Transient Pore Pressure Research Articles

Rainfall-induced landslide stability analysis in response to transient pore pressure??A case study of natural terrain landslide in Hong Kong

Transient pore pressure in response to short intense rainfall process plays an important role in shallow landslide occurrence. Using GIS technology, we carry out the rainfall-induced landslide stability analysis in response to transient pore pressure by means of transient and unsaturated rainfall infiltration modeling. A case study is performed on the shallow landslide stability analysis in Hong Kong. Detailed analysis and discussion reached some useful conclusions on the tempo-spatial behavior and characteristics of slope stability response and pore pressure response to typical rainfall process. Comparison analysis is performed on some important issues including landslide stability response in different types of slopes with different hydraulic properties, antecedent rainfall and landslide stability, and the nature of pore pressure response time. These studies might give us an important insight into landslide tringgering mechanism and the hydrological process in response to rainfall, and provide systematic information and evidences for effective risk assessment and warning system establishment.

Post-earthquake ground movements correlated to pore-pressure transients.

Large earthquakes alter the stress in the surrounding crust, leading to triggered earthquakes and aftershocks. A number of time-dependent processes, including afterslip, pore-fluid flow and viscous relaxation of the lower crust and upper mantle, further modify the stress and pore pressure near the fault, and hence the tendency for triggered earthquakes. It has proved difficult, however, to distinguish between these processes on the basis of direct field observations, despite considerable effort. Here we present a unique combination of measurements consisting of satellite radar interferograms and water-level changes in geothermal wells following two magnitude-6.5 earthquakes in the south Iceland seismic zone. The deformation recorded in the interferograms cannot be explained by either afterslip or visco-elastic relaxation, but is consistent with rebound of a porous elastic material in the first 1-2 months following the earthquakes. This interpretation is confirmed by direct measurements which show rapid (1-2-month) recovery of the earthquake-induced water-level changes. In contrast, the duration of the aftershock sequence is projected to be approximately 3.5 years, suggesting that pore-fluid flow does not control aftershock duration. But because the surface strains are dominated by pore-pressure changes in the shallow crust, we cannot rule out a longer pore-pressure transient at the depth of the aftershocks. The aftershock duration is consistent with models of seismicity rate variations based on rate- and state-dependent friction laws.

Poroelastic analysis of cover-collapse sinkhole formation by piezometric surface drawdown

Where the water table is above the soil–rock contact in karst regions, cover-collapse sinkholes in the soil and soft sediment above the rock commonly occur as a result of drawdown of the piezometric surface in the karst aquifer. Transient stresses and pore pressures around soil voids at the soil–rock contact can cause hydraulic fracturing of the soil near the wall of the void. After the first such fracture, successive sloughing of soil propagates the soil void rapidly to the surface, resulting in a cover-collapse sinkhole. Sinkhole formation by this mechanism should be strongly a function of rate and magnitude of piezometric surface drawdown, permeability and tensile strength of the soil, and the size, depth, and geometry of the initial soil void. Large soil voids and those with walls that are partly planar or of low curvature are most susceptible to hydraulic fracture and the resulting progression to sinkhole formation.

Understanding Chemical-Potential-Related Transient Pore-Pressure Response To Improve Real-Time Borehole (In)Stability Predictions

