Fault Instability Research Articles

Implications of sepiolite dehydration for earthquake nucleation in the Galera Fault Zone: A thermodynamic approach

A new thermodynamic model for the Mg-phyllosilicate sepiolite was developed and used to calculate its P-T stability conditions and water content for different bulk rock compositions. The standard-state thermodynamic properties, entropy (S°) and enthalpy (H°), were initially estimated by oxide summation taking into account the different entropic and enthalpic contributions of the three types of water in sepiolite: zeolitic water, bound water, and structural OH groups. The starting model was then refined with experimental data. The dehydration process follows a step function that allowed us to define “end-members” with decreasing hydration states in a theoretical solid solution.The stability field of sepiolite is ultimately limited by the reaction sepiolite = talc + quartz + H2O, which is located at about 325 °C at 1–500 MPa. The large thermodynamic stability field of this clay mineral suggests that it could control the mechanical behaviour of crustal faults to 325 °C. The refined model was then applied to the natural case of the Galera Fault Zone (SE Spain) using thermogravimetric analysis and X-ray fluorescence data of the bulk rock composition of the fault core gouge, which is mainly composed of sepiolite. The dehydration of sepiolite at T < 300 °C is modest compared to that occurring during its breakdown, where 62% of water molecules leave the structure to form the association talc + quartz + H2O; a reaction that also involves a reduction of around 31% of the original solid volume. These significant hydration and volume changes of sepiolite can have important implications on the strength and mechanical stability of the Galera Fault and other sepiolite-bearing gouges. We compare triaxial friction experiments on the natural sepiolite-rich gouge with a synthetic gouge to estimate strength changes on the gouge with depth. The synthetic gouge represents the breakdown association above 325 °C, and was constructed following the mineral phases predicted by the thermodynamic model (93 wt% talc and 7 wt% quartz). The contrasting frictional strength and mechanical stability of sepiolite in comparison to talc could be one of the causes of fault instability and earthquake nucleation seen to be concentrated in the upper 13 km of the Galera Fault.

Unloading-induced instability of a simulated granular fault and implications for excavation-induced seismicity

Deep excavation in rock masses has the potential to break the frictional equilibrium of nearby faults, resulting in induced seismicity. We conducted the experimental and numerical studies on a simulated granular fault to uncover the mechanism of excavation-induced seismicity. A series of laboratory experiments were used to investigate the effects of initial shear stress, initial normal stress and its unloading rate on the frictional instability of the fault, and a numerical simulation was carried out to interpret stress variation and particle evolution during the unloading process. Our results show that both normal and shear stresses sharply drop when the fault is approaching a critical stress state. The stress reduction is due to interparticle force decrease and particle contact breakage. The evolution of fault state depends on the initial stress condition and excavation process. A greater initial normal stress and a lower initial shear stress provide a favorable environment to accumulate higher strain energy in adjacent rock, leading to larger slip displacement. A larger normal stress unloading rate can also cause higher strain energy and larger slip displacement. Understanding unloading-induced instability of a simulated fault allows us to interpret the seismic events occurred during the excavation of the Gotthard Base Tunnel in Switzerland. The reduction of normal and shear stresses associated with the excavation work decreases the differential stress applied to a natural fault zone, and subsequently results in the occurrence of induced earthquakes.

Investigation of Intrinsic and External Factors Contributing to the Occurrence of Coal Bumps in the Mining Area of Western Beijing, China

