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All the significant (M L ≥ 4) events in the 2012 Haida Gwaii earthquake sequence are systematically relocated, and their moment tensor solutions are determined from waveform inversion. The focal mechanism of the mainshock shows lowangle thrust faulting along a shallowly dipping plane with a strike parallel to the Queen Charlotte fault (QCF), consistent with the inference of Pacific plate underthrusting beneath the overriding North American plate. The epicenter of the mainshock is located ∼5 km landward (northeast) of the surface trace of the QCF, suggesting the nucleation of the rupture was near the bottom of the seismogenic (locked) interface. Significant aftershocks appear to cluster on the periphery of the main rupture zone with most events located immediately seaward of the deformation front. The majority of these events show normal-faulting mechanisms that are probably associated with the bending stress within the Pacific plate near the deformation front. Several normal and strike-slip events at greater depths within the subducted Pacific slab show a consistent pattern of T-axis in the down-dip direction, implying the subducted plate is under a stress regime of down-dip extension. Only a few strike-slip events were observed along or near the QCF. The limited size and distribution of these events suggest that most of the elastic strain accumulated along the QCF was not released during the 2012 Haida Gwaii sequence. Major strike-slip earthquakes are likely to occur along the southernmost part of the QCF system in the future.

International audienceThe nature of the transition between the Zagros intra-continental collision and the Makran oceanic subduction is a matter of debate: either a major fault cutting the whole lithosphere or a more progressive transition associated with a shallow gently dipping fault restricted to the crust. Microearthquake seismicity located around the transition between the transition zone is restricted to the west of the Jaz-Murian depression and the Jiroft fault. No shallow micro-earthquakes seem to be related to the NNW–SSE trending Zendan–Minab–Palami active fault system. Most of the shallow seismicity is related either to the Zagros mountain belt, located in the west, or to the NS trending Sabzevaran–Jiroft fault system, located in the north. The depth of microearthquakes increases northeastwards to an unusually deep value (for the Zagros) of 40 km. Two dominant types of focal mechanisms are observed in this region: low-angle thrust faulting, mostly restricted to the lower crust, and strike-slip at shallow depths, both consistent with NS shortening. The 3-D inversion of P traveltimes suggests a high-velocity body dipping northeastwards to a depth of 25 km. This high-velocity body, probably related to the lower crust, is associated with the deepest earthquakes showing reverse faulting. We propose that the transition between the Zagros collision and the Makran subduction is not a sharp lithospheric-scale transform fault associated with the Zendan–Minab–Palami fault system. Instead it is a progressive transition located in the lower crust. The oblique collision results in partial partitioning between strike-slip and shortening components within the shallow brittle crust because of the weakness of the pre-existing Zendan–Minab–Palami faults

We systematically re-analyzed historical seismograms to verify the existence of background seismicity in the Horn River Basin of northeast British Columbia before the start of regional shale gas development. We also carefully relocated local earthquakes that occurred between December 2006 and December 2011 to delineate their spatiotemporal relationship with hydraulic fracturing (HF) operations in the region. Scattered seismic events were detected in the Horn River Basin throughout the study periods. The located seismicity within 100 km of the Fort Nelson seismic station had a clearly increasing trend, specifically in the Etsho area where most local HF operations were performed. The number of events was increased from 24 in 2002 -2003 to 131 in 2011 (peak period of HF operations). In addition, maximum magnitude of the events was shifted from M L 2.9 to M L 3.6 as the scale of HF operation expanded from 2006-2007 to 2011. Based on our relocated earthquake catalog, the overall b value is estimated at 1.21, which is higher than the average of tectonic/natural earthquakes of ϳ1.0. Our observations highly support the likelihood of a physical relationship between HF operation and induced seismicity in the Horn River Basin. Unfortunately, due to the sparse station density in the region, depth resolution is poor for the vast majority of events in our study area. As new seismograph stations are established in northeast British Columbia, both epicentral mislocation and depth uncertainty for future events are expected to improve significantly.Résumé : Nous analysons à nouveau et de manière systématique des sismogrammes historiques dans le but de vérifier l'existence d'une sismicité de fond dans le bassin de Horn River, nord-est de la Colombie-Britannique, avant le début d'un développement régional de gaz de shale. Nous avons aussi soigneusement relocalisé les séismes locaux survenus entre décembre 2006 et décembre 2011 afin de délimiter leurs relations spatiotemporelles par rapport aux opérations de fracturation hydraulique (FH) dans la région. Des événements sismiques dispersés ont été détectés dans le bassin de Horn River durant toutes les périodes d'étude. La sismicité localisée dans un rayon de 100 km de la station sismique de Fort Nelson montrait nettement une tendance croissante, surtout dans le secteur d'Etsho où se déroulaient la plupart des opérations de FH. Le nombre d'événements a crû de 24 en 2002-2003 (avant les opérations de FH) à 131 en 2011 (période de pointe des opérations de FH). De plus, la magnitude maximale des événements est passée de M L 2,9 à M L 3,6 à mesure qu'augmentait l'échelle des opérations de FH de 2006de FH de -2007de FH de à 2011. En se basant sur notre catalogue des séismes relocalisés, la valeur b générale est estimée à 1,21, ce qui est supérieur à la moyenne des séismes tectoniques/naturels de ϳ1,0. Nos observations supportent fortement la possibilité d'une relation physique entre les opérations de FH et la sismicité induite dans le bassin de Horn River. Malheureusement, en raison de ...