Pore Fluid Pressure Buildup Research Articles

Margin-wide variations in slab dehydration in Cascadia and their relationship to slow slip

Slow slip events are observed in subduction zones globally, with a wide range of variations in behaviour, including the periodicity and magnitude of events. These variations are observed both within and across different subduction zones. Due to their role in controlling strain accumulation and potential nucleation of regular earthquakes, understanding the physical environment of slow slip events and the source of their observed variability is critical to understanding seismogenic processes. Here we calculate thermal models for 10 trench-normal profiles along the entirety of the Cascadia subduction zone. The models incorporate lateral changes in incoming plate age and sediment input, as well as slab geometry and overriding plate structure. Temperatures at the location of peak slow slip and tremor occurrence (depths of 30–40 km) vary from 525 °C in southern Cascadia, to 475 °C in central Cascadia, to 550 °C in northern Cascadia. The variability in thermal structure and slab geometry leads to changes in the rate of calculated fluid flux in the vicinity of the mantle wedge corner where slow slip events are observed. Fluid flux rates calculated within 25 km and 50 km downdip of the mantle wedge corner show an inverse relationship with the recurrence time of slow slip with highest value observed in southern Cascadia, followed by northern Cascadia, and then central Cascadia. Other along-strike variability in tremor behaviour, such as tremor strength, correlates spatially with changes in calculated fluid flux pattern but the relationship between these parameters is not readily defined. This study provides support for models where variations in the rate of fluid production in the vicinity of the mantle wedge corner are a potential mechanism controlling the rate of pore fluid pressure buildup and fluid release during slow slip. • Changes in subduction parameters affect fluid release along Cascadia. • Major sites of slab dewatering are observed beneath the mantle wedge corner. • Fluid flux rates show an inverse relationship with the recurrence time of slow slip. • Variability in tremor strength and density correlate spatially with flux pattern.

Tectonic pressure gradients during viscous creep drive fluid flow and brittle failure at the base of the seismogenic zone

AbstractFluid-pressure cycles are commonly invoked to explain alternating frictional and viscous deformation at the base of the seismogenic crust. However, the stress conditions and geological environment of fluid-pressure cycling are unclear. We address this problem by detailed structural investigation of a vein-bearing shear zone at Sagelvvatn, northern Norwegian Caledonides. In this dominantly viscous shear zone, synkinematic quartz veins locally crosscut mylonitic fabric at a high angle and are rotated and folded with the same sense of shear as the mylonite. Chlorite thermometry indicates that both veining and mylonitization occurred at ∼315–400 °C. The vein-filled fractures are interpreted as episodically triggered by viscous creep in the mylonite, where quartz piezometry and brittle failure modes are consistent with low (18–44 MPa) differential stress. The Sagelvvatn shear zone is a stretching shear zone, where elevated pressure drives a hydraulic gradient that expels fluids from the shear zone to the host rocks. In low-permeability shear zones, this hydraulic gradient facilitates build-up of pore-fluid pressure until the hydrofracture criterion is reached and tensile fractures open. We propose that hydraulic gradients established by local and cyclic pressure variations during viscous creep can drive episodic fluid escape and result in brittle-viscous fault slip at the base of the seismogenic crust.

Time-resolved grain-scale 3D imaging of hydrofracturing in halite layers induced by gypsum dehydration and pore fluid pressure buildup

Fluid release from dehydration reactions is considered to have significant effects on the strength and dynamics of tectonic faults at convergent plate boundaries. It is classically assumed that the production of fluid leads to increased pore fluid pressures that perturb a fault's stress state and thereby facilitates and enhances deformation. This important assumption has never been supported by direct microstructural observations. Here, we investigate the role of gypsum dehydration in the deformation of evaporitic rocks using synchrotron-based time-resolved X-ray computed microtomography (4D) imaging. This approach enables the documentation of coupled chemical, hydraulic and mechanical processes on the grain scale. In our experiments with deforming halite-gypsum-halite sandwiches we observe that the fluid released by dehydrating gypsum accumulates at the gypsum-halite interface before a distributed hydraulic failure of the halite layer drains the fluid. From our observations we conclude that perceivedly impermeable halite layers in evaporites are unlikely to trap overpressured fluid, e.g., in thin-skinned tectonic detachment horizons. Moreover, as the hydraulic failure is diffuse and not localized, our experiments suggest that dehydration reactions alone may not explain intermediate depth seismicity in subduction zones. Our data demonstrate the significant potential that in-situ 4D imaging has for the grain-scale investigation of fundamental tectonic processes.