An investigation has been made to relate the occurrence of coal bumps to specific geological and mining conditions to the mining area of western Beijing. This investigation demonstrates that the high frequency of coal bumps in this area is due to four localized conditions, namely intrinsic coal properties, the presence of overturned strata and thrust faults, high in situ stress and the extraction of coal from island mining faces. Laboratory tests of coal samples indicated that the coals have a short duration of dynamic fracture, high bursting energy and high elastic strain energy, indicating that the coal is intrinsically prone to the occurrence of coal bumps. This investigation has also revealed that there are overturned strata and well-developed large- and medium-scale thrust faults in this area, and the presence of these structures results in plastic flow, severe discontinuities, rapid changes in overburden thickness and dipping of the coal seams. Well-developed secondary fold structures are also present in the axes and limbs of the primary folds. The instability of thrust faults, in combination with large-scale intrusion of igneous rocks, is closely associated with sudden roof breaking and induces sharp variations in electromagnetic radiation (EMR) and micro-seismic signals, which could be used to help predict coal bumps. In situ stress tests in the mining area demonstrate that the maximum and minimum principal stresses are nearly horizontal and that the intermediate principal stress is approximately vertical. The in situ stress level in the area is higher than the average in the Beijing area, North China and mainland China. In addition to the presence of overturned strata and thrust faults and high in situ stress levels, another external factor contributing to the frequency of coal bumps is coal extraction from island mining faces in this area. Island mining faces experience intermittent mining-induced abutment stress when a fault exists at one side of the island mining face due to reactivation of the fault, and this stress redistribution increases the likelihood of coal bumps during coal extraction from island mining faces.

Impact of fluid injection on fracture reactivation at The Geysers geothermal field

AbstractWe analyze the spatiotemporal distribution of fault geometries from seismicity induced by fluid injection at The Geysers geothermal field. The consistency of these faults with the local stress field is investigated using (1) the fault instability coefficient I comparing the orientation of a fault with the optimal orientation for failure in the assumed stress field and (2) the misfit angle β between slip vectors observed from focal mechanisms and predicted from stress tensor. A statistical approach is applied to calculate the most likely fault instabilities considering the uncertainties from focal mechanisms and stress inversion. We find that faults activated by fluid injection may display a broad range in orientations. About 72% of the analyzed seismicity occurs on faults with favorable orientation for failure with respect to the stress field. However, a number of events are observed either to occur on severely misoriented faults or to slip in a different orientation than predicted from stress field. These events mostly occur during periods of high injection rates and are located in proximity to the injection wells. From the stress inversion, the friction coefficient providing the largest overall instability is μ = 0.5. About 91% of the events are activated with an estimated excess pore pressure &lt;10 MPa, in agreement with previous models considering the combined effect of thermal and poroelastic stress changes from fluid injection. Furthermore, high seismic activity and largest magnitudes occur on favorably oriented faults with large instability coefficients and low slip misfit angles.

Reexamining the Seismological Implications of the Present‐day Stress State of the Yingxiu‐Beichuan Fault after the Wenchuan Earthquake

AbstractThe present‐day stress state of the Yingxiu‐Beichuan fault after the Wenchuan earthquake was re‐estimated using measured in‐situ stress data obtained after the Wenchuan earthquake. The results reveal that the gradient coefficients of principal stresses versus depth decrease from south to north along the Yingxiu‐Beichuan fault, revealing that the stress level decreases from south to north. The consistency between the present‐day stress levels and surface ruptures generated during the earthquake indicates that the accumulated tectonic stress beneath the Yingxiu‐Beichuan fault before the Wenchuan earthquake was relieved in form of surface ruptures. This resulted in the stress remaining high in the southern section of the Yingxiu‐Beichuan fault but relatively low in the northern section. Abnormal high pore pressure conditions and an extremely low frictional coefficient play important role in the interpretation of the stress field adjustment and seismic events observed after the Wenchuan earthquake along this fault, according to the estimation results using the Coulomb frictional‐failure theory incorporating frictional coefficients ranging from 0.4 to 1.0. To accurately estimate the seismological hazard of the Yingxiu‐Beichuan fault by analyzing fault instability using the Coulomb frictional‐failure theory, much attention should be focused on the pore pressure conditions and the evolution state of the frictional coefficient under the present‐day stress state.

Experimental research on shear-slip characteristics of simulated fault with zigzag-type gouge

We sheared layers of polymethyl methacrylate (PMMA) by traditional double-direct shear tests, and synchronously measured the acoustic emissions (AE) generated by shear-slip and friction. Laboratory observations of simulated fault by PMMA materials with zigzag-type gouge showing repetitive, shear-slip events that are reminiscent of earthquakes were verified. In addition, the influencing rules of zigzag size and normal force on shear-slip process and intensity, as well as AE were revealed. Especially, prior to final failure of zigzag, the duration among shear-slip events gradually decreases and shows significant steadiness, which may prove the probability of the obvious foreshock effect to a larger earthquake. Maybe, this work can interpret the shear-slip mechanism of earthquakes and even provide warnings of failure and instability of fault.