Chemical potential related borehole (in)stabilities in the field are predominantly time-dependent. With the intention of developing a real-time wellbore (in)stability modelling capability, experimental work was undertaken to investigate the role of the chemical potential of drilling fluids on transient pore pressure and time-dependent rock property alterations of shale formations. The current work presents the concept, and discusses the results of these undertakings.The experiments using a Pore Pressure Transmission Test (PPT) apparatus expose formation (shale) cores under simulated downhole conditions to various salt solutions and drilling fluids. The uniqueness of this study comes from the fact that timedependent alteration in the pore pressure, acoustic and rock properties of formations subjected to compressive tri-axial stress are recorded during the PPT experiments. This eliminates the anisotropy-associated differences obtained when different samples are used to determine the relationship between these characteristic parameters of shale formations.The main objective of this effort is to translate the results of the PPT tests to actual drilling conditions. At that point, the formation- drilling fluid chemical potential borehole instability model would be validated and updated in real-time to predict borehole (in)stabilities.Introduction. Borehole instability in shales is a continuing problem that results in a substantial annual expenditure by the petroleum industry ($700 million according to conservative estimates). In the past, oil-based muds (OBM) have been the system of choice for difficult drilling. As environmental concerns restrict the use of oil-based muds, the petroleum service industry must provide innovative means to obtain OBM performance without negatively impacting the environment. Water-based muds (WBM) are attractive replacements from a direct cost viewpoint. However, conventional WBM systems have failed to meet key performance measures met with OBMs, especially while drilling high-angle, extended-reach well trajectories going through water-sensitive shale formations.Historically, wellbore (in)stability problems have been approached on a trial-and-error basis, going through a costly multiwell learning curve before arriving at reasonable solutions for optimized operations and systems. Recent studies(1, 2) of fluidshale interactions have produced fresh insights into the underlying causes of borehole (in)stability, and these studies suggest new and innovative approaches to the design of water-based drilling fluids.Since the identification of chemical-potential related instabilities in shales, a chemical-potential borehole stability analytical model has been proposed to describe the physio-chemical interaction(1, 3).Advances in technology are providing new downhole sensors and tools that are increasingly able to collect and transmit information to the surface in real time, thus enabling field personnel to make faster and more informed decisions.There are increasing economic demands on reducing rig downtimes associated with borehole instability problems. Thus, the anticipation and resolution of instabilities in the field using MWD/LWD data in conjunction with prior geophysical and shale/fluid interaction knowledge is becoming more critical.In this study, transient pore pressure, compressional and shear wave velocities, and the deformation of shale cores under geostatic stress are measured.

Embracing Today's Interpretation Technologies

Abstract Chemical potential related borehole (in)stabilities in the field are predominantly time-dependent. With the intention of developing a real-time wellbore (in)stability modelling capability, experimental work was undertaken to investigate the role of the chemical potential of drilling fluids on transient pore pressure and time-dependent rock property alterations of shale formations. The current work presents the concept, and discusses the results of these undertakings. The experiments using a Pore Pressure Transmission Test (PPT) apparatus expose formation (shale) cores under simulated downhole conditions to various salt solutions and drilling fluids. The uniqueness of this study comes from the fact that timedependent alteration in the pore pressure, acoustic and rock properties of formations subjected to compressive tri-axial stress are recorded during the PPT experiments. This eliminates the anisotropy-associated differences obtained when different samples are used to determine the relationship between these characteristic parameters of shale formations. The main objective of this effort is to translate the results of the PPT tests to actual drilling conditions. At that point, the formation- drilling fluid chemical potential borehole instability model would be validated and updated in real-time to predict borehole (in)stabilities. Introduction Borehole instability in shales is a continuing problem that results in a substantial annual expenditure by the petroleum industry ($700 million according to conservative estimates). In the past, oil-based muds (OBM) have been the system of choice for difficult drilling. As environmental concerns restrict the use of oil-based muds, the petroleum service industry must provide innovative means to obtain OBM performance without negatively impacting the environment. Water-based muds (WBM) are attractive replacements from a direct cost viewpoint. However, conventional WBM systems have failed to meet key performance measures met with OBMs, especially while drilling high-angle, extended-reach well trajectories going through water-sensitive shale formations. Historically, wellbore (in)stability problems have been approached on a trial-and-error basis, going through a costly multiwell learning curve before arriving at reasonable solutions for optimized operations and systems. Recent studies(1, 2) of fluidshale interactions have produced fresh insights into the underlying causes of borehole (in)stability, and these studies suggest new and innovative approaches to the design of water-based drilling fluids. Since the identification of chemical-potential related instabilities in shales, a chemical-potential borehole stability analytical model has been proposed to describe the physio-chemical interaction(1, 3). Advances in technology are providing new downhole sensors and tools that are increasingly able to collect and transmit information to the surface in real time, thus enabling field personnel to make faster and more informed decisions. There are increasing economic demands on reducing rig downtimes associated with borehole instability problems. Thus, the anticipation and resolution of instabilities in the field using MWD/LWD data in conjunction with prior geophysical and shale/fluid interaction knowledge is becoming more critical. In this study, transient pore pressure, compressional and shear wave velocities, and the deformation of shale cores under geostatic stress are measured.

