Electronic Theses and Dissertations

Author

Khurram Aslam

Date

2019

Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Committee Chair

Eric Daub

Committee Member

Chris Cramer

Committee Member

Eunseo Choi

Committee Member

Randel Cox

Abstract

A large earthquake triggers earthquakes on many nearby faults. Most of the triggered earthquakes (i.e. aftershocks) can be explained by the static stress increase in the region where they occur. Some aftershocks also occur in the regions of static stress decrease or stress shadows. The current physical models of aftershock occurrence are not able to explain aftershocks that are observed in stress shadows. The static stress changes, following an earthquake, are calculated using slip that occurs on the main fault. The source inversions, which calculate these slips, are not able to resolve finer scale details of slip due to their coarser spatial resolution. The finer scale details of slip influence finer static stress changes, which plays an important role in the production of smaller aftershocks. These finer details of stresses may be able to better explain the occurrence of aftershocks in stress shadows. In this study, we perform dynamic earthquake rupture simulations of large earthquakes. This modeling resolves the finer scale details of slip based on elasticity and friction and hence has the ability to predict the spatial distribution of slip and stress changes. We perform numerous two dimensional (2D) earthquake rupture simulations on rough strike slip faults assuming elastic and plastic off-fault material properties. We consider many different realizations of a self-affine rough fault profile. We output the static stress changes in the off-fault medium from our simulations and use these to calculate the Coulomb failure function (CFF) in the region surrounding the fault. We use similar and variable orientations for receiver faults planes to calculate CFF values. The similar receiver fault plane orientations are chosen to be parallel to the overall trace of the main fault, while the variable receiver fault orientations are determined using the angle at which plastic shear strain is maximum. Our results show that the stresses are highly complex in the region close to the fault. This complexity reduces as the distance from the fault increases. We conclude that the stress complexity observed in the near-fault region is due to roughness of the fault profile. The complexity of stresses in the near-fault region causes the CFF to be highly heterogeneous in the near-fault region. We observe many positive CFF zones within negative CFF zones in the near-fault region. We believe that these are the potential locations of aftershocks observed in stress shadows. The areas where they appear would be seen as stress shadows in typical static stress change calculations due to insufficient resolution of the fault slip. Furthermore, we observe that the overall trend of the CFF with distance remains similar either assuming elastic or plastic off-fault material properties. In the near-fault region, we observe many more positive CFF zones when we calculate CFF values using variable receiver fault orientations. Our results suggest that the spatial aftershock distribution surrounding a fault is controlled by both stress heterogeneity as well as the co-seismic damage zone complexity. Comparing our model rupture areas of positive CFF zones with rupture areas of aftershocks and preshocks from relocated earthquake catalogs of Northern and Southern California, we conclude that the stresses in the near-fault region are dominated by the fault roughness effects throughout the seismic cycle. We model the inter-seismic period of a complex rupture by running a quasi-static model (LTM) initialized with stresses from dynamic earthquake rupture model. Our results show that the geometrical bends of the fault profile cause the plastic deformation to be localized in the co-seismic phase, which acts as a seed for the development of new shear features in the inter-seismic phase.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest

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