How Fault Surface Features Can Tell Us About Future Earthquakes

A new study suggests ways to quantify fault maturity, a property that affects earthquake characteristics.

A calculation methodology of fault relative displacement used to study the mechanical characteristic of fault slip

Fault slip caused by mining disturbance is a crucial issue that can pose considerable threats to the mine safety. This paper proposes a point-by-point integration calculated methodology of fault relative slip and studies fault instability behavior induced by coal seam mining. A physical model with the existence of a fault and an extra-thick rock stratum is constructed to simulate the fault movement and calculate relative slip using the methodology. The results indicate that the fault relative slip can be regarded as a dynamic evolution process from local slip to global slip on the fault surface. The movement of surrounding rock masses near the fault experiences three stages, including along vertical downward, parallel to the fault and then approximately perpendicular to the fault. There will be an undamaged zone in the extra-thick rock strata when the mining face is near the fault structure. The collapse and instability of this undamaged zone could induce a violent fault relative slip. In addition, the influence of dip angles on the fault relative slip is also discussed. A formula for risk of fault relative slip is further proposed by fitting the relative displacement curves with different fault dip angles.

Using Structural Geology and Cosmogenic Nuclide Dating to Infer the Slip Rate and Frictional Strength of the Active Mai’iu Low-Angle Normal Fault, Eastern Papua New Guinea

