@article{GassnerThomasKruegeretal.2015,
  author    = {Gassner, Alexandra and Thomas, Christine and Kr{\"u}ger, Frank and Weber, Michael H.},
  title     = {Probing the core-mantle boundary beneath Europe and Western Eurasia: A detailed study using PcP},
  series = {Physics of the earth and planetary interiors},
  volume    = {246},
  journal   = {Physics of the earth and planetary interiors},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0031-9201},
  doi       = {10.1016/j.pepi.2015.06.007},
  pages     = {9 -- 24},
  year      = {2015},
  abstract  = {We use PcP (the core reflected P phase) recordings of deep earthquakes and nuclear explosions from the Grafenberg (Germany) and NORSAR (Norway) arrays to investigate the core-mantle boundary region beneath Europe and western Eurasia. We find evidence for a previously unknown ultra-low velocity zone 600 km south-east of Moscow, located at the edge of a middle-size low shear- velocity region imaged in seismic tomography that is located beneath the Volga river region. The observed amplitude variations of PcP can be modelled by velocity reductions of P and S-waves of -5\% and -15\%, respectively, with a density increase of +15\%. Travel time delays of pre-and postcursors are indicating a thickness of about 13 km for this ultra-low velocity region (ULVZ). However, our modelling also reveals highly ambiguous amplitude variations of PcP and a reflection off the top of the anomaly for various ULVZs and topography models. Accordingly, large velocity contrasts of up to -10\% in V-P and -20\% in Vs cannot be excluded. In general, the whole Volga river region shows a complex pattern of PcP amplitudes caused most likely by CMB undulations. Further PcP probes beneath Paris, Kiev and northern Italy indicate likely normal CMB conditions, whereas the samples below Finland and the Hungary-Slovakia border yield strongly amplified PcP signals suggesting strong CMB topography effects. We evaluate the amplitude behaviour of PcP as a function of distance and several ULVZ models using the 1D reflectivity and the 2D Gauss beam method. The influence of the velocity and density perturbations is analysed as well as the anomaly thickness, the dominant period of the source wavelet and interface topographies. Strong variation of the PcP amplitude are obtained as a function of distance and of the impedance contrast. We also consider two types of topographies: undulations atop the CMB in the presence of flat ULVZs and vice versa. Where a broad range of CMB topography dimensions lead to large PcP amplitude variations, only large ULVZ undulations generate significant amplitude scattering. Consequently, this indicates that topography effects of anomalies may mask the true medium parameters as well as the ULVZ thickness. Moreover, there might be a possibility of misinterpreting the precursor as PcP, in particular for thin ULVZs. (C) 2015 Elsevier B.V. All rights reserved.},
  language  = {en}
}
@article{RoesslerKruegerRuempkeretal.2006,
  author    = {R{\"o}ßler, Dirk and Kr{\"u}ger, Frank and R{\"u}mpker, Georg and Psencik, Ivan},
  title     = {Tensile source components of swarm events in West Bohemia in 2000 by considering seismic anisotropy},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-12975},
  year      = {2006},
  abstract  = {Earthquake swarms occur frequently in West Bohemia, Central Europe. Their occurrence is correlated with and propably triggered by fluids that escape on the earth's surface near the epicentres. These fluids raise up periodically from a seemingbly deep-seated source in the upper mantle. Moment tensors for swarm events in 1997 indicate tensile faulting. However, they were determined under assumption of seismic isotropy although anisotropy can be observed. Anisotropy may obscure moment tensors and their interpretation. In 2000, more than 10,000 swarm earthquakes occurred near Novy Kostel, West Bohemia. Event triggering by fluid injection is likely. Activity lasted from 28/08 until 31/12/00 (9 phases) with maximum ML=3.2. High quality P-wave seismograms were used to retrieve the source mechanisms for 112 events between 28/08/00 and 30/10/00 using > 20 stations. We determine the source geometry using a new algorithm and different velocity models including anisotropy. From inversions of P waves we observe ML<3.2, strike-slip events on steep N-S oriented faults with additional normal or reverse components. Tensile components seem to be evident for more than 60\% of the processed swarm events in West Bohemia during the phases 1-7. Being most significant at great depths and at phases 1-4 during the swarm they are time and location dependent. Although tensile components are reduced when anisotropy is assumed they persist and seem to be important. They can be explained by pore-pressure changes due to the injection of fluids that raise up. Our findings agree with other observations e.g. correlation of fluid transport and seismicity, variations in b-value, forcing rate, and in pore pressure diffusion. Tests of our results show their significance.},
  language  = {en}
}