Understanding Faults: Detecting, Dating, and Modeling

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

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 8, 2019
• ISBN: 9780128159866

Book preview

Understanding Faults

Detecting, Dating, and Modeling

Edited by

David Tanner

Christian Brandes

Table of Contents

Cover image

Title page

Copyright

List of contributors

Preface

Chapter 1. Introduction

Definition of a fault surface, fault kinematics and displacement

Chapter 2. Fault mechanics and earthquakes

2.1. Introduction

2.2. Fractures

2.3. From intact rocks to opening-mode fractures to faults

2.4. Fault zone processes and structure

2.5. Fault movement and seismicity

2.6. Faults in soft-sediments

Chapter 3. Fault detection

3.1. Introduction

3.2. Active seismics

3.3. Ground-penetrating radar (GPR)

3.4. Electrical resistivity tomography (ERT)

3.5. Gravimetry and magnetics

3.6. Seismology

3.7. Remote sensing

Chapter 4. Numerical modelling of faults

4.1. Introduction

4.2. Numerical methods for hydromechanical fault zone modelling

4.3. Material parameters of fault zone rocks required for modelling

4.4. An example of numerical modelling

4.5. Conclusions

Chapter 5. Faulting in the laboratory

5.1. Fault friction in the quasi-static regime

5.2. Fault friction in the dynamic regime

5.3. Faults in scaled physical analogue models

5.4. Microstructures of laboratory faults

Chapter 6. The growth of faults

6.1. Introduction

6.2. Geometric indicators of fault growth

6.3. Direct kinematic indicators of fault growth

6.4. Displacement-length relations and fault growth

6.5. End-member fault growth models

6.6. Earthquakes and incremental growth

6.7. Concluding remarks

Chapter 7. Direct dating of fault movement

7.1. Dating of authigenic clay minerals in brittle faults

7.2. Dating methods based on thermal reset

Chapter 8. Fault sealing

8.1. Introduction

8.2. How does a fault seal?

8.3. General tools for fault seal analysis

8.4. Fault sealing in siliciclastic rocks

8.5. Fault sealing in carbonates

8.6. Evaporites and fault seals

8.7. Case studies of fault seal

Conclusions

Index

Copyright

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Notices

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List of contributors

Christian Brandes,     Institut für Geologie, Leibniz Universität Hannover, Hannover, Germany

Hermann Buness,     Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany

Conrad Childs,     Fault Analysis Group, School of Geological Sciences, UCD, Dublin, Ireland

Åke Fagereng,     School of Earth & Ocean Sciences, Cardiff University, Cardiff, United Kingdom

Gerald Gabriel,     Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany

Nicolai Gestermann,     Federal Institute for Geosciences and Natural Resources, Hannover, Germany

Thomas Günther,     Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany

Andreas Henk,     Institute of Applied Geosciences, Technical University Darmstadt, Darmstadt, Germany

Jan Igel,     Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany

Matt Ikari,     MARUM–Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, Bremen, Germany

Michael Kettermann,     Department of Geodynamics and Sedimentology, University of Vienna, Vienna, Austria

Tom Manzocchi,     Fault Analysis Group, School of Geological Sciences, UCD, Dublin, Ireland

Christopher K. Morley,     Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai, Thailand

Andrew Nicol,     Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand

Stefan Nielsen,     Department of Earth Sciences, Durham University, Durham, United Kingdom

André Niemeijer,     Department of Geoscience, Utrecht University, Utrecht, The Netherlands

Thomas Plenefisch,     Federal Institute for Geosciences and Natural Resources, Hannover, Germany

Peter Skiba,     Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany

Luca Smeraglia,     Dipartimento di Scienze della Terra, Sapienza University of Rome, Rome, Italy

Takahiro Tagami,     Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto, Japan

David C. Tanner,     Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany

Sumiko Tsukamoto,     Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany

Christoph von Hagke,     Institute of Geology & Palaeontology, RWTH Aachen University, Aachen, Germany

John Walsh,     Fault Analysis Group, School of Geological Sciences, UCD, Dublin, Ireland

Thomas R. Walter,     GFZ German Research Centre for Geosciences, Potsdam, Germany

Ernst Willingshofer,     Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands

Horst Zwingmann,     Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Kyoto, Japan

Preface

This book is the accumulation of many years of research on faults and represents our own personal views of the state of the art in fault analysis. Some years ago, we recognised that, although different branches of geoscience all regard faults and the processes of faulting as important, each group has its own methods and theories. Consequently, a book that focuses on all aspects of faults was missing on the market.

