The geodesy and geology of earthquakes and aseismic slip

In this thesis, I explore the geodetic and geologic evidence of earthquake ruptures and aseismic fault deformation in different settings in order to understand: 1) the distribution of seismic or aseismic slip, and 2) the physical factors that influence the mechanics of faulting. In a case study of the Tien Shan (Central Asia), mapping and paleoseismology of faults along a supposed 400-km signal of fault creep paints a more nuanced picture than existing regional studies. With data from the field and high-resolution digital elevation models, the 3-5 mm/yr N-S shortening gradient in GPS data appears to be distributed variably across different basins in the mountain belt. From an apparently creeping rangefront fault in Karkara, more structures to the west accommodate active deformation. Distributed folding and thrust faults in the middle of the neighboring wide basin of Issyk-Kul takes up the majority of internal basin shortening. In the narrow basins further southwest (Karakudzhur and eastern Naryn), fresh-looking fault scarps indicate Holocene activity, with along-strike morphology and extents that could be from rupture that have generated magnitude (M) ~7 earthquakes. Moving to the Philippines, analyzing the deformation field from satellite radar interferometry (InSAR) data and inverting for the source parameters of a series of four Mw 6.4--6.8 earthquakes in late 2019 reveals a previously unrecognized seismogenic conjugate fault system on the island of Mindanao. The distribution of blind and surface fault ruptures correlates with the heterogenous geology in the shallow crust. Finally, detailed InSAR time-series analysis and modeling of the mode of slip on the creeping segment of the Philippine Fault shows that while ~100 km of the fault creeps on Leyte island throughout the seismogenic zone, a 20-km long seismogenic asperity exists in the middle. This asperity ruptured in a Mw 6.5 earthquake in July 2017. Fluids on the faults and rate-strengthening lithologies can promote interseismic aseismic slip (and impede seismic slip), while seismogenic potential shows up in structurally complex sections of the seismogenic crust in these different cases.