TY - JOUR A1 - Krüger, Frank A1 - Dahm, Torsten T1 - Higher degree moment inversin using far-field broad-band recordings : theory and evaluation of the method with application to the 1994 Bolivia deep earthauke Y1 - 1999 ER - TY - JOUR A1 - Dahm, Torsten A1 - Krüger, Frank A1 - Essen, Heinz-Hermann A1 - Hensch, Martin T1 - Historic microseismic data and their relation to the wave-climate in the North Atlantic N2 - Microseismic data from observatories in Europe, which have been continuously recorded since about 100 years, contain information on the wave-climate in the North Atlantic. They can potentially be used as additional constraints in high-resolution temporal and spatial reconstructions of the storminess and oceanic waveheights in the past. To resolve spatial patterns data from observatories in different regions are needed. While previous recent studies analyzed only few observatory archives and relatively short time ranges, this is a first attempt to process the whole available data archive from different observatories. We correct and compare smoothed microseismic data from different stations and discuss their correlation and possible use for studies of storminess variability. Microseismic amplitudes at four seismic stations in northern Europe show amplitude peaks in 1920 and 1925, a slow decline in amplitudes till the middle of the 1930's followed by a steady increase of amplitudes till about 1990. From 1990 on microseismic amplitudes decrease. We find a good correlation between the average surface wind velocity in the North Atlantic and microseismic amplitudes at inland stations far away from the coast. Coastal stations are more influenced by local swell and are thus potentially useful to recover regional changes in wind and ocean wavefields with time. The study demonstrates that the analysis of microseismic has the potential to assess climate changes during the last 100 years Y1 - 2005 ER - TY - JOUR A1 - Dahm, Torsten A1 - Kuehn, Daniela A1 - Ohrnberger, Matthias A1 - Kroeger, Jens A1 - Wiederhold, Helga A1 - Reuther, Claus-Dieter A1 - Dehghani, Ali A1 - Scherbaum, Frank T1 - Combining geophysical data sets to study the dynamics of shallow evaporites in urban environments : application to Hamburg, Germany N2 - Shallowly situated evaporites in built-up areas are of relevance for urban and cultural development and hydrological regulation. The hazard of sinkholes, subrosion depressions and gypsum karst is often difficult to evaluate and may quickly change with anthropogenic influence. The geophysical exploration of evaporites in metropolitan areas is often not feasible with active industrial techniques. We collect and combine different passive geophysical data as microgravity, ambient vibrations, deformation and hydrological information to study the roof morphology of shallow evaporites beneath Hamburg, Northern Germany. The application of a novel gravity inversion technique leads to a 3-D depth model of the salt diapir under study. We compare the gravity-based depth model to pseudo-depths from H/V measurements and depth estimates from small-scale seismological array data. While the general range and trend of the diapir roof is consistent, a few anomalous regions are identified where H/V pseudo-depths indicate shallower structures not observed in gravity or array data. These are interpreted by shallow residual caprock floaters and zones of increased porosity. The shallow salt structure clearly correlates with a relative subsidence in the order of 2 mm yr(-1). The combined interpretation of roof morphology, yearly subsidence rates, chemical analyses of groundwater and of hydraulic head in aquifers indicates that the salt diapir beneath Hamburg is subject to significant ongoing dissolution that may possibly affect subrosion depressions, sinkhole distribution and land usage. The combined analysis of passive geophysical data may be exemplary for the study of shallow evaporites beneath other urban areas. Y1 - 2010 UR - http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-246X U6 - https://doi.org/10.1111/j.1365-246X.2010.04521.x SN - 0956-540X ER - TY - JOUR A1 - Kuhn, Daniela A1 - Ohrnberger, Matthias A1 - Dahm, Torsten T1 - Imaging a shallow salt diapir using ambient seismic vibrations beneath the densely built-up city area of Hamburg, Northern Germany JF - Journal of seismology N2 - Salt diapirs are common features of sedimentary basins. If close to the surface, they can bear a significant hazard due to possible dissolution sinkholes, karst formation and collapse dolines or their influence on ground water chemistry. We investigate the potential of ambient vibration techniques to map the 3-D roof morphology of shallow salt diapirs. Horizontal-to-vertical (H/V) spectral peaks are derived at more than 900 positions above a shallow diapir beneath the city area of Hamburg, Germany, and are used to infer the depth of the first strong impedance contrast. In addition, 15 small-scale array measurements are conducted at different positions in order to compute frequency-dependent phase velocities of Rayleigh waves between 0.5 and 25 Hz. The dispersion curves are inverted together with the H/V peak frequency to obtain shear-wave velocity profiles. Additionally, we compare the morphology derived from H/V and array measurements to borehole lithology and a gravity-based 3-D model of the salt diapir. Both methods give consistent results in agreement with major features indicated by the independent data. An important result