Pore Fluid Migration Research Articles

An Epifaunal Community Succession Sequence Driven by the Biogeochemical Footprint With Different Methane Seepage Intensity of the Deep Seafloor

AbstractMethane seepage, generated from natural processes or gas hydrate dissociation, drives the development of cold seep ecosystems and leads to atmosphere methane emission, thereby influencing climate change. Uncovering the intrinsic interactions among methane seepage intensities, biogeochemical processes in the sediment, and benthic community characteristics at the seafloor is essential to clarify and predict the ultimate fate of methane leakage from the deep sea. Here, we conducted a systematic investigation of methane intensity, pore fluid migration characteristics, sediment mineral fraction features, and the evolution of biological communities in areas with different methane intensities. Furthermore, analyses of high‐resolution images, pore fluid geochemical feature, and lithologic characteristics of the sediment in the Haima cold seep were also carried out in this study. The results showed that, in areas with different methane intensities, organic matter sulfate reduction and anaerobic oxidation of methane were heterogeneous. The heterogeneity led to the methane transformation zones at different depths, which changed the mineral composition of the sediment and biological communities in the seabed. In addition, a hypothesis of successional sequence of biomes in cold seep was established. This study revealed that the methane seepage intensity was a key factor in determining the biogeochemical process in the sediment of cold seep. The coupling effects of biogeochemical processes and methane seepage intensities dominated the community succession of cold seep. These findings could provide important implications for understanding the dynamic deep marine methane cycle mechanism and the contribution of deep‐sea methane released to climate change.

Polygons with halite-crusted floors and gypsum-raised rims in western Qaidam Basin and implications for polygonal landforms on Mars

Polygons ranging in size from a few meters to kilometers have been observed in playa fields on Earth and Mars. The historically hyperarid climate of the Qaidam Basin has allowed the development of extensive polygonal landforms with diverse geometric and genetic types. Here we report a terrain of polygons with a pan-like structure, raised rims, and a size of about 60 to 120 m, and spatial variation in mineral composition and geometry from the Dalangtan area of the western Qaidam Basin on the Tibetan Plateau. Spatially, the polygons in the northeastern part of the study area have complete rims, while the polygons in the southwest have incomplete rims. These polygons consist of a halite crust in the subsurface and raised rims formed mainly of gypsum. In some areas, the polygonal rims are broadened and form boundary belts that are up to ~30 m wide and about 1.2 m high. We suggest that the formation of the halite crust in the subsurface redirects upward migration of evaporitic pore fluids that accumulate gypsum deposits to form the wide polygonal boundary belts. We argue that the similarly sized polygons with raised rims on Mars have similar lateral and vertical structures caused successively by the strong evaporation of lacustrine brines and subsurface pore fluids.

Percolation characteristics of pore fluid during hydrate depressurization dissociation from multi-phase multi-field coupling analysis

The recovery of gas from hydrate-bearing reservoirs is essentially controlled by pore fluid flow. Four numerical models with different multi-phase multi-field were established in this work based on the thermo-hydro-chemical model, considering ice phase, fluidized sand phase, depositing sand phase, and ice, fluidized sand and depositing sand phases, respectively. These above models were used to simulate the dissociation of hydrate-bearing cores by depressurization and to investigate the effects of freezing and melting behavior, sand production behavior, and their combined behavior on pore fluid migration. By comparing the simulations with and without the ice phase, it can be found that the permeability decreases sharply due to ice generation resulting in a decrease of 89.16% in pore-water velocity and of 99.95% in pore-gas velocity. This indicates an inhibition of pore fluid migration by freezing behavior. Meanwhile, it can be found that the ice melting behavior can temporarily promote pore-water migration but has no obvious promotion for pore-gas migration. It is difficult for the pore fluid to percolate in the regions prone to ice generation during hydrate dissociation. In addition, the permeability at 300 min with sand detachment is 16% higher than that without sand detachment. But at the same time, the pore water velocity is 28.44% smaller since sand detachment behavior does not directly power pore fluid migration. By comparing the simulations with and without the combined behavior, it can be concluded that the freezing behavior and the sand production behavior can jointly weaken the pore fluid mobility. Significantly, the former has a stronger negative effect on the pore fluid mobility than the latter during the depressurization dissociation of the hydrate-bearing sediment.