Nucleation of frictional instability caused by fluid pressurization in subducted blueschist

AbstractPore pressure is an important factor in controlling the slip instability of faults and thus the generation of earthquakes. Particularly slow earthquakes are widespread in subduction zones and usually linked to the occurrence of high pore pressure. Yet the influence of fluid pressure and effective stress on the mechanics of earthquakes is poorly understood. Therefore, we performed shear experiments on blueschist fault rocks, which likely exist at depth in cold and old subduction zones, to investigate the influence of effective stress on frictional behavior. Our results show potentially unstable behavior at temperatures characterizing the seismogenic zone, as well as a transition from stable to unstable behavior with decreasing effective normal stress, which is mechanically equivalent to increasing fluid pressure. This transition is a prerequisite for generating slow earthquakes. Our results imply that high pore pressures are a key factor for nucleating slip leading to both megathrust and slow earthquakes.

Study on sensor fault instability prediction for the Internet of agricultural things based on largest Lyapunov exponent

Study on sensor fault instability prediction for the Internet of agricultural things based on largest Lyapunov exponent

Coupled hydro-mechanical processes and fault reactivation induced by Co2 Injection in a three-layer storage formation

The interaction between mechanical deformation and fluid flow in fault zones gives rise to a host of coupled hydro-mechanical processes fundamental to fault instability, induced seismicity, and associated fluid migration. Fault stability is studied in the context of the Heletz site which was chosen as a test site for CO2 injection experiment in the framework of the EU- MUSTANG project. The potential reservoir for CO2 storage at the Heletz site consists of three sandstone layers that are approximately one, two and nine meters in thickness, separated by impermeable shale layers of various thicknesses, and overlaid by a five-meter limestone and a thick impermeable shale, which serves as caprock. The storage formation is intersected by two pre-existing sub-vertical normal faults (F1 and F2) on two opposite sides of the injection point. A hydro-mechanical model was developed to study the interaction between mechanical deformation and fluid flow in the two faults during CO2 injection and storage. We evaluate the consequences caused by potential fault reactivation, namely, the fault slip and the CO2 leakage through the caprock. The difference in the results obtained by considering the three-layer storage formation as an equivalent single-layer storage formation is analysed. It was found that for the two cases the pore pressure evolution is similar, but the differences in the evolution of CO2 saturation are significant, which is attributed to the differences in CO2 spread in a single and three-layer storage. No fault reactivation was observed in either case. A sensitivity analysis was made to study the influence of the fault dip angle, the ratio between the horizontal and vertical stresses, the offset of the layers across fault F2, the initial permeability of the fault and the permeability of the confinement formations. Results show that reactivation of faults F1 and F2 is most sensitive to the stress ratio, the initial permeability of the faults and the permeability of the confinement formations. The offset of the layers across the fault F2 was also found to be an important parameter, mainly because an offset leads to an increase in CO2 leakage. Changes in permeability were found to be small because plastic shear strains induced by the reactivation of the faults and associated increase in volumetric strains and permeability, occur mainly in a fault section of only 10 m length, which is the approximate total thickness of the storage layers.

A laboratory acoustic emission experiment and numerical simulation of rock fracture driven by a high-pressure fluid source

In order to improve our understanding of rock fracture and fault instability driven by high-pressure fluid sources, the authors carried out rock fracture tests using granite under a confining pressure of 80 MPa with fluid injection in the laboratory. Furthermore, we tested a number of numerical models using the FLAC3D modeling software to find the best model to represent the experimental results. The high-speed multichannel acoustic emission (AE) waveform recording system used in this study made it possible to examine the total fracture process through detailed monitoring of AE hypocenters and seismic velocity. The experimental results show that injecting high-pressure oil into the rock sample can induce AE activity at very low stress levels and can dramatically reduce the strength of the rock. The results of the numerical simulations show that major experimental results, including the strength, the temporal and spatial patterns of the AE events, and the role of the fluid can be represented fairly well by a model involving (1) randomly distributed defect elements to model pre-existing cracks, (2) random modification of rock properties to represent inhomogeneity introduced by different mineral grains, and (3) macroscopic inhomogeneity. Our study, which incorporates laboratory experiments and numerical simulations, indicates that such an approach is helpful in finding a better model not only for simulating experimental results but also for upscaling purposes.

