@article{BeyreutherHammerWassermannetal.2012, author = {Beyreuther, Moritz and Hammer, Conny and Wassermann, Joachim and Ohrnberger, Matthias and Megies, Tobias}, title = {Constructing a hidden Markov Model based earthquake detector: application to induced seismicity}, series = {Geophysical journal international}, volume = {189}, journal = {Geophysical journal international}, number = {1}, publisher = {Wiley-Blackwell}, address = {Malden}, issn = {0956-540X}, doi = {10.1111/j.1365-246X.2012.05361.x}, pages = {602 -- 610}, year = {2012}, abstract = {The triggering or detection of seismic events out of a continuous seismic data stream is one of the key issues of an automatic or semi-automatic seismic monitoring system. In the case of dense networks, either local or global, most of the implemented trigger algorithms are based on a large number of active stations. However, in the case of only few available stations or small events, for example, like in monitoring volcanoes or hydrothermal power plants, common triggers often show high false alarms. In such cases detection algorithms are of interest, which show reasonable performance when operating even on a single station. In this context, we apply Hidden Markov Models (HMM) which are algorithms borrowed from speech recognition. However, many pitfalls need to be avoided to apply speech recognition technology directly to earthquake detection. We show the fit of the model parameters in an innovative way. State clustering is introduced to refine the intrinsically assumed time dependency of the HMMs and we explain the effect coda has on the recognition results. The methodology is then used for the detection of anthropogenicly induced earthquakes for which we demonstrate for a period of 3.9 months of continuous data that the single station HMM earthquake detector can achieve similar detection rates as a common trigger in combination with coincidence sums over two stations. To show the general applicability of state clustering we apply the proposed method also to earthquake classification at Mt. Merapi volcano, Indonesia.}, language = {en} } @article{SuryantoIgelWassermannetal.2006, author = {Suryanto, Wiwit and Igel, Heiner and Wassermann, Joachim and Cochard, Alain and Schuberth, B. S. A. and Vollmer, Daniel and Scherbaum, Frank and Schreiber, U. and Velikoseltsev, A.}, title = {First comparison of array-derived rotational ground motions with direct ring laser measurements}, series = {Bulletin of the Seismological Society of America}, volume = {96}, journal = {Bulletin of the Seismological Society of America}, number = {6}, publisher = {GeoScienceWorld}, address = {Alexandria, Va.}, issn = {0037-1106}, doi = {10.1785/0120060004}, pages = {2059 -- 2071}, year = {2006}, abstract = {Recently, ring laser technology has provided the first consistent observations of rotational ground motions around a vertical axis induced by earthquakes. "Consistent," in this context, implies that the observed waveforms and amplitudes are compatible with collocated recordings of translational ground motions. In particular, transverse accelerations should be in phase with rotation rate and their ratio proportional to local horizontal phase velocity assuming plane-wave propagation. The ring laser installed at the Fundamental station Wettzell in the Bavarian Forest, Southeast Germany, is recording the rotation rate around a vertical axis, theoretically a linear combination of the space derivatives of the horizontal components of motion. This suggests that, in principle, rotation can be derived from seismic-array experiments by "finite differencing." This has been attempted previously in several studies; however, the accuracy of these observations could never be tested in the absence of direct measurements. We installed a double cross-shaped array of nine stations from December 2003 to March 2004 around the ring laser instrument and observed several large earthquakes on both the ring laser and the seismic array. Here we present for the first time a comparison of array-derived rotations with direct measurements of rotations for ground motions induced by the M 6.3 Al Hoceima, Morocco, earthquake of 24 February 2004. With complete 3D synthetic seismograms calculated for this event we show that even low levels of noise may considerably influence the accuracy of the array-derived rotations when the minimum number of required stations (three) is used. Nevertheless, when using all nine stations, the overall fit between direct and array-derived measurements is surprisingly good (maximum correlation coefficient of 0.94).}, language = {en} } @article{WassermannOhrnbergerScherbaumetal.1998, author = {Wassermann, Joachim and Ohrnberger, Matthias and Scherbaum, Frank and Gossler, J. and Zschau, Jochen}, title = {Kontinuierliche seismologische Netz- und Arraymessungen am Dekadenvulkan Merapi (Java, Indonesien) : ein Zwischenres{\"u}mee = Continuous measurements at Merapi volcano (Java, Indonesia) using anetwork of small-scale seismograph arrays}, issn = {0947-1944}, year = {1998}, language = {de} } @article{Wassermann1997, author = {Wassermann, Joachim}, title = {Locating the sources of volanic explosions and volcanic tremor at stromboli volcano (Italy) : using beam- forming on diffractin hyperboloids}, year = {1997}, 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{FalsaperlaWassermannScherbaum2002, author = {Falsaperla, Susanna and Wassermann, Joachim and Scherbaum, Frank}, title = {Solid earth - 29. Polarization analyses of broadband seismic data recorded on Stromboli Volcano (Italy) from 1996 to 1999 (DOI 10.1029-2001GLO14300)}, year = {2002}, language = {en} } @article{Wassermann2001, author = {Wassermann, Joachim}, title = {Volcano seismology}, year = {2001}, language = {en} } @article{BartoschWassermann2004, author = {Bartosch, Thorsten and Wassermann, Joachim}, title = {Wavelet coherence analysis of broadband array data recorded at Stromboli volcano, Italy}, issn = {0037-1106}, year = {2004}, abstract = {We present a wavelet coherence method that is capable of displaying local coherence information between two seismic stations in the sense of a spectrogram. We have analyzed the vertical components of a 20-min-long time series from four stations that were situated in the seismic near field of Stromboli volcano. Typical volcanic seismic signals recorded in the near field of Stromboli volcano consist of continuous volcanic tremor superimposed on frequent Strombolian explosion signals. The tremor exhibits a banded and frequency-stable structure, whereas the broadband explosion signals span two or three frequency decades. We demonstrate that signals related to explosion earthquakes are strongly correlated within the network over 1.5 frequency decades. Using synthetic data, we show how coherent signal portions can be extracted out of noisy data using a coherence-filtering method. A time delay analysis using coherence information results in a coarse source location estimation that lies within the crater region. With the exception of randomly fluctuating coherence peaks, low correlations have been observed in the characteristic bands that are assumed to be generated by continuous tremor. In the low-frequency band that is related to the ocean microseisms (period approximate to 4-8 sec), we observe mostly high correlation that breaks down during the appearance of explosion earthquake signals. Based on further analysis using the inverse wavelet transformation, we propose a model that describes the breakdown phenomenon as a superposition of two independent events}, language = {en} }