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Title: Joint investigation of source parameters and seismic anisotropy using microearthquakes in Greece, Arkansas and Northeast Brazil
Author: Karnassopoulou, Aphrodite
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1996
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Initially, microearthquakes associated with an active continental region in Central Greece are studied. The distribution of epicentres is associated with the main E-W trending normal fault in the locality, and their scatter confirms a complicated velocity structure. Although shear-wave splitting is identified, it is difficult to interpret because the shear waves are frequently obscured by extensive P-wave coda. The incorporation of shear waves in the focal mechanism determination augments the available observations in the case of small local networks, but, it imposes difficulties because shear waves are sensitive to the structure along the raypath and in particular anisotropy. A procedure is developed in order to identify shear-wave splitting by testing the compatibility of the observed shear wave polarisations with those generated by a double-couple source and, if splitting exists, to remove its effect in order to reconstruct the polarisation of the first split shear wave at the source, and hence to determine valid focal mechanisms for single events. The procedure is applied to two datasets from intraplate regions, Arkansas-USA and NE Brazil, in order to exploit the simplicity of seismograms typical of such regions. The simple waveforms from the Arkansas sequence reveal that two well-distributed shear waves together with P waves can be sufficient to determine well-constrained focal mechanisms, but more may be required in order to discriminate between their interpretation as unsplit and split shear waves. However, the proximity of raypaths in this case of swarm-type activity offers the opportunity to extend the interpretation derived from one event to other events which may have too few observations to confirm anisotropy from the focal mechanism procedure. The results reveal strike-slip focal mechanisms with the P axis oriented NE-SW which is consistent with the regional stress field. The NE Brazil dataset, reveals shallow seismicity concentrated in three tight clusters. Shear-wave splitting is observed with the fast shear wave aligned approximately N-S. This agrees with previous observations at a single three-component station, but is inconsistent with conventional interpretation in terms of fluid filled cracks aligned with the known axis of maximum compressive stress; this would require an E-W alignment. It may be that the anisotropy is instead intrinsic to the rock fabric, possibly associated with known north-south Precambrian ductile shear zones. One well-constrained focal mechanism was obtained assuming the validity of the shear-wave splitting interpretation. The mechanism reveals an almost horizontal strike-slip fault plane, trending NW-SE, with a N-S oriented P axis, which contradicts other studies which reveal an E-W oriented P axis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available