Seismic responses to fluid pressure perturbations in a slipping fault

AbstractSeismicity induced by fluid injection in a natural fault is investigated in situ in the near field of the source. We present synchronous seismic and hydromechanical measurements directly recorded in the decametric injection zone. The three main types of seismic events were recorded during injection and shut‐in: high‐amplitude and short duration seismic events (SE) (i.e., microearthquakes), low to constant amplitude and 5 to 17 s long tremor‐like signals (TLS), and long period events (LP) with a narrow‐frequency band content. Seismicity first initiates with a sequence of SE and TLS, when pressure is high (~3.5 MPa), slip is activated on the fault, which experiences a twentyfold increase of permeability. Then LP events appear to be associated to fluid leakage in the fault caused by dilation during slip. During shut‐in, residual pressures as low as 0.6 MPa still trigger SE events. We show that the initial TLS sequence triggers when a progressive transition occurs from rupture controlled by effective stress variations close to the injection source to a large friction weakening‐dominated slip on the fault. We conclude that the combination of these different seismic signal types may be a proxy to monitor fault instability associated to fluid pressure perturbations.

Iterative joint inversion for stress and fault orientations from focal mechanisms

S U M M A R Y Stress inversions from focal mechanisms require knowledge of which nodal plane is the fault. If such information is missing, and faults and auxiliary nodal planes are interchanged, the stress inversions can produce inaccurate results. It is shown that the linear inversion method developed by Michael is reasonably accurate when retrieving the principal stress directions even when the selection of fault planes in focal mechanisms is incorrect. However, the shape ratio is more sensitive to the proper choice of the fault and substituting the faults by auxiliary nodal planes introduces significant errors. This difficulty is removed by modifying Michael’s method and inverting jointly for stress and for fault orientations. The fault orientations are determined by applying the fault instability constraint and the stress is calculated in iterations. As a by-product, overall friction on faults is determined. Numerical tests show that the new iterative stress inversion is fast and accurate and performs much better than the standard linear inversion. The method is exemplified on real data from central Crete and from the WestBohemia swarm area of the Czech Republic. The joint iterative inversion identified correctly 36 of 38 faults in the central Crete data. In the West Bohemia data, the faults identified by the inversion were close to the principal fault planes delineated by foci clustering. The overall friction on faults was estimated to be 0.75 and 0.85 for the central Crete and West Bohemia data, respectively.

Maskevarri Ráhppát in Finnmark, northern Norway – is it an earthquake-induced landform complex?

Abstract. The Sami word ráhppát means rough bouldery/stony terrain with sharp-relief topography in Finnmark, northern Norway. Ráhppát is a common name in the region of the Younger Dryas landforms, yet the origin of ráhppát has remained obscure. The timing of the Younger Dryas is concomitant with the maximum neotectonic fault instability in Fennoscandia. Hence, earthquake activity may have been one of the contributing factors for the Younger Dryas morphologies. Ráhppát on the Maskevarri fell, classified as a part of Tromsø-Lyngen sub-stage of the Younger Dryas, was studied by means of geomorphology and measurements of electrical-sedimentary anisotropy. Ráhppát was found to be built up of an anastomosing network of stony esker-like ridges and mounds bordered with arch-shaped and sinusoidal ridges. These bordering ridges exhibit sedimentary (azimuthal soil electrical conductivity) anisotropy parallel-to-ridge trends and were interconnected to meltwater gullies suggesting generation through short-lived conduit infills. We did not find electrical-sedimentary evidence to support the concept of englacial thrusting and/or compression, often described for Younger Dryas moraines. Maskevarri Ráhppát is typified by ~ 500 ponds and small lakes on three different elevations descending in an up-ice direction. These may have generated through late glacial earthquake(s) also contributing to subglacial deformation of Maskevarri Ráhppát.