Influences of pore fluid on gas production from hydrate-bearing reservoir by depressurization

In addition to the temperature and pressure conditions, the pore fluid composition and migration behavior are also crucial to control hydrate decomposition in the exploitation process. In this work, to investigate the effects of these factors, a series of depressurization experiments were carried out in a visible one-dimensional reactor, using hydrate reservoir samples with water saturations ranging from 20% to 65%. The results showed a linear relationship between gas production rates and gas saturations of the reservoir, suggesting that a larger gas-phase space was conducive to hydrate decomposition and gas outflow. Therefore, the rapid water production in the early stage of hydrate exploitation could release more gas-phase space in the water-rich reservoir, which in turn improved the gas production efficiency. Meanwhile, the spatiotemporal evolution of pore fluids could lead to partial accelerated decomposition or secondary formation of hydrates. In the unsealed reservoir, the peripheral water infiltration kept reservoir at a high water saturation, which hindered the overall production process and caused higher water production. Importantly, depressurization assisted with the N2 sweep could displace the pore water rapidly. According to the results, it is recommended that using the short-term N2 sweep as an auxiliary means in the early stage of depressurization to expand the gas-phase space in order to achieve the highest production efficiency.

Tectonostratigraphic controls on pore fluid pressure distribution across the Taranaki Basin, New Zealand

AbstractSignificant variations in pore pressure across the Taranaki Basin, New Zealand, are attributed to changes in lithofacies and structure, usefully illustrated in terms of ten areas that we term geopressure provinces, each displaying individual pore pressure trends. Cretaceous to Early Miocene formations in different parts of the basin can be either normally pressured (near or at hydrostatic) or significantly overpressured (up to 28 MPa) at the same depth. Variations in Eocene–Oligocene facies types and thicknesses both within and between geopressure provinces provide first‐order control on the magnitude, distribution and maintenance of overpressure across the basin. Examples of hydraulic compartmentalisation due to sealing faults and stratigraphic architecture are identified within the basin. Deep pore pressure transitions are sealed by diagenetic, structural or stratigraphic mechanisms in different places and are associated with an increase in mudrock volume (reduced permeability) or gas generation. Thus, pore pressure distribution in the Taranaki Basin is controlled by a combination of sediment loading, lithofacies variations, fault zone permeability and structural architecture. This work represents an appraisal of the pore pressure distribution across the whole of a multiphase structurally complex basin, and the approach taken provides a framework for better understanding the distribution of pore fluid pressures and pore fluid migration in other sedimentary basins.

Gas hydrate dissociation by depressurization along with ice occurrence and sand migration

The extraction efficiency and safety of hydrate reservoirs are important for the industrialization of hydrate energy production. In this connection, various depressurization schemes with different amplitudes, rates and modes have been tried in field production tests. In this work, production profiles of laboratory-scale hydrate-bearing sediments were simulated through different depressurization schemes at a constant ambient temperature of 275.45 K. The results show that there are the freezing - melting behavior (FMB) and isothermal stage in the system with the depressurization amplitude (ΔP) range of 1.27–2.67 MPa, but not in the system with a ΔP of 0.83 MPa. In addition, the lower final outlet pressure and the higher depressurization rate increase the ice content and the ice duration of the system. Based on this, the linear equation of beginning freezing time of system (each observation point) can be obtained by fitting. According to the maximum ice content of system (0.06–3.56 E−4m2) under deep depressurization mode, the function of ice content of the system with the ΔP and the production time can be obtained. Moreover, the effects of FMB and sand detaching behavior were analyzed on pore fluid migration. FMB can temporarily block 0–66.56% of the migration space of pore fluid (MSPF). It can be revealed that the pore water velocity of observation points in the absence of FMB increases first and then steadily declines over time. But the former stage due to FMB can be divided into a slight decrease/increase and a sharp increase. Meanwhile, it can be found from the sediment components that the behaviors of skeleton sand detaching and hydrate dissociation can expand the MSPF, with contribution percentages of 0–13.49% and 86.51–100%, respectively. Finally, the high depressurization rate can advance the peak of gas production and water production by 14–22 min and 13–17 min, respectively. The high ΔP can improve the production rate peaks of gas and water about 34–59% and 17.4–41%, respectively, and advance the peak of sand production by 19–60 min. The findings of this work clearly illustrate the importance of the depressurization scheme for the extraction of hydrate resources and provide a basis for the best production scheme.

