The active tectonics of Iran: InSAR-derived velocities and crustal strain

The political borders of Iran encompass one of the most tectonically-active regions in the world. Part of the larger Alpine-Himalayan orogenic belt, convergence between the Arabian and Eurasian plates is driving active deformation and seismicity throughout the Zagros Mountains, the Alborz, the Kopet Dagh, and the Makran subduction zone, posing a risk to the health and livelihoods of millions within Iran. This thesis focuses on the measurement of ground-surface velocities throughout Iran, so as to explore the country-scale kinematics of plate convergence, and to assess the local seismic hazard posed by major active faults. Previous geodetic observations have relied on Global Navigation Satellite System (GNSS) stations, which provide accurate measurements of displacement but with poor spatial coverage. The launch of the European Space Agency's Sentinel-1 radar satellite constellation has provided a wealth of imagery from which I form interferograms - measurements of apparent displacement between two given times. Through time series analysis of thousands of interferograms, I can measure the displacement of the ground surface over hundreds of kilometers, at spatial resolutions up to 100 m, and with an accuracy on the order of millimetres per year. Initially, I focus my attention on the Main Recent Fault, a major active right-lateral strike-slip fault in western Iran. Previous studies show a wide range of estimates for the rate of strain accumulation along the fault, which I seek to constrain. Using a 200 x 200 km area of line-of-sight Interferometric Synthetic Aperture Radar (InSAR) velocities, I estimate an interseismic slip rate of 2.4 ±1.2 mm/yr, one of the slowest fault slip rates measured with InSAR, and in an area with significant noise, subsiding basins, and contamination from the Mw 7.3 2017 Sarpol-e Zahab earthquake. Next, I generate an InSAR velocity field for the entirety of Iran from the processing and analysis of 85,000 Sentinel-1 interferograms. The generation of an InSAR velocity field on this scale, one of the largest to date at an area of over 2 million km^2, requires the development of a robust processing chain from radar imagery through to the estimation of East and vertical velocities from overlapping line-of-sight observations from different satellite look directions. I employ and develop existing methods for the mitigation of tropospheric noise, the removal of plate motion effects, the merging of adjacent InSAR velocities, and the conversion of relative InSAR observations into an fixed-plate reference plate through pinning to GNSS stations. The InSAR velocity field for Iran contains an abundance of tectonic (co-, post-, and interseismic deformation) and non-tectonic (groundwater extraction subsidence, salt diaprism, sediment motion) signals, which I investigate to various degrees. I focus primarily on measuring and modelling strain accumulation signals across four major faults: the Main Kopet Dagh fault, the Sharoud Fault Zone, the Doruneh fault, and the North Tabriz fault. For the Main Kopet Dagh fault, I benchmark my InSAR velocities against published Sentinel-1 velocities for the same region, demonstrating strong agreement in the estimated velocities despite differences in processing methodologies, while highlighting key differences close to the fault that influence the modelling of the rate of strain accumulation. For the Doruneh fault, I constrain variations in the rate of slip between different sections of the fault, and provide the first estimates for the locking depth of the fault - an important parameter for understanding the potential seismic hazard. Finally, I estimate high-resolution strain rates from the East InSAR velocities and interpolated North GNSS velocities, focusing on major active fault structures and exploring the differences between adjacent tectonic regions. I test two different methods of deriving strain rates, and explore how the optimal degree of spatial smoothing varies dependent upon the target tectonic signal, the spatial structure of the noise, and the proximity of non-tectonic ground motion. I bring all of these results together to discuss the active tectonics of Iran, and suggest a range of improvements for the ongoing development of country-scale InSAR velocity fields.