Understanding Chemical-Potential-Related Transient Pore-Pressure Response To Improve Real-Time Borehole (In)Stability Predictions

Abstract Chemical potential related borehole (in)stabilities in the field are predominantly time-dependent. With the intention of developing a real-time wellbore (in)stability modelling capability, experimental work was undertaken to investigate the role of the chemical potential of drilling fluids on transient pore pressure and time-dependent rock property alterations of shale formations. The current work presents the concept, and discusses the results of these undertakings. The experiments using a Pore Pressure Transmission Test (PPT) apparatus expose formation (shale) cores under simulated downhole conditions to various salt solutions and drilling fluids. The uniqueness of this study comes from the fact that timedependent alteration in the pore pressure, acoustic and rock properties of formations subjected to compressive tri-axial stress are recorded during the PPT experiments. This eliminates the anisotropy-associated differences obtained when different samples are used to determine the relationship between these characteristic parameters of shale formations. The main objective of this effort is to translate the results of the PPT tests to actual drilling conditions. At that point, the formation- drilling fluid chemical potential borehole instability model would be validated and updated in real-time to predict borehole (in)stabilities. Introduction Borehole instability in shales is a continuing problem that results in a substantial annual expenditure by the petroleum industry ($700 million according to conservative estimates). In the past, oil-based muds (OBM) have been the system of choice for difficult drilling. As environmental concerns restrict the use of oil-based muds, the petroleum service industry must provide innovative means to obtain OBM performance without negatively impacting the environment. Water-based muds (WBM) are attractive replacements from a direct cost viewpoint. However, conventional WBM systems have failed to meet key performance measures met with OBMs, especially while drilling high-angle, extended-reach well trajectories going through water-sensitive shale formations. Historically, wellbore (in)stability problems have been approached on a trial-and-error basis, going through a costly multiwell learning curve before arriving at reasonable solutions for optimized operations and systems. Recent studies(1, 2) of fluidshale interactions have produced fresh insights into the underlying causes of borehole (in)stability, and these studies suggest new and innovative approaches to the design of water-based drilling fluids. Since the identification of chemical-potential related instabilities in shales, a chemical-potential borehole stability analytical model has been proposed to describe the physio-chemical interaction(1, 3). Advances in technology are providing new downhole sensors and tools that are increasingly able to collect and transmit information to the surface in real time, thus enabling field personnel to make faster and more informed decisions. There are increasing economic demands on reducing rig downtimes associated with borehole instability problems. Thus, the anticipation and resolution of instabilities in the field using MWD/LWD data in conjunction with prior geophysical and shale/fluid interaction knowledge is becoming more critical. In this study, transient pore pressure, compressional and shear wave velocities, and the deformation of shale cores under geostatic stress are measured.

Borehole collapse analysis incorporating time-dependent pore pressure due to mud penetration in shales

This paper presents a model to predict time-dependent instability of arbitrarily oriented wellbores. The underlying factor to make the borehole collapse time-dependent is the increase of pore pressure around the wellbore due to mud penetration over time. The time-dependent pore pressure around the wellbore is estimated by analyzing two-phase fluid flow in porous media. The petrophysical parameters required for two-phase flow analysis, such as capillary pressure and absolute and relative permeabilities are characterized from experimental data. The model for time-dependent borehole collapse analysis is then developed by incorporating the transient pore pressure in the widely used wellbore failure analysis model. Potential wellbore failure modes are mathematically defined and guidance is incorporated to identify the upper and the lower bounds of the safe mud window at a particular time. The developed model is applied to wells in shale under different stress regimes. Results demonstrate that an initial safe mud window becomes narrower as the time progresses. Consequently, a mud weight, which was initially safe, may become unsafe and results in borehole collapse after a certain period of time. Such a failure is further influenced by the orientation of the wellbore and different stress regimes.