Experimental evaluation of microscopic pore structure and fluid migration characteristics of coal-measure sandstone reservoirs

Research on the microscopic migration characteristics of fluids in coal measure sandstone has always been a hot spot in the evaluation of reservoir properties. In this study, taking the Yan’an Formation sandstone reservoirs in the Block A of the Ordos Basin as an example, the pore structures and fluid migration characteristics of coal-measure sandstones are systematically studied using a large number of thin sections, SEM (Scanning Electron Microscope), NMR (Nuclear Magnetic Resonance), relative permeability and water-flooding test results. The results show that the Jurassic sandstones in the target layer mainly develop lithic quartz sandstone, and the main pore types are intergranular and dissolution pores, followed by a small amount of intercrystalline pores. The surface porosity of the target sandstones mainly ranges from 7.90 to 10.79%, with an average value of 8.78%. The good correlation between porosity and permeability indicates that the target layer is a pore-type reservoir. The T2 relaxation time of the target layer is mainly distributed within 100 ms. Moreover, the reservoir of the Yan’an Formation has a high saturation of movable fluids, which is mainly distributed in 43.17–71.24%, with an average value of 56.90%. Meanwhile, samples with fractures have higher movable fluid saturations. In addition, the average irreducible water saturation of the Yan’an Formation sandstone reservoir is 35.14%, and the final oil displacement efficiency is 51.14% on average. There is a good positive correlation between the oil displacement efficiency and the co-permeability zone. As the co-permeability zone range increased from 15 to 55%, the oil displacement efficiency increased from 30 to 65%. When the cores develop fractures, they have characteristics of high permeability, high oil recovery rate, high oil displacement efficiency in the anhydrous period, low irreducible water saturation and low residual oil saturation.

The Sr isotope signature of Wuchiapingian semi-anthracites from Chongqing, southwestern China: Indication for hydrothermal effects

The strontium (Sr) concentration and 87Sr/86Sr ratios in leachates extracted from Upper Permian (Wuchiapingian) coals from the Tianjia Mine, Chongqing, southwestern China, were investigated using a selective leaching sequential extraction technique including deionized water, dilute HCl, and a mixture of HF and HNO3. The results show that the Sr concentrations in water-leachates (mean value of 12.9 ppm) and HCl-leachates (78.4 ppm) are lower than HF/HNO3-leachates (131 ppm). The water-leachates had more radiogenic (0.707182–0.707642) 87Sr/86Sr ratios than HCl-leachates (0.707111–0.707609), while the 87Sr/86Sr ratios in the HF/HNO3-leachates had a wide range (0.706972–0.707807), and the highest 87Sr/86Sr ratios. These variations in Sr concentration and 87Sr/86Sr ratios are most likely related to the differential solubility of the different Sr-bearing minerals in the coals, in addition to the Sr associated directly with the organic components of the coals. While the Sr/Ca ratios of the HCl-leachates (median = 11.3 mmole/mole) were similar to the ratio of marine carbonate dissolution, the 87Sr/86Sr values in the HCl-leachates were higher than the expected 87Sr/86Sr range of the contemporaneous seawater (0.70700–0.70725) recorded in the Wuchiapingian marine carbonate rocks. This indicates that the carbonate minerals in the coals are not of a syngenetic marine origin, despite the indications that the coal seams were influenced by seawater. The textural properties of the cell and fracture-filling carbonate minerals provide evidence that the carbonate minerals in the Tianjia coals are of late diagenetic or epigenetic origin. The combination of isotopic evolution trends of the coal seam, the petrological characteristics (high vitrinite reflectance and bireflectance) and mineralogical assemblage of NH4-illite-paragonite and kaolinite-paragonite as well as authigenic minerals such as calcite, ferroan dolomite, chamosite, quartz, anatase, and florencite suggests that the Tianjia coals was likely affected by diagenetic processes such as sediment dewatering with increasing burial depth (i.e., upward migration of pore fluids from the mudstones interbedded with the coal seams). These conclusions agree with the regional geological history marked by large-scale uplift and denudation of the crust following regional tectonic evolution (i.e., Dongwu Movement, 258–260.5 Ma).