is that H/V and array measurements are better suited to identify weathered gypsum caprocks or gypsum floaters, while gravity-derived models better sample the interface between sediments and homogeneous salt. We further investigate qualitatively the influence of the 3-D subsurface topography of the salt diapir on the validity of local 1-D inversion results from ambient vibration dispersion curve inversion. KW - Ambient seismic vibrations KW - H/V method KW - Array measurements KW - Salt diapir KW - 3-D effects Y1 - 2011 U6 - https://doi.org/10.1007/s10950-011-9234-y SN - 1383-4649 VL - 15 IS - 3 SP - 507 EP - 531 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Maghsoudi, Samira A1 - Cesca, Simone A1 - Hainzl, Sebastian A1 - Kaiser, Diethelm A1 - Becker, Dirk A1 - Dahm, Torsten T1 - Improving the estimation of detection probability and magnitude of completeness in strongly heterogeneous media, an application to acoustic emission (AE) JF - Geophysical journal international N2 - Reliable estimations of magnitude of completeness (M-c) are essential for a correct interpretation of seismic catalogues. The spatial distribution of M-c may be strongly variable and difficult to assess in mining environments, owing to the presence of galleries, cavities, fractured regions, porous media and different mineralogical bodies, as well as in consequence of inhomogeneous spatial distribution of the seismicity. We apply a 3-D modification of the probabilistic magnitude of completeness (PMC) method, which relies on the analysis of network detection capabilities. In our approach, the probability to detect an event depends on its magnitude, source receiver Euclidian distance and source receiver direction. The suggested method is proposed for study of the spatial distribution of the magnitude of completeness in a mining environment and here is applied to a 2-months acoustic emission (AE) data set recorded at the Morsleben salt mine, Germany. The dense seismic network and the large data set, which includes more than one million events, enable a detailed testing of the method. This method is proposed specifically for strongly heterogeneous media. Besides, it can also be used for specific network installations, with sensors with a sensitivity, dependent on the direction of the incoming wave (e.g. some piezoelectric sensors). In absence of strong heterogeneities, the standards PMC approach should be used. We show that the PMC estimations in mines strongly depend on the source receiver direction, and cannot be correctly accounted using a standard PMC approach. However, results can be improved, when adopting the proposed 3-D modification of the PMC method. Our analysis of one central horizontal and vertical section yields a magnitude of completeness of about M-c approximate to 1 (AE magnitude) at the centre of the network, which increases up to M-c approximate to 4 at further distances outside the network; the best detection performance is estimated for a NNE-SSE elongated region, which corresponds to the strike direction of the low-attenuating salt body. Our approach provides us with small-scale details about the capability of sensors to detect an earthquake, which can be linked to the presence of heterogeneities in specific directions. Reduced detection performance in presence of strong structural heterogeneities (cavities) is confirmed by synthetic waveform modelling in heterogeneous media. KW - Seismic attenuation KW - Statistical seismology Y1 - 2013 U6 - https://doi.org/10.1093/gji/ggt049 SN - 0956-540X VL - 193 IS - 3 SP - 1556 EP - 1569 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Cesca, Simone A1 - Braun, Thomas A1 - Maccaferri, Francesco A1 - Passarelli, Luigi A1 - Rivalta, Eleonora A1 - Dahm, Torsten T1 - Source modelling of the M5-6 Emilia-Romagna, Italy, earthquakes (2012 May 20-29) JF - Geophysical journal international N2 - On 2012 May 20 and 29, two damaging earthquakes with magnitudes M-w 6.1 and 5.9, respectively, struck the Emilia-Romagna region in the sedimentary Po Plain, Northern Italy, causing 26 fatalities, significant damage to historical buildings and substantial impact to the economy of the region. The earthquake sequence included four more aftershocks with M-w, >= 5.0, all at shallow depths (about 7-9 km), with similar WNW-ESE striking reverse mechanism. The timeline of the sequence suggests significant static stress interaction between the largest events. We perform here a detailed source inversion, first adopting a point source approximation and considering pure double couple and full moment tensor source models. We compare different extended source inversion approaches for the two largest events, and find that the rupture occurred in both cases along a subhorizontal plane, dipping towards SSW Directivity is well detected for the May 20 main shock, indicating that the rupture propagated unilaterally towards SE. Based on the focal mechanism solution, we further estimate the co-seismic static stress change induced by the May 20 event. By using the rate-and-state model and a Poissonian earthquake occurrence, we infer that the second largest event of May 29 was induced with a probability in the range 0.2-0.4. This suggests that the segment of fault was already prone to rupture. Finally, we estimate peak ground accelerations for the two main events as occurred separately or simultaneously. For the scenario involving hypothetical rupture areas of both main events, we estimate M-w = 6.3 and an increase of ground