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In the BATHOLITHSonland seismic project, a refraction--wide-angle reflection survey was shot in 2009 across the Coast Mountains and Interior Plateau of central British Columbia. Part of the seismic profile crossed the Nechako Basin, a Jurassic-Cretaceous basin with potential for hydrocarbons within sedimentary strata that underlies widespread volcanic rocks. Along this 205 km-long line segment, eight large explosive shots were fired into 980 seismometers. Forward and inverse modelling of the traveltime data were conducted with two independent methods: ray-tracing based modelling of first and secondary arrivals, and a higher resolution wavefront-based first-arrival seismic tomography. Material with velocities less than 5.0 km/s is interpreted as sedimentary rocks of the Nechako Basin, while velocities from 5.0-6.0 km/s may correspond to interlayered sedimentary and volcanic rocks. The greatest thickness of sedimentary rocks in the basin is found in the central 110 km of the profile. Two sub-basins were identified in this region, with widths of 20-50 km and maximum sedimentary depths of 2.5 and 3.3 km. Such features are well-defined in the velocity model, since resolution tests indicate that features with widths greater than ~13 km are reliable. Beneath the sedimentary rocks, seismic velocities increase more slowly with depth--from 6.0 km/s just below the basin to 6.3 km/s at ~17 km in depth, and then to 6.8-7.0 km/s at the base of the crust. The Moho is found at a depth of 33.5-35 km beneath the profile, and mantle velocities are high at 8.058.10 km/s. En 2009, dans le cadre du projet sismique BATHOLITHSonland, un releve refraction--reflexion grand angle a ete tire a travers la Chaine cotiere et le Plateau interieur de la Colombie-Britannique. Une partie du profil sismique traversait le bassin de Nechako (Jurassique-Cretace), lequel a un potentiel pour des hydrocarbures a l'interieur de strates sedimentaires qui se retrouvent sous une grande etendue de roches volcaniques. Le long de ce segment de 205 km, huit grandes decharges explosives ont ete tirees et captees par 980 sismometres. Une modelisation directe et inversee des donnees du temps de parcours a ete effectuee par deux methodes independantes : une modelisation basee sur le trace des rayons de la premiere et de la seconde arrivee et une tomographie sismique de la premiere arrivee basee sur un front d'onde a haute resolution. Le materiel dans lequel les vitesses de propagation etaient inferieures a 5,0 km/s est interprete comme etant des roches sedimentaires du bassin de Nechako, alors que les vitesses entre 5,0-6,0 km/s pourraient correspondre a une interstratification de roches sedimentaires et volcaniques. La plus grande epaisseur de roches sedimentaires du bassin se trouve dans les 110 km au centre du profil. Deux sous-bassins ont ete identifies dans cette region; les largeurs sont de 20-50 km et ils ont des profondeurs sedimentaires maximales de 2,5 et 3,3 km. De telles caracteristiques sont bien definies dans le modele de vitesse puisque des essais de resolution sont fiables lorsqu'ils indiquent des objets dont les largeurs sont superieures a ~13 km. Sous les roches sedimentaires, les vitesses sismiques augmentent plus lentement avec la profondeur--de 6,0 km/s tout juste sous le bassin a 6,3 km/s a une profondeur ~17 km, puis de 6,8-7,0 km/s a la base de la croute. Le Moho se trouve a une profondeur de 33,5-35 km sous le profil et les vitesses dans le manteau atteignent 8,05-8,10 km/s. [Traduit par la Redaction], Introduction The Nechako basin is a large Mesozoic fore-arc sedimentary basin within the Intermontane Belt of British Columbia (Fig. 1). Following the accretion of the Intermontane Belt in the mid [...]

Three-dimensional seismic refraction data reveal the structure of the plate boundaries at the Mendocino triple junction. First arrival traveltimes were inverted to obtain a three-dimensional seismic velocity model. The tomography results indicate a strong lateral velocity contrast in the lower crust. This velocity contrast is beneath the surface trace of the San Andreas fault and tracks along the fault to the triple junction and then along the Mendocino fracture zone. Velocity values appropriate for mafic rocks are more than 5 km shallower on the Pacific plate than east of the San Andreas fault, and 1-3 km shallower than under the Cascadia accretionary prism. The model suggests that the San Andreas fault, which is growing as the triple junction migrates northward, cuts steeply through the entire crust. Pacific oceanic crust does not extend beneath North America at this location. The slabless window south of the triple junction and east of the San Andreas fault must be filled with upwelling asthenosphere. The sharp corner of the Pacific plate migrates northward, displacing the Cascadia accretionary prism and forcing the prism material to move north or east. Keywords: seismic tomography, California, plate boundary, San Andreas fault, Mendocino triple junction.

Determination of the structure and internal properties, such as geometry, mineralogy, and fluid content, of fault zones is essential to understanding the earthquake process. Exposed faults provide constraints on fault [...], Seismic reflection and refraction images illuminate the San Andreas Fault to a depth of 1 kilometer. The prestack depth-migrated reflection image contains near-vertical reflections aligned with the active fault trace. The fault is vertical in the upper 0.5 kilometer, then dips about 70 ldegrees] to the southwest to at least 1 kilometer subsurface. This dip reconciles the difference between the computed locations of earthquakes and the surface fault trace. The seismic velocity cross section shows strong lateral variations. Relatively low velocity (10 to 30%), high electrical conductivity, and low density indicate a 1-kilometer-wide vertical wedge of porous sediment or fractured rock immediately southwest of the active fault trace.

Data from a number of seismic surveys in the California Coastal Ranges have shown the existence of a mafic layer in the lower crust throughout the entire ranges. The existence and geometry of this layer place significant constraints on the tectonic models for evolution of the deep plate boundary under the influence of triple junction migration. It is possible that different tectonic models are correct in different regions. Both the Pacific slab model and the asthenospheric upwelling model are inconsistent with the seismic data.