Long-lived shallow slow-slip events on the Sunda megathrust

During most of the time between large earthquakes at tectonic plate boundaries, surface displacement time series are generally observed to be linear. This linear trend is interpreted as a result of steady stress accumulation at frictionally locked asperities on the fault interface. However, due to the short geodetic record, it is still unknown whether all interseismic periods show similar rates, and whether frictionally locked asperities remain stationary. Here we show that two consecutive interseismic periods at Simeulue Island, Indonesia experienced significantly different displacement rates, which cannot be explained by a sudden reorganization of locked and unlocked regions. Rather, these observations necessitate the occurrence of a 32-year slow-slip event on a shallow, frictionally stable area of the megathrust. We develop a self-consistent numerical model of such events driven by pore-fluid migration during the earthquake cycle. The resulting slow-slip events appear as abrupt velocity changes in geodetic time series. Due to their long-lived nature, we may be missing or mis-modelling these transient phenomena in a number of settings globally; we highlight one such ongoing example at Enggano Island, Indonesia. We provide a method for detecting these slow-slip events that will enable a substantial revision to the earthquake and tsunami hazard and risk for populations living close to these faults. A 32-year-long slow-slip event occurred on a shallow part of the Sunda megathrust, perhaps because of stress accumulation after fluid expulsion, according to an analysis of the deformation history of the area and physics-based simulations.

The spatial distribution and geochemical variation of fault and fracture hosted calcite and gypsum cements in the eastern Bristol Channel Basin

Abstract The spatial distribution and geochemical variation of fault- and fracture-hosted cements can be used to reconstruct the evolution and migration of pore fluids in sedimentary basins. Geochemical data, from stable oxygen and carbon isotope and strontium isotope analyses, of calcite and gypsum cements provide valuable insights into the source of pore fluids and their evolution in the eastern Bristol Channel Basin. Overall, at both an outcrop-scale and a basin-scale there were no observable relationships between the dip angle and direction of fractures with the oxygen and carbon isotopic composition of calcite cements, indicating that the movement and distribution of fluids in fractures is not controlled by the orientation of fracture opening. Radiogenic strontium isotopic compositions of calcite and gypsum cements indicate at least two different mineralising fluid compositions that are strongly controlled by the lithologies (fossiliferous limestones and shales, and marls with gypsum nodules, respectively) in which they occur. These fluids originated as connate pore fluids that were altered during the diagenesis of their host lithologies. Mineralising fluids within the Lower Jurassic Lias Group are characterised by enriched oxygen and strontium isotopic compositions and relatively depleted carbon isotopic compositions, associated with the alteration of labile minerals and carbonaceous fossils. Mineralising fluids within the Triassic Mercia Mudstone Group have enriched strontium isotopic compositions as a result of the interaction of fluids with evaporitic intervals. The mineralising fluids were redistributed through migration along faults and their associated damage zones. This study highlights the importance of integrating a range of geochemical analyses when attempting to understand the source of fluids and their evolution through time. The findings from this study can also be applied to nearby petroleum producing basins, e.g. the Wessex Basin, where equivalent rocks are economically significant source rocks and caprocks and have experienced similar structural histories.

Geophysical and geochemical analysis of shallow gas and an associated pockmark field in Bantry Bay, Co. Cork, Ireland

An integrated geophysical, geological, and geochemical investigation of seabed fluid venting was carried out in upper Bantry Bay, a large marine inlet on the southwest coast of Ireland. The results provide evidence of the seafloor venting of gas rich fluids, resulting in the formation of a pockmark field identified here for the first time. The pockmarks occur in an area where sub-bottom profiles provide evidence of chimney-like features interpreted to record upward gas migration through Quaternary sediments to the seafloor. Three vibrocores up to 6 m long were acquired in water depths of 24–34 m, two from the pockmark field and one from outside. Methane of predominantly biogenic origin was quantified in all three cores by headspace analysis of sediment sub-samples. Well-defined sulfate methane transition zones (SMTZs) were observed in two of the cores, the shallowest (1.25 m below sea floor (mbsf)) inside the pockmark field and the other (3.75 mbsf) outside. It is likely that an SMTZ occurs at the location of the third core, also within the pockmark field, although deeper than the samples obtained during this study. Gas migration towards the seafloor is suggested to involve both diffuse pore fluid migration across wide areas and focused flow through the pockmarks, together driven by methanogenesis of pre-glacial lacustrine sediments preserved in a bedrock basin, and possible gas release from the Owenberg River Fault. Analysis of phospholipid fatty acids (PLFAs) and archaeal isoprenoid hydrocarbons was used to investigate the microbial ecology of these sediments. Anaerobic oxidation of methane (AOM) may play a role in controlling release of CH4 to the water column and atmosphere in this shallow gas setting, potentially mediated by syntrophic sulfate reducing bacteria (SRB) and anaerobic methanotrophic archaea (ANME).

