@article{LontsiOhrnbergerKruegeretal.2016,
  author    = {Lontsi, Agostiny Marrios and Ohrnberger, Matthias and Kr{\"u}ger, Frank and S{\´a}nchez-Sesma, Francisco Jos{\´e}},
  title     = {Combining surface-wave phase-velocity dispersion curves and full microtremor horizontal-to-vertical spectral ratio for subsurface sedimentary site characterization},
  series = {Interpretation : a journal of subsurface characterization},
  volume    = {4},
  journal   = {Interpretation : a journal of subsurface characterization},
  publisher = {Society of Exploration Geophysicists},
  address   = {Tulsa},
  issn      = {2324-8858},
  doi       = {10.1190/INT-2016-0021.1},
  pages     = {SQ41 -- SQ49},
  year      = {2016},
  abstract  = {We compute seismic velocity profiles by a combined inversion of surface-wave phase-velocity dispersion curves together with the full spectrum of the microtremor horizontal-to-vertical (H/V) spectral ratio at two sediment-covered sites in Germany. The sediment deposits are approximately 100 m thick at the first test site and approximately 400 m thick at the second test site. We have used an extended physical model based on the diffuse wavefield assumption for the interpretation of the observed microtremor H/V spectral ratio. The extension includes the interpretation of the microtremor H/V spectral ratio observed at depth (in boreholes). This full-wavefield approach accounts for the energy contribution from the body and surface waves, and thus it allows for inverting the properties of the shallow subsurface. We have obtained the multimode phase velocity dispersion curves from an independent study, and a description of the extracted branches and their interpretation was developed. The inversion results indicate that the combined approach using seismic ambient noise and actively generated surface-wave data will improve the accuracy of the reconstructed near-surface velocity model, a key step in microzonation, geotechnical engineering, seismic statics corrections, and reservoir imaging.},
  language  = {en}
}
@article{LangeBedfordMorenoetal.2014,
  author    = {Lange, Dietrich and Bedford, J. R. and Moreno, M. and Tilmann, F. and B{\´a}ez, Juan Carlos and Bevis, M. and Kr{\"u}ger, Frank},
  title     = {Comparison of postseismic afterslip models with aftershock seismicity for three subduction-zone earthquakes: Nias 2005, Maule 2010 and Tohoku 2011},
  series = {Geophysical journal international},
  volume    = {199},
  journal   = {Geophysical journal international},
  number    = {2},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0956-540X},
  doi       = {10.1093/gji/ggu292},
  pages     = {784 -- 799},
  year      = {2014},
  abstract  = {We focus on the relation between seismic and total postseismic afterslip following the Maule M-w 8.8 earthquake on 2010 February 27 in central Chile. First, we calculate the cumulative slip released by aftershock seismicity. We do this by summing up the aftershock regions and slip estimated from scaling relations. Comparing the cumulative seismic slip with afterslip modelswe showthat seismic slip of individual aftershocks exceeds locally the inverted afterslip model from geodetic constraints. As the afterslip model implicitly contains the displacements from the aftershocks, this reflects the tendency of afterslip models to smear out the actual slip pattern. However, it also suggests that locally slip for a number of the larger aftershocks exceeds the aseismic slip in spite of the fact that the total equivalent moment of the afterslip exceeds the cumulative moment of aftershocks by a large factor. This effect, seen weakly for the Maule 2010 and also for the Tohoku 2011 earthquake, can be explained by taking into account the uncertainties of the seismicity and afterslip models. In spite of uncertainties, the hypocentral region of the Nias 2005 earthquake is suggested to release a large fraction of moment almost purely seismically. Therefore, these aftershocks are not driven solely by the afterslip but instead their slip areas have probably been stressed by interseismic loading and the mainshock rupture. In a second step, we divide the megathrust of the Maule 2010 rupture into discrete cells and count the number of aftershocks that occur within 50 km of the centre of each cell as a function of time. We then compare this number to a time-dependent afterslip model by defining the 'afterslip to aftershock ratio' (ASAR) for each cell as the slope of the best fitting line when the afterslip at time t is plotted against aftershock count. Although we find a linear relation between afterslip and aftershocks for most cells, there is significant variability in ASAR in both the downdip and along-strike directions of the megathrust. We compare the spatial distribution of ASAR with the spatial distribution of seismic coupling, coseismic slip and Bouguer gravity anomaly, and in each case we find no significant correlation.},
