Spatiotemporal Evolution of Ground-Motion Intensity at the Irpinia Near-Fault Observatory, Southern Italy

ABSTRACT Fault zones are major sources of hazard for many populated regions around the world. Earthquakes still occur unanticipated, and research has started to observe fault properties with increasing spatial and temporal resolution, having the goal of detecting signs of stress accumulation and strength weakening that may anticipate the rupture. The common practice is monitoring source parameters retrieved from measurements; however, model dependence and strong uncertainty propagation hamper their usage for small and microearthquakes. Here, we decipher the ground motion (i.e., ground shaking) variability associated with microseismicity detected by dense seismic networks at a near-fault observatory in Irpinia, Southern Italy, and obtain an unprecedentedly sharp picture of the fault properties evolution both in time and space. We discuss the link between the ground-motion intensity and the source parameters of the considered microseismicity, showing a coherent spatial distribution of the ground-motion intensity with that of corner frequency, stress drop, and radiation efficiency. Our analysis reveals that the ground-motion intensity presents an annual cycle in agreement with independent geodetic displacement observations from two Global Navigation Satellite System stations in the area. The temporal and spatial analyses also reveal a heterogeneous behavior of adjacent fault segments in a high seismic risk Italian area. Concerning the temporal evolution of fault properties, we highlight that the fault segment where the 1980 Ms 6.9 Irpinia earthquake nucleated shows changes in the event-specific signature of ground-motion signals since 2013, suggesting changes in their frictional properties. This evidence, combined with complementary information on the earthquake frequency–magnitude distribution, reveals differences in fault segment response to tectonic loading, suggesting rupture scenarios of future moderate and large earthquakes for seismic hazard assessment.

Cretaceous–Quaternary seismic stratigraphy of the Tanga offshore Basin in Tanzania and its petroleum potential

In this study, the available 2D seismic lines have been interpreted to understand the basin development and petroleum potential of the Late Cretaceous–Quaternary stratigraphy of the Tanga offshore Basin in Tanzania. Conventional seismic interpretation has delineated eight sedimentary fill geometries, fault properties, stratal termination patterns and unconformities characterizing the studied stratigraphy. The Late Cretaceous was found to be characterized by tectonic quiescence and uniform subsidence where slope induced gravity flows that resulted during the Miocene block movements was the major mechanism of sediment supply into the basin. The Quaternary was dominated by extensional regime that created deep N-S to NNE-SSW trending graben. The graben accommodated thick Pleistocene and Holocene successions deposited when the rate of tectonic uplift surpasses the rate of sea level rise. Thus, the deposition of lowstand system tracts characterized by debris flow deposits, slope fan turbidites, channel fill turbidites and overbank wedge deposits, known for their excellent petroleum reservoir qualities, especially where charged by Karoo Black Shales. Subsequent tectonic quiescence and transgression lead to the emplacement of deep marine deposits with characteristic seismic reflection patterns that indicate the occurrence of Quaternary shale sealing rocks in the study area. The occurrence of all the necessary petroleum play systems confirms the hydrocarbon generation, accumulations and preservation potential in the Tanga Basin.

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.

Source Time Function Clustering Reveals Patterns in Earthquake Dynamics

We cluster a global database of 3529 Mw>5.5 earthquakes in 1995–2018 based on a dynamic time warping distance between earthquake source time functions (STFs). The clustering exhibits different degrees of complexity of the STF shapes and suggests an association between STF complexity and earthquake source parameters. Most of the thrust events have simple STF shapes across all depths. In contrast, earthquakes with complex STF shapes tend to be located at shallow depths in complicated tectonic regions, exhibit long source duration compared with others of similar magnitude, and tend to have strike-slip mechanisms. With 2D dynamic modeling of dynamic ruptures on heterogeneous fault properties, we find a systematic variation of the simulated STF complexity with frictional properties. Comparison between the observed and synthetic clustering distributions provides useful constraints on frictional properties. In particular, the characteristic slip-weakening distance could be constrained to be short (<0.1 m) and depth dependent if stress drop is in general constant.

Stress field perturbations from faults

<p>Faults are crucial structures in the subsurface with respect to seismic hazards or the exploitation of the subsurface. However, even though it is clear that the released elastic energy changes the stress field, it is not well known at what distance these change leave a significant imprint on the stress tensor components. In particular, it is assumed that stress tensor rotations are a measure of these changes. Furthermore, from a technical point of view, the implementation of faults in geomechanical models is a challenging task. There are several implementation concepts are to mimic faults in geomechanical models. The two main classes are the continuous approach (soft of low plastic elements) and the discontinuous approach (contact surfaces). However, only partial aspects of the complex behaviour of faults or fault zones are represented by these techniques.</p><p>Knowing this limitation, we investigate the influence of the implementation concepts, fault properties and numerical resolution on the resulting stress field in the vicinity of a fault. The main focus of the generic models is to investigate, up to which distance from a fault, significant stress changes of the stress tensor components can be observed. In doing so, the respective models undergo a deformation that produces a similar stress state. The resulting stress magnitudes are investigated along a horizontal line at a depth of 660m, parallel to the shortening direction.</p><p>The result indicates, that stress magnitude pattern varies significantly close to the modelled fault, depending on the used implementation concept. However, beyond 500 m distance from the fault, the changes in stresses are < 0.5 MPa, regardless of the concept. Even a significant coarser resolution causes comparable stress patterns and magnitudes away from the implemented fault. Similarly, the dip angle, as well as the strike angle, have little effect on the observed distance effect. For stiff rocks having a higher Young's modulus, significant stress changes can also exceed the distance of 1000 m away from the fault.</p><p>The results indicate, that faults alone have limited effect on the far-field stress pattern. On the other hand, data of stress magnitudes or the stress tensor orientation close to a fault (< 500 m) are most likely affected by the particular fault geometry and fault characteristics. This is also the case for the vertical stress magnitude.</p>

