@article{GrigoliCescaRinaldietal.2018, author = {Grigoli, Francesco and Cesca, Simone and Rinaldi, Antonio Pio and Manconi, Andrea and Lopez-Comino, Jos{\´e} {\´A}ngel and Clinton, John F. and Westaway, Rob and Cauzzi, Carlo and Dahm, Torsten and Wiemer, Stefan}, title = {The November 2017 M-w 5.5 Pohang earthquake}, series = {Science}, volume = {360}, journal = {Science}, number = {6392}, publisher = {American Assoc. for the Advancement of Science}, address = {Washington}, issn = {0036-8075}, doi = {10.1126/science.aat2010}, pages = {1003 -- 1006}, year = {2018}, abstract = {The moment magnitude (M-w) 5.5 earthquake that struck South Korea in November 2017 was one of the largest and most damaging events in that country over the past century. Its proximity to an enhanced geothermal system site, where high-pressure hydraulic injection had been performed during the previous 2 years, raises the possibility that this earthquake was anthropogenic. We have combined seismological and geodetic analyses to characterize the mainshock and its largest aftershocks, constrain the geometry of this seismic sequence, and shed light on its causal factors. According to our analysis, it seems plausible that the occurrence of this earthquake was influenced by the aforementioned industrial activities. Finally, we found that the earthquake transferred static stress to larger nearby faults, potentially increasing the seismic hazard in the area.}, language = {en} } @article{CescaGrigoliHeimannetal.2016, author = {Cesca, Simone and Grigoli, Francesco and Heimann, Sebastian and Dahm, Torsten and Kriegerowski, Marius and Sobiesiak, M. and Tassara, C. and Olcay, M.}, title = {The M-w 8.1 2014 Iquique, Chile, seismic sequence: a tale of foreshocks and aftershocks}, series = {Geophysical journal international}, volume = {204}, journal = {Geophysical journal international}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0956-540X}, doi = {10.1093/gji/ggv544}, pages = {1766 -- 1780}, year = {2016}, abstract = {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.}, language = {en} } @article{delFresnoDominguezCerdenaCescaetal.2015, author = {del Fresno, Carmen and Dominguez Cerdena, Itahiza and Cesca, Simone and Buforn, Elisa}, title = {The 8 October 2011 Earthquake at El Hierro (M-w 4.0): Focal Mechanisms of the Mainshock and Its Foreshocks}, series = {Bulletin of the Seismological Society of America}, volume = {105}, journal = {Bulletin of the Seismological Society of America}, number = {1}, publisher = {Seismological Society of America}, address = {Albany}, issn = {0037-1106}, doi = {10.1785/0120140151}, pages = {330 -- 340}, year = {2015}, abstract = {We have studied the focal mechanism of an M-w 4.0 earthquake that occurred on 8 October 2011 in the southwest of El Hierro (Canary Islands), the largest shock of the swarm that preceded the submarine eruption of El Hierro 2011-2012. The joint focal mechanism solution of 34 foreshocks has also been obtained. The results for the mainshock are consistent with a pure double-couple mechanism of a strike-slip motion with a reverse component and a focal depth of 12-13 km. The stress pattern obtained from the focal mechanism indicates horizontal compression in a north-northwest-south-southeast direction, parallel to the southern ridge of the island, and a quasi-horizontal extension in an east-west direction. Similar stress pattern is derived from the joint solution of the foreshocks. The occurrence of this family of earthquakes at the moment of the maximum strain rate of the pre-eruptive swarm suggests that their rupture process is related to tectonic stress, which led to the eruption only two days later, 5 km away from the mainshock epicenter.}, language = {en} } @article{CescaGrigoliHeimannetal.2014, author = {Cesca, Simone and Grigoli, Francesco and Heimann, Sebastian and Gonzalez, Alvaro and Buforn, Elisa and Maghsoudi, Samira and Blanch, Estefania and Dahm, Torsten}, title = {The 2013 September-October seismic sequence offshore Spain: a case of seismicity triggered by gas injection?}, series = {Geophysical journal international}, volume = {198}, journal = {Geophysical journal international}, number = {2}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0956-540X}, doi = {10.1093/gji/ggu172}, pages = {941 -- 953}, year = {2014}, abstract = {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.