Surficial geology indicates early Holocene faulting and seismicity, central Sweden

In Sweden, knowledge of the location and timing of glacially induced faulting and seismicity is critical to effective engineering of a long-term nuclear disposal facility. To improve understanding and modeling of the complex ice-induced and tectonic stresses associated with glacially induced faulting, field studies detailing the location and timing of movement of such structures are required. Although the fault has not been confirmed in the bedrock, multi-proxy surficial geologic evidence indicates that the recently discovered scarp in Bollnas is such a structure. Machine-excavated trenches across the scarp reveal landsliding down the scarp and, in one location, faulted and vertically offset fine-grained glacial sediments. The presence of water-escape structures in trenches excavated on a topographic high strongly suggests a co-seismic origin derived from earthquake magnitudes >5.5. Numerous landslides in till exist in the region as well. Four slopes with landslides were examined in detail, and the factors of safety for these slopes indicate stable conditions and suggest a seismic trigger. Basal radiocarbon dates from peat bogs located stratigraphically above the landslides provide minimum limiting ages for the co-seismic landslides. The oldest date indicates sliding prior to 10,180 calendar years before the present. The proposed Bollnas Fault is 400 km south of the so called Lapland Fault Province. To date, it is the southernmost confirmed glacially induced fault in Sweden. The results of this study are consistent with existing modeling results that indicate fault instability in this region of central Sweden following deglaciation.

CO2 storage and potential fault instability in the St. Lawrence Lowlands sedimentary basin (Quebec, Canada): Insights from coupled reservoir-geomechanical modeling

Coupled reservoir-geomechanical (TOUGH-FLAC) modeling is applied for the first time to the St. Lawrence Lowlands region to evaluate the potential for shear failure along pre-existing high-angle normal faults, as well as the potential for tensile failure in the caprock units (Utica Shale and Lorraine Group). This activity is part of a general assessment of the potential for safe CO2 injection into a sandstone reservoir (the Covey Hill Formation) within an Early Paleozoic sedimentary basin. Field and subsurface data are used to estimate the sealing properties of two reservoir-bounding faults (Yamaska and Champlain faults). The spatial variations in fluid pressure, effective minimum horizontal stress, and shear strain are calculated for different injection rates, using a simplified 2D geological model of the Becancour area, located ∼110km southwest of Quebec City. The simulation results show that initial fault permeability affects the timing, localization, rate, and length of fault shear slip. Contrary to the conventional view, our results suggest that shear failure may start earlier for a permeable fault than for a sealing fault, depending on the site-specific geologic setting. In simulations of a permeable fault, shear slip is nucleated along a 60m long fault segment in a thin and brittle caprock unit (Utica Shale) trapped below a thicker and more ductile caprock unit (Lorraine Group) – and then subsequently progresses up to the surface. In the case of a sealing fault, shear failure occurs later in time and is localized along a fault segment (300m) below the caprock units. The presence of the inclined low-permeable Yamaska Fault close to the injection well causes asymmetric fluid-pressure buildup and lateral migration of the CO2 plume away from the fault, reducing the overall risk of CO2 leakage along faults. Fluid-pressure-induced tensile fracturing occurs only under extremely high injection rates and is localized below the caprock units, which remain intact, preventing upward CO2 migration.