We had two aims when compiling this book. First, we wanted to provide a holistic view on all facets of faulting, with a focus on fault processes and fault detection. Second, this book presents a transdisciplinary approach that unites the different geoscience sub-disciplines that are concerned with faults, in particular showing the advantages of combining the methods. For instance, it is important to connect the way faults are treated in structural geology with seismological methods of fault analysis. We believe that this holistic treatment is the key to understand faults, and to develop advanced predictive fault models. We have attempted to keep the style of the book so that students from any geo-relevant background can read it. Nevertheless, we also tried reach a level between textbook and research article to make the book interesting for the advanced reader. In addition, for reasons of brevity, some chapters are shorter than we would like; therefore we made an effort to cite the background and advanced reading in these subjects.

Discussion with many colleagues has shaped the book. In addition to the authors in the book, they include; Peter Eichhubl, Bob Holdsworth, Catherine Homberg, Christopher A.-L. Jackson, Rüdiger Killian, Charlotte M. Krawczyk, Katharina Müller, Anne Pluymakers, Janos Urai, Jennifer Ziesch. We humbly claim any mistakes for ourselves.

We are very grateful to the reviewers who have critically read certain chapters and provided constructive reviews, namely: István Dunkl, Ingo Heyde, Inga Moeck, Andy Nicol, Shigeru Sueoka, Martin Schöpfer. Till Schierer and Lotta Hanzelmann are thanked for redrawing some of the figures.

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

Definition of a fault surface, fault kinematics and displacement

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.

The amount of knowledge that is not known about a fault can be shown by the surprises that have resulted from scientific deep-drilling projects that aimed to cross major faults. For instance, the SAFOD project was designed with the purpose of drilling through the San Andreas Fault at a depth of 2.7   km (e.g., Hickman and Zoback, 2004; Zoback et al., 2011). The fault was found to be profoundly weak (coefficient of friction   =   0.15; Lockner et al., 2011; Carpenter et al., 2015), which can be attributed to the presence of smectite in shear fractures (Warr et al., 2014).

The German Continental Deep Drilling Program (Abbreviation in German, KTB) drilled through the Franconian Lineament, a major strike-slip fault of western border of the Bohemian Massif in NE Bavaria, Germany (see Section 3.4; Fig. 3.16), The borehole probably crossed the fault at a depth of 6850–7950   m (Emmermann and Lauterjung, 1997). Most surprising was the presence of graphite along the fault plane, making electromagnetics the best geophysical method to determine the position of the fault at depth (Rath et al., 2001). All the basement rocks drilled also contained a surprising amount of free fluids (Emmermann and Lauterjung, 1997), but significant inflows of fluids were noted along the fault zones (Huenges et al., 1997), even at depths down to 9   km.

The Deep Fault Drilling Project (DFDP) has drilled through an inactive portion of the Alpine Fault, New Zealand (Toy et al., 2015). Here, cataclastic processes, in particular, powder lubrication and grain rolling, are considered responsible for deformation on the fault instead of frictional processes (Schuck et al., 2018).

Definition of a fault surface, fault kinematics and displacement

We define a fault surface as:

‘A structural discontinuity in a given volume of rock, along which movement has taken place’.

This simple definition implies that the discontinuity is a sharp, three-dimensional surface, i.e., the rock volume underwent brittle deformation. However, the fault surface will acquire, after sufficient movement, a zone around it that will also differ from the original unfaulted medium. During the evolution of a fault, the fault develops into a complex fault zone with a fault core where the slip is concentrated and the surrounding damage zone, which is often characterized by an increase in fracture density towards the fault core. This complex and heterogeneous zone has a strong impact of earthquake behaviour and fluid flow in the lithosphere.

Because a fault, or at least a segment of a fault, is usually sub-planar, it can be described by the strike and dip (or dip and dip azimuth) of the plane (Fig. 1.4). Miners, who worked ore bodies that followed the fault plane, coined the terms footwall for the block they were walking on and hanging-wall for the beds above them. It is this information, together with the sense of fault block movement, that, in the first instance, classifies the kinematics of the fault (Fig. 1.5).