Sources of seeping fluids, and distribution and migration of pore fluids, inferred by chemical compositions of pore water from sediment at cold seep sites off Kumano

Sources of seeping fluids, and distribution and migration of pore fluids, inferred by chemical compositions of pore water from sediment at cold seep sites off Kumano

Understanding the microscopic moisture migration in pore space using DEM simulation

The deformation of soil skeleton and migration of pore fluid are the major factors relevant to the triggering of and damages by liquefaction. The influence of pore fluid migration during earthquake has been demonstrated from recent model experiments and field case studies. Most of the current liquefaction assessment models are based on testing of isotropic liquefiable materials. However the recent New Zealand earthquake shows much severer damages than those predicted by existing models. A fundamental cause has been contributed to the embedded layers of low permeability silts. The existence of these silt layers inhibits water migration under seismic loads, which accelerated liquefaction and caused a much larger settlement than that predicted by existing theories. This study intends to understand the process of moisture migration in the pore space of sand using discrete element method (DEM) simulation. Simulations were conducted on consolidated undrained triaxial testing of sand where a cylinder sample of sand was built and subjected to a constant confining pressure and axial loading. The porosity distribution was monitored during the axial loading process. The spatial distribution of porosity change was determined, which had a direct relationship with the distribution of excess pore water pressure. The non-uniform distribution of excess pore water pressure causes moisture migration. From this, the migration of pore water during the loading process can be estimated. The results of DEM simulation show a few important observations: (1) External forces are mainly carried and transmitted by the particle chains of the soil sample; (2) Porosity distribution during loading is not uniform due to non-homogeneous soil fabric (i.e. the initial particle arrangement and existence of particle chains); (3) Excess pore water pressure develops differently at different loading stages. At the early stage of loading, zones with a high initial porosity feature higher porosity changes under the influence of external loading, which leads to a larger pore pressure variation (increase or decrease) in such zones. As the axial strain increases, particle rearrangement occurs and final porosity distribution has minor relationship with the initial condition, and the pore pressure distribution becomes irregular. The differences in the pore pressure development imply that water will migrate in the pore space in order to balance the pore water pressure distribution. The results of this simulation offer an insight on the microscale water migration in the soil pore space, which is important for holistic description of the triggering of soil liquefaction in light of its microstructure.

Dewatering structure and soft-sediment deformation controlled by slope instability: examples from the late Miocene to Pliocene Miura–Boso accretionary prism and trench-slope basin, central Japan

Abstract Systematic arrays of dewatering structure and soft-sediment deformation (e.g. convolute laminations and flame structures possibly associated with dewatering) were examined in the late Miocene to Pliocene Miura–Boso accretionary prism and Plio-Pleistocene trench-slope basin, central Japan, with respect to paleoslope inclination. Numerous preferred orientations of these structures strike perpendicular to the slumping direction. The observed features, including the measured orientations and field observations that dewatering-induced foliations terminated by the slip plane of a slope failure deposit, indicate that the structures formed under the influence of slope instability. The foliations from both the upper part of the accretionary prism and the slope basin correspond to plausible “pseudo-hydraulic fracture” planes, parallel to σ 1 and perpendicular to σ 3 , and are associated with slope instability. Pore fluids or liquidized silt escaped along the foliations until pore fluid pressures along the uppermost and lowermost levels of the foliated zone were balanced. Consequently, pore fluid pressure increased just below the relatively impermeable shear planes. This pore fluid migration and increase in pore fluid pressure would have been critical for slope instability and could have triggered failures, especially within sediments close to the critical state. The findings indicate that pore-fluid behavior associated with shear stress controls the failure patterns and their locations.

