Fast to Slow Megathrust Slip and Fault Strength at Seismogenic Depths of the Cascadia Subduction Zone PDF Download
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Author: Duo Li
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
"The Cascadia subduction zone is short of modern seismological records of megathrust earthquakes, which makes it difficult to investigate the potential of fault ruptures directly. At the very beginning of the 21st century, a type of quasi-static fault deformation was observed around the downdip end of seismogenic zones. These aseismic transient events are called slow slip events (SSEs). SSEs accommodate a fraction of the plate convergence and may affect the stress loading at the megathrust depths. The discovery of SSEs sheds light on our knowledge of megathrust faults. This thesis aims to investigate physical constraints for subducting faults at depths of both megathrust earthquakes and slow slip events. Chapter 1 gives an introduction of the Cascadia megathrust fault and the current understanding of the physics of SSEs.In Chapter 2, I study the physics of the deep SSEs by investigating the effects of the megathrust fault geometry and overlying continental plate. I incorporate a realistic fault geometry of the northern Cascadia in the framework of rate- and state-dependent friction law, to simulate the spatiotemporal evolution of SSEs on a non-planar subduction fault. The modeled SSEs capture the major characteristics revealed by GPS observations. The along-strike distribution of SSE is inversely related to the fault local dip and strike angle of the SSE zone, suggesting a strong geometrical influence. Besides the GPS-detectable fast-spreading phase, I find that each SSE cycle consists of a deep pre-SSE preparation (nucleation) and a post-SSE relaxation (healing) phase, which may be the driving mechanism for the inter-ETS (Episodic Tremor and Slip) tremor activity that is discovered in Cascadia. In Chapter 3, I develop a 3-D episodic SSE model for the northern and the central Cascadia, incorporating both seismic and geodetic observations to constrain heterogeneous megathrust fault properties. The segmentation of SSE recurrence intervals from models that are constrained by Free-air and Bouguer gravity anomalies is equally comparable to GPS observations. However, the model constrained by Free-air anomaly does a better job in reproducing the cumulative slip as well as more consistent surface displacements with GPS observations. The modeled along-strike segmentation only represents the averaged slip release over many SSE cycles, rather than acting as permanent barriers. In Chapter 4, I study the fault shear strength at the seismogenic depths by inverting fault strength from tectonic stress tensors in the continental crust and oceanic mantle in Mendocino Triple Junction, the southern end of the Cascadia subduction zone. I obtain the fault strength for the megathrust fault in Mendocino. I use Cascadia Initiative (CI) expedition ocean bottom seismometer (OBS) data to resolve the focal mechanisms for small-to-intermediate earthquakes from 2014 to 2015. The stress orientations are obtained by combining the CI OBS resolved earthquake focal mechanisms with those reported by the Northern California Earthquake Data Center between 1980 and 2016. The fault shear strength scales with a subjective mantle strength assumed in the inversion. When the mantle strength is in the range of 50-400 MPa, the megathrust fault shear strength can be no higher than 50 MPa. The resolved friction coefficients are in the range of 0 to 0.2. In Chapter 5, I use a planar fault model with rate and state friction parameters constrained by geodetic fault locking coefficients to study megathrust earthquake cycles. The modeled coseismic fault slip can reproduce the historical coastal subsidence observations. The along-strike variation of coseismic rupture is affected by both the width of seismogenic zones and heterogeneous frictional properties (e.g., nucleation size) in Cascadia.Chapter 6 contains conclusions and future scopes." --
Author: Duo Li
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
"The Cascadia subduction zone is short of modern seismological records of megathrust earthquakes, which makes it difficult to investigate the potential of fault ruptures directly. At the very beginning of the 21st century, a type of quasi-static fault deformation was observed around the downdip end of seismogenic zones. These aseismic transient events are called slow slip events (SSEs). SSEs accommodate a fraction of the plate convergence and may affect the stress loading at the megathrust depths. The discovery of SSEs sheds light on our knowledge of megathrust faults. This thesis aims to investigate physical constraints for subducting faults at depths of both megathrust earthquakes and slow slip events. Chapter 1 gives an introduction of the Cascadia megathrust fault and the current understanding of the physics of SSEs.In Chapter 2, I study the physics of