Geodetic Imaging Of The Earthquake Cycle

In this dissertation I used Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) to recover crustal deformation caused by earthquake cycle processes. The studied areas span three different types of tectonic boundaries: a continental thrust earthquake (M7.9 Wenchuan, China) at the eastern margin of the Tibet plateau, a mega-thrust earthquake (M8.8 Maule, Chile) at the Chile subduction zone, and the interseismic deformation of the San Andreas Fault System (SAFS). A new L-band radar onboard a Japanese satellite ALOS allows us to image high-resolution surface deformation in vegetated areas, which is not possible with older C-band radar systems. In particular, both the Wenchuan and Maule InSAR analyses involved L-band ScanSAR interferometry which had not been attempted before. I integrated a large InSAR dataset with dense GPS networks over the entire SAFS. The integration approach features combining the long-wavelength deformation from GPS with the short-wavelength deformation from InSAR through a physical model. The recovered fine-scale surface deformation leads us to better understand the underlying earthquake cycle processes. The geodetic slip inversion reveals that the fault slip of the Wenchuan earthquake is maximum near the surface and decreases with depth. The coseismic slip model of the Maule earthquake constrains the down-dip extent of the fault slip to be at 45 km depth, similar to the Moho depth. I inverted for the slip rate on 51 major faults of the SAFS using Green's functions for a 3-dimensional earthquake cycle model that includes kinematically prescribed slip events for the past earthquakes since the year 1000. A 60 km thick plate model with effective viscosity of 1019 Pa · s is preferred based on the geodetic and geological observations. The slip rates recovered from the plate models are compared to the half-space model. The InSAR observation reveals that the creeping section of the SAFS is partially locked. This high-resolution deformation model will refine the moment accumulation rates and shear strain rates, which are not well resolved by previous models.

Geodetic observations provide kinematic constraints on the behavior of tectonically active fault systems. Estimates of earthquake cycle activity derived from these constraints may depend on modeling assumptions and/or regularization of a geodetic inverse problem, which is often poorly conditioned. Common model assumptions may affect kinematic solutions and conclusions about physical properties of faults and fault zones. For example, within a geometrically complex fault system, parameterization of nearby faults may affect slip estimates on an individual fault. In addition, fault slip models are often regularized by assuming that slip varies smoothly in space, which may artificially smear slip estimates beyond physical boundaries. As an alternative to smooth regularization, the applied mathematics field of compressed sensing provides a suite of methods for recovering sparse solutions. Applied to GPS observations of the 2011 Tohoku earthquake, compressed sensing algorithms enable imaging of spatially localized slip during and following the earthquake, and identification of a sharp boundary between coseismic and postseismic slip. Similar algorithms recover quantized solutions and may be applied to models of plate boundary deformation. Beginning with a dense array of tectonic micro-plates bounded by mapped faults in North America, these methods can be used to detect coherent motions of groups of micro-plates behaving as larger active blocks, effectively quantifying the complexity of North America plate boundary deformation. By improving our ability to identify and compare kinematic constraints on earthquake cycle processes, we are able to characterize the spectrum of earthquake cycle behaviors and gain a deeper understanding of earthquake phenomenology and physics.

These proceedings contain a selection of peer-reviewed papers presented at the International Symposium on Geodesy for Earthquake and Natural Hazards (GENAH), Matsushima, Japan, 22-26 July, 2014. The scientific sessions focused on monitoring temporal and spatial changes in Earth's lithosphere and atmosphere using geodetic satellite systems, high rate GNSS as well as high resolution imaging (InSAR, Lidar). Researchers in various fields of geodesy discussed the role of geodesy in disaster mitigation and how groups with different techniques can collaborate toward such a goal.

The study of the seismic cycle has many applications, from the study of faulting to the estimation of seismic hazards. It must be considered at different timescales, from that of an earthquake, the co-seismic phase (a few seconds), the post seismic phase (from months to dozens of years) and the inter-seismic phase (from dozens to hundreds of years), up to cumulative deformations due to several seismic cycles (from a few thousand to hundreds of thousands of years). The Seismic Cycle uses many different tools to approach its subject matter, from short-term geodesic, such as GPS and InSAR, and seismological observations to long-term tectonic, geomorphological, morphotectonic observations, including those related to paleoseismology. Various modeling tools such as analog experiences, experimental approaches and mechanical modeling are also examined. Different tectonic contexts are considered when engaging with the seismic cycle, from continental strike-slip faults to subduction zones such as the Chilean, Mexican and Ecuadorian zones. The interactions between the seismic cycle and magmatism in rifts and interactions with erosion in mountain chains are also discussed.

