Retrieval of Earthquake Source Parameters in Inhomogeneous Anisotropic Media with Application to Swarm Events in WestBohemia in 2000

Dirk Rössler

Research output: Book/ReportPh.D. thesis

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Abstract

Currently used standard techniques for the retrieval of earthquake source mecha- nisms assume isotropic rock properties. By means of moment tensors, equivalent forces acting at the source are used to explain the radiated wavefield. Usually, shear sources have been observed. Besides, in certain areas such as in West Bohemia at the German- Czech border and other volcanic areas earthquakes have been reported that apparently comprise additional volumetric changes (tensile earthquakes). In contrast to the assumption of isotropy, anisotropy, i.e. directional dependence of elastic parameters, has been often observed as a property of the earth’s crust and mantle, such as in West Bohemia. In comparison to isotropic media, anisotropy causes changes in wave amplitudes and polarisations, and shear-wave splitting. In this work, effects of seismic anisotropy on true or apparent tensile source compo- nents of earthquakes are investigated. Earthquake source parameters are determined considering anisotropy. It is shown that moment tensors and radiation patterns due to shear sources in anisotropic media may be similar to those of tensile sources in isotropic media. Conversely, similarities between tensile earthquakes in anisotropic rocks and shear sources in isotropic media may exist. As a consequence, the interpretation of tensile source components is ambiguous. The effects that are due to anisotropy depend on the orientation of the earthquake source and the degree of anisotropy. In addition, the moment of an earthquake is influenced by anisotropy. The orientation of fault planes can be reliably determined even if isotropy instead of anisotropy is assumed and if the spectra of the compressional waves are used. Greater difficulties arise when the spectra of shear waves are additionally included. If anisotropy is restricted to the region in the vicinity of the source, the anisotropic elastic properties may be determined from retrieved moment tensors of differently oriented sources. Examples show that the tensile source components determined for events in West Bohemia in 1997 can only partly be attributed to the effects of moderate anisotropy. Furthermore, moment tensors determined earlier for earthquakes induced at the Ger- man Continental Deep Drilling Program (KTB) were reinterpreted under assumption of anisotropic rock properties near the borehole. The events can be consistently identi- fied as shear sources, although their moment tensors comprise tensile components that are considered to be apparent. These results show the necessity to consider anisotropy if a unique determination of tensile source parameters is desired. Therefore, a new inversion algorithm has been de- veloped, tested, and successfully applied to 112 earthquakes. Their source mechanisms have been retrieved using isotropic and anisotropic velocity models. The earthquakes occurred during the most recent intense swarm episode in West Bohemia in 2000. The determined local magnitudes are in the range between 1.6 and 3.2. Fault-plane so- lutions are similar to each other and characterised by left-lateral faulting on steeply dipping, roughly North-South oriented rupture planes. Their dip angles decrease above a depth of about 8.4 km. Together with the moment tensor the slip inclination, i.e. the angle between the direction of rupture and the normal to the fault plane, is retrieved as an important parameter to quantify volume changes at the source. Tensile source components indi- cating positive volume changes are found for more than 60% of the considered earth- quakes. Their size depends on source time and location. They are significant at the beginning of the swarm and at depths below 8.4 km but they decrease in importance later in the course of the swarm. Determined principle stress axes include P axes striking Northeast and T axes striking Southeast. They resemble those found earlier in Central Europe. However, depth-dependence in plunge is observed. Plunge angles of the P axes decrease gradually from 50◦ towards shallow angles with increasing depth. In contrast, the plunge angles of the T axes change rapidly from about 8◦ above a depth of 8.4 km to 21◦ below this depth. By this thesis, spatial and temporal variations in tensile source components and stress conditions have been reported for the first time for swarm earthquakes in West Bohemia in 2000. They also persist, when anisotropy is assumed and can be explained by intrusion of fluids into the opened cracks during tensile faulting.
Original languageEnglish
Place of PublicationPotsdam
PublisherGeoForschungsZentrum
Number of pages189
Publication statusPublished - 2006
Externally publishedYes
SeriesScientific Technical Report
ISSN1610-0956

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