The results of three seismological investigations of the crust in the seismically active La Malbaie Region are reported. First arrivals from a reversed reflection-refraction profile within the Charlevoix structure indicate a uniform P, velocity of 6.08 f 0.04 kmls. Secondary arrivals forming discontinuous, sometimes arcuate segments on the record sections are interpreted as subcritical reflections from structural contortions related to impact of the Charlevoix meteorite. A coherent event at 14s may signal reflection from the M-discontinuity near 45 km depth.Analysis of travel times from calibration shots recorded by a network of stations spanning the St. Lawrence River in terms of time-distance profiles, travel time residuals, and numerical models provides excellent support for a model of the Precambrian-Paleozoic contact striking along the north shore and dipping to the southeast about 20" beneath the wedge of 5.5 kmls sediments.Unreversed profiles obtained by recording timed Thetford Mines blasts across a similar network suggests a comparable deep structure beneath the north and south shores, with average crustal velocitiesof 6.8and 6.7 kmls, respectively. A minor mid-crustaldiscontinuity is suggested, and Moho depth is estimated at 42-43 km.The 6.2 kmls upper crustal velocity found for the crater and immediately surrounding area is anomalously low compared to the 6.4-6.5 kmls reported for the eastern shield region. Structural lineations gleaned from satellite imagery suggest an elongated area of impact-related disruption much larger than previously recognized. This expanded area of crustal weakening encompasses and may partially explain the zone of continuing microseismic activity, as well as the low upper crustal velocity of the La Malbaie Region.On rapporte les resultgts de trois etudes sismologiques de la croDte dans la region sismiquement active de La Malbaie. Les premieres arrivkes d'un profil renverse reflexion-refraction a l'interieur de la structure de Charlevoix indiquent une vitesse P,uniforme de 6.08 +_ 0.04 kmls. On interprete les anivees secondaires formant des segments discontinus, quelquefois arques sur des portions de l'enregistrement, comme des reflexions subcritiques des contorsions structurales reliees a l'impact meteoritique de Charlevoix. Un episode coherent de 14 s peut signaler une reflexion de la discontinuite-M a environ 45 km de profondeur. L'analyse de la duree des trajets a partir d'explosions de calibration enregistres par un reseau de stations s'etendant de part et d'autre du fleuve Saint-Laurent en termes des profils temps-distance, des valeurs residuelles de la durte des trajets, et de modeles numeriques fournit un excellent appui pour un modele de contact Precambrien-Paleozo'ique dont la direction est parallele a la rive nord avec un pendage au sud-est a environ 20" sous un coin de sediments dont la vitesse est de 5.5 kmls.Les profils non renverses obtenus par enregistrement des explosions de dates connues a Thetford Mines a travers un reseau semblable suggerent une structure p...

Crustal-scale seismic refraction data obtained in the Beaufort Sea during 1976 reveal a 4–5 km thick sedimentary layer overlying an oceanic crust that thickens rapidly as it approaches the continental terrace of Alaska. A synthetic seismogram analysis of multiple reflected, water-wave events indicates that the upper sedimentary layer has a compressional velocity of 1.8 km/s and a shear velocity of about 0.2 km/s. An oceanic layer 2A of 4.3 km/s, a layer 3A of 6.6 km/s, and a layer 3B of 7.6 km/s overlying an anisotropic (3%) upper mantle with a median velocity of 8.3 km/s are interpreted. The direction of maximum upper-mantle velocity appears to be approximately north–south in the area surveyed, suggesting that a rotation of the Northwind Ridge–Chukchi Plateau away from the Barrow–Martin Point sector of Alaska may have occurred.

dT/dA and azimuth measurements at the Yellowknife Seismic Array (YKA) for P and pP phases that have penetrated to depths greater than 1850 km show systematic bias relative to those values calculated from spherically symmetric Earth models. Moreover, the corresponding arrival vectors for the phases PcP, ScP, PnKP and SnKP (n > 2) approaching YKA from within the same narrow azimuth ranges as the P andpP phases show related systematic errors. From an analysis of combinations of all these phases, we show that most of the observed bias for P and pP is produced along the upgoing portion of the ray path, and that any heterogeneities and velocity anomalies near the base of the mantle cannot be clearly elucidated by a dT/dA study of the P phase alone. P wave data must be supplemented by other phases that traverse similar paths within the upper mantle beneath arrays. Evidence for regional variations in structure at depths greater than 1850 km is presented. An abrupt drop in dT/dA between distances of 87" and 90" is reported for two different regions of the Earth and possibly a third; this drop represents a fairly rapid increase in velocity gradients at about 2700 km depth that may be a world-wide phenomenon. There are also strong lateral variations in the upper mantle to the north-east and east of YKA. Array observations of precursors to PP, 1 ' ' P' and PKP indicate small-scale lateral variations in the crust and upper mantle and in the lowest 200 km of the mantle that give rise to extensive scattered P wave energy. We initially try to relate our data on lateral variations to regional variations in travel times and mantle convection. Our main concern is the problem of motion within the lower mantle, and how dT/dA measurements in combination with theoretical studies of the properties of the scattering regions might indicate flow. The combined P wave data on small-scale and large-scale lateral variations suggest that the mantle is most homogeneous between depths of 800 km and 2500 km; consequently, we propose that mantle convection is confined to the upper mantle and the lowest 400 km of the mantle, and that convection plumes, if they exist, do not originate at depths greater than 800 km. * Contribution from the Earth Physics Branch No.: 533 115 c c h,

A wide‐angle reflection/refraction survey, conducted near Yellowknife, N.W.T., Canada has revealed a complex crust in contrast to the simple model indicated by a large‐scale refraction survey. A near‐vertical reflection survey, conducted in north‐central British Columbia, using dynamite detonated in a shallow lake, recorded coherent energy at two‐way times of up to 11 sec. A reflector at a depth of 30–35 km is thus indicated which corresponds to a refraction‐determined Moho depth for the area. CRP stacking attempts have been unsuccessful for both of these surveys due mainly to the complex overburden encountered but possibly due in part to the nature of the deep reflectors themselves. A reflector comprised of a series of thin layers of alternating high and low velocities will produce a composite reflected pulse, whose amplitude, frequency content, and apparent arrival time will change as the reflecting angle changes. Successful application of the CRP method would require restricting its use to a much narrower range of incident angles than we have employed. A Vibroseis survey, duplicating the coverage of the B.C. reflection profile, indicated the same reflector at 11 sec. The Vibroseis section remains superior despite exhaustive attempts at enhancement of the dynamite‐produced data.

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