The Corinth Rift Laboratory, Greece (CRL): A Multidisciplinary Near Fault Observatory (NFO) on a Fast Rifting System

The western rift of Corinth (Greece) is one of the most active tectonic structures of the euro-mediterranean area. Its NS opening rate is 1.5 cm/yr ( strain rate of 10-6/yr) results into a high microseismicity level and a few destructive, M>6 earthquakes per century, activating a system of mostly north dipping normal faults. Since 2001, monitoring arrays of the European Corinth Rift Laboratory (CRL, www.crlab.eu) allowed to better track the mechanical processes at work, with short period and broad band seismometers, cGPS, borehole strainmeters, EM stations, …). The recent (300 kyr) tectonic history has been revealed by onland (uplifted fan deltas and terraces) and offshore geological studies (mapping, shallow seismic, coring), showing a fast evolution of the normal fault system. The microseismicity, dominated by swarms lasting from days to months, mostly clusters in a layer 1 to 3 km thick, between 6 and 9 km in depth, dipping towards north, on which most faults are rooting. The diffusion of the microseismicity suggests its triggering by pore pressure transients, with no or barely detected strain. Despite a large proportion of multiplets, true repeaters seem seldom, suggesting a minor contribution of creep in their triggering, although transient or steady creep is clearly detected on the shallow part of some majors faults. The microseismic layer may thus be an immature, downward growing detachment, and the dominant rifting mechanism might be a mode I, anelastic strain beneath the rift axis , for which a mechanical model is under development. Paleoseismological (trenching, paleoshorelines, turbidites), archeological and historical studies completed the catalogues of instrumental seismicity, motivating attempts of time dependent hazard assessment. The Near Fault Observatory of CRL is thus a multidisciplinary research infrastructure aiming at a better understanding and modeling of multiscale, coupled seismic/aseismic processes on fault systems.Grant for Researchers (CC) ID 188753

Fault Delineation and Regional Stress Direction from the Analysis of Background Microseismicity in the southern Apennines, Italy

The present active faults and stress field acting in the southern Apennines (Italy), a structurally complex area characterized by high seismic potential, are studied by analyzing the background microseismicity (M <= 3). We used a microearthquake data set consisting of 1312 events that occurred from August 2005 to April 2011 by integrating the data recorded at 42 seismic stations of various networks. The refined seismicity locations and focal mechanisms delineate a system of northwest-southeast striking normal faults along the Apenninic chain and an approximately east-west oriented strike-slip fault transversely cutting the belt. The seismicity along the chain does not occur on a single fault but in a volume, delimited by the faults activated during the 1980 Irpinia M 6.9 earthquake on subparallel predominantly normal faults. Results show that the recent low magnitude earthquakes belong to the background seismicity, and they are likely generated along the major fault segments activated during the most recent earthquakes, suggesting that they are still active today, 30 years after the main-shock occurrences. The stress inversion from the whole data set suggests that a unique anti-Apenninic extensional regional stress field could explain the two different faulting styles characterizing the earthquakes that occur along the chain and the east-west fault dissecting the belt. On the other hand, the results obtained by separately inverting the Irpinia and the Potenza clusters indicate a more complex model that would predict a change from a normal-faulting regime, acting in the inner sector of the chain, to a strike-slip regime moving eastward and down-depth in the Potenza area

LASTQUAKE, UN SYSTEME D'INFORMATION MULTICANAL POUR LA REDUCTION DU RISQUE SISMIQUE GLOBAL

International audienceSummary LastQuake is a multichannel (websites, Twitter and Telegram quakebots, smartphone app) real time information system developed by the European Mediterranean Seismological Centre (EMSC), one of the top global earthquake information centre and targeting eyewitnesses. Based on visual communication, LastQuake informs in real time eyewitnesses on earthquakes and their effects, massively crowdsource their experiences (felt reports, geo-located pics, open comments) and offers geo-targeted safety tips immediately after violent shaking. We show how these affordable and standard Internet and smartphone technologies fulfill eyewitnesses' needs during earthquake emergencies and how they complement existing strategies for contributing to immediate reduction of seismic risk at global scaleLastQuake est un système multicanal (sites Internet, robot de publication Twitter et Telegram, application smartphone) d'information sismologique en temps réel à destination du public développé par le Centre Sismologique Euro-Méditerranéen (CSEM), un des principaux centres d'information sismologique au monde. Basé sur la communication visuelle, LastQuake informe ses utilisateurs sur les séismes et leurs effets, collecte massivement leur témoignages et, en cas de violente séisme, leur indique les comportements à privilégier et éviter immédiatement après la secousse. Nous montrons comment ces outils numériques simples et bon marché répondent à un besoin des citoyens et peuvent contribuer, en complément des stratégies existantes à la réduction du risque sismique global

The 2013 earthquake swarm in Helike, Greece: Seismic activity at the root of old normal faults