  language  = {en}
}
@article{KastleWeberKrueger2017,
  author    = {Kastle, Emanuel D. and Weber, Michael and Kr{\"u}ger, Frank},
  title     = {Complex Deep Structure of the African Low-Velocity Zone},
  series = {Bulletin of the Seismological Society of America},
  volume    = {107},
  journal   = {Bulletin of the Seismological Society of America},
  publisher = {Seismological Society of America},
  address   = {Albany},
  issn      = {0037-1106},
  doi       = {10.1785/0120160215},
  pages     = {1688 -- 1703},
  year      = {2017},
  abstract  = {We use recently deployed seismological arrays in Africa to sample a 2D cross section through the mantle down to the core-mantle boundary (CMB). By making use of travel-time residuals of S, ScS, and SKS phases, a new shear-velocity model of the African low-velocity zone (ALVZ) is derived. Our model suggests between 1.2\% shear-velocity reduction at the top and 5\% at the bottom with respect to 1D reference models. The average reduction over the whole low-velocity zone (LVZ) amounts to 2\% in the presented model and is therefore about twice as strong as values found in global tomographic models. The top of the LVZ reaches up to 1200-km depth, and its lateral extent at the CMB is about 35 degrees. We propose the existence of a gap of 300 km, splitting the structure into two blocks. Our results are based on remarkable differences in SK(K) S travel-time residuals over a few degrees distance. The complexity of the structure could provide a key to an improved understanding of the deep-mantle LVZ dynamics and composition by comparison to geodynamic models. The gap in the model might suggest that the 2D cross section is cutting through a 3D indentation in the boundary of the ALVZ but may also be interpreted as a sign of two individual plumes, rather than one large homogeneous upwelling.},
  language  = {en}
}
@article{CzubaGradMjeldeetal.2011,
  author    = {Czuba, Wojciech and Grad, Marek and Mjelde, Rolf and Guterch, Aleksander and Libak, Audun and Kr{\"u}ger, Frank and Murai, Yoshio and Schweitzer, Johannes},
  title     = {Continent-ocean-transition across a trans-tensional margin segment: off Bear Island, Barents Sea},
  series = {Geophysical journal international},
  volume    = {184},
  journal   = {Geophysical journal international},
  number    = {2},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  organization    = {IPY Project Grp},
  issn      = {0956-540X},
  doi       = {10.1111/j.1365-246X.2010.04873.x},
  pages     = {541 -- 554},
  year      = {2011},
  abstract  = {P>A 410 km long Ocean Bottom Seismometer profile spanning from the Bear Island, Barents Sea to oceanic crust formed along the Mohns Ridge has been modelled by use of ray-tracing with regard to observed P-waves. The northeastern part of the model represents typical continental crust, thinned from ca. 30 km thickness beneath the Bear Island to ca. 13 km within the Continent-Ocean-Transition. Between the Hornsund FZ and the Kn circle divide legga Fault, a 3-4 km thick sedimentary basin, dominantly of Permian/Carboniferous age, is modelled beneath the ca. 1.5 km thick layer of volcanics (Vestbakken Volcanic Province). The P-wave velocity in the 3-4 km thick lowermost continental crust is significantly higher than normal (ca. 7.5 km s-1). We interpret this layer as a mixture of mafic intrusions and continental crystalline blocks, dominantly related to the Paleocene-Early Eocene rifting event. The crystalline portion of the crust within the south-western part of the COT consists of a ca. 30 km wide and ca. 6 km thick high-velocity (7.3 km s-1) body. We interpret the body as a ridge of serpentinized peridotites. The magmatic portion of the ocean crust accreted along the Knipovich Ridge from continental break-up at ca. 35 Ma until ca. 20 Ma is 3-5 km thicker than normal. We interpret the increased magmatism as a passive response to the bending of this southernmost part of the Knipovich Ridge. The thickness of the magmatic portion of the crust formed along the Mohns Ridge at ca. 20 Ma decreases to ca. 3 km, which is normal for ultra slow spreading ridges.},
  language  = {en}
}
@article{DahmCescaHainzletal.2015,
  author    = {Dahm, Torsten and Cesca, Simone and Hainzl, Sebastian and Braun, Thomas and Kr{\"u}ger, Frank},
  title     = {Discrimination between induced, triggered, and natural earthquakes close to hydrocarbon reservoirs: A probabilistic approach based on the modeling of depletion-induced stress changes and seismological source parameters},
  series = {Journal of geophysical research : Solid earth},
  volume    = {120},
  journal   = {Journal of geophysical research : Solid earth},