acceleration by 50 per cent. The approach we propose may help to quantify rapidly which regions are invested by a significant increase of the hazard, bearing the potential for large aftershocks or even a second main shock. KW - Earthquake dynamics KW - Earthquake source observations Y1 - 2013 U6 - https://doi.org/10.1093/gji/ggt069 SN - 0956-540X VL - 193 IS - 3 SP - 1658 EP - 1672 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Sen, Ali Tolga A1 - Cesca, Simone A1 - Bischoff, Monika A1 - Meier, Thomas A1 - Dahm, Torsten T1 - Automated full moment tensor inversion of coal mining-induced seismicity JF - Geophysical journal international N2 - Seismicity induced by coal mining in the Ruhr region, Germany, has been monitored continuously over the last 25 yr. In 2006, a dense temporary network (HAMNET) was deployed to locally monitor seismicity induced by longwall mining close to the town of Hamm. Between 2006 July and 2007 July, more than 7000 events with magnitudes M-L from -1.7 to 2.0 were detected. The spatiotemporal distribution of seismicity shows high correlation with the mining activity. In order to monitor rupture processes, we set up an automated source inversion routine and successfully perform double couple and full moment tensor (MT) inversions for more than 1000 events with magnitudes above M-L -0.5. The source inversion is based on a full waveform approach, both in the frequency and in the time domain, providing information about the centroid location, focal mechanism, scalar moment and full MT. Inversion results indicate a strong dominance of normal faulting focal mechanisms, with a steeper plane and a subhorizontal one. Fault planes are oriented parallel to the mining stopes. We classify the focal mechanisms based on their orientation and observe different frequency-magnitude distributions for families of events with different focal mechanisms; the overall frequency-magnitude distribution is not fitting the Gutenberg-Richter relation. Full MTs indicate that non-negligible opening tensile components accompanied normal faulting source mechanisms. Finally, extended source models are investigated for largest events. Results suggest that the rupture processes mostly occurred along the subvertical planes. KW - Geomechanics KW - Fracture and flow KW - Earthquake source observations KW - Seismicity and tectonics Y1 - 2013 U6 - https://doi.org/10.1093/gji/ggt300 SN - 0956-540X SN - 1365-246X VL - 195 IS - 2 SP - 1267 EP - 1281 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Heimann, Sebastian A1 - Gonzalez, Alvaro A1 - Wang, Rongjiang A1 - Cesca, Simone A1 - Dahm, Torsten T1 - Seismic characterization of the Chelyabinsk Meteor's terminal explosion JF - Seismological research letters Y1 - 2013 U6 - https://doi.org/10.1785/0220130042 SN - 0895-0695 VL - 84 IS - 6 SP - 1021 EP - 1025 PB - Seismological Society of America CY - Albany ER - TY - JOUR A1 - Grigoli, Francesco A1 - Cesca, Simone A1 - Vassallo, Maurizio A1 - Dahm, Torsten T1 - Automated seismic event location by travel-time stacking an application to mining induced seismicity JF - Seismological research letters Y1 - 2013 U6 - https://doi.org/10.1785/0220120191 SN - 0895-0695 VL - 84 IS - 4 SP - 666 EP - 677 PB - Seismological Society of America CY - Albany ER - TY - JOUR A1 - Dahm, Torsten A1 - Fischer, Tomas T1 - Velocity ratio variations in the source region of earthquake swarms in NW Bohemia obtained from arrival time double-differences JF - Geophysical journal international N2 - Crustal earthquake swarms are an expression of intensive cracking and rock damaging over periods of days, weeks or month in a small source region in the crust. They are caused by longer lasting stress changes in the source region. Often, the localized stressing of the crust is associated with fluid or gas migration, possibly in combination with pre-existing zones of weaknesses. However, verifying and quantifying localized fluid movement at depth remains difficult since the area affected is small and geophysical prospecting methods often cannot reach the required resolution. We apply a simple and robust method to estimate the velocity ratio between compressional (P) and shear (S) waves (upsilon(P)/upsilon(S)-ratio) in the source region of an earthquake swarm. The upsilon(P)/upsilon(S)-ratio may be unusual small if the swarm is related to gas in a porous or fractured rock. The method uses arrival time difference between P and S waves observed at surface seismic stations, and the associated double differences between pairs of earthquakes. An advantage is that earthquake locations are not required and the method seems lesser dependent on unknown velocity variations in the crust outside the source region. It is, thus, suited for monitoring purposes. Applications comprise three natural, mid-crustal (8-10 km) earthquake swarms between 1997 and 2008 from the NW-Bohemia swarm region. We resolve a strong temporal decrease of upsilon(P)/upsilon(S) before and during the main activity of the swarm, and a recovery of upsilon(P)/upsilon(S) to background levels at the end of the swarms. The anomalies are interpreted in terms of the Biot-Gassman equations, assuming the presence of oversaturated fluids degassing during the beginning phase of the swarm activity. KW - Tomography KW - Earthquake source observations KW - Volcano seismology Y1 - 2014 U6 - https://doi.org/10.1093/gji/ggt410 SN - 0956-540X SN - 1365-246X VL - 196 IS - 2 SP - 957 EP - 970 PB - Oxford Univ. Press CY - Oxford ER -