Summary
Ten seismograms, due to rockbursts at Lake Shore Mines, Kirkland Lake, Ontario, were recorded on a Benioff seismograph at Ottawa at a distance of 450 km. (279 mi.). The center of each burst was located within a few feet; but, for the purpose of preparing travel-time tables, they may all be considered to have occurred at a single point at the surface. Two of the bursts were accurately timed on the seismograph at the mine.
Six phases were registered on each seismogram, being more sharply marked on some records than on others. Five of these are well defined on nearly all the records. It is thus possible to deduce a set of arrival times at a distance of 450 km. for a burst (or earthquake) occurring at the surface; and this set of times is known with fair precision, since all the readings may be combined. The distance is determined within one part in 7000, the depth within 2000 ft. and the travel times with an error of ±.5 sec.
These travel times have been compared with those obtained by Joliat in computing his Tables for Near Earthquakes, based on the velocities deduced by Jeffreys for northern Europe and arbitrarily assuming an earth structure with two layers above the Mohorovičić discontinuity. The differences are minor and are to be explained as chiefly owing to the fact that Joliat assumed the focus to lie at the bottom instead of the top of the upper layer.
On the strength of the comparisons afforded by the ten seismograms, the focal time of each burst may be considered as known within ±.5 sec. One of the bursts was so severe that it was registered also at Shawinigan Falls, Quebec (Δ = 576 km., 358 mi.) and at Weston, Massachusetts (Δ = 935 km., 581 mi.). These records will afford a means of deducing the earth structure and velocities in the vicinity of Ottawa, and will permit the construction of tables for rock-bursts and blasts in that area up to 10° (1110 km., 690 mi.). These will be prepared and issued, together with corrections permitting their being used for local earthquakes with finite focal depth.
Should other bursts occur later at Kirkland Lake, timed by the mine seismograph and registered at Ottawa or the other stations, the data so made available may be directly used to check and add precision to the deductions made from the seismograms already in hand. Such further data would be most valuable.
Waves reflected at the “surface” of the earth: P'P'P'P'