the deep SSEs by investigating the effects of the megathrust fault geometry and overlying continental plate. I incorporate a realistic fault geometry of the northern Cascadia in the framework of rate- and state-dependent friction law, to simulate the spatiotemporal evolution of SSEs on a non-planar subduction fault. The modeled SSEs capture the major characteristics revealed by GPS observations. The along-strike distribution of SSE is inversely related to the fault local dip and strike angle of the SSE zone, suggesting a strong geometrical influence. Besides the GPS-detectable fast-spreading phase, I find that each SSE cycle consists of a deep pre-SSE preparation (nucleation) and a post-SSE relaxation (healing) phase, which may be the driving mechanism for the inter-ETS (Episodic Tremor and Slip) tremor activity that is discovered in Cascadia. In Chapter 3, I develop a 3-D episodic SSE model for the northern and the central Cascadia, incorporating both seismic and geodetic observations to constrain heterogeneous megathrust fault properties. The segmentation of SSE recurrence intervals from models that are constrained by Free-air and Bouguer gravity anomalies is equally comparable to GPS observations. However, the model constrained by Free-air anomaly does a better job in reproducing the cumulative slip as well as more consistent surface displacements with GPS observations. The modeled along-strike segmentation only represents the averaged slip release over many SSE cycles, rather than acting as permanent barriers. In Chapter 4, I study the fault shear strength at the seismogenic depths by inverting fault strength from tectonic stress tensors in the continental crust and oceanic mantle in Mendocino Triple Junction, the southern end of the Cascadia subduction zone. I obtain the fault strength for the megathrust fault in Mendocino. I use Cascadia Initiative (CI) expedition ocean bottom seismometer (OBS) data to resolve the focal mechanisms for small-to-intermediate earthquakes from 2014 to 2015. The stress orientations are obtained by combining the CI OBS resolved earthquake focal mechanisms with those reported by the Northern California Earthquake Data Center between 1980 and 2016. The fault shear strength scales with a subjective mantle strength assumed in the inversion. When the mantle strength is in the range of 50-400 MPa, the megathrust fault shear strength can be no higher than 50 MPa. The resolved friction coefficients are in the range of 0 to 0.2. In Chapter 5, I use a planar fault model with rate and state friction parameters constrained by geodetic fault locking coefficients to study megathrust earthquake cycles. The modeled coseismic fault slip can reproduce the historical coastal subsidence observations. The along-strike variation of coseismic rupture is affected by both the width of seismogenic zones and heterogeneous frictional properties (e.g., nucleation size) in Cascadia.Chapter 6 contains conclusions and future scopes." --
Author: Timothy H. Dixon
Publisher: Columbia University Press
ISBN: 0231512015
Category : Science
Languages : en
Pages : 691
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Book Description
Subduction zones, one of the three types of plate boundaries, return Earth's surface to its deep interior. Because subduction zones are gently inclined at shallow depths and depress Earth's temperature gradient, they have the largest seismogenic area of any plate boundary. Consequently, subduction zones generate Earth's largest earthquakes and most destructive tsunamis. As tragically demonstrated by the Sumatra earthquake and tsunami of December 2004, these events often impact densely populated coastal areas and cause large numbers of fatalities. While scientists have a general understanding of the seismogenic zone, many critical details remain obscure. This volume attempts to answer such fundamental concerns as why some interplate subduction earthquakes are relatively modest in rupture length (greater than 100 km) while others, such as the great (M greater than 9) 1960 Chile, 1964 Alaska, and 2004 Sumatra events, rupture along 1000 km or more. Contributors also address why certain subduction zones are fully locked, accumulating elastic strain at essentially the full plate convergence rate, while others appear to be only partially coupled or even freely slipping; whether these locking patterns persist through the seismic cycle; and what is the role of sediments and fluids on the incoming plate. Nineteen papers written by experts in a variety of fields review the most current lab, field, and theoretical research on the origins and mechanics of subduction zone earthquakes and suggest further areas of exploration. They consider the composition of incoming plates, laboratory studies concerning sediment evolution during subduction and fault frictional properties, seismic and geodetic studies, and regional scale deformation. The forces behind subduction zone earthquakes are of increasing environmental and societal importance.