Flow modelling to quantify structural control on CO2 migration and containment, CCS South West Hub, Australia

In order to reduce uncertainties around CO2 containment for the South West Hub CCS site (Western Australia), conceptual fault hydrodynamic models were defined and numerical simulations were carried out. These simulations model worst-case scenarios with a plume reaching a main compartment-bounding fault near the proposed injection depth and at the faulted interface between the primary and secondary containment interval.The conceptual models incorporate host-rock and fault properties accounting for fault-zone lithology, cementation and cataclastic processes but with no account made for geomechanical processes as the risk of reactivation is perceived as low. Flow simulations were performed to assess cross-fault and upfault migration in the case of plume–faults interaction.Results near the injection depth suggest that the main faults are likely to experience a significant reduction in transmissivity and impede CO2 flow. This could promote the migration of CO2 vertically or along the stratigraphic dip.Results near the interface between the primary and secondary containment intervals show that none of the main faults would critically control CO2 flow nor would they act as primary leakage pathways. CO2 flow is predicted to be primarily controlled by the sedimentological morphology. The presence of baffles in the secondary containment interval is expected to be associated with local CO2 accumulations; additional permeability impacts introduced by faults are minor.Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

Is complex fault zone behaviour a reflection of rheological heterogeneity?

Fault slip speeds range from steady plate boundary creep through to earthquake slip. Geological descriptions of faults range from localized displacement on one or more discrete planes, through to distributed shearing flow in tabular zones of finite thickness, indicating a large range of possible strain rates in natural faults. We review geological observations and analyse numerical models of two-phase shear zones to discuss the degree and distribution of fault zone heterogeneity and effects on active fault slip style. There must be certain conditions that produce earthquakes, creep and slip at intermediate velocities. Because intermediate slip styles occur over large ranges in temperature, the controlling conditions must be effects of fault properties and/or other dynamic variables. We suggest that the ratio of bulk driving stress to frictional yield strength, and viscosity contrasts within the fault zone, are critical factors. While earthquake nucleation requires the frictional yield to be reached, steady viscous flow requires conditions far from the frictional yield. Intermediate slip speeds may arise when driving stress is sufficient to nucleate local frictional failure by stress amplification, or local frictional yield is lowered by fluid pressure, but such failure is spatially limited by surrounding shear zone stress heterogeneity. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

Three-dimensional deformation field analysis of the 2016 Kumamoto Mw 7.1 earthquake

The Kumamoto earthquake is analyzed mainly with InSAR data combined with strong earthquake and GPS data, using a variety of joint InSAR technology methods and multisource data solution methods and comprehensively considering the normalization and weighting of multisource data. The three-dimensional (3D) deformation field is determined. The results show that the joint solution of multisource data can improve the accuracy of the 3D solution deformation results to a certain extent. From the 3D solution results, the maximum east-west deformation caused by the 2016 Kumamoto earthquake is approximately 2 m; the north-south direction mainly manifests expansion and stretching; the northwestern side subsides vertically, with a maximum subsidence of 2 m; and the southeastern side is uplifted. The horizontal deformation characteristics show that the earthquake is dominated by right-lateral strike-slip; the strike is NE-SW, the dip of the seismogenic fault is nearly vertical, and the Futagawa fault has a few normal fault properties. By analyzing the coseismic 3D deformation field, the seismogenic fault can be better understood, which provides a foundation for studying seismic mechanisms.

Elastic and Frictional Properties of Fault Zones in Reservoir-Scale Hydro-Mechanical Models—A Sensitivity Study

The proper representation of faults in coupled hydro-mechanical reservoir models is challenged, among others, by the difference between the small-scale heterogeneity of fault zones observed in nature and the large size of the calculation cells in numerical simulations. In the present study we use a generic finite element (FE) model with a volumetric fault zone description to examine what effect the corresponding upscaled material parameters have on pore pressures, stresses, and deformation within and surrounding the fault zone. Such a sensitivity study is important as the usually poor data base regarding specific hydro-mechanical fault properties as well as the upscaling process introduces uncertainties, whose impact on the modelling results is otherwise difficult to assess. Altogether, 87 scenarios with different elastic and plastic parameter combinations were studied. Numerical modelling results indicate that Young’s modulus and cohesion assigned to the fault zone have the strongest influence on the stress and strain perturbations, both in absolute numbers as well as regarding the spatial extent. Angle of internal friction has only a minor and Poisson’s ratio of the fault zone a negligible impact. Finally, some general recommendations concerning the choice of mechanical fault zone properties for reservoir-scale hydro-mechanical models are given.