TY - JOUR A1 - Al-Halbouni, Djamil A1 - Holohan, Eoghan P. A1 - Taheri, Abbas A1 - Schöpfer, Martin P. J. A1 - Emam, Sacha A1 - Dahm, Torsten T1 - Geomechanical modelling of sinkhole development using distinct elements BT - model verification for a single void space and application to the Dead Sea area JF - Solid earth N2 - Mechanical and/or chemical removal of material from the subsurface may generate large subsurface cavities, the destabilisation of which can lead to ground collapse and the formation of sinkholes. Numerical simulation of the interaction of cavity growth, host material deformation and overburden collapse is desirable to better understand the sinkhole hazard but is a challenging task due to the involved high strains and material discontinuities. Here, we present 2-D distinct element method numerical simulations of cavity growth and sinkhole development. Firstly, we simulate cavity formation by quasi-static, stepwise removal of material in a single growing zone of an arbitrary geometry and depth. We benchmark this approach against analytical and boundary element method models of a deep void space in a linear elastic material. Secondly, we explore the effects of properties of different uniform materials on cavity stability and sinkhole development. We perform simulated biaxial tests to calibrate macroscopic geotechnical parameters of three model materials representative of those in which sinkholes develop at the Dead Sea shoreline: mud, alluvium and salt. We show that weak materials do not support large cavities, leading to gradual sagging or suffusion-style subsidence. Strong materials support quasi-stable to stable cavities, the overburdens of which may fail suddenly in a caprock or bedrock collapse style. Thirdly, we examine the consequences of layered arrangements of weak and strong materials. We find that these are more susceptible to sinkhole collapse than uniform materials not only due to a lower integrated strength of the overburden but also due to an inhibition of stabilising stress arching. Finally, we compare our model sinkhole geometries to observations at the Ghor Al-Haditha sinkhole site in Jordan. Sinkhole depth / diameter ratios of 0.15 in mud, 0.37 in alluvium and 0.33 in salt are reproduced successfully in the calibrated model materials. The model results suggest that the observed distribution of sinkhole depth / diameter values in each material type may partly reflect sinkhole growth trends. Y1 - 2018 U6 - https://doi.org/10.5194/se-9-1341-2018 SN - 1869-9510 SN - 1869-9529 VL - 9 IS - 6 SP - 1341 EP - 1373 PB - Copernicus CY - Göttingen ER - TY - GEN A1 - Al-Halbouni, Djamil A1 - Holohan, Eoghan P. A1 - Taheri, Abbas A1 - Schöpfer, Martin P. J. A1 - Emam, Sacha A1 - Dahm, Torsten T1 - Geomechanical modelling of sinkhole development using distinct elements BT - model verification for a single void space and application to the Dead Sea area T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Mechanical and/or chemical removal of material from the subsurface may generate large subsurface cavities, the destabilisation of which can lead to ground collapse and the formation of sinkholes. Numerical simulation of the interaction of cavity growth, host material deformation and overburden collapse is desirable to better understand the sinkhole hazard but is a challenging task due to the involved high strains and material discontinuities. Here, we present 2-D distinct element method numerical simulations of cavity growth and sinkhole development. Firstly, we simulate cavity formation by quasi-static, stepwise removal of material in a single growing zone of an arbitrary geometry and depth. We benchmark this approach against analytical and boundary element method models of a deep void space in a linear elastic material. Secondly, we explore the effects of properties of different uniform materials on cavity stability and sinkhole development. We perform simulated biaxial tests to calibrate macroscopic geotechnical parameters of three model materials representative of those in which sinkholes develop at the Dead Sea shoreline: mud, alluvium and salt. We show that weak materials do not support large cavities, leading to gradual sagging or suffusion-style subsidence. Strong materials support quasi-stable to stable cavities, the overburdens of which may fail suddenly in a caprock or bedrock collapse style. Thirdly, we examine the consequences of layered arrangements of weak and strong materials. We find that these are more susceptible to sinkhole collapse than uniform materials not only due to a lower integrated strength of the overburden but also due to an inhibition of stabilising stress arching. Finally, we compare our model sinkhole geometries to observations at the Ghor Al-Haditha sinkhole site in Jordan. Sinkhole depth ∕ diameter ratios of 0.15 in mud, 0.37 in alluvium and 0.33 in salt are reproduced successfully in the calibrated model materials. The model results suggest that the observed distribution of sinkhole depth ∕ diameter values in each material type may partly reflect sinkhole growth trends. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1061 KW - rock mass KW - karst KW - dissolution KW - reflection KW - subsidence KW - subrosion KW - collapse KW - simulation KW - scale KW - fault Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-468435 SN - 1866-8372 IS - 1061 ER - TY - JOUR A1 - Al-Halbouni, Djamil A1 - Holohan, Eoghan P. A1 - Taheri, Abbas A1 - Watson, Robert A. A1 - Polom, Ulrich A1 - Schoepfer, Martin P. J. A1 - Emam, Sacha A1 - Dahm, Torsten T1 - Distinct element geomechanical modelling of the formation of sinkhole clusters within large-scale karstic depressions JF - Solid earth N2 - The 2-D distinct element method (DEM) code (PFC2D_V5) is used here to simulate the evolution of subsidence-related karst landforms, such as single and clustered sinkholes, and associated larger-scale depressions. Subsurface material in the DEM model is removed progressively to produce an array of cavities; this simulates a network of subsurface groundwater conduits growing by chemical/mechanical erosion. The growth of the cavity array is coupled mechanically to the gravitationally loaded surroundings, such that cavities can grow also in part by material failure at their margins, which in the limit can produce individual collapse sinkholes. Two end-member growth scenarios of the cavity array and their impact on surface subsidence were examined in the models: (1) cavity growth at the same depth level and growth rate; (2) cavity growth at progressively deepening levels with varying growth rates. These growth scenarios are characterised by differing stress patterns across the cavity array and its overburden, which are in turn an important factor for the formation of sinkholes and uvalalike depressions. For growth scenario (1), a stable compression arch is established around the entire cavity array, hindering sinkhole collapse into individual cavities and favouring block-wise, relatively even subsidence across the whole cavity array. In contrast, for growth scenario (2), the stress system is more heterogeneous, such that local stress concentrations exist around individual cavities, leading to stress interactions and local wall/overburden fractures. Consequently, sinkhole collapses occur in individual cavities, which results in uneven, differential subsidence within a larger-scale depression. Depending on material properties of the cavity-hosting material and the overburden, the larger-scale depression forms either by sinkhole coalescence or by widespread subsidence linked geometrically to the entire cavity array. The results from models with growth scenario (2) are in close agreement with surface morphological and subsurface geophysical observations from an evaporite karst area on the eastern shore of the Dead Sea. Y1 - 2019 U6 - https://doi.org/10.5194/se-10-1219-2019 SN - 1869-9510 SN - 1869-9529 VL - 10 IS - 4 SP - 1219 EP - 1241 PB - Copernicus CY - Göttingen 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 - Cesca, Simone A1 - Grigoli, Francesco A1 - Heimann, Sebastian A1 - Dahm, Torsten A1 - Kriegerowski, Marius A1 - Sobiesiak, M. A1 - Tassara, C. A1 - Olcay, M. T1 - The M-w 8.1 2014 Iquique, Chile, seismic sequence: a tale of foreshocks and aftershocks JF - Geophysical journal international N2 - The 2014 April 1, M-w 8.1 Iquique (Chile) earthquake struck in the Northern Chile seismic gap. With a rupture length of less than 200 km, it left unbroken large segments of the former gap. Early studies were able to model the main rupture features but results are ambiguous with respect to the role of aseismic slip and left open questions on the remaining hazard at the Northern Chile gap. A striking observation of the 2014 earthquake has been its extensive preparation phase, with more than 1300 events with magnitude above M-L 3, occurring during the 15 months preceding the main shock. Increasing seismicity rates and observed peak magnitudes accompanied the last three weeks before the main shock. Thanks to the large data sets of regional recordings, we assess the precursor activity, compare foreshocks and aftershocks and model rupture preparation and rupture effects. To tackle inversion challenges for moderate events with an asymmetric network geometry, we use full waveforms techniques to locate events, map the seismicity rate and derive source parameters, obtaining moment tensors for more than 300 events (magnitudes M-w 4.0-8.1) in the period 2013 January 1-2014 April 30. This unique data set of fore- and aftershocks is investigated to distinguish rupture process models and models of strain and stress rotation during an earthquake. Results indicate that the spatial distributions of foreshocks delineated the shallower part of the rupture areas of the main shock and its largest aftershock, well matching the spatial extension of the aftershocks cloud. Most moment tensors correspond to almost pure double couple thrust mechanisms, consistent with the slab orientation. Whereas no significant differences are observed among