When the hanging-wall moves downwards, relative to the footwall, the fault is termed a normal fault. The opposite sense of movement defines a reverse fault. A reverse fault with low angle of dip < 45° is known as a thrust. If the fault blocks move laterally, relative to each other, the fault is a strike-slip fault (Fig. 1.5). Normal faults place younger beds over older (i.e., maintain the superposition); reverse fault place older beds over younger (i.e., reverse the superposition). These three main fault types represent end members, and real faults may have two components of each kind of movement. Fig. 1.5 also shows that a borehole through a normal fault will ‘miss’ part of the stratigraphy (this is known as fault cut), while the reverse fault replicates the strata at any point on the fault.

The amount of displacement (slip, heave and throw) on a fault can be calculated in two dimensions from the cutoffs of bedding (Fig. 1.6). However, true analysis in two dimensions can only be achieved if the cross-section is parallel to the 3-D fault displacement vector. Otherwise, the displacement vectors will be underestimated (i.e., it is an apparent displacement). This can be avoided if the fault is observed in three-dimensional space (Fig. 1.7). However, it is not enough to know only the cutoffs of a planar feature, such as a bed. If piecing points (i.e. where linear structures on both sides of the fault are known, such as a fold axis, sedimentary feature or a dyke crosscutting bedding) are recognised, then it is possible to calculate the true slip vector (Fig. 1.7). Of course, such data is rare! In outcrop or 2D seismics, it is usual to determine only the apparent slip.

Fig. 1.4 A cutaway faulted block to show the conventions used to measure a fault plane. A fault surface can be represented either by its strike or dip direction and dip, or by the dip and dip azimuth. Note that both dip direction and dip azimuth are both perpendicular to the strike direction. Linear objects on the fault, such as slickensides, can be described the rake angle, i.e., the angle the lineation makes with the strike direction. Rake is measured anticlockwise from the strike direction, thus upward and downward rake angles are positive and negative, respectively. The dark grey side of the block is perpendicular to the strike and therefore shows the true dip angle. Other cross-section directions would give shallower apparent fault dip angles and must be corrected to obtain the true dip angle. The trace of the fault is the intersection of the fault with the topographic surface.

Fig. 1.5 Different classification of faults depending on the sense of movement. (A) a normal fault, (B) a reverse fault, (C) a left-lateral strike-slip fault. Note that in (A) the borehole misses part of the stratigraphy, whereas in (B) the stratum is doubled, and in (C) the borehole sees no change.

Fig. 1.6 Calculation of the amount of slip (displacement on the fault surface), heave (horizontal component of displacement) and throw (vertical component of displacement) in two dimensions, from the footwall and hanging-wall cutoffs of bed A. α is the fault dip. Since slip, heave and throw, in this case, form a right-angled triangle, it is geometrically easy to derive all the values, if the fault dip and one length are known.

Fig. 1.7 (A) Oblique slip of two fault blocks (a combination of normal and sinistral strike-slip movement). A-A′ represent piecing points caused by the intersection of a dyke with bed A. (B) Decomposition of the slip vector into two heave vectors (h1 parallel to the strike, h2 parallel to the dip direction, with respect to the fault orientation) and the throw vector. The heave vector is also known as the slip azimuth.

Fault rarely maintain the same geometry along strike or dip, nor does the amount of fault slip remain constant. The former can be shown as depth or contour maps, whereas the latter is more often shown as heave maps or throw (juxtaposition) diagrams (see Chapters 4–7).

This book follows a systematic approach. In Chapter 2 - Fault mechanics and earthquakes, the reader will first gain knowledge on fault evolution from an intact rock volume over initial fracture formation to the establishment of a fault that finally separates a rock volume into two individual compartments. This chapter then focuses on the structure of the fault zone and shows how the view on faults has changed over the last three decades from the simple geometric treatment of faults to the modern view of faults as complex zones that are composed of different structural domains. Based on the mechanics of faulting, earthquake processes are explained. The aim of this chapter is to present the basics of fault mechanics and to connect the theoretical models with field observations. The chapter links the geological outcrop-based fault characterization and the geophysical way of dealing with earthquakes, and thus forms the foundation for the following chapters.

