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Title: Assessing the role of post-seismic viscoelastic relaxations in earthquake triggering
Author: Sunbul, Fatih
ISNI:       0000 0004 7654 0724
Awarding Body: Ulster University
Current Institution: Ulster University
Date of Award: 2017
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Analyses of deformations associated with earthquakes are calculated in terms of plate motions and co-seismic stress changes, which assume the crust responds as an elastic medium. However, the lower crust and region of ductile mantle behaves viscoealastically over much longer time scales. In other words, lithospheric stress change following earthquakes is time dependent process. In order to draw a complete picture of total deformation over a seismic cycle, we cannot ignore the considerable influence of the viscoelastic relaxation processes. The objective of this thesis is to assess the role of post-seismic viscoelastic relaxation processes following earthquakes. First, I investigate the post-seismic viscoelastic flow in the lower crust and upper mantle of large earthquakes in eastern Turkey, during the 19th and 20th century. Using three possible rheological models, the viscoelastic, post-seismic deformation is analysed to assess the extent to which these events influence the velocity fields at GPS sites in the region. Models show that the post-seismic signal currently contributes to the observed deformation in the eastern part of the North Anatolian fault and northern and middle parts of the East Anatolian Fault Zone, primarily due to the long-lasting effect of the Ms 7.9 1939 earthquake. Study results identify a post-seismic signal in the region and could contribute to between 3% and 25% of the observed GPS measurements. Next, in order to assess the importance of the post-seismic viscoelastic relaxations over the last seismic cycle, I investigate stress accumulation over the East Anatolian Fault Zone by modelling the stress changes associated with 16 large (M>6.8) earthquakes that occurred in the region since 1822. Two areas of increased stress in the region are identified; the first in a fault segment that runs from the southern part of the city of Kahramanmaras to the southern part of the city of Malatya (KM Segment) and the second is the Yedisu segment at the eastern terminus of the North Anatolian Fault Zone. It is also observed that the stress accumulation associated with the post-seismic viscoelastic stress equals or larger than the co-seismic stress values in these segments. Finally, I assess the extent of time dependent viscoelastic post-seismic stress influences on the static shear stress change on the rate of triggered events that located in ± 2° after a main shock. Receiver faults that have calculated shear stress changes (on their nodal planes) between ±0.01 and ±1 MPa are considered as potentially triggered by the source earthquakes. The stress changes associated with 244 main shocks (M>7.0) from Global Centroid Moment Tensor Catalogue are modelled. A 10-year time window after the main shocks is used for the subsequent analysis to allow the numbers of aftershocks to decaying to the background seismicity rate. Regional variations in the rate of triggered aftershocks are observed ranging from 35% (Mediterranean and Tibet) to 65% (Western Pacific, Sumatra, Central and South America). When time-dependent viscoelastic post-seismic effects are incorporated into the calculations, the global rate remains unchanged. However, the total number of triggered earthquakes in areas of increased positive and negative stresses rises by up to 13% and 12% respectively. After excluding earthquakes whose Moment Tensor solutions having large uncertainties from the catalogue, the global rate of positively triggered events has increased considerably by 3%, which illustrates 63% of potentially triggered events are associated with shear stress increase due to prior events while 37% of events with shear stress decrease.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available