thrust mechanisms in different areas, nor among thrust foreshocks and aftershocks, the early aftershock sequence is characterized by the presence of normal fault mechanisms, striking parallel to the trench but dipping westward. These events likely occurred in the shallow wedge structure close to the slab interface and are consequence of the increased extensional stress in this region after the largest events. The overall stress inversion result suggests a minor stress rotation after the main shock, but a significant release of the deviatoric stress. The temporal change in the distribution of focal mechanisms can also be explained in terms of the spatial heterogeneity of the stress field: under such interpretation, the potential of a large megathrust earthquake breaking a larger segment offshore Northern Chile remains high. KW - Earthquake source observations KW - South America Y1 - 2016 U6 - https://doi.org/10.1093/gji/ggv544 SN - 0956-540X SN - 1365-246X VL - 204 SP - 1766 EP - 1780 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Cesca, Simone A1 - Grigoli, Francesco A1 - Heimann, Sebastian A1 - Gonzalez, Alvaro A1 - Buforn, Elisa A1 - Maghsoudi, Samira A1 - Blanch, Estefania A1 - Dahm, Torsten T1 - The 2013 September-October seismic sequence offshore Spain: a case of seismicity triggered by gas injection? JF - Geophysical journal international N2 - A spatially localized seismic sequence originated few tens of kilometres offshore the Mediterranean coast of Spain, close to the Ebro river delta, starting on 2013 September 5, and lasting at least until 2013 October. The sequence culminated in a maximal moment magnitude M-w 4.3 earthquake, on 2013 October 1. The most relevant seismogenic feature in the area is the Fosa de Amposta fault system, which includes different strands mapped at different distances to the coast, with a general NE-SW orientation, roughly parallel to the coastline. However, no significant known historical seismicity has involved this fault system in the past. The epicentral region is also located near the offshore platform of the Castor project, where gas is conducted through a pipeline from mainland and where it was recently injected in a depleted oil reservoir, at about 2 km depth. We analyse the temporal evolution of the seismic sequence and use full waveform techniques to derive absolute and relative locations, estimate depths and focal mechanisms for the largest events in the sequence (with magnitude mbLg larger than 3), and compare them to a previous event (2012 April 8, mbLg 3.3) taking place in the same region prior to the gas injection. Moment tensor inversion results show that the overall seismicity in this sequence is characterized by oblique mechanisms with a normal fault component, with a 30A degrees low-dip angle plane oriented NNE-SSW and a subvertical plane oriented NW-SE. The combined analysis of hypocentral location and focal mechanisms could indicate that the seismic sequence corresponds to rupture processes along shallow low-dip surfaces, which could have been triggered by the gas injection in the reservoir, and excludes the activation of the Amposta fault, as its known orientation is inconsistent with focal mechanism results. An alternative scenario includes the iterated triggering of a system of steep faults oriented NW-SE, which were identified by prior marine seismics investigations. KW - Earthquake dynamics KW - Earthquake source observations Y1 - 2014 U6 - https://doi.org/10.1093/gji/ggu172 SN - 0956-540X SN - 1365-246X VL - 198 IS - 2 SP - 941 EP - 953 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Cesca, Simone A1 - Heimann, Sebastian A1 - Kriegerowski, Marius A1 - Saul, Joachim A1 - Dahm, Torsten T1 - Moment tensor inversion for nuclear explosions BT - what can we learn from the 6 January and 9 September 2016 Nuclear Tests, North Korea? JF - Seismological research letters N2 - Two nuclear explosions were carried out by the Democratic People’s Republic of North Korea in January and September 2016. Epicenters were located close to those of the 2006, 2009, and 2013 previous explosions. We perform a seismological analysis of the 2016 events combining the analysis of full waveforms at regional distances and seismic array beams at teleseismic distances. We estimate the most relevant source parameters, such as source depth, moment release, and full moment tensor (MT). The best MT solution can be decomposed into an isotropic source, directly related with the explosion and an additional deviatoric term, likely due to near‐source interactions with topographic and/or underground facilities features. We additionally perform an accurate resolution test to assess source parameters uncertainties and trade‐offs. This analysis sheds light on source parameters inconsistencies among studies on previous shallow explosive sources. The resolution of the true MT is hindered by strong source parameters trade‐offs, so that a broad range of well‐fitting MT solutions can be found, spanning from a dominant positive isotropic term to a dominant