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
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.
|Place of Publication||Potsdam|
|Number of pages||189|
|Publication status||Published - 2006|
|Series||Scientific Technical Report|