Landslide triggering by rain infiltration

Landsliding in response to rainfall involves physical processes that operate on disparate timescales. Relationships between these timescales guide development of a mathematical model that uses reduced forms of Richards equation to evaluate effects of rainfall infiltration on landslide occurrence, timing, depth, and acceleration in diverse situations. The longest pertinent timescale is A/D0, where D0 is the maximum hydraulic diffusivity of the soil and A is the catchment area that potentially affects groundwater pressures at a prospective landslide slip surface location with areal coordinates x, y and depth H. Times greater than A/D0 are necessary for establishment of steady background water pressures that develop at (x, y, H) in response to rainfall averaged over periods that commonly range from days to many decades. These steady groundwater pressures influence the propensity for landsliding at (x, y, H), but they do not trigger slope failure. Failure results from rainfall over a typically shorter timescale H2/D0 associated with transient pore pressure transmission during and following storms. Commonly, this timescale ranges from minutes to months. The shortest timescale affecting landslide responses to rainfall is , where g is the magnitude of gravitational acceleration. Postfailure landslide motion occurs on this timescale, which indicates that the thinnest landslides accelerate most quickly if all other factors are constant. Effects of hydrologic processes on landslide processes across these diverse timescales are encapsulated by a response function, , which depends only on normalized time, t*. Use of R(t*) in conjunction with topographic data, rainfall intensity and duration information, an infinite‐slope failure criterion, and Newton's second law predicts the timing, depth, and acceleration of rainfall‐triggered landslides. Data from contrasting landslides that exhibit rapid, shallow motion and slow, deep‐seated motion corroborate these predictions.

Fluid- and reaction-assisted low-angle normal faulting: evidence from rift-related brittle fault rocks in the Alps (Err Nappe, eastern Switzerland)

Brittle fault rocks associated with a rift-related low-angle detachment system, exposed in the Lower Austroalpine Err Nappe (eastern Switzerland), exhibit systematic changes in mineralogy, fabrics, and structures, which are reflected by the transition from an undeformed granite to a cataclasite and a phyllosilicate-rich gouge across the fault zone. Cross-cutting relationships between the brittle fault rocks and syn-kinematic quartz veins suggest that a major part of the 11 km (min.) of displacement along the fault zone was accommodated in the thin, continuous gouge layer. Strain localization in the fault zone is controlled by fluid-assisted deformation which enhances break-down reactions of feldspar to phyllosilicates. An increasing proportion of phyllosilicates decreases the strength and the permeability of the fault zone and may facilitate inter-crystalline deformation mechanisms such as grain-boundary sliding in phyllosilicate-rich zones. Normal slip of the hanging wall relative to the footwall at low angles of inclination may be facilitated by weakening of the fault zone and transient high pore pressure events.

Mechanics of upward propagation of cover-collapse sinkholes

Cover collapse sinkholes occur by failure of the walls of a soil void above cavernous bedrock. I examine conditions for failure in compression by crack propagation and coalescence, and for failure by sloughing in which pore water pressure produces a net tension perpendicular to the wall of the void. Compressive failure without relaxation of stress does not explain observed long-term stability of thick soils, nor failures in thin residual soils. If stress relaxation occurs with compressive failure, soil voids tend to stabilize and sinkholes are not predicted. Sloughing failure is not predicted for steady state pore pressure fields, even for a perched water table at the surface over a drained void. Transient pore pressure conditions that could produce sloughing may result from: (1) rapid drawdown causing soil consolidation and transient high pore pressure gradients; (2) approach of a wetting front in unsaturated soil, with high permeability behind the front; (3) dissolution of air from bubbles in the soil and exsolution near the void producing a low permeability zone near the void.

Earthquake-related deformation beneath gently inclined ground

The experimental data from an investigation of the response of sand slopes to centrifuge model earthquakes are presented. In each test a condition of steady state seepage was established parallel to the base of the model slope before triggering a single earthquake. Discrete slip planes are not observed; the predominant form of instability beneath both steeper and more gentle slopes is shear strain deformation. The deformation is continuous, though not linear, with depth. The data recorded by acceleration and pore pressure transducers are used to establish time histories of effective stress beneath slopes of different gradient. The mechanisms of deformation are then identified through the concept of characteristic state theory (phase transformation). It is shown that inertial loading is predominant in mechanisms associated with the deformation of slopes of moderate inclination, while transient pore pressure increase (with loss of shear strength and subsequent gravity effects) predominates in mechanisms associated with the deformation of more gentle slopes. Ĺauteur présente les données expérimentales de recherches sur la réaction de talus de sable &garve; des modélisations de tremblement de terre en centrifugeuse. Dans chaque essai, on a établi un régime permanent de suintement parallèlement à la base du talus modé1isé avant de déclencher un tremblement de terre. On ńobserve aucun plan de glissement distinct, et le cisaillement est la forme prédominante ´instabilité sous les pentes abruptes comme sous les pentes plus douces. La déformation est continue, mais pas linéaire, en profondeur. Les données enregistrées par les capteurs d'accé1ération et de pression ´ean interstitielle servent à établir les diagrammes ´évolution des contraintes intergranulaires sous des talus de pente variée. Les m6canismes de déformation sont alors identifiés à ĺaide du concept de la Worie de ĺétat caractéristique (transition de phase). On montre que le chargement inertiel est prédominant dans les mécanismes associés à la déformation des talus de pente modérée, tandis que ĺaugmentation transitoire de la pression ´ean interstitielle (avec perte de r6sistance au cisaillement et effets de gravité subséquents) prédomine dans les mé- canismes associés à la déformation de talus à pente plus modérée.

