Space Geodetic Constraints on Plate Motion, Fault Creep, and Megathrust Coupling

Earthquakes release centuries of accumulated elastic strain in seconds, yet where and how this strain builds up across fault systems remain only partly understood. The contrast between seismogenic asperities—regions that lock and rupture in earthquakes—and aseismic barriers that creep or remain stable defines the scale, frequency, and segmentation of major ruptures. Constraining the spatial extent and temporal persistence of these regions requires geodetic observations that resolve deformation from local fault zones to plate-wide strain fields. In this thesis, I use spaceborne geodesy, specifically Interferometric Synthetic Aperture Radar (InSAR), to complement sparse Global Navigation Satellite System (GNSS) networks in quantifying crustal deformation and inferring tectonic processes across a continuum of spatial and temporal scales. We strive to understand how strain accumulates, transfers, and is released along major plate boundary systems within a probabilistic framework that explicitly incorporates uncertainty and prior assumptions. I begin with the central San Andreas Fault (Chapter 1), where locked, transitional, and creeping segments coexist within a narrow zone of complex mechanical interaction. By jointly analyzing geodetic coupling and seismicity, I find that the fraction of background earthquakes scales with the aseismic slip rate normalized by the plate rate, suggesting a phenomenological link between fault coupling and micro-earthquake clustering. I then address a broader methodological challenge in InSAR geodesy—the treatment of reference-frame motion and its influence on long-wavelength deformation gradients (Chapter 2). After applying refined tropospheric and ionospheric corrections and inversion of large-scale InSAR velocity fields, I show that the absolute Euler rotation of a tectonic plate can be directly estimated from InSAR data, offering an independent constraint on plate kinematics. This capability extends InSAR beyond local fault studies, enabling continental and even global mapping of tectonic deformation, which is a prospect that becomes increasingly viable with the advent of modern L-band radar missions capable of penetrating vegetation and maintaining coherence over wide spatial and temporal baselines. Next, I examine the southern Dead Sea Transform and Gulf of Aqaba, where the Arabian and Nubian plates undergo transtensional motion as the transform transitions into the Red Sea rift (Chapter 3). By compiling a decade of Sentinel-1 data, I map along-strike variations in fault coupling and localized extension toward the rift triple junction, delineating the gradual shift from strike-slip shear to crustal rifting. Finally, I combine InSAR and GNSS time series to construct a spatially continuous model of interseismic coupling along the Nazca-South America margin (Chapter 4). The results reveal strong spatial correlations between heterogeneity in fault coupling and subducted bathymetric highs, underscoring the mechanical role of subducted topography in segmenting megathrust behavior and strain accumulation. Through these studies, I demonstrate how integrating long-wavelength geodetic imaging allows one to map crustal deformation continuously from fault to plate scales with quantified confidence, while explicitly identifying regions where the data remain uninformative or ambiguous. In doing so, we aim to bridge observation and theory to connect the patterns we see from orbit to the physics that govern the accumulation of strain and seismic release of elastic strain within the Earth’s dynamic lithosphere.