TY - JOUR A1 - Wassermann, Joachim A1 - Budi, E. N. A1 - Ohrnberger, Matthias A1 - Gossler, J. T1 - Long term seismicity and source changes during different activity stages of Mt. Merapi (Indonesia) using a two scale seismic array Y1 - 1999 ER - TY - JOUR A1 - Wassermann, Joachim T1 - Locating the sources of volanic explosions and volcanic tremor at stromboli volcano (Italy) : using beam- forming on diffractin hyperboloids Y1 - 1997 ER - TY - JOUR A1 - Wassermann, Joachim T1 - Volcano seismology Y1 - 2001 UR - http://www.seismo.com/msop/nmsop/13%20volcano/volcano.html ER - TY - JOUR A1 - Suryanto, Wiwit A1 - Igel, Heiner A1 - Wassermann, Joachim A1 - Cochard, Alain A1 - Schuberth, B. S. A. A1 - Vollmer, Daniel A1 - Scherbaum, Frank A1 - Schreiber, U. A1 - Velikoseltsev, A. T1 - First comparison of array-derived rotational ground motions with direct ring laser measurements JF - Bulletin of the Seismological Society of America N2 - 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). Y1 - 2006 U6 - https://doi.org/10.1785/0120060004 SN - 0037-1106 SN - 1943-3573 VL - 96 IS - 6 SP - 2059 EP - 2071 PB - GeoScienceWorld CY - Alexandria, Va. ER - TY - JOUR A1 - Falsaperla, Susanna A1 - Wassermann, Joachim A1 - Scherbaum, Frank T1 - Solid earth - 29. Polarization analyses of broadband seismic data recorded on Stromboli Volcano (Italy) from 1996 to 1999 (DOI 10.1029-2001GLO14300) Y1 - 2002 ER - TY - JOUR A1 - Beyreuther, Moritz A1 - Hammer, Conny A1 - Wassermann, Joachim A1 - Ohrnberger, Matthias A1 - Megies, Tobias T1 - Constructing a hidden Markov Model based earthquake detector: application to induced seismicity JF - Geophysical journal international N2 - 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. KW - Time-series analysis KW - Neural networks KW - fuzzy logic KW - Seismic monitoring and test-ban treaty verification KW - Volcano seismology Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-246X.2012.05361.x SN - 0956-540X VL - 189 IS - 1 SP - 602 EP - 610 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Bartosch, Thorsten A1 - Wassermann, Joachim T1 - Wavelet coherence analysis of broadband array data recorded at Stromboli volcano, Italy N2 - 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 Y1 - 2004 SN - 0037-1106 ER -