pore microstructure of the tight sandstone, dispersion and attenuation are induced predominantly by the squirt‐flow stiffening effect from cracks to pores.

Gravitational slope-deformation of a resurgent caldera: New insights from the mechanical behaviour of Mt. Nuovo tuffs (Ischia Island, Italy)

Ischia Island (Italy) is an impressive example of the rare phenomenon of caldera resurgence. The emplacement and replenishment of magmas at shallow depth resulted in a vertical uplift of about 900m, concentrated in the western portion of Mt. Epomeo (789m a.s.l.). As a consequence of this uplift, the island has experienced several slope instabilities at different scales since the Holocene, from shallow mass movements to large rock and debris avalanches. These mass wasting events, which mobilised large volumes of greenish alkali-trachytic tuff (the Mt. Epomeo Green Tuff, MEGT), were strictly related to volcano-tectonic activity and the interaction between the volcanic slopes and the hydrothermal system beneath the island. Deep-Seated Gravitational Slope Deformation (DSGSD) at Mt. Nuovo, located adjacent to densely populated coastal villages, is an ongoing process that covers an area of 1.6km2. The Mt. Nuovo DSGSD involves a rock mass volume of 190Mm3 and is accommodated by a main shear zone and a series of sub-vertical fault zones associated with high-angle joint sets. To improve our understanding of this gravity-induced process, we performed a physical (porosity and permeability) and mechanical (uniaxial and triaxial deformation experiments) characterisation of two ignimbrite deposits - both from the MEGT - that form a significant component of the NW sector of Mt. Epomeo. The main conclusions drawn from our experiments are twofold. First, the presence of water dramatically reduces the strength of the tuffs, suggesting that the movement of fluids within the hydrothermal system could greatly impact slope stability. Second, the transition from brittle (dilatant) to ductile (compactant) behaviour in the tuffs of the MEGT occurs at a very low effective pressure, analogous to a depth of a couple of hundred metres, and that this transition is likely moved closer to the surface in the presence of water. We hypothesise that compactant (porosity decreasing) behaviour at the base of the layer could therefore facilitate slope instability. Although our results show that transient exposure to 300°C does not influence the short-term strength of the tuff, we speculate that the high in-situ temperature could increase the efficiency of brittle and compactant creep and therefore increase the rate of slope deformation. Taken together, our experimental data highlight a potentially important role for the hydrothermal system (that reaches a minimum depth of ~1km) in dictating the DSGSD at Mt. Nuovo. An understanding of this deformation process is not only important for the proximal coastal villages, at risk of engulfment by a large debris avalanche, but also for the towns and cities along the coast of the Gulf of Naples that are at risk to a secondary consequence of such an avalanche - a tsunami wave.

Coupling between a glacier and a soft bed: II Model results

The relation between the local effective pressure and shear stress on till beneath Storglaciaren, Sweden, discussed in Iverson and others (1999), provides an empirical basis for studying the processes that control the strength of the ice/bed coupling. Particles in the bed that protrude into the glacier sole support shear stresses that are limited by either ploughing or the traditional sliding mechanisms. Model calculations, based on studies of cone penetration through fine-grained sediment and sliding theory, agree with the observed relation between shear stress and effective pressure if the water layer at the ice/bed interface is assumed to thicken rapidly as the effective pressure approaches zero. Studies of the hydraulics of linked cavities provide support for this assumption, if the mean thickness of the water layer reflects the extent of microcavity development at the interface. Comparison of the calculated shear stress with the ultimate strength of till suggests that bed deformation limits the shear stress on till beneath Storglaciaren only at intermediate effective pressures; at very low effective pressures, like those inferred at the site of the tiltmeter discussed in Iverson and others (1999), and at sufficiently high effective pressures, ploughing and sliding should focus motion near the glacier sole. A calculation using parameter values appropriate for Ice Stream B, West Antarctica, suggests that ploughing may occur there at shear stresses not sufficient to deform the bed at depth. This conclusion is reinforced by the likelihood that pore pressures in excess of hydrostatic should develop down-glacier from ploughing particles, thereby weakening the bed near the glacier sole. However, given the apparent sensitivity of the ice/bed coupling to basal conditions that may be highly variable, any blanket assumption regarding the flow mechanism of ice masses on soft beds should probably be viewed with skepticism.

The state of stress in the Earth's crust along the northwestern flank of the Pacific seismic focal zone before the Tohoku earthquake of 11 March 2011

This paper presents a detailed reconstruction of the state of stress in the Earth's crust in the Japanese seismic focal zone before the Tohoku earthquake of March 11, 2011. The stress parameters are determined using the method of cataclastic analysis (MCA) of discontinuous displacements. This method integrates some of the main principles of the theory of plastic deformation and a generalization of the results of rock failure experiments. The reconstruction of the state of stress is based on earthquake focal mechanism data for two magnitude ranges, Mw: 1) 3.0–5.5 and 2) 5.0–7.0. The calculations for the first data set are performed at six depths. Stress reconstruction for the second data set is performed from 0 to 50km. The focal mechanism data provides reasonably stable parameters of the stress field for magnitudes in this range. A large amount of data on the focal mechanisms of earthquakes with magnitudes between 3.0 and 5.5 has allowed us to obtain new data on changes in the stress field, which were missed in the previous calculation for the second data set. It is shown that in the upper part of the lithosphere (up to 90km) the axis of maximum compression is generally oriented slightly (25–35°) under the oceanic plate west of the Japan Trench, and it dips subvertically east of the trench. The upper part of the tectonosphere is divided into two geodynamic regimes: subhorizontal compression west of the trench axis and subhorizontal extension east of the trench axis. West of the Japan Trench, the axis of maximum horizontal compression (SHmax) is orthogonal to the trench axis, but east of the Japan Trench, its orientation has changed (~90°). The underthrusting shear stress that acts normal to the horizontal planes has a similar west and west-southwest direction in the upper layers (0–50km). Deviations in the orientation are observed at depths between 70 and 90km at the junction between the Japan Trench and the Nankai Trough and in the crust of the northeastern part of the Tohoku region. Obtained results are summarized in 2 D-pattern, which can be used to explain the loading mechanism in the region of the Tohoku earthquake. The results of the stress reconstruction also allowed us to obtain the new specific features of the field stress: the oceanic trench divides lithosphere into the areas of high effective pressure west of the trench axis and the areas of low effective pressure east of the trench axis. The rupture developed in the area of the maximum stress gradient and it stopped in the north-east and the south-west directions, where there were areas of high effective pressure.