@article{KoehlerOhrnbergerScherbaum2009, author = {Koehler, Andreas and Ohrnberger, Matthias and Scherbaum, Frank}, title = {Unsupervised feature selection and general pattern discovery using Self-Organizing Maps for gaining insights into the nature of seismic wavefields}, issn = {0098-3004}, doi = {10.1016/j.cageo.2009.02.004}, year = {2009}, abstract = {This study presents an unsupervised feature selection and learning approach for the discovery and intuitive imaging of significant temporal patterns in seismic single-station or network recordings. For this purpose, the data are parametrized by real-valued feature vectors for short time windows using standard analysis tools for seismic data, such as frequency-wavenumber, polarization, and spectral analysis. We use Self-Organizing Maps (SOMs) for a data-driven feature selection, visualization and clustering procedure, which is in particular suitable for high-dimensional data sets. Our feature selection method is based on significance testing using the Wald-Wolfowitz runs test for-individual features and on correlation hunting with SOMs in feature subsets. Using synthetics composed of Rayleigh and Love waves and real-world data, we show the robustness and the improved discriminative power of that approach compared to feature subsets manually selected from individual wavefield parametrization methods. Furthermore, the capability of the clustering and visualization techniques to investigate the discrimination of wave phases is shown by means of synthetic waveforms and regional earthquake recordings.}, language = {en} } @article{DiGiulioSavvaidisOhrnbergeretal.2012, author = {Di Giulio, Giuseppe and Savvaidis, Alexandros and Ohrnberger, Matthias and Wathelet, Marc and Cornou, Cecile and Knapmeyer-Endrun, Brigitte and Renalier, Florence and Theodoulidis, Nikos and Bard, Pierre-Yves}, title = {Exploring the model space and ranking a best class of models in surface-wave dispersion inversion application at European strong-motion sites}, series = {Geophysics}, volume = {77}, journal = {Geophysics}, number = {3}, publisher = {Society of Exploration Geophysicists}, address = {Tulsa}, issn = {0016-8033}, doi = {10.1190/GEO2011-0116.1}, pages = {B147 -- B166}, year = {2012}, abstract = {The inversion of surface-wave dispersion curve to derive shear-wave velocity profile is a very delicate process dealing with a nonunique problem, which is strongly dependent on the model space parameterization. When independent and reliable information is not available, the selection of most representative models within the ensemble produced. by the inversion is often difficult. We implemented a strategy in the inversion of dispersion curves able to investigate the influence of the parameterization of the model space and to select a "best" class of models. We analyzed surface-wave dispersion curves measured at 14 European strong..-motion sites within the NERIES EC-Project. We focused on the inversion task exploring the model space by means of four distinct pararneterization classes composed of layers progressively added over a half-space. The classes differ in the definition of the shear-wave velocity profile; we considered models with uniform velocity as well as models with increasing velocity with depth. At each site and for each model parameterization, we performed an extensive surface-wave inversion (200,100 models for five seeds) using the conditional neighborhood algorithm. We addressed the model evaluation following the corrected Akaike's information criterion (AlCc) that combines the concept of misfit to the number of degrees of freedom of the system. The misfit was computed as least-squares estimation between theoretical and observed dispersion curve. The model complexity was accounted in a penalty term by AlCc. By applying such inversion strategy on 14 strong-motion sites, we found that the best parameterization of the model space is mostly three to four layers over a half-space: where the shear-wave velocity of the uppermost layers can follow uniform or power-law dependence with depth. The shear-wave velocity profiles derived by inversion agree with shear-wave velocity profiles provided by borehole surveys at approximately 80\% of the sites.}, language = {en} } @article{HobigerCornouWatheletetal.2013, author = {Hobiger, M. and Cornou, C. and Wathelet, M. and Di Giulio, G. and Knapmeyer-Endrun, B. and Renalier, F. and Bard, Pierre-Yves and Savvaidis, Alexandros and Hailemikael, S. and Le Bihan, N. and Ohrnberger, Matthias and Theodoulidis, N.