Geophysical early warning of salt precipitation during geological carbon sequestration

Sequestration of industrial carbon dioxide (CO2) in deep geological saline aquifers is needed to mitigate global greenhouse gas emissions; monitoring the mechanical integrity of reservoir formations is essential for effective and safe operations. Clogging of fluid transport pathways in rocks from CO2-induced salt precipitation reduces injectivity and potentially compromises the reservoir storage integrity through pore fluid pressure build-up. Here, we show that early warning of salt precipitation can be achieved through geophysical remote sensing. From elastic P- and S-wave velocity and electrical resistivity monitoring during controlled laboratory CO2 injection experiments into brine-saturated quartz-sandstone of high porosity (29%) and permeability (1660 mD), and X-ray CT imaging of pore-scale salt precipitation, we were able to observe, for the first time, how CO2-induced salt precipitation leads to detectable geophysical signatures. We inferred salt-induced rock changes from (i) strain changes, (ii) a permanent ~ 1.5% decrease in wave velocities, linking the geophysical signatures to salt volume fraction through geophysical models, and (iii) increases of porosity (by ~ 6%) and permeability (~ 7%). Despite over 10% salt saturation, no clogging effects were observed, which suggests salt precipitation could extend to large sub-surface regions without loss of CO2 injectivity into high porosity and permeability saline sandstone aquifers.

Experimental and numerical investigation of time-delayed pressurization for the manipulation of breakdown pressure and acoustic emissions during hydraulic stimulation

For the mitigation of micro-seismicity during hydraulic stimulation, we introduce the concept of time-delayed pressurization (TDP) which involves arresting the constant-rate process of borehole pressurization for a given delay period, and then resuming it until breakdown occurs. The additional fluid permeation into a porous media by the TDP scheme allows for the manipulation of pore fluid pressure buildup near a borehole, thereby resulting in the mechanical and seismic behaviors at failure in a controlled manner. Laboratory fracturing experiments with acoustic emission monitoring were conducted for several delay periods using mortar specimens to evaluate the breakdown pressure and acoustic emission (AE) in the TDP fracturing process. In addition, the morphological characteristics of induced fractures were visually investigated by 3D X-ray CT analysis on the fractured specimens. Numerical simulations using a phase-field approach were also performed to further examine the evolution of pore pressure during the fracturing process. The results show the considerable volume of permeated fluid and the elevated pore pressure during the delay period are the main factors responsible for the decreased breakdown pressure and AE.

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.

Permeability control on transient slip weakening during gypsum dehydration: Implications for earthquakes in subduction zones

A conflict has emerged from recent laboratory experiments regarding the question of whether or not dehydration reactions can promote unstable slip in subduction zones leading to earthquakes. Although reactions produce mechanical weakening due to pore-fluid pressure increase, this weakening has been associated with both stable and unstable slip. Here, new results monitoring strength, permeability, pore-fluid pressure, reaction progress and microstructural evolution during dehydration reactions are presented to identify the conditions necessary for mechanical instability. Triaxial experiments are conducted using gypsum and a direct shear sample assembly with constant normal stress that allows the measurement of permeability during sliding. Tests are conducted with temperature ramp from 70 to 150 °C and with different effective confining pressures (50, 100 and 150 MPa) and velocities (0.1 and 0.4 μm s−1). Results show that gypsum dehydration to bassanite induces transient stable-slip weakening that is controlled by pore-fluid pressure and permeability evolution. At the onset of dehydration, the low permeability promoted by pore compaction induces pore-fluid pressure build-up and stable slip weakening. The increase of bassanite content during the reaction shows clear evidence of dehydration related with the development of R1 Riedel shears and P foliation planes where bassanite is preferentially localized along these structures. The continued production of bassanite, which is stronger than gypsum, provides a supporting framework for newly formed pores, thus resulting in permeability increase, pore-fluid pressure drop and fault strength increase. After dehydration reaction, deformation is characterized by unstable slip on the fully dehydrated reaction product, controlled by the transition from velocity-strengthening to velocity-weakening behaviour of bassanite at temperature above ∼140 °C and the localization of deformation along narrow Y-shear planes. This study highlights the generic conditions required to trigger instabilities during dehydration reactions. It shows that pore-fluid pressure build-up during dehydration reactions associated with the localization of a velocity-weakening reacting or dehydrated phase along shear planes is necessary for earthquake triggering.