}, language = {en} } @article{PassarelliRivaltaCescaetal.2015, author = {Passarelli, Luigi and Rivalta, Eleonora and Cesca, Simone and Aoki, Yosuke}, title = {Stress changes, focal mechanisms, and earthquake scaling laws for the 2000 dike at Miyakejima (Japan)}, series = {Journal of geophysical research : Solid earth}, volume = {120}, journal = {Journal of geophysical research : Solid earth}, number = {6}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9313}, doi = {10.1002/2014JB011504}, pages = {4130 -- 4145}, year = {2015}, abstract = {Faulting processes in volcanic areas result from a complex interaction of pressurized fluid-filled cracks and conduits with the host rock and local and regional tectonic setting. Often, volcanic seismicity is difficult to decipher in terms of the physical processes involved, and there is a need for models relating the mechanics of volcanic sources to observations. Here we use focal mechanism data of the energetic swarm induced by the 2000 dike intrusion at Miyakejima (Izu Archipelago, Japan), to study the relation between the 3-D dike-induced stresses and the characteristics of the seismicity. We perform a clustering analysis on the focal mechanism (FM) solutions and relate them to the dike stress field and to the scaling relationships of the earthquakes. We find that the strike and rake angles of the FMs are strongly correlated and cluster on bands in a strike-rake plot. We suggest that this is consistent with optimally oriented faults according to the expected pattern of Coulomb stress changes. We calculate the frequency-size distribution of the clustered sets finding that focal mechanisms with a large strike-slip component are consistent with the Gutenberg-Richter relation with a b value of about 1. Conversely, events with large normal faulting components deviate from the Gutenberg-Richter distribution with a marked roll-off on its right-hand tail, suggesting a lack of large-magnitude events (M-w>5.5). This may result from the interplay of the limited thickness and lower rock strength of the layer of rock above the dike, where normal faulting is expected, and lower stress levels linked to the faulting style and low confining pressure.}, language = {en} } @article{CescaBraunMaccaferrietal.2013, author = {Cesca, Simone and Braun, Thomas and Maccaferri, Francesco and Passarelli, Luigi and Rivalta, Eleonora and Dahm, Torsten}, title = {Source modelling of the M5-6 Emilia-Romagna, Italy, earthquakes (2012 May 20-29)}, series = {Geophysical journal international}, volume = {193}, journal = {Geophysical journal international}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0956-540X}, doi = {10.1093/gji/ggt069}, pages = {1658 -- 1672}, year = {2013}, abstract = {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.}, language = {en} } @article{ValenzuelaMalebranCescaLopezCominoetal.2022, author = {Valenzuela-Malebran, Carla and Cesca, Simone and Lopez-Comino, Jos{\´e} {\´A}ngel and Zeckra, Martin and Kr{\"u}ger, F. and Dahm, Torsten}, title = {Source mechanisms and rupture processes of the Jujuy seismic nest, Chile-Argentina border}, series = {Journal of South American earth sciences}, volume = {117}, journal = {Journal of South American earth sciences}, publisher = {Elsevier}, address = {Oxford}, issn = {0895-9811}, doi = {10.1016/j.jsames.2022.103887}, pages = {13}, year = {2022}, abstract = {The Altiplano-Puna plateau, in Central Andes, is the second-largest continental plateau on Earth, extending between 22 degrees and 27 degrees S at an average altitude of 4400 m. The Puna plateau has been formed in consequence of the subduction of the oceanic Nazca Plate beneath the continental South American plate, which has an average crustal thickness of 50 km at this location. A large seismicity cluster, the Jujuy cluster, is observed at depth of 150-250 km beneath the central region of the Puna plateau. The cluster is seismically very active, with hundreds of earthquakes reported and a peak magnitude MW 6.6 on 25th August 2006. The cluster is situated in one of three band of intermediate-depth focus seismicity, which extend parallel to the trench roughly North to South. It has been hypothesized that the Jujuy cluster could be a seismic