The Corinth Rift in Central Greece has been studied extensively during the past decades, as it is one of the most seismically active regions in Europe. It is characterized by normal faulting and extension rates between 6 and 15 mm yr-1 in an approximately N10E° direction. On 2013 May 21, an earthquake swarm was initiated with a series of small events 4 km southeast of Aigion city. In the next days, the seismic activity became more intense, with outbursts of several stronger events of magnitude between 3.3 and 3.7. The seismicity migrated towards the east during June, followed by a sudden activation of the western part of the swarm on July 15th. More than 1500 events have been detected and manually analysed during the period between 2013 May 21 and August 31, using over 15 local stations in epicentral distances up to 30 km and a local velocity model determined by an error minimization method. Waveform similarity-based analysis was performed, revealing several distinct multiplets within the earthquake swarm. High-resolution relocation was applied using the double-difference algorithm HypoDD, incorporating both catalogue and cross-correlation differential traveltime data, which managed to separate the initial seismic cloud into several smaller, densely concentrated spatial clusters of strongly correlated events. Focal mechanism solutions for over 170 events were determined using P-wave first motion polarities, while regional waveform modelling was applied for the calculation of moment tensors for the 18 largest events of the sequence. Selected events belonging to common spatial groups were considered for the calculation of composite mechanisms to characterize different parts of the swarm. The solutions are mainly in agreement with the regional NNE-SSW extension, representing typical normal faulting on 30-50° north-dipping planes, while a few exhibit slip in an NNE-SSW direction, on a roughly subhorizontal plane. Moment magnitudes were calculated by spectral analysis of S waves, yielding b-values between 1.1 and 1.2 in their frequency-magnitude distribution. The seismic moment release history indicates swarm-like activity during the first phase, which could have acted as a preparatory stage for the second phase (after 12 July) that presented a more typical main-shock-aftershock behaviour. The spatiotemporal analysis reveals that the swarm has occurred in a volume that is likely related with the extension at depth of the NNE-dipping Pirgaki normal fault, outcropping ~8 km to the south. The slow velocity of eastward migration of the epicentres during June implies triggering by fluids. The situation appears different in the second phase of the sequence, which was probably triggered by a build-up of stress during the first one. The relatively deep hypocentres of the 2013 swarm, compared to the shallower seismic layer within the rift, and their coincidence with the steeply dipping Pirgaki fault, favour an immature rift detachment model. Previous results from instrumental data indicate that approximately the same region had been activated during July-August 1991. The availability of the dense permanent seismological network data thus allowed for a detailed analysis of this crisis, a better understanding of its mechanical context and of the earlier events. © The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved

The 2013 earthquake swarm in Helike, Greece: seismic activity at the root of old normal faults

International audienceThe Corinth Rift in Central Greece has been studied extensively during the past decades, as it is one of the most seismically active regions in Europe. It is characterized by normal faulting and extension rates between 6 and 15 mm yr−1 in an approximately N10E° direction. On 2013 May 21, an earthquake swarm was initiated with a series of small events 4 km southeast of Aigion city. In the next days, the seismic activity became more intense, with outbursts of several stronger events of magnitude between 3.3 and 3.7. The seismicity migrated towards the east during June, followed by a sudden activation of the western part of the swarm on July 15th. More than 1500 events have been detected and manually analysed during the period between 2013 May 21 and August 31, using over 15 local stations in epicentral distances up to 30 km and a local velocity model determined by an error minimization method. Waveform similarity-based analysis was performed, revealing several distinct multiplets within the earthquake swarm. High-resolution relocation was applied using the double-difference algorithm HypoDD, incorporating both catalogue and cross-correlation differential traveltime data, which managed to separate the initial seismic cloud into several smaller, densely concentrated spatial clusters of strongly correlated events. Focal mechanism solutions for over 170 events were determined using P-wave first motion polarities, while regional waveform modelling was applied for the calculation of moment tensors for the 18 largest events of the sequence. Selected events belonging to common spatial groups were considered for the calculation of composite mechanisms to characterize different parts of the swarm. The solutions are mainly in agreement with the regional NNE–SSW extension, representing typical normal faulting on 30–50° north-dipping planes, while a few exhibit slip in an NNE–SSW direction, on a roughly subhorizontal plane. Moment magnitudes were calculated by spectral analysis of S waves, yielding b-values between 1.1 and 1.2 in their frequency–magnitude distribution. The seismic moment release history indicates swarm-like activity during the first phase, which could have acted as a preparatory stage for the second phase (after 12 July) that presented a more typical main-shock–aftershock behaviour. The spatiotemporal analysis reveals that the swarm has occurred in a volume that is likely related with the extension at depth of the NNE-dipping Pirgaki normal fault, outcropping ∼8 km to the south. The slow velocity of eastward migration of the epicentres during June implies triggering by fluids. The situation appears different in the second phase of the sequence, which was probably triggered by a build-up of stress during the first one. The relatively deep hypocentres of the 2013 swarm, compared to the shallower seismic layer within the rift, and their coincidence with the steeply dipping Pirgaki fault, favour an immature rift detachment model. Previous results from instrumental data indicate that approximately the same region had been activated during July–August 1991. The availability of the dense permanent seismological network data thus allowed for a detailed analysis of this crisis, a better understanding of its mechanical context and of the earlier events