Inversion for local stress field inhomogeneities

Abstract

In this thesis, the 1997 Vogtland/NW-Bohemia swarm has been selected for the analysis of inhomogeneities in the stress field because two predominant nearly perpendicular flat zones of seismicity are visible in the hypocentre distribution implying inhomogeneities in the stress field. This is unusual compared to other swarms originating from this area. An existing dataset of waveform data, P- and S-phase picks, and master event locations has been analysed regarding similarity of waveforms, location refinement, and estimation of relative moment tensors. The latter are used together with a regional dataset of 50 single focal mechanisms and 125 focal mechanisms from the 2000 hydraulic fracturing experiment at the KTB for an estimate of the regional homogeneous and the locally inhomogeneous stress field.

An automated processing procedure consisting of coherence analysis,precise relocation, relative moment tensor inversion, and stress trajectory determination has been set up. The coherence analysis has been successfully applied using a new method that uses three component seismograms. 457 events are separated into 13 multiplets of similar waveforms of at least size 8. Another result are precise relative arrival time measurements which are fed into the precise relocation program "hypoDD". Two nearly perpendicular structures are found in the hypocentre distribution. 352 moment tensors are estimated using a relative moment tensor inversion. Three different algorithms to distinguish between fault plane and auxiliary plane are successfully applied to them. A regional homogeneous stress inversion using the focal mechanisms of the single events and selected events from the other datasets has been applied yielding (azimuth/plunge) sigma1=(147°/9°), sigma2=(10°/78°), and sigma3=(238°/8°). The dense population of moment tensors for the 1997 swarm inside a volume of aproximately 1 km^3 has been systematically subdivided using a moving box technique. A locally homogeneous stress inversion has been applied to each subset that consists of at least 10 measurements. The resulting deviatoric stress tensors are arranged on a regular grid and their components are smoothed using Non Uniform Rational B-Splines (NURBS) depending on three spatial parameters. The resulting sigma1 and sigma3 trajectories are visualised by projecting part of them into the The plane structures derived from the hypocentre distribution are consistent with the fault planes that have been selected from moment tensors and with the patterns found in the stress trajectories. Neutral points regarding the maximum and minimum horizontal stresses, sigmaH and sigmah, respectively,are identified. The results of this work support the model of fluid induced seismicity and migrating fluids.