A program has been written which determines the best positions of the P-nodal planes and B axis for any set of observations and defines the variations possible in these potiitions. In the best and in the extreme positions a number of parameters are calculated: (1) with Model I assumed, and each plane taken in turn as the fault, the strike and dip components are computed; (2) with Model II assumed, the orientation of the pressure and tension axes is calculated. This program has been applied to the data 011 618 earthquakes; these include all earthquakes for which solutions have previously been published anywhere in the world and for which the data are available, together with a large number of European earthquakes for which data have been collected through the European Seismological Commission's Working Group on Earthquake Mechanism (Dr. A. R. Ritsema, chairman) . Where more than one author has considered an earthquake, the data of each are treated separately and are then combined to produce a final solution. The entire print-out is included in the paper. Of the 618 earthquakes, 70 provide unique solutions in which none of the parameters vary by more than 10 degrees. For these the paper provides a listing of the observations on which the solution is based, together with the constants of the observing stations, the theorctical amplitude and polarity for each of these stations reduced to the focal sphere, and a plot of the data and solution on a Byerly projection.
Between 1938 and 1943 a number of large rockbursts at Kirkland Lake were recorded at permanent seismological observatories in eastern Canada and New England. Analysis of these records suggested that rockbursts enjoy certain unique advantages as an energy source in crustal studies, since they are susceptible of precise location and timing and yet have about the same energy distribution as earthquakes. Immediately after the war steps were taken lo set out a refraction profile for detailed studies of the crust. A seismograph was installed at Kirkland Lake to time the bursts at their source and 14 stations were occupied at distances varying from 8 to 174 km. The locations were occupied one or two at a lime, the instruments being moved to new locations after bursts had been satisfactorily recorded. The stations were housed in portable prefabricated buildings especially designed for the purpose. Several types of instruments were used during the life of the project, those finally selected being of a type designed by Willmore for use in South African crustal studies. Particular attention was given to accurate timing and it is shown that the elapsed time of a single event has an accuracy of the order of ± 0.06 seconds (p.e.). The refraction profile obtained consists of the 14 field stations already mentioned plus 5 distant stations which recorded the earlier large bursts. First arrivals in the P and S groups suggest that the crust is single layered, and the point at which P is first observed provides confirmation of this conclusion. In the analysis of secondary arrivals it becomes necessary to conclude that the crust, by lateral variation in rock types and by variable thickness, provides several alternate paths for each ray, so that groups of phases are obtained rather than single distinct phases. Within these limits it is possible to account qualitatively for the secondary arrivals and lo conclude that reflections are obtained from the base of the crust as the critical angle is approached and exceeded. The records of the distant stations show a very large amplitude for about 10 sec. following the expected arrival time of the direct S waves. This group appears to be identical with that called Lg by Ewing and Press. P and S velocities in the crustal layer are 6.246 ± 0.015 and 3.544 ± 0.023 km/ sec., the uncertainties being probable errors. The P velocity below the Mohorovicic discontinuity is 7.913 ± 0.125 km/ sec. if near-station data are used, and 8.176 ± 0.013 km/ sec. combining data of near and distant stations. This suggests an increase of velocity with with depth. Sn velocity, based on the records of the distant stations only, is 4.85 ± 0.10 km/ sec. The mean thickness of the crust, based on the P waves, is 35.4 ± 5.5 km., the uncertainty being the result of the uncertainties in the velocities, and not a true probable error. It is suggested that this uncertainty corresponds to the actual variation in crustal thickness.
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