Measurement of hydraulic properties in deep lake sediments using a tethered pore pressure probe: Applications in the Hamilton Harbour, western Lake Ontario

Estimates of groundwater seepage flux in lake bottom sediments require knowledge of the hydraulic gradient at the sediment‐surface water interface and the hydraulic conductivity of the lake‐bottom materials. In deep waters, in situ measurement of these parameters can be accomplished through the use of piezometer probes lowered and monitored remotely from a surface vessel. In this research work a new tethered piezometer probe was developed and tested for use in collecting hydraulic property data in deep‐lake bottom sediments. The probe uses a variable‐reluctance transducer to measure the differential sediment pore pressure between two ports spaced 100 cm apart. The dissipation of pore pressure transients that develop during rapid emplacement of the probe were extrapolated in time to estimate equilibrium hydraulic gradients. In addition, various data analysis techniques were evaluated for determining sediment hydraulic conductivity and specific storage through interpretation of the pore‐pressure dissipation data. The probe was used to estimate groundwater seepage in the bottom sediments of the Hamilton Harbour, at the western end of Lake Ontario. Upward gradients were measured at nine locations within the harbor ranging from 0.010 to 0.425 and a downward gradient of −0.015 was recorded at one site along the harbor's eastern boundary. Hydraulic conductivities determined from pore‐pressure dissipation over time ranged from 6.9 × 10−9to 4.8 × 10−7 m/s. Specific storage values ranged from 0.08 to 0.19 m−1. Calculated average linear seepage velocities ranged from 4.3 × 10−8 to −8.5 × 10−9 m/s. The groundwater contribution to the harbor through the deeper, fine‐grained sediments was estimated to be 9.1 × 10−2 m3/s, or 2.9 × 106 m3/yr. This represents approximately 1.0% of the harbor basin's total volume, 15% of precipitation's contribution, 1.2% of the contribution of surface inflows (excluding the Burlington ship canal) and 0.22% of the total surface outflow passing through the Burlington shipping canal, which connects the harbor to Lake Ontario.

Common cyclicities in the seismicity and water level fluctuations at the charlevoix seismic zone on the St Lawrence River, Quebec, Canada

Intraplate seismicity has no widely accepted explanation for its origin. The hydroseismicity hypothesis suggests that natural increases in hydraulic head, caused by transient increases in water table elevation, can be transmitted to hypocentral depths (10–25 km) in a fractured, prestressed, near-failure crust, and, along with long term hydrolytic weakening of rocks, contribute to the triggering of earthquakes. In this study, the temporal characteristics of seismicity and water level fluctuations at the Charlevoix seismic zone on the Saint Lawrence River in Quebec, Canada, are determined using Fourier spectral analysis and are investigated for possible temporal relationships, as predicted by hydroseismicity, using group delay analysis. Fourier spectral analysis of the temporal release of seismic energy indicates the presence of short term cyclicities of periods of 13–14 yr, along with longer term periods of approx. 55–70 yr. Spectral analysis of the water level data indicates cyclicities with periods of 1, 23, 14 and 8 years. Group delay analysis of the 14 yr periodicities in the water level and seismicity time series yields a time lag of 2.3–2.6 yr, with the water levels leading the seismicity time series. Crustal diffusivity values estimated from the group delay analysis range between 0.3 and 2.7 m 2/s, for a depth interval from 7 to 20 km, which is within the range of published values of diffusivities for the crust (0.1–100 m 2/sec). The present study suggests that common cyclicities are present in both the water levels and seismicity in the Charlevoix region. Such cyclicities are an essential element of the hydroseismicity hypothesis for the diffusion of surficial fluid pressure variations to hypocentral depths. Furthermore, the temporal relationship between the cyclicities observed allows for a possible causal relationship between repetitive mechanical effects of pore pressure transients and seismicity in the Charlevoix region.