negative vertical compensated linear vector dipole. The true mechanism can be discriminated by additionally modeling first‐motion polarities at seismic arrays at teleseismic distances. A comparative assessment of the 2016 explosion with earlier nuclear tests documents similar vertical waveforms but a significant increase of amplitude for the 2016 explosions, which proves that the 9 September 2016 was the largest nuclear explosion ever performed in North Korea with a magnitude Mw 4.9 and a shallow depth of less than 2 km, although there are no proofs of a fusion explosion. Modeling transversal component waveforms suggests variable size and orientation of the double‐couple components of the 2009, 2013, and 2016 sources. Y1 - 2017 U6 - https://doi.org/10.1785/0220160139 SN - 0895-0695 SN - 1938-2057 VL - 88 IS - 2A SP - 300 EP - 310 PB - Seismological Society of America CY - Albany ER - TY - JOUR A1 - Cesca, Simone A1 - Sen, Ali Tolga A1 - Dahm, Torsten T1 - Seismicity monitoring by cluster analysis of moment tensors JF - Geophysical journal international N2 - We suggest a new clustering approach to classify focal mechanisms from large moment tensor catalogues, with the purpose of automatically identify families of earthquakes with similar source geometry, recognize the orientation of most active faults, and detect temporal variations of the rupture processes. The approach differs in comparison to waveform similarity methods since clusters are detected even if they occur in large spatial distances. This approach is particularly helpful to analyse large moment tensor catalogues, as in microseismicity applications, where a manual analysis and classification is not feasible. A flexible algorithm is here proposed: it can handle different metrics, norms, and focal mechanism representations. In particular, the method can handle full moment tensor or constrained source model catalogues, for which different metrics are suggested. The method can account for variable uncertainties of different moment tensor components. We verify the method with synthetic catalogues. An application to real data from mining induced seismicity illustrates possible applications of the method and demonstrate the cluster detection and event classification performance with different moment tensor catalogues. Results proof that main earthquake source types occur on spatially separated faults, and that temporal changes in the number and characterization of focal mechanism clusters are detected. We suggest that moment tensor clustering can help assessing time dependent hazard in mines. KW - Persistence KW - memory KW - correlations KW - clustering KW - Earthquake source observations Y1 - 2014 U6 - https://doi.org/10.1093/gji/ggt492 SN - 0956-540X SN - 1365-246X VL - 196 IS - 3 SP - 1813 EP - 1826 PB - Oxford Univ. Press CY - Oxford ER - TY - GEN A1 - Dahm, Torsten A1 - Becker, Dirk A1 - Bischoff, Monika A1 - Cesca, Simone A1 - Dost, B. A1 - Fritschen, R. A1 - Hainzl, Sebastian A1 - Klose, C. D. A1 - Kuhn, D. A1 - Lasocki, S. A1 - Meier, Thomas A1 - Ohrnberger, Matthias A1 - Rivalta, Eleonora A1 - Wegler, Ulrich A1 - Husen, Stephan T1 - Recommendation for the discrimination of human-related and natural seismicity T2 - Journal of seismology N2 - Various techniques are utilized by the seismological community, extractive industries, energy and geoengineering companies to identify earthquake nucleation processes in close proximity to engineering operation points. These operations may comprise fluid extraction or injections, artificial water reservoir impoundments, open pit and deep mining, deep geothermal power generations or carbon sequestration. In this letter to the editor, we outline several lines of investigation that we suggest to follow to address the discrimination problem between natural seismicity and seismic events induced or triggered by geoengineering activities. These suggestions have been developed by a group of experts during several meetings and workshops, and we feel that their publication as a summary report is helpful for the geoscientific community. Specific investigation procedures and discrimination approaches, on which our recommendations are based, are also published in this Special Issue (SI) of Journal of Seismology. KW - Triggered seismicity KW - Induced seismicity Y1 - 2013 U6 - https://doi.org/10.1007/s10950-012-9295-6 SN - 1383-4649 VL - 17 IS - 1 SP - 197 EP - 202 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Dahm, Torsten A1 - Cesca, Simone A1 - Hainzl, Sebastian A1 - Braun, Thomas A1 - Krüger, Frank T1 - 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 JF - Journal of geophysical research : Solid earth N2 - 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. KW - induced seismicity KW - probabilistic discrimination KW - hydrocarbon field KW - triggered earthquake KW - seismic hazard KW - earthquake Y1 - 2015 U6 - https://doi.org/10.1002/2014JB011778 SN - 2169-9313 SN - 2169-9356 VL - 120 IS - 4 SP - 2491 EP - 2509 PB - American Geophysical Union CY - Washington ER -