Brittle failure and fracture reactivation in sandstone by fluid injection

We performed laboratory experiments on sandstone specimens to study brittle failure and the reactivation of an experimentally produced failure plane induced by pore-pressure perturbations using constant force control in high compressive stress states. Here, we focus on the shear failure of a dry sample and the later on induced fracture plane reactivation due to water injection. Acoustic Emission (AE) monitoring has been used during both experiments. We also used ultrasonic wave velocities to monitor pore fluid migration through the initially dry specimen. To characterise AE source mechanisms, we analysed first motion polarities and performed full moment tensor inversion at all stages of the experiments. For the case of water injection on the dry specimen that previously failed in shear, AE activity during formation of new fractures is dominated by tensile and shear sources as opposed to the fracture plane reactivation, when compressive and shear sources are most frequent. Furthermore, during the reactivation of the latter, compressive sources involve higher compressive components compared to the shear failure case. The polarity method and the moment tensor inversion reveal similar source mechanisms but the latter provides more information on the source components.

Inverse modelling and seismic data constraints on overpressure generation by disequilibrium compaction and aquathermal pressuring: application to the Eastern Black Sea Basin

SUMMARY Pore pressure above the hydrostatic (overpressure) is common in deep basins. It plays an important role in pore fluid migration, represent a significant drilling hazard, and is one of the factors controlling slope stability and deformation in seismically active areas. Here, we present an inverse model to calculate overpressure due to disequilibrium compaction and aquathermal pressuring. We minimize a function that contains the misfits between estimates from our forward model and observed values using a non-linear least squares approach. The inverse model allows the introduction of observed seismic and geological constraints such as P-wave velocity (Vp) and density data, and depth of the layer boundaries, for a better pore-pressure prediction. The model output also provides estimates of: (1) surface porosity, (2) compaction factor, (3) intrinsic permeability at surface conditions, (4) a parameter controlling the evolution of the intrinsic permeability with porosity, (5) the ratio between horizontal and vertical permeability and (6) uncompacted thickness (so sedimentation rate assuming known time intervals), for each sedimentary layer. We apply our inverse approach to the centre of the Eastern Black Sea Basin (EBSB) where the Vp structure has been inferred from wide-angle seismic data. First, we present results from a 1-D inverse model and an uncertainty analysis based on the Monte Carlo error propagation technique. To represent the observed rapid change from low Vp to normal Vp below the Maikop formation, we impose a zero overpressure bottom boundary, and subdivide the layer below the Maikop formation into two sublayers: an upper layer where the rapid change is located and a lower layer where the Vp is normal. Secondly, we present the results from a 2-D inverse model for the same layers using two alternative bottom boundary conditions, zero overpressure and zero flow. We are able to simulatetheobserved Vp,suggestingthatthelowvelocityzone(LVZ)at ∼3500–6500mdepth below the seabed (mbsf) can be explained by overpressure generated due to disequilibrium compaction (>90 per cent) and to aquathermal pressuring (<10 per cent). Our results suggest thattheuppersublayer,belowtheMaikopformation,behavesasasealduetoitslowpermeability ∼0.3–2 × 10 −14 ms –1 . This seal layer does not allow the fluids to escape downwards, and hence overpressure develops in the Maikop formation and not in the layers below. This overpressure was mainly generated by the relatively high sedimentation rate of ∼0.29 m ka –1 of the Maikop formation at 33.9–20.5Ma and an even higher sedimentation rate of ∼0.93 m ka –1 at 13–11 Ma. We estimate a maximum ratio of overpressure to vertical effective stress in hydrostatic conditions (λ ∗ )o f∼0.62 at ∼5200 mbsf associated with an overpressure

Applying Wedge Theory to Dynamic Rupture Modeling of Fault Junctions

Wedges, such as accretionary prisms and thin‐skinned fold‐and‐thrust belts, occur frequently in nature and can be the site of devastating earthquakes. Critical wedge theory can be applied to these settings, but this steady‐state description of wedge deformation is at odds with the periodic occurrence of earthquakes. We discuss how critical wedge theory applies to the seismic cycle, and we use elastic wedge theory to constrain realistic stress states. Our goal is to determine the rupture behavior of an earthquake in a wedge. If rupture initiates on the basal sliding surface, will it stay confined to the basal surface, or will it propagate onto a fault branch interior to the wedge? This information can significantly alter the seismic hazard in areas where fault intersections occur. We answer this question using numerical models of dynamic rupture propagation through branched geometries for which the stress state is a pivotal input parameter. We apply wedge theory to constrain the stress state, but inherent to this theory is the assumption of a weak basal fault. We investigate the role of this assumption and determine that rupture is unlikely to propagate from a weak basal fault onto a strong branch fault without the aid of a physical process such as pore fluid migration along the branch. This framework is applied to the rupture of the 2008 Wenchuan earthquake. We find that we are able to reproduce the behavior at some fault intersections, but our 2D model is not able to reproduce all the behaviors, possibly due to the oblique nature of this event.