In this dissertation, I use space-based geodetic data to study the ground deformation caused by the earthquake cycle processes. Chapter 1 is an introduction to the data I used and the motivation on each of the following chapters. Chapter 2 focuses on investigating a controversial problem brought up by several co-seismic inversions using geodetic data, which is called the shallow slip deficit. I explored whether this problem is largely an artifact of inversion due to incomplete data and refined the magnitude of this deficit. Chapter 3, following Chapter 2 develops a new data-driven spectral expansion approach for co-seismic slip inversion using geodetic data. Compared to traditional method, it isolates the unstable part of the model and only solves for the well-determined part. Meanwhile we also developed a 1-D thermal model to understand the different down-dip rupture limits of continental-continental and continental-oceanic megathrust events. Chapter 4 aims at understanding the new TOPS mode data from Sentinel-1 satellites and fully testing the capability of this dataset. A subsidence of about 160 mm/yr at the Cerro Prieto Geothermal Field is recovered together with a 40 mm/yr tectonic fault parallel motion at the nearby region. Chapter 5 further develops the processing algorithm used in Chapter 4 and uses Sentinel-1 data to reveal both tectonic and anthropogenic deformation along the San Andreas Fault System.

The work presented in my dissertation focuses on the crustal deformation during the co- and postseismic periods in earthquake cycles. I use geodetic and seismic data to constrain and better understand the behavior of the earthquake source during the coseismic period. For the postseismic period, I use geodetic data to observe the surface displacements from centimeter-scale to millimeter-scale from an Mw 7.9 and Mw 6.9 event, respectively. I model different mechanisms to explain the postseismic deformation and to further constrain the crustal and upper mantle rheology. For the coseismic earthquake source study, I explore the source of the 2010 Mw 6.3 Jia-Shian, Taiwan earthquake. I develop finite-source models using a combination of seismic data (strong motion and broadband) and geodetic data (InSAR and GPS) to understand the rupture process and slip distribution of this event. The main shock is a thrust event with a small left-lateral component. Both the main shock and aftershocks are located in a transition zone where the depth of seismicity and an inferred regional basal detachment increases from central to southern Taiwan. The depth of this event and the orientation of its compressional axis suggest that this event involves the reactivation of a deep and weak pre-existing NW-SE geological structure. The 1989 Mw 6.9 Loma Prieta earthquake provides the first opportunity since the 1906 San Francisco (Mw 7.9) earthquake to study postseismic relaxation processes and estimate rheological parameters in the region with modern space geodetic tools. The first five years postseismic displacements can be interpreted to be due to aseismic right-oblique fault slip on or near the coseismic rupture, as well as thrusting up-dip of the rupture within the Foothills thrust belt. However, continuing transient surface displacements (d"5 mm/yr) until 2002 revealed by PSInSAR and GPS in the northern Santa Cruz Mountains may indicate a longer-term postseismic deformation. I model the viscoelastic relaxation of the lower crust and upper mantle following the Loma Prieta earthquake to explain the surface displacement. A 14-km-thick lower crust (16 - 30 km depth) viscosity of> 1019 Pa s and an upper mantle viscosity of ~1018 Pa s best explain the geodetic data. The weak upper mantle viscosity in this area is in good agreement with upper mantle rheology in southern California (0.46 - 5 × 1019 Pa s) using a similar approach from studying the postseismic deformation following the 1999 (Mw 7.1) Hector Mine earthquake. Periods of accelerated postseismic deformation following large earthquakes reflect the response of the Earth's lithosphere to sudden coseismic stress changes. I investigate postseismic displacements following the 2008 Wenchuan (Mw 7.9), China earthquake in eastern Tibet and probe the differences in rheological properties across the edge of the Tibetan Plateau. Based on nearly two years of GPS and InSAR measurements, I find that the shallow afterslip on the Beichuan Fault can explain the near-field displacements, and the far-field displacements can be explained by a viscoelastic lower crust beneath Tibet with an initial effective viscosity of 4.4 × 1017 Pa s and a long-term viscosity of 1018 Pa s. On the other hand, the Sichuan Basin block has a high-viscosity upper mantle (> 1020 Pa s) underlying an elastic 35-km-thick crust. The inferred strong contrast in lithospheric rheologies between the Tibetan Plateau and the Sichuan Basin is consistent with models of ductile lower crustal flow that predict maximum topographic gradients across the Plateau margins where viscosity differences are greatest. With additional 6-year-long continuous GPS measurements deployed in the eastern Tibetan Plateau and the Sichuan Basin, viscoelastic relaxation models with the same geometry setups suggests Tibetan lower crust with an initial effective viscosity of 9 × 1017 Pa s and steady-state viscosity of 1019 Pa s. I also use the laboratory experiments derived power law flow model to fit the postseismic deformation. The viscosity estimated from this model varies with material parameters (e.g. grain size, water content, etc.) as well as environmental parameters (temperature, pressure, background strain rate, etc.). The diffusion creep refers to the power law flow mainly controlled by the mineral grain size, and the dislocation creep refers to it mainly controlled by the background stress level. For a diffusion creep type of power law flow, a Tibetan crust composed of wet feldspar (water content = 1000 H/106Si; grain size = 1 - 4 mm) and upper mantle composed of wet olivine (water content = 200 H/106Si; grain size = ~2 mm) can predict the 6-year-long poseismic time series well. This result roughly agrees with rock mechanics laboratory experiments. The channel flow model predicts the plateau margins are steepest where the viscosity of the surrounding blocks are highest. The low viscosity in the Tibetan lower crust and the contrasting rheology across the plateau margin derived from postseismic deformation are consistent with the channel flow model.