Chapter 3 - Fault Detection suggests a number of geophysical methods to detect faults. This includes active methods like reflection seismics, ground penetrating Radar, Electrical Resistivity Tomography surveys and gravimetry, but also passive methods, where the seismic waves that are emitted from a fault during an earthquake are used to detect the presence of a fault and to derive its geometry and kinematics. Remote sensing, thanks to satellites, can be used to detect faulting in any part of the world, but also smaller-scale approaches, such as drone technology, is handled.

In Chapter 4 - Numerical modelling of faults different desktop numeral modelling approaches are presented and their application to faults is shown. In the last decades, numerical tools such as the finite difference, finite element and distinct element methods have rapidly spread into geoscience research and application. They allow the simulation of the mechanical development of faults and their effect on the fluid flow in the lithosphere. Finally, the reader is taken through a case study of modelling a fault.

Chapter 5 - Faulting in the laboratory describes the growing field of analogue fault (rock) analysis using laboratory experiments. The chapter is subdivided into four sections that focus on frictional experiments and relate the experimental results to natural earthquake processes. In addition, faults in scaled analogue models are discussed in this chapter as well as fault lubrication processes due to frictional melt generation.

In Chapter 6 - The growth of faults, different models of the spatial evolution of faults are explained. This chapter summarises more than three decades of research in to this subject. It shows how faults grow laterally and how they may interact with each other. Understanding fault growth is important for the characterization of the subsurface structure and for earthquake geology.

Chapter 7 - Direct dating of fault movement gives an overview on the different methods that can be utilized to date fault rocks and therefore the fault activity. The focus is set on analytical approaches to derive the age of fault movement based on the minerals that can grow on the fault surface as well as on thermochronological methods.

Finally, Chapter 8 - Fault seal deals with fault sealing and permeability. Faults can have a strong impact on the fluid flow in the lithosphere. The basic concepts of fault seal processes and their impact on the permeability are explained. This is flanked by a wide range of case studies that allow the reader to connect the theory with field examples. The reader will learn about the effect that the different host-rock types (clastic rocks, carbonates, evaporites) can have on the structure and composition of a fault that develops within these rocks. Understanding fault seal processes has a strong practical application, especially in hydrocarbon exploration projects and in the growing field of geothermal play assessment.

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Curewitz D, Karson J.A. Structural settings of hydrothermal outflow: Fracture permeability maintained by fault propagation and interaction.  J. Volcanol. Geothermal Res.  1997;79:149–168.

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Onderdonk N. The role of the Hot Springs Fault in the development of the San Jacinto Fault Zone and uplift of the San Jacinto mountains. In:  Palms to Pines: Geological and Historical Excursions through the Palm Springs Region, Riverside County, California . San Diego Association of Geologists and South Coast Geological Society Field Trip Guidebook; 2012.

Onderdonk N, Mazzini A, Shafer L, Svensen H. Controls on the geomorphic expression and evolution of gryphons, pools, and caldera features at hydrothermal seeps in the Salton Sea Geothermal Field, southern California.  Geomorphology . 2011;130:327–342. doi: 10.1016/j.geomorph.2011.04.014.

Rath V, Schwalenberg K, Brasse H. A detailed electromagnetic investigation of the Franconian Lineament/Bavaria. In: Hördt A, Stoll J.B, eds.  19. Kolloquium Elektromagnetische Tiefenforschung . Burg Ludwigstein; 2001:335–339 1.10.-5.10.2001.

Schuck B, Janssen C, Schleicher A.M, Toy V.G, Dresen G. Microstructures imply cataclasis and authigenic mineral formation control geomechanical properties of New Zealand's Alpine Fault.  J. Struct. Geol.  2018;110:172–186. doi: 10.1016/j.jsg.2018.03.001.

Sorkhabi R, Tsuji Y. The place of faults in petroleum traps. In: Sorkhabi R, Tsuji Y, eds.  Faults, Fluid Flow, and Petroleum Traps . vol. 85. AAPG Memoir; 2005:1–31.

Toy V.G, Boulton C.J, Sutherland R, Townend J, Norris R.J, Little T.A, Prior D.J, Mariani E, Faulkner D, Menzies C.D, Scott H, Carpenter B.M.Fault rock lithologies and architecture of the central Alpine fault, New Zealand, revealed by DFDP-1 drilling.  Lithosphere . 2015;7/2:155–173. doi: 10.1130/L395.1.