Experimental evidence for seismically initiated gas bubble nucleation and growth in groundwater as a mechanism for coseismic borehole water level rise and remotely triggered seismicity

AbstractChanges in borehole water levels and remotely triggered seismicity occur in response to near and distant earthquakes at locations around the globe, but the mechanisms for these phenomena are not well understood. Experiments were conducted to show that seismically initiated gas bubble growth in groundwater can trigger a sustained increase in pore fluid pressure consistent in magnitude with observed coseismic borehole water level rise, constituting a physically plausible mechanism for remote triggering of secondary earthquakes through the reduction of effective stress in critically loaded geologic faults. A portion of the CO2 degassing from the Earth's crust dissolves in groundwater where seismic Rayleigh and P waves cause dilational strain, which can reduce pore fluid pressure to or below the bubble pressure, triggering CO2 gas bubble growth in the saturated zone, indicated by a spontaneous buildup of pore fluid pressure. Excess pore fluid pressure was measured in response to the application of 0.1–1.0 MPa, 0.01–0.30 Hz confining stress oscillations to a Berea sandstone core flooded with initially subsaturated aqueous CO2, under conditions representative of a confined aquifer. Confining stress oscillations equivalent to the dynamic stress of the 28 June 1992 Mw 7.3 Landers, California, earthquake Rayleigh wave as it traveled through the Long Valley caldera, and Parkfield, California, increased the pore fluid pressure in the Berea core by an average of 36 ± 15 cm and 23 ± 15 cm of equivalent freshwater head, respectively, in agreement with 41.8 cm and 34 cm rises recorded in wells at those locations.

Transient pore pressure response to confining stress excursions in Berea sandstone flooded with an aqueous solution of CO2

AbstractWe measured the pore pressure response due to carbon dioxide (CO2) gas bubble nucleation and growth in a Berea sandstone core flooded with an initially subsaturated aqueous solution of CO2, in response to a rapid drop in confining stress, under conditions representative of a confined aquifer. A portion of the CO2 in the Earth's crust, derived from volcanic, magmatic, and biogenic sources, dissolves in groundwater. Sudden reductions in confining stress in the Earth's crust occur due to dilational strain generated by the propagation of seismic Rayleigh and P waves, or aseismic slip in the near field of earthquakes. A drop in confining stress produces a proportional drop in pore fluid pressure. When the pore fluid contains dissolved CO2, the pore pressure responds to a drop in confining stress like it does in the dissolved gas‐free case, until the pore pressure falls below the bubble pressure. Gas bubble nucleation and diffusive growth in the pore space trigger spontaneous, transient buildup of the pore fluid pressure, and reduction of effective stress. We measured the rate of pore fluid pressure buildup in the 100 s immediately following the confining stress drop, as a function of the saturation with respect to CO2 at the lowest pore pressure realized during the confining stress drop, using five different CO2 partial pressures. The rate scales with the saturation with respect to dissolved CO2, from 10 kPa/min at 1.25 to 166 kPa/min at 1.8. The net pore pressure rise was as large as 0.7 MPa (100 psi) over 5 h.

Observations of static Coulomb stress triggering of the November 2011M5.7 Oklahoma earthquake sequence

AbstractIn November 2011, aM5.0 earthquake occurred less than a day before aM5.7 earthquake near Prague, Oklahoma, which may have promoted failure of the mainshock and thousands of aftershocks along the Wilzetta fault, including aM5.0 aftershock. TheM5.0 foreshock occurred in close proximity to active fluid injection wells; fluid injection can cause a buildup of pore fluid pressure, decrease the fault strength, and may induce earthquakes.Keranen et al. [] links theM5.0 foreshock with fluid injection, but the relationship between the foreshock and successive events has not been investigated. Here we examine the role of coseismic Coulomb stress transfer on earthquakes that follow theM5.0 foreshock, including theM5.7 mainshock. We resolve the static Coulomb stress change onto the focal mechanism nodal plane that is most consistent with the rupture geometry of the threeM ≥ 5.0 earthquakes, as well as specified receiver fault planes that reflect the regional stress orientation. We find that Coulomb stress is increased, e.g., fault failure is promoted, on the nodal planes of ~60% of the events that have focal mechanism solutions, and more specifically, that theM5.0 foreshock promoted failure on the rupture plane of theM5.7 mainshock. We test our results over a range of effective coefficient of friction values. Hence, we argue that theM5.0 foreshock, induced by fluid injection, potentially triggered a cascading failure of earthquakes along the complex Wilzetta fault system.

