TY - JOUR A1 - Passarelli, Luigi A1 - Hainzl, Sebastian A1 - Cesca, Simone A1 - Maccaferri, Francesco A1 - Mucciarelli, Marco A1 - Rößler, Dirk A1 - Corbi, Fabio A1 - Dahm, Torsten A1 - Rivalta, Eleonora T1 - Aseismic transient driving the swarm-like seismic sequence in the Pollino range, Southern Italy JF - Geophysical journal international N2 - Tectonic earthquake swarms challenge our understanding of earthquake processes since it is difficult to link observations to the underlying physical mechanisms and to assess the hazard they pose. Transient forcing is thought to initiate and drive the spatio-temporal release of energy during swarms. The nature of the transient forcing may vary across sequences and range from aseismic creeping or transient slip to diffusion of pore pressure pulses to fluid redistribution and migration within the seismogenic crust. Distinguishing between such forcing mechanisms may be critical to reduce epistemic uncertainties in the assessment of hazard due to seismic swarms, because it can provide information on the frequency-magnitude distribution of the earthquakes (often deviating from the assumed Gutenberg-Richter relation) and on the expected source parameters influencing the ground motion (for example the stress drop). Here we study the ongoing Pollino range (Southern Italy) seismic swarm, a long-lasting seismic sequence with more than five thousand events recorded and located since October 2010. The two largest shocks (magnitude M-w = 4.2 and M-w = 5.1) are among the largest earthquakes ever recorded in an area which represents a seismic gap in the Italian historical earthquake catalogue. We investigate the geometrical, mechanical and statistical characteristics of the largest earthquakes and of the entire swarm. We calculate the focal mechanisms of the M-l > 3 events in the sequence and the transfer of Coulomb stress on nearby known faults and analyse the statistics of the earthquake catalogue. We find that only 25 per cent of the earthquakes in the sequence can be explained as aftershocks, and the remaining 75 per cent may be attributed to a transient forcing. The b-values change in time throughout the sequence, with low b-values correlated with the period of highest rate of activity and with the occurrence of the largest shock. In the light of recent studies on the palaeoseismic and historical activity in the Pollino area, we identify two scenarios consistent with the observations and our analysis: This and past seismic swarms may have been 'passive' features, with small fault patches failing on largely locked faults, or may have been accompanied by an 'active', largely aseismic, release of a large portion of the accumulated tectonic strain. Those scenarios have very different implications for the seismic hazard of the area. KW - Seismicity and tectonics KW - Statistical seismology KW - Dynamics: seismotectonics Y1 - 2015 U6 - https://doi.org/10.1093/gji/ggv111 SN - 0956-540X SN - 1365-246X VL - 201 IS - 3 SP - 1553 EP - 1567 PB - Oxford Univ. Press CY - Oxford ER - TY - GEN A1 - Passarelli, Luigi A1 - Hainzl, Sebastian A1 - Cesca, Simone A1 - Maccaferri, Francesco A1 - Mucciarelli, Marco A1 - Roessler, Dirk A1 - Corbi, Fabio A1 - Dahm, Torsten A1 - Rivalta, Eleonora T1 - Aseismic transient driving the swarm-like seismic sequence in the Pollino range, Southern Italy (vol 201, pg 1553, 2015) T2 - Geophysical journal international KW - Seismicity and tectonics KW - Statistical seismology KW - Dynamics: seismotectonics Y1 - 2016 U6 - https://doi.org/10.1093/gji/ggv425 SN - 0956-540X SN - 1365-246X VL - 204 SP - 365 EP - 365 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 - Petersen, Gesa Maria A1 - Cesca, Simone A1 - Kriegerowski, Marius T1 - Automated quality control for large seismic networks BT - implementation and application to the AlpArray seismic network JF - Seismological research letters N2 - As a consequence of the rapid growing worldwide seismic data set, a huge variety of automatized data-processing methods have been developed. To perform automatized waveform-based seismological studies aiming for magnitudes or source process inversion, it is crucial to identify network stations with erroneous transfer functions, gain factors, or component orientations. We developed a new tool dedicated to automated station quality control of dense seismic networks and arrays. The python-based AutoStatsQ toolbox uses the pyrocko seismic data-processing environment. The toolbox automatically downloads data and metadata for selected teleseismic events and performs different tests. As a result, relative gain factors, sensor orientation corrections, and reliable frequency bands are computed for all stations in a chosen time period. Relative gain factors are calculated for all stations and events in a time domain based on maximum P-phase amplitudes. A Rayleigh-wave polarization analysis is used to identify deviating sensor orientations. The power spectra of all stations in a given frequency range are compared with synthetic ones, accessing Global Centroid