<p>Low-angle normal faults (LANFs) have induced debate due to their apparent non -Andersonian behaviour and lack of significant seismicity associated with slip. Dipping 21°/N, the Mai’iu Fault, located in the Woodlark Rift, Eastern Papua New Guinea is an active LANF that occupies a position at the transition between continental extension and seafloor spreading. Surface geomorphology indicates that the Mai’iu Fault scarp is not significantly eroded despite high rainfall and ~2900 m of relief. Based on modelling of regional campaign GPS data (Wallace et al., 2014) the Mai’iu Fault is thought to accommodate rapid (7–9 mm/yr) horizontal extension; however the slip rate of the Mai’iu Fault has not been directly validated. I use a range of methodologies, including field mapping, cosmogenic exposure dating, cosmogenic burial dating, and Mohr-Coulomb modelling, in order to provide new constraints on LANF strength and slip behaviour. I analyse the structure of conglomeratic strata within a back -rotated rider block atop the Mai’iu Fault surface. The Gwoira rider block is a large fault-bounded sedimentary rock slice comprising the Gwoira Conglomerate, located within a large synformal megamullion in the Mai’iu Fault surface. The Gwoira Conglomerate was originally deposited on the Mai’iu Fault hanging wall concurrent with extension, and has since been buried to a maximum depth of ~2 km (evidenced by modelling of vitrinite reflectance data, and structural analysis), back-tilted, and synformally folded. The Mai’iu Fault is also overlain by a large fault slice (the Gwoira rider block), that has been transferred from the previous LANF hanging wall to the current footwall by the initiation of the younger Gwoira Fault. Both the Gwoira Conglomerate (former hanging wall) and mylonitic foliation (footwall) of the Mai’iu Fault have been shortened ~E-W, perpendicular to the extension direction. I show that N-S trending synformal folding of the Gwoira Conglomerate was concurrent with on-going sedimentation and extension on the Mai’iu Fault. Structurally shallower Gwoira Conglomerate strata are folded less than deeper strata, indicating that folding was progressively accrued concurrent with ~N -S extension. I also show that abandonment of the inactive strand of the Mai’iu Fault in favour of the Gwoira Fault, which resulted in formation of the Gwoira rider block, occurred in response to progressive megamullion amplification and resultant misorientation of the inactive strand of the Mai’iu Fault. I attribute N-S trending synformal folding to extension-perpendicular constriction. This is consistent with numerous observations of outcrop-scale conjugate strike-slip faults that deform the footwall and hanging wall of the Mai’iu Fault (Little et al., 2015), and accommodate E-W shortening. Constrictional folding remains active in the near-surface as evidenced by synformal tilting of inferred Late Quaternary fluvial terraces atop the Gwoira rider block. In order to date this sequence of progressive constrictional folding, I have processed ten ²⁶Al/¹⁰Be terrestrial cosmogenic nuclide burial samples obtained from the Gwoira Conglomerate; unfortunately these data were not yet available at the time of printing, due to reasons outside of my control. I also present terrestrial cosmogenic nuclide (TCN) exposure ages for ten rock samples obtained from the lowermost Mai’iu Fault scarp at Biniguni Falls, in order to determine the Holocene slip-rate and style using cosmogenic ¹⁰Be in quartz. I model exposure age data after the approach of Schlagenhauf et al. (2011), using a Monte-Carlo simulation in which fault slip rate, the period of last slip on the fault, and local erosion rate are allowed to vary. Modelling evidences that the Mai’iu Fault at Biniguni Falls is active and slipping at 13.9±4.0 mm/yr (1σ), resolved over the last 13.2±2.7 ka (1σ). Modelling constrains the time of last slip to 2.9±1.4 ka (1σ); this is consistent with a seismic event at that time, followed by non-slip on the Mai’iu Fault until the present day. Finally, because rider block formation records abandonment of the uppermost part of a LANF, Coulomb fault mechanical analysis can be applied to field observations to provide an upper limit on LANF frictional strength (µf). Calculations are made in terms of Mohr-Coulomb mechanics, after the framework of Choi and Buck (2012). The lock-up (abandonment) orientation at any particular position on the Mai’iu Fault is principally a function of fault friction (µf), crustal friction (µc), fault cohesion (Cf), crustal cohesion (Cc), depth, fault orientation, fluid pressure, and the orientation of the greatest principle stress. Model results suggest that fault friction for the active Gwoira-Mai’iu Fault surface is 0.128≤μf≤0.265 for Cf<1.8 MPa, and 0.2≤μf≤0.265 for Cf≤0.5 MPa. Modelling of abandonment of the inactive Mai’iu Fault suggests that 0.26≤μf≤0.309 for Cf<1.8 MPa. This suggests that past slip on the inactive Mai’iu Fault, and continued slip on the active Gwoira-Mai’iu Fault, were enabled by low fault frictional strength. I also model the strength of the active Mai’iu Fault at Biniguni Falls; results suggest greater LANF friction (μf≥0.32) than the Gwoira-Mai’iu Fault surface, and inactive Mai’iu Fault. In order to explain active slip on the LANF at Biniguni Falls concurrent with widespread field observations of outcrop-scale faulting of the LANF footwall, I suggest a process whereby overall the LANF remains viable and active, but locally stress conditions exceed the LANF abandonment criteria; this results in highly localised and temporary ‘footwall damage’ where the LANF footwall is locally dissected by outcrop-scale faulting.</p>

Using Structural Geology and Cosmogenic Nuclide Dating to Infer the Slip Rate and Frictional Strength of the Active Mai’iu Low-Angle Normal Fault, Eastern Papua New Guinea