  number    = {4},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {2169-9313},
  doi       = {10.1002/2014JB011778},
  pages     = {2491 -- 2509},
  year      = {2015},
  abstract  = {Earthquakes occurring close to hydrocarbon fields under production are often under critical view of being induced or triggered. However, clear and testable rules to discriminate the different events have rarely been developed and tested. The unresolved scientific problem may lead to lengthy public disputes with unpredictable impact on the local acceptance of the exploitation and field operations. We propose a quantitative approach to discriminate induced, triggered, and natural earthquakes, which is based on testable input parameters. Maxima of occurrence probabilities are compared for the cases under question, and a single probability of being triggered or induced is reported. The uncertainties of earthquake location and other input parameters are considered in terms of the integration over probability density functions. The probability that events have been human triggered/induced is derived from the modeling of Coulomb stress changes and a rate and state-dependent seismicity model. In our case a 3-D boundary element method has been adapted for the nuclei of strain approach to estimate the stress changes outside the reservoir, which are related to pore pressure changes in the field formation. The predicted rate of natural earthquakes is either derived from the background seismicity or, in case of rare events, from an estimate of the tectonic stress rate. Instrumentally derived seismological information on the event location, source mechanism, and the size of the rupture plane is of advantage for the method. If the rupture plane has been estimated, the discrimination between induced or only triggered events is theoretically possible if probability functions are convolved with a rupture fault filter. We apply the approach to three recent main shock events: (1) the M-w 4.3 Ekofisk 2001, North Sea, earthquake close to the Ekofisk oil field; (2) the M-w 4.4 Rotenburg 2004, Northern Germany, earthquake in the vicinity of the Sohlingen gas field; and (3) the M-w 6.1 Emilia 2012, Northern Italy, earthquake in the vicinity of a hydrocarbon reservoir. The three test cases cover the complete range of possible causes: clearly human induced, not even human triggered, and a third case in between both extremes.},
  language  = {en}
}
@article{ScherbaumKruegerWeber1997,
  author    = {Scherbaum, Frank and Kr{\"u}ger, Frank and Weber, Michael H.},
  title     = {Double beam imaging : mapping lower mantle heterogeneities using combinations of source and receiver arrays},
  year      = {1997},
  language  = {en}
}
@article{KaramzadehToularoudHeimannDahmetal.2020,
  author    = {Karamzadeh Toularoud, Nasim and Heimann, Sebastian and Dahm, Torsten and Kr{\"u}ger, Frank},
  title     = {Earthquake source arrays},
  series = {Geophysical journal international},
  volume    = {221},
  journal   = {Geophysical journal international},
  number    = {1},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0956-540X},
  doi       = {10.1093/gji/ggaa002},
  pages     = {352 -- 370},
  year      = {2020},
  abstract  = {A collection of earthquake sources recorded at a single station, under specific conditions, are considered as a source array (SA), that is interpreted as if earthquake sources originate at the station location and are recorded at the source location. Then, array processing methods, that is array beamforming, are applicable to analyse the recorded signals. A possible application is to use source array multiple event techniques to locate and characterize near-source scatterers and structural interfaces. In this work the aim is to facilitate the use of earthquake source arrays by presenting an automatic search algorithm to configure the source array elements. We developed a procedure to search for an optimal source array element distribution given an earthquake catalogue including accurate origin time and hypocentre locations. The objective function of the optimization process can be flexibly defined for each application to ensure the prerequisites (criteria) of making a source array. We formulated four quantitative criteria as subfunctions and used the weighted sum technique to combine them in one single scalar function. The criteria are: (1) to control the accuracy of the slowness vector estimation using the time domain beamforming method, (2) to measure the waveform coherency of the array elements, (3) to select events with lower location error and (4) to select traces with high energy of specific phases, that is, sp- or ps-phases. The proposed procedure is verified using synthetic data as well as real examples for the Vogtland region in Northwest Bohemia. We discussed the possible application of the optimized source arrays to identify the location of scatterers in the velocity model by presenting a synthetic test and an example using real waveforms.},