Warr L.N, Wojatschke J, Carpenter B.M, Marone C, Schleicher A.M, van der Pluijm B.A. A slice-and-view (FIBeSEM) study of clay gouge from the SAFOD creeping section of the San Andreas Fault at ∼2.7 km depth.  J. Struct. Geol.  2014;69:234–244.

Zoback M, Hickman S, Ellsworth W. Scientific drilling into the San Andreas Fault Zone – an overview of SAFOD's first five years.  Sci. Drill.  2011;11:14–28.

Chapter 2

Fault mechanics and earthquakes

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

Abstract

This chapter describes the processes that drive and control fault formation and evolution, with a focus on the mechanics of fracturing and faulting. The structure of this chapter allows the reader to understand the mechanics of fault evolution from an intact rock volume over initial fracture formation to the establishment of a fault that finally separates a rock volume into two individual parts. Subsequently, the reader will learn about the processes that control fault behaviour after a fault is established. The reader will gain knowledge on the structure of fault zones that represent the finite stage of fault development. Finally, based on fault behaviour and fault zone composition, rupture processes and earthquake origins are portrayed.

Keywords

Fault mechanics; Fault zone; Fault gouge; Earthquakes; Rupture; Fault creep; Deformation bands

2.1 Introduction

2.2 Fractures

2.3 From intact rocks to opening-mode fractures to faults

2.3.1 Griffith cracks

2.3.2 The Coulomb failure criterion and the Mohr circle

2.3.3 Hydrofractures

2.3.4 Stress state and dynamic fault classification of Anderson

2.3.5 Wallace-Bott hypothesis

2.4 Fault zone processes and structure

2.4.1 The fault zone

2.4.2 Principal slip surface

2.4.3 Pseudotachylites

2.4.4 Strain hardening/strain softening of the fault core

2.4.5 Fault surface geometry and roughness

2.4.6 The process zone

2.4.7 Deformation bands

2.4.8 Fault groups and their characterization

2.4.8.1 Fault arrangement and fractal geometry

2.4.9 Fault evolution with depth

2.4.10 Fault-related folding

2.5 Fault movement and seismicity

2.5.1 Fault rupture

2.5.1.1 The seismic cycle

2.5.1.2 Barriers and asperities

2.5.2 Fault creep

2.5.3 Slow earthquakes

2.5.4 The Cosserat theory as a concept to describe fault and deformation band behaviour

2.5.5 Large overthrusts and the effect of fluid pressure

2.6 Faults in soft-sediments

References

2.1. Introduction

Faults can be treated from different perspectives. Faults as static features can be described as, continuum-Euclidean, fractal or granular (Ben-Zion and Sammis, 2003) (Fig. 2.1). These three individual views strongly depend on the scale of observation. The standard way faults are described is the continuum-Euclidean view, where faults are smooth and continuous geometric objects in a solid continuum. This view can explain many observations on a first-order scale. The fractal view is largely based on the observation of rough fault surfaces, fault branching and fracture patterns as well as the distribution of earthquakes (Gillespie et al., 1993). The granular view takes observations such as fault block rotation and the development of fault breccias and fault gouge into account (Ben-Zion and Sammis, 2003). The dynamics of faulting on the other hand can be regarded as an energy transformation process, where fault motion extracts strain energy from the surroundings of a fault and transforms it into frictional heat, fracture energy and emitted seismic energy (Husseini, 1977). Movement along a fault is an energy budget that involves work against gravity, internal work within the fault system, work against frictional resistance along the fault surface, energy needed for fault propagation, and energy that is radiated as seismic waves (Cooke and Madden, 2014).

Because the same basic mechanical laws control all fault processes from micro-scale to kilometre-scale, fault mechanics is the key to understanding the formation, development and the long-term behaviour of faults. Treating faults from a mechanical point of view allows to link their static characteristics (fault structure derived from outcrops or reflection seismic) with their dynamic behaviour (fault slip and related seismicity derived from geodetic measurements or the record of seismic signals), and enables us to bridge the gaps in the spatial and temporal scale. This could potentially lead to a unified fault model that integrates the different views of faults. Several grand challenges have been formulated in the last years for the research fields of structural geology and seismology ( Lay, 2009; Forsyth et al., 2009; Gudmundsson, 2013; Huntington and Klepeis with 66 community contributors, 2018). Some of these challenges contain questions related to the mechanics of faults:

Fig. 2.1 Fault classification, based on the scale of observation. (A) Trace of the San Andreas Fault. In map view and from a large distance, the fault can be regarded as a straight, discrete line/surface (Euclidean view). Trace of the San Andreas Fault based on  van der Pluijm and Marshak (2004) . (B) Shear-deformation band network. The deformation bands branch and the branches split into smaller branches (fractal view) (a one euro coin for scale). (C) Fault breccia. The core of this fault consists of rock fragments bounded by slip surfaces (granular view) (pen for scale).