nest, a compact seismogenic region characterized by a high stationary activity relative to its surroundings. In this study, we collected more than 40 years of data from different catalogs and proof that the cluster meets the three conditions of a seismic nest. Compared to other known intermediate depth nests at Hindu Kush (Afganisthan) or Bucaramanga (Colombia), the Jujuy nest presents an outstanding seismicity rate, with more than 100 M4+ earthquakes per year. We additionally performed a detailed analysis of the rupture process of some of the largest earthquakes in the nest, by means of moment tensor inversion and directivity analysis. We focused on the time period 2017-2018, where the seismic monitoring was the most extended. Our results show that earthquakes in the nest take place within the eastward subducting oceanic plate, but rupture along sub-horizontal planes dipping westward. We suggest that seismicity at Jujuy nest is controlled by dehydration processes, which are also responsible for the generation of fluids ascending to the crust beneath the Puna volcanic region. We use the rupture plane and nest geometry to provide a constraint to maximal expected magnitude, which we estimate as MW -6.7.}, language = {en} } @article{KaramzadehKuehnKriegerowskietal.2019, author = {Karamzadeh, Nasim Toularoud and K{\"u}hn, Daniela and Kriegerowski, Marius and L{\´o}pez-Comino, Jos{\´e} {\´A}ngel and Cesca, Simone and Dahm, Torsten}, title = {Small-aperture array as a tool to monitor fluid injection- and extraction-induced microseismicity}, series = {Acta Geophysica}, volume = {67}, journal = {Acta Geophysica}, number = {1}, publisher = {Springer}, address = {Cham}, issn = {1895-6572}, doi = {10.1007/s11600-018-0231-1}, pages = {311 -- 326}, year = {2019}, abstract = {The monitoring of microseismicity during temporary human activities such as fluid injections for hydrofracturing, hydrothermal stimulations or wastewater disposal is a difficult task. The seismic stations often cannot be installed on hard rock, and at quiet places, noise is strongly increased during the operation itself and the installation of sensors in deep wells is costly and often not feasible. The combination of small-aperture seismic arrays with shallow borehole sensors offers a solution. We tested this monitoring approach at two different sites: (1) accompanying a fracking experiment in sedimentary shale at 4km depth and (2) above a gas field under depletion. The small-aperture arrays were planned according to theoretical wavenumber studies combined with simulations considering the local noise conditions. We compared array recordings with recordings available from shallow borehole sensors and give examples of detection and location performance. Although the high-frequency noise on the 50-m-deep borehole sensors was smaller compared to the surface noise before the injection experiment, the signals were highly contaminated during injection by the pumping activities. Therefore, a set of three small-aperture arrays at different azimuths was more suited to detect small events, since noise recorded on these arrays is uncorrelated with each other. Further, we developed recommendations for the adaptation of the monitoring concept to other sites experiencing induced seismicity.}, language = {en} } @article{CescaSenDahm2014, author = {Cesca, Simone and Sen, Ali Tolga and Dahm, Torsten}, title = {Seismicity monitoring by cluster analysis of moment tensors}, series = {Geophysical journal international}, volume = {196}, journal = {Geophysical journal international}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0956-540X}, doi = {10.1093/gji/ggt492}, pages = {1813 -- 1826}, year = {2014}, abstract = {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.}, language = {en} } @article{HeimannGonzalezWangetal.2013, author = {Heimann, Sebastian and Gonzalez, Alvaro and Wang, Rongjiang and Cesca, Simone and Dahm, Torsten}, title = {Seismic characterization of the Chelyabinsk Meteor's terminal explosion}, series = {Seismological research letters}, volume = {84}, journal = {Seismological research letters}, number = {6}, publisher = {Seismological Society of America}, address = {Albany}, issn = {0895-0695}, doi = {10.1785/0220130042}, pages = {1021 -- 1025}, year = {2013}, language = {en} }