  language  = {en}
}
@misc{DonnerStreckerRoessleretal.2009,
  author    = {Donner, Stefanie and Strecker, Manfred and R{\"o}ßler, Dirk and Ghods, Abdolreza and Kr{\"u}ger, Frank and Landgraf, Angela and Ballato, Paolo},
  title     = {Earthquake source models for earthquakes in Northern Iran},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-32581},
  year      = {2009},
  abstract  = {The complex system of strike-slip and thrust faults in the Alborz Mountains, Northern Iran, are not well understood yet. Mainly structural and geomorphic data are available so far. As a more extensive base for seismotectonic studies and seismic hazard analysis we plan to do a comprehensive seismic moment tensor study also from smaller magnitudes (M < 4.5) by developing a new algorithm. Here, we present first preliminary results.},
  language  = {en}
}
@misc{KriegerowskiCescaOhrnbergeretal.2018,
  author    = {Kriegerowski, Marius and Cesca, Simone and Ohrnberger, Matthias and Dahm, Torsten and Kr{\"u}ger, Frank},
  title     = {Event couple spectral ratio Q method for earthquake clusters},
  series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe},
  number    = {683},
  doi       = {10.25932/publishup-42602},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-426029},
  pages     = {12},
  year      = {2018},
  abstract  = {We develop an amplitude spectral ratio method for event couples from clustered earthquakes to estimate seismic wave attenuation (Q-1) in the source volume. The method allows to study attenuation within the source region of earthquake swarms or aftershocks at depth, independent of wave path and attenuation between source region and surface station. We exploit the high-frequency slope of phase spectra using multitaper spectral estimates. The method is tested using simulated full wave-field seismograms affected by recorded noise and finite source rupture. The synthetic tests verify the approach and show that solutions are independent of focal mechanisms but also show that seismic noise may broaden the scatter of results. We apply the event couple spectral ratio method to northwest Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth. Our method indicates a strong anomaly of high attenuation in the source region of the swarm with an averaged attenuation factor of Qp < 100. The application to S phases fails due to scattered P-phase energy interfering with S phases. The Qp anomaly supports the common hypothesis of highly fractured and fluid saturated rocks in the source region of the swarms in northwest Bohemia. However, high temperatures in a small volume around the swarms cannot be excluded to explain our observations.},
  language  = {en}
}
@article{KriegerowskiCescaOhrnbergeretal.2019,
  author    = {Kriegerowski, Marius and Cesca, Simone and Ohrnberger, Matthias and Dahm, Torsten and Kr{\"u}ger, Frank},
  title     = {Event couple spectral ratio Q method for earthquake clusters},
  series = {Solid Earth},
  journal   = {Solid Earth},
  number    = {10},
  publisher = {Copernicus Publications},
  address   = {G{\"o}ttingen},
  issn      = {1869-9529},
  doi       = {10.5194/se-10-317-2019},
  pages     = {317 -- 328},
  year      = {2019},
  abstract  = {We develop an amplitude spectral ratio method for event couples from clustered earthquakes to estimate seismic wave attenuation (Q-1) in the source volume. The method allows to study attenuation within the source region of earthquake swarms or aftershocks at depth, independent of wave path and attenuation between source region and surface station. We exploit the high-frequency slope of phase spectra using multitaper spectral estimates. The method is tested using simulated full wave-field seismograms affected by recorded noise and finite source rupture. The synthetic tests verify the approach and show that solutions are independent of focal mechanisms but also show that seismic noise may broaden the scatter of results. We apply the event couple spectral ratio method to northwest Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth. Our method indicates a strong anomaly of high attenuation in the source region of the swarm with an averaged attenuation factor of Qp < 100. The application to S phases fails due to scattered P-phase energy interfering with S phases. The Qp anomaly supports the common hypothesis of highly fractured and fluid saturated rocks in the source region of the swarms in northwest Bohemia. However, high temperatures in a small volume around the swarms cannot be excluded to explain our observations.},
  language  = {en}
}