}, title = {Ground structure imaging by inversions of Rayleigh wave ellipticity sensitivity analysis and application to European strong-motion sites}, series = {Geophysical journal international}, volume = {192}, journal = {Geophysical journal international}, number = {1}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0956-540X}, doi = {10.1093/gji/ggs005}, pages = {207 -- 229}, year = {2013}, abstract = {The knowledge of the local soil structure is important for the assessment of seismic hazards. A widespread, but time-consuming technique to retrieve the parameters of the local underground is the drilling of boreholes. Another way to obtain the shear wave velocity profile at a given location is the inversion of surface wave dispersion curves. To ensure a good resolution for both superficial and deeper layers, the used dispersion curves need to cover a wide frequency range. This wide frequency range can be obtained using several arrays of seismic sensors or a single array comprising a large number of sensors. Consequently, these measurements are time-consuming. A simpler alternative is provided by the use of the ellipticity of Rayleigh waves. The frequency dependence of the ellipticity is tightly linked to the shear wave velocity profile. Furthermore, it can be measured using a single seismic sensor. As soil structures obtained by scaling of a given model exhibit the same ellipticity curve, any inversion of the ellipticity curve alone will be ambiguous. Therefore, additional measurements which fix the absolute value of the shear wave velocity profile at some points have to be included in the inversion process. Small-scale spatial autocorrelation measurements or MASW measurements can provide the needed data. Using a theoretical soil structure, we show which parts of the ellipticity curve have to be included in the inversion process to get a reliable result and which parts can be omitted. Furthermore, the use of autocorrelation or high-frequency dispersion curves will be highlighted. The resulting guidelines for inversions including ellipticity data are then applied to real data measurements collected at 14 different sites during the European NERIES project. It is found that the results are in good agreement with dispersion curve measurements. Furthermore, the method can help in identifying the mode of Rayleigh waves in dispersion curve measurements.}, language = {en} } @article{RiggelsenOhrnberger2014, author = {Riggelsen, Carsten and Ohrnberger, Matthias}, title = {A machine learning approach for improving the detection capabilities at 3C Seismic Stations}, series = {Pure and applied geophysics}, volume = {171}, journal = {Pure and applied geophysics}, number = {3-5}, publisher = {Springer}, address = {Basel}, issn = {0033-4553}, doi = {10.1007/s00024-012-0592-3}, pages = {395 -- 411}, year = {2014}, abstract = {We apply and evaluate a recent machine learning method for the automatic classification of seismic waveforms. The method relies on Dynamic Bayesian Networks (DBN) and supervised learning to improve the detection capabilities at 3C seismic stations. A time-frequency decomposition provides the basis for the required signal characteristics we need in order to derive the features defining typical "signal" and "noise" patterns. Each pattern class is modeled by a DBN, specifying the interrelationships of the derived features in the time-frequency plane. Subsequently, the models are trained using previously labeled segments of seismic data. The DBN models can now be compared against in order to determine the likelihood of new incoming seismic waveform segments to be either signal or noise. As the noise characteristics of seismic stations varies smoothly in time (seasonal variation as well as anthropogenic influence), we accommodate in our approach for a continuous adaptation of the DBN model that is associated with the noise class. Given the difficulty for obtaining a golden standard for real data (ground truth) the proof of concept and evaluation is shown by conducting experiments based on 3C seismic data from the International Monitoring Stations, BOSA and LPAZ.}, language = {en} } @article{GarofaloFotiHollenderetal.2016, author = {Garofalo, F. and Foti, S. and Hollender, F. and Bard, Pierre-Yves and Cornou, C. and Cox, B. R. and Ohrnberger, Matthias and Sicilia, D. and Asten, M. and Di Giulio, G. and Forbriger, T. and Guillier, B. and Hayashi, K. and Martin, A. and Matsushima, Satoru and Mercerat, D. and Poggi, V. and Yamanaka, H.