<p>Low-angle normal faults (LANFs) have induced debate due to their apparent non -Andersonian behaviour and lack of significant seismicity associated with slip. Dipping 21°/N, the Mai’iu Fault, located in the Woodlark Rift, Eastern Papua New Guinea is an active LANF that occupies a position at the transition between continental extension and seafloor spreading. Surface geomorphology indicates that the Mai’iu Fault scarp is not significantly eroded despite high rainfall and ~2900 m of relief. Based on modelling of regional campaign GPS data (Wallace et al., 2014) the Mai’iu Fault is thought to accommodate rapid (7–9 mm/yr) horizontal extension; however the slip rate of the Mai’iu Fault has not been directly validated. I use a range of methodologies, including field mapping, cosmogenic exposure dating, cosmogenic burial dating, and Mohr-Coulomb modelling, in order to provide new constraints on LANF strength and slip behaviour. I analyse the structure of conglomeratic strata within a back -rotated rider block atop the Mai’iu Fault surface. The Gwoira rider block is a large fault-bounded sedimentary rock slice comprising the Gwoira Conglomerate, located within a large synformal megamullion in the Mai’iu Fault surface. The Gwoira Conglomerate was originally deposited on the Mai’iu Fault hanging wall concurrent with extension, and has since been buried to a maximum depth of ~2 km (evidenced by modelling of vitrinite reflectance data, and structural analysis), back-tilted, and synformally folded. The Mai’iu Fault is also overlain by a large fault slice (the Gwoira rider block), that has been transferred from the previous LANF hanging wall to the current footwall by the initiation of the younger Gwoira Fault. Both the Gwoira Conglomerate (former hanging wall) and mylonitic foliation (footwall) of the Mai’iu Fault have been shortened ~E-W, perpendicular to the extension direction. I show that N-S trending synformal folding of the Gwoira Conglomerate was concurrent with on-going sedimentation and extension on the Mai’iu Fault. Structurally shallower Gwoira Conglomerate strata are folded less than deeper strata, indicating that folding was progressively accrued concurrent with ~N -S extension. I also show that abandonment of the inactive strand of the Mai’iu Fault in favour of the Gwoira Fault, which resulted in formation of the Gwoira rider block, occurred in response to progressive megamullion amplification and resultant misorientation of the inactive strand of the Mai’iu Fault. I attribute N-S trending synformal folding to extension-perpendicular constriction. This is consistent with numerous observations of outcrop-scale conjugate strike-slip faults that deform the footwall and hanging wall of the Mai’iu Fault (Little et al., 2015), and accommodate E-W shortening. Constrictional folding remains active in the near-surface as evidenced by synformal tilting of inferred Late Quaternary fluvial terraces atop the Gwoira rider block. In order to date this sequence of progressive constrictional folding, I have processed ten ²⁶Al/¹⁰Be terrestrial cosmogenic nuclide burial samples obtained from the Gwoira Conglomerate; unfortunately these data were not yet available at the time of printing, due to reasons outside of my control. I also present terrestrial cosmogenic nuclide (TCN) exposure ages for ten rock samples obtained from the lowermost Mai’iu Fault scarp at Biniguni Falls, in order to determine the Holocene slip-rate and style using cosmogenic ¹⁰Be in quartz. I model exposure age data after the approach of Schlagenhauf et al. (2011), using a Monte-Carlo simulation in which fault slip rate, the period of last slip on the fault, and local erosion rate are allowed to vary. Modelling evidences that the Mai’iu Fault at Biniguni Falls is active and slipping at 13.9±4.0 mm/yr (1σ), resolved over the last 13.2±2.7 ka (1σ). Modelling constrains the time of last slip to 2.9±1.4 ka (1σ); this is consistent with a seismic event at that time, followed by non-slip on the Mai’iu Fault until the present day. Finally, because rider block formation records abandonment of the uppermost part of a LANF, Coulomb fault mechanical analysis can be applied to field observations to provide an upper limit on LANF frictional strength (µf). Calculations are made in terms of Mohr-Coulomb mechanics, after the framework of Choi and Buck (2012). The lock-up (abandonment) orientation at any particular position on the Mai’iu Fault is principally a function of fault friction (µf), crustal friction (µc), fault cohesion (Cf), crustal cohesion (Cc), depth, fault orientation, fluid pressure, and the orientation of the greatest principle stress. Model results suggest that fault friction for the active Gwoira-Mai’iu Fault surface is 0.128≤μf≤0.265 for Cf<1.8 MPa, and 0.2≤μf≤0.265 for Cf≤0.5 MPa. Modelling of abandonment of the inactive Mai’iu Fault suggests that 0.26≤μf≤0.309 for Cf<1.8 MPa. This suggests that past slip on the inactive Mai’iu Fault, and continued slip on the active Gwoira-Mai’iu Fault, were enabled by low fault frictional strength. I also model the strength of the active Mai’iu Fault at Biniguni Falls; results suggest greater LANF friction (μf≥0.32) than the Gwoira-Mai’iu Fault surface, and inactive Mai’iu Fault. In order to explain active slip on the LANF at Biniguni Falls concurrent with widespread field observations of outcrop-scale faulting of the LANF footwall, I suggest a process whereby overall the LANF remains viable and active, but locally stress conditions exceed the LANF abandonment criteria; this results in highly localised and temporary ‘footwall damage’ where the LANF footwall is locally dissected by outcrop-scale faulting.</p>