Accelerated synergism along a fault: A possible indicator for an impending major earthquake

It is generally accepted that crustal earthquakes are caused by sudden displacement along faults, which rely on two primary conditions. One is that the fault has a high degree of synergism, so that once the stress threshold is reached, fault segments can be connected rapidly to facilitate fast slip of longer fault sections. The other is sufficient strain accumulated at some portions of the fault which can overcome resistance to slip of the high-strength portions of the fault. Investigations to such processes would help explore how to detect short-term and impending precursors prior to earthquakes. A simulation study on instability of a straight fault is conducted in the laboratory. From curves of stress variations, the stress state of the specimen is recognized and the meta-instability stage is identified. By comparison of the observational information from the press machine and physical parameters of the fields on the sample, this work reveals differences of temporal-spatial evolution processes of fault stress in the stages of stress deviating from linearity and meta-instability. The results show that due to interaction between distinct portions of the fault, their independent activities turn gradually into a synergetic activity, and the degree of such synergism is an indicator for the stress state of the fault. This synergetic process of fault activity includes three stages: generation, expansion and increase amount of strain release patches, and connection between them.. The first stage begins when the stress curve deviates from linearity, different strain variations occur at every portions of the fault, resulting in isolated areas of stress release and strain accumulation. The second stage is associated with quasi-static instability of the early meta-instability when isolated strain release areas of the fault increase and stable expansion proceeds. And the third stage corresponds to the late meta-instability, i.e. quasi-dynamic instability as both the expansion of strain release areas and rise of strain level of strain accumulation areas are accelerated. The synergism is accelerated when the quasi-static expansion transforms into quasi-dynamic expansion, with interaction between fault segments as its mechanism. The essence of such transformation is that the expansion mechanism has changed, i.e. expansion of isolated fault segments is replaced by linkage of the interacting segments when the fault enters the critical state of a potential earthquake. Based on the experimental results, coupled with data on the temporal-spatial evolution of earthquakes along the Laohushan-Maomaoshan fault, west of the Haiyuan fault zone in northwestern China, the synergism process of this fault before the 6 June 2000 M6.2 earthquake is analyzed.

Failure time variation derived from R–S relation: the role of the static stress perturbation

In general, earthquake cycle related to earthquake faulting could include four major processes which could be described by (1) fault locking, (2) self-acceleration or nucleation (possible foreshocks), (3) coseismic slip, and (4) post-stress relaxation and afterslip. A sudden static stress change/perturbation in the surrounding crust can advance/delay the fault instability or failure time and modify earthquake rates. Based on a simple one-dimensional spring-slider block model with the combination of rate-and-state-dependent friction relation, in this study, we have approximately derived the simple analytical solutions of clock advance/delay of fault failures caused by a sudden static Coulomb stress change applied in the different temporal evolution periods during an earthquake faulting. The results have been used in the physics-based explanation of delayed characteristic earthquake in Parkfield region, California, in which the next characteristic earthquake ofM 6.0 after 1966 occurred in 2004 instead of around 1988 according to its characteristic return period of 22 years. At the same time, the analytical solutions also indicate that the time advance/delay in Coulomb stress change derived by the dislocation model has a certain limitation and fundamental flaw. Furthermore, we discussed the essential difference between rate- and state-variable constitutive (R–S) model and Coulomb stress model used commonly in current earthquake triggering study, and demonstrated that, in fact, the Coulomb stress model could be involved in the R–S model. The results, we have obtained in this study, could be used in the development of time-dependent fault interaction model and the probability calculation related to the time-dependent and renewal earthquake prediction model.

On the implementation of faults in finite-element glacial isostatic adjustment models

Stresses induced in the crust and mantle by continental-scale ice sheets during glaciation have triggered earthquakes along pre-existing faults, commencing near the end of the deglaciation. In order to get a better understanding of the relationship between glacial loading/unloading and fault movement due to the spatio-temporal evolution of stresses, a commonly used model for glacial isostatic adjustment (GIA) is extended by including a fault structure. Solving this problem is enabled by development of a workflow involving three cascaded finite-element simulations. Each step has identical lithospheric and mantle structure and properties, but evolving stress conditions along the fault.The purpose of the first simulation is to compute the spatio-temporal evolution of rebound stress when the fault is tied together. An ice load with a parabolic profile and simple ice history is applied to represent glacial loading of the Laurentide Ice Sheet. The results of the first step describe the evolution of the stress and displacement induced by the rebound process. The second step in the procedure augments the results of the first, by computing the spatio-temporal evolution of total stress (i.e. rebound stress plus tectonic background stress and overburden pressure) and displacement with reaction forces that can hold the model in equilibrium. The background stress is estimated by assuming that the fault is in frictional equilibrium before glaciation. The third step simulates fault movement induced by the spatio-temporal evolution of total stress by evaluating fault stability in a subroutine. If the fault remains stable, no movement occurs; in case of fault instability, the fault displacement is computed.We show an example of fault motion along a 45°-dipping fault at the ice-sheet centre for a two-dimensional model. Stable conditions along the fault are found during glaciation and the initial part of deglaciation. Before deglaciation ends, the fault starts to move, and fault offsets of up to 22m are obtained. A fault scarp at the surface of 19.74m is determined. The fault is stable in the following time steps with a high stress accumulation at the fault tip. Along the upper part of the fault, GIA stresses are released in one earthquake.