Anisotropic Plasticity Model for Undrained Cyclic Behavior of Clays. I: Theory

A constitutive model for the stress‐strain‐pore‐pressure behavior of fluid‐saturated cohesive soils under dynamic loading is developed using the concept of bounding surface plasticity. The model adopts the joint invariants of the second‐order stress tensor and clay fabric tensor as a formalism to account for material anisotropy. The model includes three internal variables: density‐hardening variable representing change in void ratio; rotational hardening variable depicting fabric ellipsoid changes; and, finally, distortional hardening variable controlling the shape of bounding surface. The corresponding evolution laws are formulated using micromechanics arguments and phenomenological observations. In a companion paper, the resulting constitutive equation is shown to capture the essential features of undrained cyclic behavior of clays, including: (1) Degradation of undrained strength and stiffness; (2) development of transient and residual pore pressure and deformation; (3) effect of initial consolidation ...

Anisotropic Plasticity Model for Undrained Cyclic Behavior of Clays. II: Verification

A constitutive model for the stress‐strain‐pore pressure behavior of saturated cohesive soils under both monotonic and cyclic loadings has been presented in the companion paper. The model has been implemented into a numerical program to facilitate evaluation of the model's predictive capability. Procedures for model parameter determination are given. Numerical simulations are compared with a set of data encompassing a wide range of stress history, consolidation‐stress ratio, and stress path. In addition, both strain‐controlled and stress‐controlled cyclic tests are considered. The comparisons show that the model is capable of capturing the essential features of undrained cyclic behavior of clays, including: (1) Degradation of undrained strength and stiffness; (2) development of transient and residual pore pressure and deformation; (3) influence of initial consolidation stress condition and stress history; and (4) effect of loading path involving rotation of principal stress direction.

Detection of geotechnical parameters from transient pore pressures in the field : Barends, B J, Proc Indian Geotechnical Conference, 1989, Geotechniques of Problematic Soils and Rocks, Visakhapatnam, 14–16 December 1989V1, P3–8. Publ New Delhi: Sarita Prakashan, 1989

Detection of geotechnical parameters from transient pore pressures in the field : Barends, B J, Proc Indian Geotechnical Conference, 1989, Geotechniques of Problematic Soils and Rocks, Visakhapatnam, 14–16 December 1989V1, P3–8. Publ New Delhi: Sarita Prakashan, 1989

Correlations between streamflow and intraplate seismicity in the central Virginia, U.S.A., seismic zone: evidence for possible climatic controls

There is no widely accepted explanation for the origin of intraplate earthquakes. The central Virginia seismic zone, like other seismically active intraplate areas, is a spatially isolated area of persistent, diffuse earthquake activity. We suggested earlier that rainfall plays a key role in the generation of intraplate seismicity (“hydroseismicity”). Observed long-period (10–30 years) changes in streamflow (rainfall) are hypothesized to generate intraplate seismicity by diffusion of pore pressure transients from recharge areas of groundwater basins to depth as deep as the brittle-ductile transition. Streamflow and earthquake strain for a 62-year sample from 1925 to 1987 in the central Virginia seismic zone were cumulated, and a least-squares straight-line fit was subtracted to obtain residuals of streamflow and strain. Residual streamflow was differentiated to obtain the rate of change of residual streamflow. We observed common cyclicities with periods of 10–30 years for residual streamflow and strain. From the one-dimensional diffusion equation, we determined the time response of fluid pressure at depths, ψ, in a hydraulically diffusive crust to an impulsive change in fluid pressure at the surface of a groundwater basin. These responses were convolved with the surface streamflow residuals, or, because of results from reservoir-induced seismicity, with the derivatives of these residuals. Root-mean-square values (rms) of the convolutions were computed for ψ = 5, 10, 15 and 20 km and various values of D. For central Virginia, the number of earthquakes, N, within a crustal slice centered on a depth, ψ, was found to be proportional to the rms value of the convolution, suggesting that the number of intraplate earthquakes generated is directly proportional to the magnitude of the rms changes in fluid pore pressure within the crustal slice. These fluctuations in pore pressure, in concert with stress corrosion and hydrolytic weakening, are hypothesized to trigger intraplate earthquakes in a crust stressed by ridge-push. Crosscorrelation of the residual streamflow convolutions with residual strain, provides a measure of similarity between the streamflow convolution and the strain. Optimum values of crustal diffusivity, D, were considered to be those values for which the Crosscorrelation function looks approximately even. That this occurs only over a reasonable range of diffusivities (near D = 50 km 2/ year) is consistent with the hypothesized causal relationship between streamflow and intraplate seismicity. Finally, others have shown a clear relationship between the 11-year solar cycle and storm track latitudes in the North Atlantic. Storm tracks over the eastern United States and Canada indicate that north of 50 ° N, the average latitude of storm tracks during December, January, and February is about 2.5 ° farther south at sunspot maximum than at sunspot minimum. A correlation with the 11-year solar cycle was discovered by Labitzke (1987). Significantly, we recognize a clear peak near 11 years in the Fourier spectra of actual (not residual) streamflow in the James River in the central Virginia seismic zone.

