Cross-scale modeling of mountain building and the seismic cycle: From Alps to Himalaya

Abstract

Orogenesis—the process of mountain building—forms some of the most spectacular features of the Earth's surface. This complex geological process operates on a wide range of time and length scales and is characterized by fold-and-thrust-belts that grow through sequential stacking of thrust sheets. These faults trigger large earthquakes, often close to densely populated areas. Understanding the dynamics and deformation of collisional orogens thus constitutes an important and challenging step towards improving seismic hazard assessment. By integrating the approaches of numerical modeling, seismology, geodesy, and tectonophysics, this thesis provides a thorough and advanced analysis to improve our understanding of the seismotectonics evolution of collisional mountain ranges. Generic models as well as specific examples—from the Alps to the Himalayas—are explored to evaluate the relation between long-term mountain building processes and short-term seismogenesis. First, I present a novel model to investigate the partitioning between tectonic and kinematic processes and assess seismic behaviour of mountain belts. These models obtain a Gutenberg–Richter frequency magnitude distribution due to spontaneous events occurring throughout the orogen. I propose that both the corresponding slope (b value) and maximum earthquake magnitude correlate linearly with plate convergence rate through a rheological feedback with temperature and strain rate. This is in agreement with earthquakes recorded across the Alps, Apennines, Zagros and Himalaya. I then explore the self-driven evolution of a convergent margin, from subduction to collisional orogeny and spontaneous slab breakoff. I show how slow—but persistent—bending of a post-collisional residual slab controls the latest evolution of the orogen, including crustal delamination, the construction of the foreland basin, and the seismicity pattern throughout the orogen. Based on these new insights, I argue that tectonic processes across the Central Alps are related to vertical forces driven by a Slab Rollback Orogeny model. To facilitate a comparison to natural settings, I develop a tool for the design of realistic and accurate model setups. By including a number of tectonic constraints, I use this tool to set a rigorous setup of the present-day lithospheric structure of the Nepal Himalaya. This model allows me to explore the conditions that could explain the bimodal seismicity (Mw≤7.8 vs. M8+) of large Himalayan earthquakes. Results reproduce realistic earthquake sequence of irregular magnitude—including events similar to the 2015 Mw 7.8 Gorkha earthquake—and provides an excellent match to the interseismic observations. Most importantly, this model shows that fault frictional and non-planar geometry of the Himalayan megathrust introduce a shallow region of large strength excess, which can only be activated once enough stress is transferred by partial (blind) ruptures. Finally, I use a Bayesian sampling method to propose a new coupling model of the Himalayan megathrust. This inversion benefits from an objective weighting of the various datasets by combining observational and modelling errors. In particular, this approach does not include any contamination from the smoothing regularization used in standard approaches. The impact of this model is distinct, as it shows three potential barriers of low coupling separating discrete and large highly coupled patches. These new findings raise the possibility of a heterogeneous coupling distribution of the Himalayan megathrust, both along-strike and down-dip. Overall, this thesis provides a better understanding of the interaction between mountain building processes and seismicity, and a valuable improvement to this fast-moving cross-field of seismotectonics, seismology and geodynamics.