Author: Harmony Colella
Publisher:
ISBN: 9781267132116
Category : Earthquakes
Languages : en
Pages : 91
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Book Description
The recent discovery of slow slip events (SSEs) in subduction zones has resulted in a variety of new observations that are modeled using the new, physics-based, computationally efficient, earthquake simulation code, RSQSim. RSQSim fully incorporates 3D elastic stress interactions and employs rate- and-state constitutive properties for the sliding strength of faults. RSQSim is capable of generating 100,000s of slip events, which is ideal to understand the long-term characteristics of SSEs and their interactions with adjacent sections of the megathrust. For the simulations presented here, I adopt a Cascadia-like model of the subduction zone interface, where the megathrust is divided into three sections with different sliding characteristics: locked, transition, and continuous creep. The locked zone (25 km depth) corresponds to the section of the megathrust that generates great earthquakes, the transition zone (~25-45 km depth) corresponds to the section of the megathrust that generates SSEs, and the continuous creep zone (45 km depth) corresponds to the section at depth that slides continuously. Results from the simulations are in broad agreement with the characteristics of observed SSEs, for example, their average durations, inter-event times, and slip. The simulations produce complex, high-resolution slip patterns that are remarkably similar to tremor migration patterns observed during SSEs in Cascadia and Nankai. Additionally, the results show a depth-dependence of the characteristics of slip in the transition zone, where the frequency of slip increases with increasing depth. The depth-dependence of slip, and subsequently stress, suggests a spectrum of behaviors along a subduction zone interface, is, in part, related to the creeping zone adjacent to, and below, the transition zone and, in part, related to the constitutive properties in the transition zone. The stressing rate on the seismogenic zone is ~100x higher during a SSE than during the inter-SSE period, which may give rise to increased activity in the highly stressed region or may initiate nucleation of a great earthquake. Finally, the simulations show a significant slip deficit in the transition zone, which may have significant implications for seismic hazards for coastal cities near subduction zones.
Author: Justin R. Sweet
Publisher:
ISBN:
Category :
Languages : en
Pages : 81
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Book Description
Recent discoveries in subduction zones worldwide--including here in Cascadia--have illuminated the once shrouded process of plate convergence below the seismogenic zone. Early geodetic [Dragert, et al., 2001] and seismic [Obara, 2002] signals were observed to correlate in space and time, and were associated with periodic episodes of deep slow slip, termed Episodic Tremor and Slip (ETS) [Rogers and Dragert, 2003]. In this dissertation, I present evidence further detailing the process of where, how, and how often deep slow slip occurs using several catalogs of low-frequency earthquakes (LFEs) as slow slip indicators. In the first section I compare four distinct LFE families that span the range of the ETS zone beneath western Washington State. I find that LFE behavior varies systematically with depth: LFE moments, swarm durations, and swarm recurrence intervals are all largest in the updip portion of the ETS zone, and smallest in the downdip portion. I interpret these systematic differences as a result of variation in fault strength on the subduction interface--with the strongest coupling found updip (near the seismogenic zone), and the weakest coupling found downdip. In the second section I look within individual LFE families and perform double-difference event relocations to map out the spatial extent of the LFE patch (or patches) responsible for LFE generation. I determine LFE locking efficiency from estimates of LFE density and released seismic moment. I also track LFE migrations over time in an effort to map the progression of slow slip fronts, rapid tremor reversals (RTRs), and other phenomena.