}, title = {InterPACIFIC project: Comparison of invasive and non-invasive methods for seismic site characterization. Part I: Intra-comparison of surface wave methods}, series = {Soil Dynamics and Earthquake Engineering}, volume = {82}, journal = {Soil Dynamics and Earthquake Engineering}, publisher = {Elsevier}, address = {Oxford}, issn = {0267-7261}, doi = {10.1016/j.soildyn.2015.12.010}, pages = {222 -- 240}, year = {2016}, abstract = {The main scope of the InterPACIFIC (Intercomparison of methods for site parameter and velocity profile characterization) project is to assess the reliability of in-hole and surface-wave methods, used for estimating shear wave velocity. Three test-sites with different subsurface conditions were chosen: a soft soil, a stiff soil and a rock outcrop. This paper reports the surface-wave methods results. Specifically 14 teams of expert users analysed the same experimental surface-wave datasets, consisting of both passive and active data. Each team adopted their own strategy to retrieve the dispersion curve and the shear-wave velocity profile at each site. Despite different approaches, the dispersion curves are quite in agreement with each other. Conversely, the shear-wave velocity profiles show a certain variability that increases in correspondence of major stratigraphic interfaces. This larger variability is mainly due to non-uniqueness of the solution and lateral variability. As expected, the observed variability in V-s,V-30 estimatesis small, as solution non-uniqueness plays a limited role. (C) 2015 Elsevier Ltd. All rights reserved.}, language = {en} } @article{TranThanhTuanPhamChiVinhOhrnbergeretal.2016, author = {Tran Thanh Tuan, and Pham Chi Vinh, and Ohrnberger, Matthias and Malischewsky, Peter and Aoudia, Abdelkrim}, title = {An Improved Formula of Fundamental Resonance Frequency of a Layered Half-Space Model Used in H/V Ratio Technique}, series = {Pure and applied geophysics}, volume = {173}, journal = {Pure and applied geophysics}, publisher = {Springer}, address = {Basel}, issn = {0033-4553}, doi = {10.1007/s00024-016-1313-0}, pages = {2803 -- 2812}, year = {2016}, abstract = {The resonance frequency of the transmission response in layered half-space model is important in the study of site effect because it is the frequency where the shake-ability of the ground is enhanced significantly. In practice, it is often determined by the H/V ratio technique in which the peak frequency of recorded H/V spectral ratio is interpreted as the resonance frequency. Despite of its importance, there has not been any formula of the resonance frequency of the layered half-space structure. In this paper, a simple approximate formula of the fundamental resonance frequency is presented after an exact formula in explicit form of the response function of vertically SH incident wave is obtained. The formula is in similar form with the one used in H/V ratio technique but it reflects several major effects of the model to the resonance frequency such as the arrangement of layers, the impedance contrast between layers and the half-space. Therefore, it could be considered as an improved formula used in H/V ratio technique. The formula also reflects the consistency between two approaches of the H/V ratio technique based on SH body waves or Rayleigh surface waves on the peak frequency under high impedance contrast condition. This formula is in explicit form and, therefore, may be used in the direct and inverse problem efficiently. A numerical illustration of the improved formula for an actual layered half-space model already investigated by H/V ratio technique is presented to demonstrate its new features and its improvement to the currently used formula.}, language = {en} } @article{WassermannBudiOhrnbergeretal.1999, author = {Wassermann, Joachim and Budi, E. N. and Ohrnberger, Matthias and Gossler, J.}, title = {Long term seismicity and source changes during different activity stages of Mt. Merapi (Indonesia) using a two scale seismic array}, year = {1999}, language = {en} } @article{OhrnbergerWassermannScherbaumetal.1999, author = {Ohrnberger, Matthias and Wassermann, J{\"u}rgen and Scherbaum, Frank and Budi, E. N. and Gossler, J.