A seismic survey in the Canadian Shield, I: refraction studies based on rockbursts at Kirkland Lake, Ontario

Abstract

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.

A simplified method for the analysis and synthesis of dispersed wave trains

“…the group-velocity data of Press and others[1956]. This result for crustal thickness also agrees closely with the seismic refraction results thatHodgson [1953] obtained for the Canadian shield areas which are probably similar in structure to the average path between Hudson Bay and Palisades. The mean thickness of the crust, based on P wave arrivals, was found by Hodgson to be 35 q-5 km.…”

Crustal structure of the New York-Pennsylvania area

“…First, there is a general tendency for the experimental data, particularly toward the long-period end of the graph (Fig. 6) There is no serious disagreement between this structure and that deduced by Hodgson [1953] surface waves. The present study gives a structure that is in agreement with this conclusion.…”

“…This triangle is small enough so that curvature of the earth may be neglected for the present study. Also shown in Figure I and indicated by the dashed and the dashed-crossed lines are the locations of seismic refraction profiles by Katz [1955] and by Hodgson [1953]. Their resttits are summarized in Table 2 and compared with the results for the phase-velocity method in a later section.…”

“…The new experimental data consist of phase velocities of long-period Rayleigh waves which have crossed the Waynesburg, Pennsylvania-Ottawa, Ontario-Palisades, New York, tripartite array. Previously published seismic refraction results of Katz [1955] are also used as, to a lesser extent, are those of Hodgson [1953]. When combined, these data permit the deduction of a crustal structure which is more clearly defined than if either of the two kinds of information had been used independently.…”