April 5, 1958 Tsunami: Ripple Rock, Bc, Canada

Reference #4153:

On April 5, 1958, one of the world's largest man-made non-nuclear explosions was fired at Ripple Rock in Seymour Narrows on the west coast of Canada. Briefly, the object of the explosion, which was carried out for the Department of Public Works, Canada, was to remove a twin peaked underwater rock from the centre of a narrow seaway [Wright et al., 1958].

The third attempt [of removal] was fixed for April 5, 1958, at 9.31 A.M. LT. This moment corresponded to the best tidal conditions to give a minimum load of water over the rock, and at the same time the maximum current to promote rock dispersal. The north flowing ebb was chosen to minimize the effect of shock waves travelling southwards towards the town of Campbell River, about 15 km to the south.

The blast of 1375 tons had all the characteristics of a small earthquake with the added advantage that the exact time, location and depth within the earth were known. There were 5 permanent and 5 temporary seismic stations of the Seismological Service located along the east and west lines across British Columbia and in Alberta.

The seismic wave took 16 s to reach the westernmost permanent recording station in Port Alberni, BC and 90 s to reach the easternmost permanent station of Banff, Alberta. The latter took 2 s longer than expected, which, according to the seismologists, is an indication that the high mountains along this route may have roots extending beneath the earth crust into the mantel.

Shock waves, created by the blast. Wave recording stations were established at Bloedel Wharf in Menzies Bay, 22 km SW of the blast, in Brown Bay 3.2 km N of the blast, and at Duncan Bay 8 km SE of the blast. The maximum wave recorded on Bloedel Wharfrecorder was the forth wave, 3 min 45.5 s after the blast. Its height was 0.46 m. Waves recorded in Brown Bay were negligible, the maximum being the eight after the blast with a recorded height of 0.07m. No record of any wave was obtained at Duncan Bay nor was any evidence of the blast experienced other than a very slight earth shock, which might have passed unnoticed, had the time of the blast not been known [Rutley, 1959]