Thermal pressurization during earthquake nucleation and dynamic rupture [electronic resource]
Keywords: Geophysics.

Understanding the strength of faults over the course of the seismic cycle is an essential element in developing physical models of earthquakes. This thesis presents results from numerical modeling of shear heating-induced thermal pressurization of pore fluid, which is one of the most-often proposed mechanisms for reducing the strength of faults during earthquakes. Significant fault weakening during dynamic slip is necessary to reconcile several observations. Laboratory rock-sliding experiments over the past 40 years have shown that the static frictional strength of faults is likely to be large. On the other hand, stress and heat-flow observations near faults indicate that shear tractions on faults are much smaller than fault static strength estimated with laboratory friction coefficients. Provided that shear traction somewhere on a fault reaches its static strength in order to nucleate slip, a dynamic weakening mechanism such as thermal pressurization is capable of sustaining an earthquake even as rupture propagates into weakly stressed regions. In the first major section of this thesis, earthquake nucleation (in 2D) is modeled on planar faults with coupled rate-state friction and shear heating-induced thermal pressurization. Diffusive transport of heat and pore pressure is coupled to the shear heating and pressurization. Thermal pressurization increases pore pressure, decreasing effective normal stress and thereby reducing frictional resistance to shear. Depending on the material properties and the friction evolution law used, thermal pressurization may overwhelm frictional weakening before seismic radiation occurs. Even if not the dominant weakening mechanism, thermal pressurization is likely to be significant at or before the onset of seismic radiation. The second section of this thesis continues the investigation by permitting nucleation to proceed into the dynamic rupture phase. Mean shear stresses on the fault are low, but local stress heterogeneities are introduced in order to allow for nucleation. Two endmember stress heterogeneities are considered. In the first case, the "high stress" model, shear stress is locally elevated while initial effective normal stress is uniformly high. In the second case, the "low strength" model, shear stress is uniformly low while nominal effective normal stress is locally depressed. Outside the slip-nucleating stress heterogeneity, background shear stress is low in both cases. The level of background shear stress controls whether the dynamic rupture arrests or is sustained. Using typical values for stresses and material properties corresponding to 7 km depth, sustained rupture occurs at shear stress levels as low as 0.13 times effective normal stress in the high-stress case and 0.15 times the effective normal stress in the low-stress case. Most sustained ruptures are crack-like, but pulse-like ruptures occur for both stress models over a small range of background shear stress just below the threshold for crack-like rupture. At lower values of background shear stress, ruptures arrest. For arresting ruptures, earthquake source parameters such as moment, rupture length, fracture energy, and stress drop are compared to values observed for small earthquakes and are found to be generally consistent. The final portion of this thesis is similar to the preceding section but also includes flash heating of asperity contacts, an additional dynamic weakening mechanism. In this mechanism, shear heating of microscopic asperity contacts reduces their strength, leading to a dramatic reduction of the friction coefficient. Flash heating becomes significant at comparable slip speeds to those at which thermal pressurization becomes significant, about 0.1 m/s. The spatial scale of weakening associated with flash heating is much smaller than that of thermal pressurization, which leads to computational challenges that limit the scope of the investigation to a few high-stress cases. Combined, the two effects sustain rupture at background shear stress levels lower than those at which either weakening mechanism alone can sustain rupture. At a given background shear stress, the two effects combine to allow rupture to propagate greater distances. For arresting ruptures, the comparison of modeled earthquake source parameters to observed values is improved over the models that neglect flash heating. Flash heating alone, however, fails to predict values of fracture energy consistent with those inferred for real earthquakes. Therefore, both flash heating and thermal pressurization together yield the most realistic model for dynamic weakening during earthquakes on weakly stressed faults.