}, title = {Detection and classification of seismic signals of volcanic origin at Mt. Merapi (Indonesia)}, year = {1999}, language = {en} } @article{RichterWassermannZimmeretal.2004, author = {Richter, Gudrun and Wassermann, J{\"u}rgen and Zimmer, Martin and Ohrnberger, Matthias}, title = {Correlation of seismic activity and fumarole temperature at the Mt. Merapi volcano (Indonesia) in 2000}, issn = {0377-0273}, doi = {10.1016/j.jvolgeores.2004.03.006}, year = {2004}, abstract = {In this paper we present densely sampled fumarole temperature data, recorded continuously at a high-temperature fumarole of Mt. Merapi volcano (Indonesia). These temperature time series are correlated with continuous records of rainfall and seismic waveform data collected at the Indonesian - German multi-parameter monitoring network. The correlation analysis of fumarole temperature and precipitation data shows a clear influence of tropical rain events on fumarole temperature. In addition, there is some evidence that rainfall may influence seismicity rates, indicating interaction of meteoric water with the volcanic system. Knowledge about such interactions is important, as lava dome instabilities caused by heavy-precipitation events may result in pyroclastic flows. Apart from the strong external influences on fumarole temperature and seismicity rate, which may conceal smaller signals caused by volcanic degassing processes, the analysis of fumarole temperature and seismic data indicates a statistically significant correlation between a certain type of seismic activity and an increase in fumarole temperature. This certain type of seismic activity consists of a seismic cluster of several high-frequency transients and an ultra-long-period signal (< 0.002 Hz), which are best observed using a broadband seismometer deployed at a distance of 600 m from the active lava dome. The corresponding change in fumarole temperature starts a few minutes after the ultra-long-period signal and simultaneously with the high-frequency seismic cluster. The change in fumarole temperature, an increase of 5 degreesC on average, resembles a smoothed step. Fifty-four occurrences of simultaneous high-frequency seismic cluster, ultra-long period signal and increase of fumarole temperature have been identified in the data set from August 2000 to January 2001. The observed signals appear to correspond to degassing processes in the summit region of Mt. Merapi. (C) 2004 Elsevier B.V. All rights reserved}, language = {en} } @article{BlaserKruegerOhrnbergeretal.2010, author = {Blaser, Lilian and Kr{\"u}ger, Frank and Ohrnberger, Matthias and Scherbaum, Frank}, title = {Scaling relations of earthquake source parameter estimates with special focus on subduction environment}, issn = {0037-1106}, doi = {10.1785/0120100111}, year = {2010}, abstract = {Earthquake rupture length and width estimates are in demand in many seismological applications. Earthquake magnitude estimates are often available, whereas the geometrical extensions of the rupture fault mostly are lacking. Therefore, scaling relations are needed to derive length and width from magnitude. Most frequently used are the relationships of Wells and Coppersmith (1994) derived on the basis of a large dataset including all slip types with the exception of thrust faulting events in subduction environments. However, there are many applications dealing with earthquakes in subduction zones because of their high seismic and tsunamigenic potential. There are no well-established scaling relations for moment magnitude and length/width for subduction events. Within this study, we compiled a large database of source parameter estimates of 283 earthquakes. All focal mechanisms are represented, but special focus is set on (large) subduction zone events, in particular. Scaling relations were fitted with linear least-square as well as orthogonal regression and analyzed regarding the difference between continental and subduction zone/oceanic relationships. Additionally, the effect of technical progress in earthquake parameter estimation on scaling relations was tested as well as the influence of different fault mechanisms. For a given moment magnitude we found shorter but wider rupture areas of thrust events compared to Wells and Coppersmith (1994). The thrust event relationships for pure continental and pure subduction zone rupture areas were found to be almost identical. The scaling relations differ significantly for slip types. The exclusion of events prior to 1964 when the worldwide standard seismic network was established resulted in a remarkable effect on strike-slip scaling relations: the data do not show any saturation of rupture width of strike- slip earthquakes. Generally, rupture area seems to scale with mean slip independent of magnitude. The aspect ratio L/W, however, depends on moment and differs for each slip type.}, language = {en} }