Slip synergism of planar strike-slip fault during meta-instable state: Experimental research based on digital image correlation analysis

The meta-instable state (MIS) is the final stage before fault instability during stick-slip movement. Thus, identification of MIS is of great significance for assessing earthquake hazard in fault zones. A rock sample with a precut planar fault was loaded on a horizontally biaxial servo-controlled press machine to create stick-slip conditions. Digital images of the sample surface were taken by a high-speed camera at a rate of 1000 frames per second during the stick-slip motion and processed using a 2D digital image correlation method to obtain the displacement field. We define a synergism coefficient that describes the relative dispersion of the accumulative fault slip. The results reveal that: (1) a local pre-slip area spreads very slowly along the fault before the MIS develops. It extends at a higher but still slow speed during meta-instable state I (MIS-I). During the final ∼1.5% of MIS, in meta-instable state II (MIS-II), the local pre-slip area first extends at a speed of ∼0.9 m/s, and then expands out of the observed image area at a very high speed. These results indicate that the local pre-slip area transforms from a state of quasi-static extension in MIS-I to quasi-dynamic extension in MIS-II. (2) The synergism coefficient of the fault slip decreases to half of its original value in MIS-I and to a quarter of its original value in MIS-II. This continuous decrease of synergism coefficient indicates that the strengthening of fault slip synergism is a characteristic of MIS. (3) Furthermore, the unstable sliding stage includes three sliding processes: initial-, fast-, and adjusted-sliding. There are two pauses between the three sliding processes.

Effective stress, porosity, velocity and abnormal pore pressure prediction accounting for compaction disequilibrium and unloading

Abnormal pore pressures, mostly overpressures, exist in many sedimentary formations. The overpressures deteriorate drilling safety, causing borehole influx, kicks, and even blowout, if the pressures are not accurately predicted prior to drilling. Highly anomalous overpressures may also induce instability and reactivation of faults, causing fault weakness. Formation overpressures are primarily generated by compaction disequilibrium, which is often recognized by higher than expected porosities at a given depth and the porosities deviated from the normal porosity trend. Based on this mechanism, the paper proposes a new generalized theoretical model for porosity–depth relationship for both normally compacted and abnormally compacted formations, i.e., ϕ=ϕ0e−cZ(σe/σn). This model leads to a new method for calculating effective stress and pore pressure in subsurface formations using porosity and compressional velocity. A new relationship of the transit time and depth has also been derived which extends the existing model (Chapman's model). It demonstrates that the sonic/seismic travel time and effective stress have an exponential relationship (i.e., Δt=Δtm+(Δtml−Δtm)e−cZ(σe/σn)).Stress unloading caused by formation uplift has a different path compared to compaction/loading curve of the stress and velocity, thus a different compaction constant. This causes a smaller effective stress and lower porosity than those in the loading case; i.e., unloading causes pore pressure increase. Effective stress and pore pressure calculations accounting for unloading are also proposed. Field data in several petroleum basins are analyzed and verify the theoretical relationship between effective stress and sonic transit time. Lab experimental data in sonic velocity and effective stress in both loading and unloading cases also verify the proposed effective stress and velocity relationship. Case study in an oil field is presented to examine the proposed model for pore pressure analysis in subsalt formations.