CO2-Plume Geothermal (CPG) Heat Extraction in Multi-layered Geologic Reservoirs

Abstract CO2-Plume Geothermal (CPG) technology involves injecting CO2 into natural, highly permeable geologic units to extract energy. The subsurface CO2 absorbs heat from the reservoir, buoyantly rises to the surface, and drives a power generation system. The CO2 is then cooled and reinjected underground. Here, we analyze the effects of multi-layered geologic reservoirs on CPG system performance by examining the CO2 mass fraction in the produced fluid, pore-fluid pressure buildup during operation, and heat energy extraction rates. The produced CO2 mass fraction depends on the stratigraphic positions of highly permeable layers which also affect the pore-fluid pressure drop across the reservoir.

Introduction to special section on Phenomenology, Underlying Processes, and Hazard Implications of Aseismic Slip and Nonvolcanic tremor

This paper introduces the special section on the “phenomenology, underlying processes, and hazard implications of aseismic slip and nonvolcanic tremor” by highlighting key results of the studies published in it. Many of the results indicate that seismic and aseismic manifestations of slow slip reflect transient shear displacements on the plate interface, with the outstanding exception of northern Cascadia where tremor sources have been located on and above the plate interface (differing models of the plate interface there also need to be reconciled). Slow slip phenomena appear to result from propagating deformation that may develop with persistent gaps and segment boundaries. Results add to evidence that when tectonic deformation is relaxed via slow slip, most relaxation occurs aseismically but with seismic signals providing higher‐resolution proxies for the aseismic slip. Instead of two distinct slip modes as suggested previously, lines between “fast” and “slow” slip more appropriately may be described as blurry zones. Results reported also show that slow slip sources do not coincide with a specific temperature or metamorphic reaction. Their associations with zones of high conductivity and low shear to compressional wave velocity ratios corroborate source models involving pore fluid pressure buildup and release. These models and spatial anticorrelations between earthquake and tremor activity also corroborate a linkage between slow slip and frictional properties transitional between steady state and stick‐slip. Finally, this special section highlights the benefits of global and multidisciplinary studies, which demonstrate that slow phenomena are not confined to beneath the locked zone but exist in many settings.

Slab morphology in the Cascadia fore arc and its relation to episodic tremor and slip

Episodic tremor and slip (ETS) events in subduction zones occur in the general vicinity of the plate boundary, downdip of the locked zone. In developing an understanding of the ETS phenomenon it is important to relate the spatial occurrence of nonvolcanic tremor to the principal structural elements within the subduction complex. In Cascadia, active and passive source seismic data image a highly reflective, dipping, low‐velocity zone (LVZ) beneath the fore‐arc crust; however, its continuity along the margin is not established with certainty, and its interpretation is debated. In this work we have assembled a large teleseismic body wave data set comprising stations from northern California to northern Vancouver Island. Using stacked receiver functions we demonstrate that the LVZ is well developed along the entire margin from the coast eastward to the fore‐arc basins (Georgia Strait, Puget Sound, and Willamette Valley). Combined with observations and predictions of intraslab seismicity, seismic velocity structure, and tremor hypocenters, our results support the thesis that the LVZ represents the signature of subducted oceanic crust, consistent with thermal‐petrological modeling of subduction zone metamorphism. The location of tremor epicenters along the revised slab contours indicates their occurrence close to but seaward of the wedge corner. Based on evidence for high pore fluid pressure within the oceanic crust and a downdip transition in permeability of the plate interface, we propose a conceptual model for the generation of ETS where the occurrence and recurrence of propagating slow slip and low‐frequency tremor are explained by episodic pore fluid pressure buildup and fluid release into or across the plate boundary.