Author: Carolyn Nuyen
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
In this dissertation, I study the style and distribution of interseismic strain release along a major convergent plate boundary to better understand how faults accommodate the relative motion between tectonic plates. Focusing on the Cascadia Subduction Zone, I utilize geodetic data, specifically in the form of Global Navigation Satellite System (GNSS) data, to measure crustal deformation and relate this deformation to slip processes along the megathrust and crustal faults. In particular, I focus on characterizing the spatiotemporal pattern of slow slip events (SSEs) across the margin and examine how conditions along the plate interface may influence the distribution of these events. I also analyze how strain is accommodated by crustal forearc faults in southern Cascadia near the Mendocino triple junction (MTJ) and consider how slip on these faults reduces the buildup of strain along the megathrust fault. The release of interseismic strain via SSEs and slip on crustal faults impacts the distribution of accumulated strain within the 'locked zone', a portion of the megathrust fault that is capable of producing large catastrophic earthquakes. Therefore, by characterizing SSE behavior and slip rates on crustal faults, this work provides important insights on the state of strain along the megathrust and how it relates to seismic hazards in Cascadia. Furthermore, this work serves as a case study for strain release in subduction zone systems and broadens our understanding of seismic and aseismic deformational processes within these environments. Chapter 1 provides an introduction to the themes and concepts that are discussed in subsequent chapters. In Chapter 2, I focus on a mode of strain release associated with long-duration SSEs. I conduct a systematic analysis of 13 years of GNSS time series data from 2006 to 2019 and present evidence of at least one low-amplitude long-term SSE on the Cascadia subduction zone, with the possibility of others that are less resolved. This 1.5-year transient is observed in southern Cascadia, and the data are modeled as a Mw 6.4 slow slip event occurring at 15-35 km depth on the plate interface, just updip of previously recognized short-term slow slip and tremor. The event shares many characteristics with similar long-term transient events on the Nankai subduction zone. However, the maximum horizontal surface displacements and total fault slip amplitudes for this event are an order of magnitude smaller than most other long-term SSEs at other subduction zones. Therefore, I propose that the frequency and size of long-term SSEs in Cascadia is limited by the width of the semi-frictional zone along the plate interface. In Chapter 3, I examine how plate boundary strain is partitioned across shallow crustal faults in the forearc of the Cascadia subduction zone. I construct elastic block models of the MTJ region and leverage GNSS velocity data to constrain deformation on the Little Salmon fault, Mad River fault zone (MRFZ) and Grogan fault, which are all identified as quaternary-active crustal structures. I evaluate models with various forearc fault structures and apply a bootstrap analysis to provide histograms of the long-term slip rate for each fault. Model results indicate that the Little Salmon fault zone is an important structure that accommodates both thrust and right-lateral shear motion in the forearc. Appreciable right-lateral slip rates on the MRFZ indicate that this system plays an important role in facilitating translation of the forearc and may serve as an extension of the northern San Andreas fault system. In contrast, the models place very little strike-slip motion on the Grogan fault and prefer this structure to mainly host reverse slip. The highest slip rates are observed on fault structures immediately to the north of the MTJ, indicating significant strain on the Bear River fault zone or nearby fault strands. Overall, these results help to constrain the seismic hazards associated with crustal faults in this region. In Chapter 4, I characterize strain release along the plate boundary in a special setting where slow slip and tremor are observed simultaneously during episodic tremor and slip (ETS) events. I use tremor and GNSS time series data to identify nineteen of the largest ETS events in southern Cascadia between 2016.5-2022 and document source properties. Distributed slip models for these events show that cumulative fault slip along the megathrust reaches a maximum near 40.5° N latitude and that large ETS events accommodate up to 80% of plate convergence at this location on the plate interface. However, ETS fault slip and tremor terminate farther to the south near ~40° N latitude, some 50 km before the southern lateral edge of the subducting Gorda plate. I explore possible controls on the distribution of strain release from ETS in southern Cascadia, including changes in the slab geometry and thermal gradient near the southern edge of the subduction zone. After exploring possible controls on the distribution of ETS, I propose that the heating of the downgoing slab edge and complex slab geometry inhibit ETS behavior in southernmost Cascadia. In particular, I demonstrate that seismic deformation in the form of tremor does not take place along the plate interface in the southernmost 50 km of Cascadia below 35 km depth, which is distinct from the rest of the subduction zone. In Chapter 5, I summarize the primary results and highlight the significance of my work. I also identify what future study is needed to resolve any lingering scientific questions and test outstanding hypotheses.