Experimental investigation of transient pore pressure behaviour in soils due to gas exsolution : Sobkowicz, J C; Morgenstern, N R Proc International Symposium on Prediction and Performance in Geotechnical Engineering, Calgary, 17–19 June 1987P267–275. Publ Rotterdam: A A Balkema, 1987

Experimental investigation of transient pore pressure behaviour in soils due to gas exsolution : Sobkowicz, J C; Morgenstern, N R Proc International Symposium on Prediction and Performance in Geotechnical Engineering, Calgary, 17–19 June 1987P267–275. Publ Rotterdam: A A Balkema, 1987

Hydroselsmicity: A Hypothesis for The Role of Water in the Generation of Intraplate Seismicity

Abstract A new hypothesis termed Hydroseismicity that has hydrologic (diffusion of pore pressure transients from recharge areas of groundwater basins), geologic (rifted, fractured crust), and chemical (solubility of minerals) elements is proposed to explain the role of water in the generation of intraplate seismicity. Its basis is a spatial correlation in the southeastern U. S. between 1) seismogenic crustal volumes of high seismicity, 2) large gravity-driven river basins that can provide an adequate supply of water to the upper- and mid-crust, and 3) a permeable crust that is tectonically stressed close to failure. It is suggested that in crustal volumes with a combination of connected fractures and adequate groundwater, natural transient increases in hydraulic head in recharge areas of groundwater basins can be transmitted to depths of 10–20 km, and thereby trigger earthquakes, via a flow-path geometry that resembles except for scale the model familiar to groundwater hydrologists for near-surface flow. Possible trigger mechanisms for Hydroseismicity include small increases in fluid pressure at hypocentral depths caused by such transient increases, and hydrolytic weakening of minerals that leads to structural weakening. Implicit in the model is a diffuse distribution of epicenters (as is observed in the region) rather than concentrations along discrete geologie (faults) or geomorphic (rivers) elements. Open fractures imply fracture roughness, i.e., asperities under a higher stress that keep fractures open even in an ambient tectonic stress field. Intraplate earthquakes in a fractured crust prestressed to near-failure are thus postulated to be triggered by small transient increases in fluid pressure transmitted along preexisting fractures in a rock fabric weakened by stress corrosion of asperities. Abundant petrologic evidence is available to justify an assumption of fracture permeability to depths of 20 km near passive rifted margins. All four principal seismogenic volumes in the southeastern U. S. are within gravity-driven groundwater basins that can provide an abundant supply of water to the ernst, and that intersect known or suspected Eocambrian or Mesozoic rifted crust. The host basins have the largest surface recharge areas and contain rivers with the highest average stream gradients as measured from their headwaters to the Fall Line. Seismicity in the region is characterized by steeply dipping focal mechanism nodal planes and diffuse alignments and/or clusters of epicenters. These characteristics are compatible with a steep to vertical fracture fabric currently being reactivated by porc pressure diffusion from surface recharge of groundwater basins.

Finite element formulation for transient pore pressure dissipation: a variational approach : Int J Solids StructV22, N11, 1986, P1201–1211

Finite element formulation for transient pore pressure dissipation: a variational approach : Int J Solids StructV22, N11, 1986, P1201–1211