Dissociation Behavior of CO2 Hydrate in Sediments during Isochoric Heating

As CO2 is sequestered into sediments in the oceanic environment, CO2 hydrate can form as a byproduct. This study explored the dissociation behavior of CO2 hydrate in sediments in relation to pore fluid pressure evolution and sediment particle size. We synthesized CO2 hydrate in three types of particulate sediments: glass beads, fine sand, and crushed silt. We then dissociated them through isochoric heating. We observed the excess pore fluid pressure build-up and self-preservation behavior, in which the pressure-temperature state evolves along the hydrate phase boundary until either it reaches the second quadruple point or all hydrates dissociate. The pore fluid pressure evolution is limited, however, by the CO2 vapor-liquid phase equilibrium boundary due to the liquefaction of CO2. The presence of CO2 liquid in sediments forces the pressure-temperature evolution to follow the CO2 vapor-liquid phase equilibrium boundary, regardless of hydrate formation and dissociation processes. CO2 hydrate in fine-grained sediments experiences capillary pressure-induced melting point depression, but this effect vanishes when the pores exceed approximately 1 microm, such as in coarse-grained sediments. In particular, any fracture generation in sediments which involves the local release of confinement eliminates the melting point depression induced by the capillary effect.

Time-Dependent Poromechanical Responses of Saturated Cylinders

The uniaxial, hydrostatic, and triaxial tests of saturated cylindrical rock samples are very common in a rock mechanics laboratory. The conventional solution for the mechanical responses of the sample under such testing conditions (an axial load and a confining pressure) is trivial within the elastic range. For saturated samples, however, these elastic solutions can only be applied to the drained or undrained cases, and there has been a lack of transient analyses of cylindrical samples under such tests, taking into account the pore fluid pressure buildup and the coupling effects, especially for samples with low permeability. In this paper, poroelasticity is employed to develop the solution for saturated cylindrical samples subjected to an axial load and a confining pressure. Significant poroelastic effects on the tests were observed through the analyses of a uniaxial test and a triaxial test based on the proposed poroelastic solution. Without consideration of the poroelastic effects, erroneous interpretations of the testing results can be expected.

Emplacement of foreland thrust systems

Emplacement‐driving mechanisms for thrust systems have been briefly reviewed and compared with distinctive structural styles in thrust belts. A B‐type mode, which implies a push from the rear and an elastic‐brittle behavior of the rock mass, appears to be suitable as a general model for the emplacement of thrust sheets in thin‐skinned fold and thrust belts where stratal shortening of sedimentary covers and imbrication of upper level crustal rocks occur. In such an environment that the mechanical behavior of the crustal section involved the mode, and timing of deformation, and the geometry of thrust systems are all strongly dependent upon the coupling between induced tectonic strain and transient, close to lithostatic pore fluid pressure buildup over a critical areal extent. The model presented here does not require a weak basal horizon for the detachment of thrust sheets, and pore pressure Pp does not need to be high at once under the entire length of the thrust wedge. Rather, the transient buildup of Pp and hence the spatial and temporal progressive weakening of the rock mass are directly coupled with deformation.

Reservoir induced seismicity in the vicinity of Lake Bhatsa, Maharashtra, India

The township of Khardi, located 7 km northwest of the Bhatsa dam, in the western part of the Deccan Volcanic Province, was subjected to earthquakes of magnitudes 4.0 and 4.8 on August 17, 1983 and September 15, 1983, respectively. The Khardi-Bhatsa area is located to the east of the Panvel flexure axis and on a northwest-southeast trending belt, forming an extension of the Ghod lineament of the Deccan Plateau. The earth tremors at Khardi followed impoundment of water, during June–July 1983, behind the recently constructed Bhatsa dam, and the rise in lake-level from about 92 to 110 m above MSL. It is suggested that percolation of water during this period along fractures led to a build-up of pore-fluid pressure which was transmitted down to a stress zone with a potential to slip. Consequent reduction in effective stress (tectonic stress-pore fluid pressure) induced the earthquakes. Available seismic data for the Khardi-Bhatsa area suggest that b-values can also be used as a ‘short term’ precursor for detecting impending, strong reservoir-induced earth tremors in the area.