Slip tendency analysis of major faults in Germany

Natural seismicity and tectonic activity are important processes for the site-selection and for the long-term safety assessment of a nuclear waste repository, as they can influence the integrity of underground structures significantly. Therefore, it is crucial to gain insight into the reactivation potential of faults. The two key factors that control the reactivation potential are (a) the geometry and properties of the fault such as strike direction and friction angle, and (b) the orientations and magnitudes of the recent stress field and future changes to it due to exogenous processes such as glacial loading as well as anthropogenic activities in the subsurface. One measure of the reactivation potential of faults is the ratio of resolved shear stress to normal stresses at the fault surface, which is called slip tendency. However, the available information on fault properties and the stress field in Germany is sparse. Geomechanical numerical modelling can provide a prediction of the required 3D stress tensor in places without stress data. Here, we present slip tendency calculations on major faults based on a 3D geomechanical numerical model of Germany and adjacent regions of the SpannEnD project (Ahlers et al., 2021). Criteria for the selection of faults relevant to the scope of the SpannEnD project were identified and 55 faults within the model area were selected. For the selected faults, simplified geometries were created. For a subset of the selected faults, vertical profiles and seismic sections could be used to generate semi-realistic 3D fault geometries. Slip tendency calculations using the stress tensor from the SpannEnD model were performed for both 3D fault sets. The slip tendencies were calculated without factoring in pore pressure and cohesion, and were normalized to a coefficient of friction of 0.6. The resulting values range mainly between 0 and 1, with 6 % of values larger than 0.4. In general, the observed slip tendency is slightly higher for faults striking in the NW and NNE directions than for faults of other strikes. Normal faults show higher slip tendencies than reverse and strike slip faults for the majority of faults. Seismic events are generally in good agreement with the regions of elevated slip tendencies; however, not all seismicity can be explained through the slip tendency analysis.

Modelling of faults in LoopStructural 1.0

Without properly accounting for both fault kinematics and observations of a faulted surface, it is challenging to create 3D geological models of faulted geological units. Geometries where multiple faults interact, where the faulted surface geometry significantly deviate from a flat plane and where the geological interfaces are poorly characterised by sparse datasets are particular challenges. There are two existing approaches for incorporating faults into geological surface modelling. One approach incorporates the fault displacement into the surface description but does not incorporate fault kinematics and in most cases will produce geologically unexpected results such as shrinking intrusions, fold hinges without offset and layer thickness growth in flat oblique faults. The second approach builds a continuous surface without faulting and then applies a kinematic fault operator to the continuous surface to create the displacement. Both approaches have their strengths; however, neither approach can capture the interaction of faults within complicated fault networks, e.g. fault duplexes, flower structures and listric faults because they either (1) impose an incorrect (not defined by data) fault slip direction or (2) require an over-sampled dataset that describes the faulted surface location. In this study, we integrate the fault kinematics into the implicit surface, by using the fault kinematics to restore observations, and the model domain prior to interpolating the faulted surface. This new approach can build models that are consistent with observations of the faulted surface and fault kinematics. Integrating fault kinematics directly into the implicit surface description allows for complexly faulted stratigraphy and fault–fault interactions to be modelled. Our approach shows significant improvement in capturing faulted surface geometries, especially where the intersection angle between the faulted surface and the fault surface varies (e.g. intrusions, fold series) and when modelling interacting faults (fault duplex).

