Numerical Modelling of Aftershock Sequences, Volcano-Tectonic Interactions, and Pandemic Propagation

Abstract

The mystery of the dominant mechanism driving aftershocks is almost 130 years old. In 1894, Omori first empirically established that the rate of aftershocks for most aftershock sequences decreases approximately as 1/time. This is known as Omori's law. Many other empirical fits have been attempted, but the most commonly cited manifestation of this law is the modified Omori-Utsu Law, which can fit most aftershock sequences using fitting parameters that include aftershock productivity and a time lag until the onset of a power law decay in time. Few physical models exist, but the one most relevant to this thesis was first proposed in 1972 by Nur and Booker, who suggested that the high-pressure fluid induced by stress drops during an earthquake could trigger subsequent seismicity by reducing the effetive normal stress of neighboring regions. Recent studies showed that a physical, fluid-based model can successfully reproduce Omori-type behavior by arguing that the aftershock decay rate reflects the tectonic ability to heal the co-seismically generated permeable network. More precisely, the mainshock creates permeable networks that provide escape pathways for high-pressure fluids and generate aftershocks along these flow paths. Like dominoes, the aftershocks create new pathways. Postseismic deformation and chemical precipitation processes reseal the permeable network and influence aftershock decay rates. However, not all earthquake sequences follow the typical Omori behavior; some examples are the 2011 Tohoku earthquake sequence (Mw 9.1) in northern Japan, the 2016 Amatrice-Visso-Norcia (AVN) earthquake sequence in the central Apennines in Italy (Mw 6.5), and the 2014 Iquique earthquake sequence in Chile (Mw 8.1). These earthquakes triggered hundreds of thousands of aftershocks in the first year and showed dramatic differences in aftershock rates along strike, with non-Omori type aftershock behavior. In this thesis, I present a hypothesis for a fluid-driven aftershock cycle in which the differential decay rate is explained by continuous fluid source generation through thermal decomposition. The spread of the COVID19 virus first demonstrates the importance of an internal source. Using a non-linear diffusion model with a source term, I calculate the diffusion of infection pressure through a porous society. The internal source mimics the ongoing transmission of the virus from a human to a human. In earthquake physics, dehydration and decarbonization of H2O and CO2 at depth provide continuous fluid sources that trigger thousands of aftershocks. Using the non-linear diffusion model, I compare the temporal and spatial evolution of the 2016 earthquake-rich AVN sequence with numerically triggered aftershocks and show that aftershocks are driven by co-seismically generated (high-pressure) fluid sources through thermal decomposition. Earthquakes without trapped fluid sources at depth or without thermal decomposition generate few, if any, aftershocks. The spatial distribution of evolving high-pressure fluid required developing a 3D dimensional model that includes (i) high-pressure fluid generation by thermal decomposition and (ii) the dynamics of permeability that plays a significant role. Higher permeability allows flow paths to remain open for extended periods of time, contribute to the aftershock sequence, and strongly influence the spatial and temporal variation of aftershocks. In this work, I show excellent spatial and temporal correlations between model results and observations for the AVN (2016) and L'Aquila (2009) earthquake sequences in the Apennines. Furthermore, seismic activity opens pre-existing vertical fractures that provide pathways for upwelling high-temperature magma, which is beneficial for geothermal production. The interplay between the penultimate eruption of Japan's Aso volcano in 2016, the associated Kumamoto sequence, and its impact on local geothermal power plants are explored using a nonlinear advection/diffusion model for pore pressure and heat flow. The results interpretations of the model presented in this thesis have potentially far-reaching implications in geophysics, geodynamics, and earthquake physics.