Moment Tensor (CMT) solutions. Frequency ranges of coinciding synthetic and recorded power spectral densities (PSDs) may serve as guidelines for choosing band-pass filters for moment tensor (MT) inversion and help confirm the corner frequency of the instrument. The toolbox was applied to the permanent and temporary AlpArray networks as well as to the denser SWATH-D network, a total of over 750 stations. Stations with significantly deviating gain factors were identified, as well as stations with inverse polarity and misorientations of the horizontal components. The tool can be used to quickly access network quality and to omit or correct stations before MT inversion. Electronic Supplement: List of teleseismic events and tables of median, mean, and standard deviation of relative gain factors, and figures of relative gain factors of all event-station pairs, waveform example showing inverse polarity of horizontal components on ZS.D125, histograms of median, mean, and standard deviation of the correction angles, examples of synthetic and recorded frequency spectra of ZS.D046 and NI.VINO. Y1 - 2019 U6 - https://doi.org/10.1785/0220180342 SN - 0895-0695 SN - 1938-2057 VL - 90 IS - 3 SP - 1177 EP - 1190 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 - Grigoli, Francesco A1 - Cesca, Simone A1 - Amoroso, Ortensia A1 - Emolo, Antonio A1 - Zollo, Aldo A1 - Dahm, Torsten T1 - Automated seismic event location by waveform coherence analysis JF - Geophysical journal international N2 - Automated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring. Since microseismic records are generally characterized by low signal-to-noise ratio, automated location methods are requested to be noise robust and sufficiently accurate. Most of the standard automated location routines are based on the automated picking, identification and association of the first arrivals of P and S waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases. Although current methods can accurately pick P onsets, the automatic picking of the S onset is still problematic, especially when the P coda overlaps the S wave onset. In this paper, we propose a picking free earthquake location method based on the use of the short-term-average/long-term-average (STA/LTA) traces at different stations as observed data. For the P phases, we use the STA/LTA traces of the vertical energy function, whereas for the S phases, we use the STA/LTA traces of a second characteristic function, which is obtained using the principal component analysis technique. In order to locate the seismic event, we scan the space of possible hypocentral locations and origin times, and stack the STA/LTA traces along the theoretical arrival time surface for both P and S phases. Iterating this procedure on a 3-D grid, we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial coordinates of the seismic event. A pilot application was performed in the Campania-Lucania region (southern Italy) using a seismic network (Irpinia Seismic Network) with an aperture of about 150 km. We located 196 crustal earthquakes (depth < 20 km) with magnitude range 1.1 < M-L < 2.7. A subset of these locations were compared with accurate manual locations refined by using a double-difference technique. Our results indicate a good agreement with manual locations. Moreover, our method is noise robust and performs better than classical location methods based on the automatic picking of the P and S waves first arrivals. KW - Time-series analysis KW - Inverse theory KW - Earthquake source observations KW - Seismicity and tectonics KW - Early warning Y1 - 2014 U6 - https://doi.org/10.1093/gji/ggt477 SN - 0956-540X SN - 1365-246X VL - 196 IS - 3 SP - 1742 EP - 1753 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Negi, Sanjay S. A1 - Paul, Ajay A1 - Cesca, Simone A1 - Kamal, A1 - Kriegerowski, Marius A1 - Mahesh, P. A1 - Gupta, Sandeep T1 - Crustal velocity structure and earthquake processes of Garhwal-Kumaun Himalaya: Constraints from regional waveform inversion and array beam modeling JF - Tectonophysics : international journal of geotectonics and the geology and physics of the interior of the earth N2 - In order to understand present day earthquake kinematics at the Indian plate boundary, we analyse seismic broadband data recorded between 2007 and 2015 by the regional network in the Garhwal-Kumaun region, northwest Himalaya. We first estimate a local 1-D velocity model for the computation of reliable Green's functions, based on 2837 P-wave and 2680 S-wave arrivals from 251 well located earthquakes. The resulting 1-D crustal structure yields a 4-layer velocity model down to the depths of 20 km. A fifth homogeneous layer extends down to 46 km, constraining the Moho using travel-time distance curve method. We then employ a multistep moment tensor (MT) inversion algorithm to infer seismic moment tensors of 11 moderate earthquakes with Mw magnitude in the range 4.0–5.0. The method provides a fast MT inversion for future monitoring of local seismicity, since Green's functions database has