• How do fault zones behave from the Earth's surface to the base of the lithosphere?

• How are new faults initiated and how are they reactivated throughout Earth history?

• How do fault networks evolve on different spatial and temporal scales?

• How do faults slip?

• What causes large seismogenic fault-slip?

• How do ruptures stop?

• What controls the transition from seismic rupturing to aseismic fault creep?

• How do the mechanical properties of a fault evolve over an earthquake cycle and also over the lifetime of a fault?

All these questions are directly related to fault mechanics and the understanding of the physical processes that control their behaviour. Extensive outcrop analysis of the fault structure and the lithology of fault-rocks, coupled with numerical modelling techniques can help to develop more realistic fault models and to solve these questions.

2.2. Fractures

Fractures are fissures in rocks that include joints and faults. We use the term fracture (as most of the literature does) to describe a mechanical discontinuity in rocks that results from tectonic forces and is characterized by no or very low offset. In contrast, a fault has a distinct offset. The commonly-used term joint describes mode I fractures, i.e., opening of the fracture perpendicular to the fracture wall. The term vein describes the mineral fill between fracture walls, with a composition different from the host rock (Bonnet et al., 2001). The term crack is used to describe an idealized elastic discontinuity (Vermilye and Scholz, 1998).

Fracturing represents the dominant mechanism of rock failure at shallow depths in the upper crust under low confining pressures (Friedman, 1975). A brittle fracture is defined as a macroscopic planar discontinuity in a rock volume (Nelson, 1979, 2001) that has lost most or all of its cohesion due to failure, but can at any time be healed and become completely coherent again (Price and Cosgrove, 1990).

Following the work of Peacock (2001), fractures can pre-date faults, they can develop synchronous to faults or act as their precursors, and they can post-date faults. Fractures that evolve as precursors to faults play a major role in fault development. To fracture an intact rock, the failure strength of the rock volume must be exceeded. This material failure is driven by the stresses acting upon it. Stress is defined as a force per unit area. If the rock undergoes deformation in response to the applied stress, this is known as strain. Rock deformation can be best explained in a stress/strain diagram (Fig. 2.2).

Fig. 2.2 Stress/strain diagram. Stress is shown on the vertical axis and strain on the horizontal axis. The elastic part of the deformation is displayed by the green curve. Plastic deformation is shown by the purple curve (yield stress). Material that undergoes plastic deformation with strain hardening behaviour is represented by the red curve; plastic deformation with strain softening is shown by the blue curve. 

Figure is modified after Hajiabdolmajid et al. (2002).

A summary of deformation features that can be seen in triaxial experiments in which the rock fractures or undergoes plastic strain, after Hajiabdolmajid et al. (2002), are shown in Fig. 2.2. As stress increases, the rock goes through four stages of linear elastic deformation (green line; Fig. 2.2); true elastic behaviour is first reached when existing cracks and pore space are closed. In the elastic domain, the rock follows Hook's Law, for which strain is proportional to stress. The gradient of this part of the curve is:

where E is Young's Modulus. Increasing the stress beyond the failure stress (tensile strength) leads to permanent deformation. This is either manifest as fracturing (black line; Fig. 2.2) or the material continues to deform at a steady, yield stress, lower than the failure stress, which is termed plastic behaviour (purple line). If the material requires increasing or decreasing stress to maintain plastic strain, this is termed strain hardening (red line) and strain softening (blue line), respectively (Fig. 2.2).

In general, three types of fractures can be distinguished by their movement vectors (Lawn, 1993); tensile fractures (mode I; extension or opening) and two types of shear fracture (mode II; in-plane or sliding, and mode III; out-of-plane or tearing mode, Atkinson, 1982; 2001; Fig. 2.3). Hybrid fractures are also possible, for instance under increasing compressive stress fractures show a continuous transition from extension to shear (Ramsey and Chester, 2004).