Author: Harmony Colella
Publisher:
ISBN:
Category : Earthquakes
Languages : en
Pages : 91
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Book Description
The recent discovery of slow slip events (SSEs) in subduction zones has resulted in a variety of new observations that are modeled using the new, physics-based, computationally efficient, earthquake simulation code, RSQSim. RSQSim fully incorporates 3D elastic stress interactions and employs rate- and-state constitutive properties for the sliding strength of faults. RSQSim is capable of generating 100,000s of slip events, which is ideal to understand the long-term characteristics of SSEs and their interactions with adjacent sections of the megathrust. For the simulations presented here, I adopt a Cascadia-like model of the subduction zone interface, where the megathrust is divided into three sections with different sliding characteristics: locked, transition, and continuous creep. The locked zone (25 km depth) corresponds to the section of the megathrust that generates great earthquakes, the transition zone (~25-45 km depth) corresponds to the section of the megathrust that generates SSEs, and the continuous creep zone (45 km depth) corresponds to the section at depth that slides continuously. Results from the simulations are in broad agreement with the characteristics of observed SSEs, for example, their average durations, inter-event times, and slip. The simulations produce complex, high-resolution slip patterns that are remarkably similar to tremor migration patterns observed during SSEs in Cascadia and Nankai. Additionally, the results show a depth-dependence of the characteristics of slip in the transition zone, where the frequency of slip increases with increasing depth. The depth-dependence of slip, and subsequently stress, suggests a spectrum of behaviors along a subduction zone interface, is, in part, related to the creeping zone adjacent to, and below, the transition zone and, in part, related to the constitutive properties in the transition zone. The stressing rate on the seismogenic zone is ~100x higher during a SSE than during the inter-SSE period, which may give rise to increased activity in the highly stressed region or may initiate nucleation of a great earthquake. Finally, the simulations show a significant slip deficit in the transition zone, which may have significant implications for seismic hazards for coastal cities near subduction zones.
Author: Kelley Hall
Publisher:
ISBN:
Category :
Languages : en
Pages : 85
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Book Description
This thesis seeks to integrate geodetic and seismic observations to explore the relationship between tremor and slow slip on subduction zones. In particular, I evaluate the one-to-one relationship of tremor and slip in space and time, and test various hypotheses that describe their interaction and scaling. This work adds insight into the state of the Cascadia subduction zone and the slow-slip seismic cycle. In the first chapter, I use the surface displacements measured by GPS stations to analyze six major episodic tremor and slip (ETS) events from 2007 to 2016 in northern Cascadia and invert for slip on a realistic plate interface. Tremor is typically constrained to a relatively narrow band along dip that is downdip of the inferred locked megathrust. My results indicate that slow slip extends updip of tremor by about 15 km beneath the Olympic Peninsula. Additionally, I find that along-strike variations in the amount of slow slip updip of tremor correspond to changes in lithology of the overlying crust. In these ETS events, slow slip extends from the downdip portion of the tremorgenic region beyond the updip extent of tremor, although still downdip of the inferred locked megathrust. Slip updip of tremor is a persistent feature of all six ETS events at this along-strike location. Inversions that restrict slip to occur only in regions that generated tremor produced slip distributions with unphysical characteristics and unsustainable concentrations at the updip part of the tremor footprint. Updip slow slip without tremor may suggest that the gap between stress and strength widens updip above the observed limit of tremor. In these ETS events, the regions updip of tremor may undergo ductile failure surrounding potentially tremorgenic patches. A widening gap between stress and strength in the updip direction is consistent with an observed along-dip dependence of LFE occurrence and numerical simulations of slow slip. Alternatively, rheological properties in the region updip of tremor may favor stable slip and not permit seismic slip (i.e. tremor). In the second chapter, I explore the evolution of slow slip on the Cascadia megathrust during two large ETS events and compare stress changes to the spatial evolution of tremor from PNSN tremor locations. I use displacement time series from GPS stations, along with the Extended Network Inversion