been prepared. To further support the moment tensor solutions, we additionally model P phase beams at seismic arrays at teleseismic distances. The MT inversion result reveals the presence of dominant thrust fault kinematics persisting along the Himalayan belt. Shallow low and high angle thrust faulting is the dominating mechanism in the Garhwal-Kumaun Himalaya. The centroid depths for these moderate earthquakes are shallow between 1 and 12 km. The beam modeling result confirm hypocentral depth estimates between 1 and 7 km. The updated seismicity, constrained source mechanism and depth results indicate typical setting of duplexes above the mid crustal ramp where slip is confirmed along out-of-sequence thrusting. The involvement of Tons thrust sheet in out-of-sequence thrusting indicate Tons thrust to be the principal active thrust at shallow depth in the Himalayan region. Our results thus support the critical taper wedge theory, where we infer the microseismicity cluster as a result of intense activity within the Lesser Himalayan Duplex (LHD) system. KW - Critical taper wedge KW - Lesser Himalayan Duplex KW - Out-of-sequence thrust Y1 - 2017 U6 - https://doi.org/10.1016/j.tecto.2017.05.007 SN - 0040-1951 SN - 1879-3266 VL - 712 SP - 45 EP - 63 PB - Elsevier CY - Amsterdam 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 - TY - GEN A1 - Kriegerowski, Marius A1 - Cesca, Simone A1 - Ohrnberger, Matthias A1 - Dahm, Torsten A1 - Krüger, Frank T1 - Event couple spectral ratio Q method for earthquake clusters BT - application to northwest Bohemia T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - We develop an amplitude spectral ratio method for event couples from clustered earthquakes to estimate seismic wave attenuation (Q-1) in the source volume. The method allows to study attenuation within the source region of earthquake swarms or aftershocks at depth, independent of wave path and attenuation between source region and surface station. We exploit the high-frequency slope of phase spectra using multitaper spectral estimates. The method is tested using simulated full wave-field seismograms affected by recorded noise and finite source rupture. The synthetic tests verify the approach and show that solutions are independent of focal mechanisms but also show that seismic noise may broaden the scatter of results. We apply the event couple spectral ratio method to northwest Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth. Our method indicates a strong anomaly of high attenuation in the source region of the swarm with an averaged attenuation factor of Qp < 100. The application to S phases fails due to scattered P-phase energy interfering with S phases. The Qp anomaly supports the common hypothesis of highly fractured and fluid saturated rocks in the source region of the swarms in northwest Bohemia. However, high temperatures in a small volume around the swarms cannot be excluded to explain our observations. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 683 KW - west bohemia KW - attenuation tomography KW - swarm earthquakes KW - focal zone KW - parameters KW - locations KW - fault Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-426029 IS - 683 ER - TY - JOUR A1 - Kriegerowski, Marius A1 - Cesca, Simone A1 - Ohrnberger, Matthias A1 - Dahm, Torsten A1 - Krüger, Frank T1 - Event couple spectral ratio Q method for earthquake clusters BT - application to northwest Bohemia JF - Solid Earth N2 - We develop an amplitude spectral ratio method for event couples from clustered earthquakes to estimate seismic wave attenuation (Q-1) in the source volume. The method allows to study attenuation within the source region of earthquake swarms or aftershocks at depth, independent of wave path and attenuation between source region and surface station. We exploit the high-frequency slope of phase spectra using multitaper spectral estimates. The method is tested using simulated full wave-field seismograms affected by recorded noise and finite source rupture. The synthetic tests verify the approach and show that solutions are independent of focal mechanisms but also show that seismic noise may broaden the scatter of results. We apply the event couple spectral ratio method to northwest Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth. Our method indicates a strong anomaly of high attenuation in the source region of the swarm with an averaged attenuation factor of Qp < 100. The application to S phases fails due to scattered P-phase energy interfering with S phases. The Qp anomaly supports the common hypothesis of highly fractured and fluid saturated rocks in the source region of the swarms in northwest Bohemia. However, high temperatures in a small volume around the swarms cannot be excluded to explain our observations. KW - west bohemia KW - attenuation tomography KW - swarm earthquakes KW - focal zone KW - parameters KW - locations KW - fault Y1 - 2019 U6 - https://doi.org/10.5194/se-10-317-2019 SN - 1869-9529 IS - 10 SP - 317 EP - 328 PB - Copernicus Publications CY - Göttingen ER -