Location of Thesis Examination

Room 1027 B&GS

Degree

Doctor of Philosophy

Program

Geophysics

Supervisor

Dr. Kristy Tiampo

Abstract

Natural earthquake fault systems are composed of a variety of materials with different spatial configurations a complicated, inhomogeneous fault surface. The associated inhomogeneities with their physical properties can result in a variety of spatial and temporal behaviors. As a result, understanding the dynamics of seismic activity in an inhomogeneous environment is fundamental to the investigation of the earthquakes processes.

This study presents the results from an inhomogeneous earthquake fault model based on the Olami-Feder-Christensen (OFC) and Rundle-Jackson-Brown (RJB) cellular automata models with long-range interactions that incorporates a fixed percentage of stronger sites, or ‘asperity cells’, into the lattice. These asperity cells are significantly stronger than the surrounding lattice sites but eventually rupture when the applied stress reaches their higher threshold stress.

The introduction of these spatial heterogeneities results in temporal clustering in the model that mimics that seen in natural fault systems. Sequences of activity that start with a gradually accelerating number of larger events (foreshocks) prior to a mainshock that is followed by a tail of decreasing activity (aftershocks) are observed for the first time in simple models of this type. These recurrent large events occur at regular intervals, similar to characteristic earthquakes frequently observed in historic seismicity, and the time between events and their magnitude are a function of the stress dissipation parameter. The relative length of the foreshock to aftershock sequences can vary and also depends on the amount of stress dissipation in the system.

The magnitude-frequency distribution of events for various amounts of inhomogeneities (asperity sites) in the lattice is investigated in order to provide a better understanding of Gutenberg-Richter (GR) scaling. The spatiotemporal clustering of events in systems with different spatial distribution of asperities and the Thirumalai and Mountain (TM) metric behaviour, an indicator of changes in activity before the main event in the sequence, also are investigated. Accelerating Moment Release (AMR) is observed before the mainshock. The Omori law behaviour for foreshocks and aftershocks is quantified for the model in this study.

Finally, a fixed percentage of randomly distributed asperity sites were aggregated into bigger asperity blocks in order to investigate the effect of changing the spatial configuration of stronger sites. The results show that the larger block of asperities generally increases the capability of the fault system to generate larger events, but the total percentage of asperities is important as well. The increasing number of larger events is also associated with an increase in the total number of asperities in the lattice.

This work provides further evidence that the spatial and temporal patterns observed in natural seismicity may be controlled by the underlying physical properties and are not solely the result of a simple cascade mechanism and, as a result, may not be inherently unpredictable.