Filter to solve for the time-dependent fault slip on the megathrust. The 2010 (Mw 6.8) and 2012 (Mw 6.8) slow slip events propagated northward and southward, respectively, allowing us to assess directional effects on slip behavior. I observe that tremor occurs on the leading edge of propagating slipping regions, well ahead of the highest slip rates, independent of the along-strike propagation direction. Using the tremor distribution to generate synthetic surface displacement data, resolution tests show that the result of peak tremor rates leading peak slip rates is not due to biases introduced by temporal smoothing. Calculated stress changes due to the time-dependent fault slip distributions imply that tremor is sensitive to kPa of stress, consistent with studies of tidally-triggered tremor. Within the resolution of our model, these results are consistent with the hypothesis that significant tremor is triggered by stresses ahead of the highest slip rates. I also observe ongoing slip continuing several days after tremor has passed. Our observations are consistent with some numerical models of tremor patches that suggest that this behavior can be explained by densely packed asperities, which act to widen the length scale of the slip pulse, rather than a narrow slip pulse. In the third chapter, I explore small slow slip events (SSEs), with Mw 6.0, and assesses whether fault slip and tremor detections scale linearly. Under the assumption that tremor and slip are spatially and temporally related during slow slip events, I develop a scaling relationship between tremor counts and slip based on known large slow slip events (SSEs) in Cascadia. I use the existing tremor catalog in Cascadia to cluster tremor detections into distinct events that can be scaled into slip distributions. Using this scaling relationship on a clustered tremor catalog, I obtain event magnitudes that range from Mw 4.5 to 6.5. We also find that the larger (Mo 3*1017 Nm) clustered events follow a Mo ~ T scaling. This catalog partially fills the long-standing observational gap between seismically detectable events and geodetically detectable events. GPS and strainmeters are used as an independent check of the scaling relationship. Using this clustered catalog as a guide, I identify a patch of repeating events beneath the Olympic Peninsula that produces frequent small SSEs. We stack the daily GPS time series for seven small slow slip events, aligning each record on peak tremor activity. We then estimate the average surface displacement and find the average moment. The GPS-based average moment for events in this patch is Mw 5.5 with peak fault slip reaching 0.6 cm and only 30% of the tremoring area slipping, compared to Mw 5.8 predicted by scaling the number of tremor detections. For further validation of our scaling relationship, I compare the scaled-tremor models to observed strainmeter records. We find that our empirical scaling relationship for large SSEs accurately predicts the strain for several small SSEs.
Author: Gabriele Morra
Publisher: John Wiley & Sons
ISBN: 1118888995
Category : Science
Languages : en
Pages : 208
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Book Description
Subduction dynamics has been actively studied through seismology, mineral physics, and laboratory and numerical experiments. Understanding the dynamics of the subducting slab is critical to a better understanding of the primary societally relevant natural hazards emerging from our planetary interior, the megathrust earthquakes and consequent tsunamis. Subduction Dynamics is the result of a meeting that was held between August 19 and 22, 2012 on Jeju island, South Korea, where about fifty researchers from East Asia, North America and Europe met. Chapters treat diverse topics ranging from the response of the ionosphere to earthquake and tsunamis, to the origin of mid-continental volcanism thousands kilometers distant from the subduction zone, from the mysterious deep earthquakes triggered in the interior of the descending slabs, to the detailed pattern of accretionary wedges in convergent zones, from the induced mantle flow in the deep mantle, to the nature of the paradigms of earthquake occurrence, showing that all of them ultimately are due to the subduction process. Volume highlights include: Multidisciplinary research involving geology, mineral physics, geophysics and geodynamics Extremely large-scale numerical models with sliate-of-the art high performance computing facilities Overview of exceptional three-dimensional dynamic representation of the evolution of the Earth interiors and of the earthquake and subsequent tsunami dynamics Global risk assessment strategies in predicting natural disasters This volume is a valuable contribution in earth and environmental sciences that will assist with understanding the mechanisms behind plate tectonics and predicting and mitigating future natural hazards like earthquakes, volcanoes and tsunamis.