In the last decade of the 20th century, there has been great progress in the physics of earthquake generation; that is, the introduction of laboratory-based fault constitutive laws as a basic equation governing earthquake rupture, quantitative description of tectonic loading driven by plate motion, and a microscopic approach to study fault zone processes. The fault constitutive law plays the role of an interface between microscopic processes in fault zones and macroscopic processes of a fault system, and the plate motion connects diverse crustal activities with mantle dynamics. An ambitious challenge for us is to develop realistic computer simulation models for the complete earthquake process on the basis of microphysics in fault zones and macro-dynamics in the crust-mantle system. Recent advances in high performance computer technology and numerical simulation methodology are bringing this vision within reach. The book consists of two parts and presents a cross-section of cutting-edge research in the field of computational earthquake physics. Part I includes works on microphysics of rupture and fault constitutive laws, and dynamic rupture, wave propagation and strong ground motion. Part II covers earthquake cycles, crustal deformation, plate dynamics, and seismicity change and its physical interpretation. Topics in Part II range from the 3-D simulations of earthquake generation cycles and interseismic crustal deformation associated with plate subduction to the development of new methods for analyzing geophysical and geodetical data and new simulation algorithms for large amplitude folding and mantle convection with viscoelastic/brittle lithosphere, as well as a theoretical study of accelerated seismic release on heterogeneous faults, simulation of long-range automaton models of earthquakes, and various approaches to earthquake predicition based on underlying physical and/or statistical models for seismicity change.

Geodesy is the science of accurately measuring and understanding three fundamental properties of Earth: its geometric shape, its orientation in space, and its gravity field, as well as the changes of these properties with time. Over the past half century, the United States, in cooperation with international partners, has led the development of geodetic techniques and instrumentation. Geodetic observing systems provide a significant benefit to society in a wide array of military, research, civil, and commercial areas, including sea level change monitoring, autonomous navigation, tighter low flying routes for strategic aircraft, precision agriculture, civil surveying, earthquake monitoring, forest structural mapping and biomass estimation, and improved floodplain mapping. Recognizing the growing reliance of a wide range of scientific and societal endeavors on infrastructure for precise geodesy, and recognizing geodetic infrastructure as a shared national resource, this book provides an independent assessment of the benefits provided by geodetic observations and networks, as well as a plan for the future development and support of the infrastructure needed to meet the demand for increasingly greater precision. Precise Geodetic Infrastructure makes a series of focused recommendations for upgrading and improving specific elements of the infrastructure, for enhancing the role of the United States in international geodetic services, for evaluating the requirements for a geodetic workforce for the coming decades, and for providing national coordination and advocacy for the various agencies and organizations that contribute to the geodetic infrastructure.