Determining Stress State of Source Media with Identified Difference between Groundwater Level during Loading and Unloading Induced by Earth Tides

The groundwater level might be adopted as a useful tool to explore pre-seismic stress change in the earth crust, because it circulates in the deep crust and should be altered by the processes associated with the preparation of earthquakes. This work makes a new attempt that applies the load/unload response ratio (LURR) technique to study the stress state of the source media before the large earthquakes by calculating the ratio between the water levels during the loading and unloading phases. The change of Coulomb failure stress induced by earth tides in the tectonically preferred slip direction on the fault surface of the mainshock is adopted for differentiating the loading and unloading periods. Using this approach, we tested the groundwater level in the wells near the epicenters of some large earthquakes that occurred in the Sichuan-Yunnan region of southwest China. Results show that the LURR time series fluctuated narrowly around 1.0 for many years and reached anomalously high peaks 2~8 months prior to the mainshocks. For the earthquakes with multiple observation wells, the magnitude of the maximum values decreases with the distance from the epicenter. The underlying physics of these changes should be caused by the pre-seismic dilatancy. The corresponding volume variations in the crust could be observed in the geodetic time series in the same neighborhoods and during the same period.

Improved Workflow for Fault Detection and Extraction Using Seismic Attributes and Orientation Clustering

Faults represent important analytical targets for the identification of perceptual ground motions and associated seismic hazards. In particular, during oil production, important data such as the path and flow rate of fluid flows can be obtained from information on fault location and their connectivity. Seismic attributes are conventional methods used for fault detection, whereby information obtained from seismic data are analyzed using various property processing methods. The analyzed data eventually provide information on fault properties and imaging of fault surfaces. In this study, we propose an efficient workflow for fault detection and extraction of requisite information to construct a fault surface model using 3D seismic cubes. This workflow not only improves the ability to detect faults but also distinguishes the edges of a fault more clearly, even with the application of fewer attributes compared to conventional workflows. Thus, the computing time of attribute processing is reduced, and fault surface cubes are generated more rapidly. In addition, the reduction in input variables reduces the effect of the interpreter’s subjective intervention on the results. Furthermore, the clustering method can be applied to the azimuth and dip of the fault to be extracted from the complexly intertwined fault faces and subsequently imaged. The application of the proposed workflow to field data obtained from the Vincentian oil field in Australia resulted in a significant reduction in noise compared to conventional methods. It also led to clearer and continuous edge detection and extraction.

Experimental Study on Formation Slip under Injection-Production Interregional Pressure Difference Based on the Abnormal Similarity Theory

In oilfield development, the pore pressure difference between adjacent areas leads to cracks and slipping in the weak structural surface layer, which triggers the shear failure of the casing. The formation slip involves a large range of formation, and its amount is not proportional to the size of the slipping rock mass, which conventional physical models cannot simulate. In this study, based on the abnormal similarity theory, we derived the similarity coefficients of mechanical parameters with different horizontal and vertical proportions. Furthermore, an experimental device for simulating the formation crack and slip under interregional formation pressure difference was developed. Through the experiments, we obtained slip conditions under different pressure differences between adjacent areas and different oil layers and fault surface depths. The study shows that the pore pressure difference between adjacent areas is the driving force of the formation slips. The slip zone is located in the middle of two abnormal pressure zones, and the distance between the adjacent areas can affect the slip range. The deep burial of the oil layer and shallow depth of the weak structural surface can trigger a more significant formation slip. The experimental method proposed in this paper provides an experimental device and method for understanding the formation of cracks and slips on weak structural surfaces. The experimental results provide a theoretical basis for the prevention of shear-type casing damage caused by formation slip.