Author: Paul Segall
Publisher: Princeton University Press
ISBN: 140083385X
Category : Science
Languages : en
Pages : 465
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Book Description
Earthquake and Volcano Deformation is the first textbook to present the mechanical models of earthquake and volcanic processes, emphasizing earth-surface deformations that can be compared with observations from Global Positioning System (GPS) receivers, Interferometric Radar (InSAR), and borehole strain- and tiltmeters. Paul Segall provides the physical and mathematical fundamentals for the models used to interpret deformation measurements near active faults and volcanic centers. Segall highlights analytical methods of continuum mechanics applied to problems of active crustal deformation. Topics include elastic dislocation theory in homogeneous and layered half-spaces, crack models of faults and planar intrusions, elastic fields due to pressurized spherical and ellipsoidal magma chambers, time-dependent deformation resulting from faulting in an elastic layer overlying a viscoelastic half-space and related earthquake cycle models, poroelastic effects due to faulting and magma chamber inflation in a fluid-saturated crust, and the effects of gravity on deformation. He also explains changes in the gravitational field due to faulting and magmatic intrusion, effects of irregular surface topography and earth curvature, and modern concepts in rate- and state-dependent fault friction. This textbook presents sample calculations and compares model predictions against field data from seismic and volcanic settings from around the world. Earthquake and Volcano Deformation requires working knowledge of stress and strain, and advanced calculus. It is appropriate for advanced undergraduates and graduate students in geophysics, geology, and engineering. Professors: A supplementary Instructor's Manual is available for this book. It is restricted to teachers using the text in courses. For information on how to obtain a copy, refer to: http://press.princeton.edu/class_use/solutions.html
Author: Agathe M. Eijsink
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Over the last decades, new types of earthquakes have been discovered. The most well-known group of ordinary earthquakes might be the most dangerous as they emit the largest amount of seismic radiation and cause ground-shaking, but repeating slow earthquakes can also damage buildings and infrastructure. Ordinary earthquakes occur when movement on a fault is unstable and a run-away process accelerates the movement to seismogenic velocities. During slow earthquakes, there are also clearly defined phases of faster slip along the fault, but the maximum slip velocity reached during these phases is lower. Then, there are aseismic faults, where slip accumulates constantly by stable creep at a rate close to the far-field stressing rate. The mechanisms that control the nature of sliding behavior of faults are multiple and studied in more or less detail. In this thesis, I explore how three factors influence fault stability: fault surface roughness and roughness anisotropy, fault-normal stiffness and stiffness contrasts across a fault, and the lithological controls on the extraordinary shallow slow slip events in the Hikurangi subduction zone margin (New-Zealand). Here, I present results using direct shear experiments, while varying one of the studied variables. To study the influence of fault surface morphology, I use two materials; a velocity-weakening and therefore potentially unstable pure quartz powder, and Rochester shale powder, which is velocity-strengthening and therefore likely to show stable sliding. Fault surface morphology evolves with displacement and its influence on frictional behavior is therefore studied by varying the amount of displacement on the samples. To test the influence of host-rock stiffness, the testing device is fitted with springs of variable stiffness in both the shear-parallel and fault-normal directions. Testing occurs on the intrinsically unstable quartz powder and I analyze both the frictional properties as well as the slip instabilities that occur. For the study about the Hikurangi margin, I use samples of the sediments on the incoming plate and use realistically low deformation rates, to study the frictional behavior and the occurrence of spontaneous slow slip events during the experiments. The results show rough, isotropic faults can host slip instabilities, because these show the required velocity-weakening frictional behavior. Striated, smooth surfaces are velocity-strengthening and promote stable sliding. The formed fault surfaces obey the typical self-affine fractal scaling, that make these results directly applicable to natural faults. Reducing the fault-normal stiffness causes the fault to become less velocity-weakening and would therefore promote stable sliding. However, slip instabilities occur when the fault-normal stiffness is reduced, which I explain by a different mechanism that requires a stiffness asymmetry. The asymmetry is the result of reducing the fault-normal stiffness on one side of the fault. The plate-rate shear experiments on Hikurangi sediments show spontaneous slow slip events occur in the calcite-rich lithologies, whereas the weakest lithologies are velocity-strengthening. Altogether, the results presented in this thesis suggest unstable sliding will occur on rough, isotropic fault patches. The slow slip events in the Hikurangi margin can only occur when the slow slip event-hosting lithologies are introduced into the deformation zone. This could be explained by a geometrically complex deformation zone due to subducting seamounts. Stiffness contrasts, due to lithological contrast across a fault or due to asymmetric damage, may cause slip instabilities that are not explained by the traditional critical stiffness theory. I show the three studied variables are closely linked and fault surface roughness, fault stiffness and stiffness contrast, as well as fault zone lithology may affect each other.
