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Title: Fracture mechanics and the evolution of seismicity in an intra-plate setting
Author: Henderson, Jeremy
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1992
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This thesis is concerned with the evolution of seismicity as described by the value of b in the Gutenberg-Richter relation, and the fractal dimension, D, of the distribution of earthquake epicenters. It is found that, in the case of two areas of California, there is a negative correlation between the b-value and D. These observations lead to the development of a one-dimensional fracture-mechanical model of seismicity which includes both negative and positive feedback processes. The fracture toughness of the fault elements is initially generated by a random walk function, but as the applied remote stress is increased, the fracture system becomes organised into a fractal set. The values of b and the fractal dimension of the crack population are measured at each step. It is found that the model predicts the observed negative correlation between the b-value and the D of earthquake epicenters. The short-range interactions described in the model give rise to a system with a long-range order, similar to current models of critical processes. It is found that in the long term there is an extremely irregular evolution of b-value. A preliminary extension of the model to two dimensions using a cellular automaton is presented. The model is further tested using a high quality dataset collected in a structurally simple area in Brazil. Two clusters of seismicity are examined in detail. It is found that in one cluster there is a strong negative correlation between b and D, whereas in the other cluster, which is characterised by a relatively small number of large events, there is a weak positive correlation. It is suggested that this positive correlation arises from processes connected with fluid migration in the crust. The fractal nature of heterogeneity in the crust in western Norway is examined by measuring coda-Q. It is found that the attenuation in this area is very low, with Q values of 1400 at 10Hz, and that the fractal dimension of lithospheric heterogeneity is low. The origin of the stresses responsible for intra-plate seismicity is studied by estimating the stress tensor in Sweden by inversion of data from earthquake fault-plane solutions. The stresses in the southern part of Sweden are found to be uniform and suggest a NW-SE compression, as predicted from present-day plate motions. In the north of the country, however, the stress varies over a short distance, and appears to have been significantly perturbed by recent deglaciation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available