Control of pore fluid pressure diffusion on fault failure mode: Insights from the 2009 L'Aquila seismic sequence

The MW 6.13 L'Aquila earthquake ruptured the Paganica fault on 2009/04/06 at 01:32 UTC, and started a strong sequence of aftershocks. For the first four days, the region north of the hypocenter of the main quake was shaken by three large events (MW ∼ 5.0) that ruptured different patches of the Monti della Laga fault (hereafter “Campotosto”). In our hypothesis, these aftershocks were induced by a dramatic reduction in the fault's shear strength due to a pulse of pore fluid pressure released after the L'Aquila main earthquake. Here we model the time evolution of the pore fluid pressure northward from the main hypocenter. We show that, during the sequence, the Campotosto fault failed in multiple episodes, when the specific patches/asperities underwent fluid pressure‐related strength reductions of 7–10 MPa. Although such drops in strength are very large in amplitude, the contribution of other weakening mechanisms (perturbations of the Coulomb shear stress, and/or dynamic stresses induced by passing seismic waves) cannot be ruled out by our observations. However, the Coulomb shear stress variations either had negative amplitudes down to −0.2 MPa (i.e., tended to inhibit further seismic activity), or had very small positive amplitudes (&lt;0.05 MPa). Paleoseismological evidence supports the hypothesis that larger events (MW 6.5–7) have occurred on the Paganica fault [EMERGEO Working Group, 2009], whereas Lucente et al. [2010] concluded that an important migration of pore fluids characterized the preparatory phase of the L'Aquila main shock. Consequently, the MW 6.13 L'Aquila earthquake may be analogous, at a larger scale, to one of the three Campotosto largest aftershocks. The complex behavior observed for the L'Aquila‐Campotosto fault system seems to be common to other seismogenic structures in the Central Apennines (e.g., the Umbria‐Marche fault system), and need to be taken into consideration for the assessment of seismic hazard.

Dynamics of fault-fluid-hydrate system around a shale-cored anticline in deepwater Nigeria

[1] Gas hydrates were recovered by coring at the eastern border of a shale-cored anticline in the eastern Niger Delta. To characterize the link between faults and fluid release and to identify the role of fluid flow in the gas hydrate dynamics, three piezometers were deployed for periods ranging from 387 to 435 days. Two of them were deployed along a major fault linked to a shallow hydrocarbon reservoir while the third monitored the fluid pressure in a pockmark aligned above the same major fault. In addition, 10 ocean-bottom seismometers (OBS) were deployed for around 60 days. The piezometers simultaneously registered a prolonged fluid flow event lasting 90 days. During this time, OBS measurements record several episodic fluid release events. By combining and analyzing existing and newly acquired data, we show that the fluid-fault system operates according to the following three stages: (1) upward pore fluid migration through existing conduits and free gas circulation within several shallow sandy layers intersecting the major fault, (2) gas accumulation and pore pressure increases within sandy-silty layers, and (3) hydrofracturing and fluid pressure dissipation through sporadic degassing events, causing pore fluid circulation through shallow sandy layers and drawing overlying seawater into the sediment. This paper clearly demonstrates how an integrated approach based on seafloor observations, in situ measurements, and monitoring is essential for understanding fault-fluid-hydrate systems.

Pore-fluid migration and the timing of the 2005 M8.7 Nias earthquake

Two great earthquakes have occurred recently along the Sunda Trench, the 2004 M9.2 Sumatra-Andaman earthquake and the 2005 M8.7 Nias earthquake. These earthquakes ruptured over 1600 km of adjacent crust within 3 mo of each other. We quantitatively present poroelastic deformation analyses suggesting that postseismic fluid flow and recovery induced by the Sumatra-Andaman earthquake advanced the timing of the Nias earthquake. Simple back-slip simulations indicate that the megapascal (MPa)–scale pore-pressure recovery is equivalent to 7 yr of interseismic Coulomb stress accumulation near the Nias earthquake hypocenter, implying that pore-pressure recovery of the Sumatra-Andaman earthquake advanced the timing of the Nias earthquake by ∼7 yr. That is, in the absence of postseismic pore-pressure recovery, we predict that the Nias earthquake would have occurred in 2011 instead of 2005.