Understanding Faults

Detecting, Dating, and Modeling

David Tanner, Christian Brandes, David Tanner, Christian Brandes

• Editore: Elsevier
• Anno: 2019
• ISBN:
  9780128159866
• Argomento: Physical Sciences
• Categoria: Geophysics

Understanding Faults: Detecting, Dating, and Modeling offers a single resource for analyzing faults for a variety of applications, from hazard detection and earthquake processes, to geophysical exploration. The book presents the latest research, including fault dating using new mineral growth, fault reactivation, and fault modeling, and also helps bridge the gap between geologists and geophysicists working across fault-related disciplines. Using diagrams, formulae, and worldwide case studies to illustrate concepts, the book provides geoscientists and industry experts in oil and gas with a valuable reference for detecting, modeling, analyzing and dating faults.

• Presents cutting-edge information relating to fault analysis, including mechanical, geometrical and numerical models, theory and methodologies
• Includes calculations of fault sealing capabilities
• Describes how faults are detected, what fault models predict, and techniques for dating fault movement
• Utilizes worldwide case studies throughout the book to concretely illustrate key concepts

Chapter 1

Introduction

David C. Tanner ^a , and Christian Brandes ^{b a} Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany ^b Institut für Geologie, Leibniz Universität Hannover, Hannover, Germany

1. Definition of a fault surface, fault kinematics and displacement
2. References

Since the advent of plate tectonics, geoscience has rapidly developed. Within the field of geoscience, tectonic research on faults represents a highly diverse sub-discipline. It underwent a transformation over the last few decades in its approach to understanding the Earth, by combining observations that are derived from natural rocks, experiments, and modelling studies. This overcame the previous, simple kinematic and steady-state fault descriptions, and allowed the analysis of dynamic and transient processes (Huntington, K.W., Klepeis, K.A., with 66 community contributors, 2018). We follow this path to present a book that delivers a holistic dynamic treatment of faults.

Faults are structural elements in the lithosphere that compensate for deformation under brittle conditions. At greater depth, faults can pass into shear zones, where plastic deformation occurs, which means that deformation mechanisms vary along a fault. Faults are very widespread in the lithosphere and they generally occur in groups, which means that the subsurface structure is often more heterogeneous than expected. In addition, faults are complex structures that are insufficiently described by simple geometrical models. Such models might work on the first-order scale, but faults (especially faults with displacements of more than several 10s of metres) tend to evolve into complex fault zones that are very heterogeneous in terms of geometry, composition and structure. As such, they have a strong control on the subsurface fluid flow and in the case of active faults, significantly influence rupture behaviour. Consequently, very different geoscience sub-disciplines, such as structural geology, geomechanics, seismology, engineering geology, petroleum geology, and Quaternary geology require profound knowledge of faults. Faults are the source of earthquakes and thus they are the connecting elements between structural geology and seismology. For instance, although both disciplines focus on faults, they often treat them from very different perspectives and investigate them on different temporal and spatial scales. Whereas, structural geology treats faults very directly, but often based on outcrop studies, seismology often concentrates on the signals (seismic waves) that are emitted during fault activity. This leads to isolated and thus restricted views on faults.

Analysing fault zone heterogeneity is a key task in characterizing a fault. In this context, there are many questions unsolved and understanding faults is a complex problem. Although there has been great progress in fault analysis over the last two decades, a unified fault model is still lacking that can serve as a predictive tool for fault zone composition, structure and for fault slip behaviour. Especially fault behaviour needs to be understood on different spatial and temporal scales.

When active faults move, they may enter a seismic phase, during which earthquakes occur. The co-related earth movements that take place during the earthquake, such as landslides, tsunami, and the destruction of infrastructure mean that earthquakes are one of the most important global geological hazards (Fig. 1.1). It is this side of faulting that is most well-known. Earthquakes, at the very best, destroy infrastructure and, at the very worst, cause loss of life. Since 1990, earthquakes have cost 27000 lives on average, each year (Guha-Sapir et al., 2011). To people who live on plate boundaries, e.g. in New Zealand, Japan, and the west coast of North America, their lives are govern by earthquakes (Fig. 1.1). Even within continental plates, there are less frequent and, for that reason, even more surprising earthquakes.

Fig. 1.1 300 m high landslip caused by the 2016 magnitude 7.8 Kaikoura Earthquake in the South Island of New Zealand. The photo was taken two years after the earthquake, which occurred 2 minutes after midnight on 14 November 2016.

Fig. 1.2 Scenarios in which faults are useful. (A) Hydrocarbons trapped by faulting. (B) fault guiding hydrothermal energy to the surface and a shallow borehole.

There is a lesser-known side of faulting, which is clearly beneficial to humankind. Many fault zones are known to act as conduits for the focused migration of fluids and clearly play a central role in determining the location, modes of transport, and emplacement of economically important hydrocarbon and hydrological reservoirs, and hydrothermal mineral deposits (Fig. 1.2A). For instance, water can migrate along a fault damage zone and appear at the surface as hot springs along the fault trace (Fig. 1.2B). The ancient Romans recognised this was the case around Aachen in Germany, and it was for this reason that they settled there (they called it “Aquae Granni“ - at the waters, Fig. 1.3). In fact, the springs around Aachen deliver far more thermal power than the SuperC borehole that was drilled in Aachen specifically for geothermal use, showing that the faults are far better at delivering thermal energy than the surrounding rocks (Dijkshoorn et al., 2013). Similar situations, where hot springs are sourced by faults, are found, for instance, in Indonesia (Brehme et al.,. 2014), along the well-named Hot Springs Fault and other faults in California (Onderdonk et al., 2011; Onderdonk, 2012), and along the Alpine Fault in New Zealand (Cox et al., 2015), to name just a few examples. This is an important observation and moves faults into focus for exploration of geothermal energy plays (Moeck, 2014).

Exploration for geothermal energy now often concentrates on finding faults at depth, preferably still active or recently active ( Barton et al., 1995; Carewitz and Karson, 1997; Huenges and Ledru, 2011 ). This is because the faults form both pathways (parallel to the fault surface) and baffles to the flow of hydrothermal water (across the fault; see Chapter 8 - Fault seal). Loveless et al. (2014) suggested that faults could even determine the success or failure of low enthalpy geothermal projects. For example, Blackwell et al. (2000) show that 90% or more of major known geothermal systems in the Basin and Range area of America are within 3 km of late Pleistocene or younger faults. Faults can also seal an otherwise open reservoir and trap hydrocarbons or ore minerals that would normally escape and dissipate (Fig. 1.2B). A majority of petroleum traps are due to fault closures and/or fault-rock seals (Sorkhabi and Tsuji, 2005).

Fig. 1.3 The Elisenbrunnen in Aachen, built in neoclassic style in 1827, allows visitors to sample the highly sulphurous, 52°C mineral water that migrates along the many faults in the area (see Chapter 8) photo: Nils Chudalla.

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