Showing papers on "Mohorovičić discontinuity published in 1969"
TL;DR: The structure of the Earth's crust and upper mantle under the Canadian Shield as determined by the Project Early Rise seismic experiment of 1966 is presented in this article, where 39 Lake Superior shots were recorded along a 1500 km line running from the shot location in a direction NNW across the Canadian shield, and in a region north and northeast of Lake Superior at distances of 300 to 600 km from a shot.
Abstract: The structure of the Earth's crust and upper mantle under the Canadian Shield as determined by the Project Early Rise seismic experiment of 1966 is presented in this paper. The 39 Lake Superior shots were recorded (1) along a 1500 km line running from the shot location in a direction NNW across the Canadian shield, and (2) in a region north and northeast of Lake Superior at distances of 300 to 600 km from the shot. The seismic line crossed the geological boundary between the rocks of the Superior structural province and the younger Churchill structural province.
The travel-time graph had 3 branches showing apparent velocities of (1) 6.3 km/sec up to 2.4° (2) 8.10 km/sec from 2.4° to 5.8° and (3) 8.54 km/sec from 5.8° to 13.4°. When corrected for curvature the latter two velocities were found to be 8.05 and 8.43 km/sec respectively with the higher velocity occurring at a depth of approximately 84 km. A detailed analysis of the residuals showed that the Mohorovicic discontinuity is relatively horizontal at a depth of 30 to 35 km under the Superior province while under the younger Churchill province it is deeper than 40 km. Since normal values of 35 km are reached in the regions of Ontario just north and east of the lake, the unusually thick 50 km crust under Lake Superior is not extensive.
Very weak amplitudes in the distance range 10° to 12.4° followed by a large amplitude indicate that there may be a slight decrease in P -wave velocity at a depth of 95 km.
87 citations
TL;DR: In this article, the Mohorovicic discontinuity has been studied by reflection seismology, gravity and magnetics, and it has been traced by gravity and magnetic trends for several hundred kilometers across Alberta and under the Rocky Mountains into British Columbia.
66 citations
TL;DR: In this paper, a two-layer crust was found, with velocities uniform both laterally and vertically within the layers, and a discontinuity separating the crustal layers (called the Intermediate discontinuity) is believed to be similar to the Conrad discontinuity.
Abstract: Deep seismic sounding of the earth's crust has been carried out between latitudes 49°30′ N and 51°30′ N, from longitude 93 °W to longitude 96 °W, by means of a refraction survey, using energy from underwater explosions. A two-layer crust was found, with velocities uniform both laterally and vertically within the layers. Velocities found were: Pg = 6.05 ± 0.05 km/s; Sg = 3.46 ± 0.05 km/s; P* = 6.85 ± 0.05 km/s; S* = 4.00 ± 0.05 km/s; Pn = 7.92 km/s; Sn = 4.60 ± 0.08 km/s. The discontinuity separating the crustal layers (called the Intermediate discontinuity) is believed to be similar to the Conrad discontinuity. Contour maps of depths to this discontinuity and the Mohorovicic discontinuity were produced. Average depths (below surface) are: Intermediate = 18.25 km; Mohorovicic = 34.28 km. Average surface elevation is 0.33 km. Velocity averaged vertically through the crust has a mean value over the area of 6.36 km/s. Structures on the discontinuities are related to at least one major surface geological featu...
39 citations
TL;DR: In this paper, the authors used recordings from a crustal seismic experiment, which was conducted in the Yellowknife area in 1966, were used for calibration of the YN seismic array.
Abstract: Summary Recordings from a crustal seismic experiment, which was conducted in the Yellowknife area in 1966, were used for calibration of the Yellowknife seismic array. In the immediate vicinity of the array the crust is found to be very uniform. A superficial layer with an intercept time of 0.172+0.012 s and unknown velocity is underlain by a crust with a P wave velocity of 6-04+0-01 km s-' near the top: assuming this velocity constant throughout the second layer, the total thickness of the crust is about 34+2 km. The Mohorovicic discontinuity is horizontal under the array within the resolution of this experiment and the apparent P, velocity is 8-15 km s-'. At a distance of a few tens of kilometres the crustal uniformity breaks down. The distances are such that, for most teleseismic signals, the effect of these inhomogeneities should be negligible.
26 citations
TL;DR: In this paper, a simple model is presented to illustrate the effect of large-scale undulations along the Mohorovicic discontinuity on P-wave amplitude and travel-time curves.
Abstract: Recent crustal and upper mantle seismic experiments such as the 1963 Lake Superior experiment and the Project Early Rise experiment showed that the Mohorovicic discontinuity may in general not be a plane layer boundary but rather a very irregular surface. In this report a simple model is presented to illustrate the effect of large-scale undulations along this discontinuity on P-wave amplitude and travel-time curves. The results show that the discontinuity in acting as a lens may (1) focus the seismic rays, in which case small regions of relatively large amplitudes are produced, (2) diverge the rays to produce large areas of weaker amplitudes. For a continental crust, the magnitude of this effect is significant when the angle of incidence of P waves at the base of the crust are relatively large (greater than 40°) and when the radius of curvature of the M discontinuity is less than 100 km. Under certain conditions seismic rays may cross below the surface of the earth to produce triplications of the travel-time graph, in which case multiple arrivals with different apparent velocities, separated from each other by fractions of a second, are to be expected. A comparison of the theory with actual amplitudes observed along the Superior-Churchill line of the Project Early Rise experiment is made.
6 citations