Week 2014.21 (May 25-31) ~ A lasting impact


Once upon a time, on a day perhaps very much like today, long before mammals appeared on Earth, when the landscape teemed with primitive insects and amphibians crawled through the ferns, a two-kilometer wide extraterrestrial object appeared over the skies of what is now eastern Canada and slammed into the Earth. The energy released from that asteroid collision was great enough to fracture the Earth’s crust and reactivate several of the area’s ancient earthquake faults. Now, some 350 million years later, the only visible artifact of that tumultuous event is a heavily eroded crater near Charlevoix, Quebec, on the north shore of the St Lawrence River. But if you listen closely to the Earth today, you can still hear the distant echoes of that impact, in the form of frequent small earthquakes along this stretch of the St Lawrence River, eastern Canada’s most seismically active region.


The St. Lawrence River from space, looking west. The site of last week’s Baie-Comeau earthquake is near dead-center of the image (red dot). Although not clearly visible in this image, the Charlevoix crater is located on the north shore of the St Lawrence, about one-third of the way across the image. The conspicuous circular structure to the right is Lac Manicouagan, a crater from a much larger impact about 135 million years later that may have been responsible for one of history’s great mass extinctions. [Image courtesy NASA.]

I am happy to report that this week there were no such cataclysmic impacts on Earth, no such messengers of doom from the skies. But, as happens every week, we received plenty of messages from below. Listen for the faint codas of distant earthquakes in Guatemala (5.1, 00:46), the Azores (5.2, 01:28), near the border of Myanmar and China (5.9, 04:04) and the Komandorsky Islands in the Bering Sea (5.4, 05:01). Note, too, the crisp, clear sound of the deep crust snapping (03:07) from a moderate undersea earthquake (5.8) in the Mona Passage, between the islands of Hispaniola and Puerto Rico. Each earthquake tells the story of stresses that accumulated over millions of years under unrelenting subterranean forces — forces powerful enough to split oceans apart, move entire continents, and raise mountains into the sky.

And occasionally there is a tiny sound that tells a truly astonishing story. Listen closely this week at 01:31 for the unremarkable little sound that resembles that of a pinch of sand tossed onto a snare drum. Regular followers of this podcast may recognize the sound from earlier programs: it is the sound of a small earthquake (3.2) in eastern Canada. This one occurred under the St. Lawrence River, near the town of Baie-Comeau, on a fault in the St Lawrence Rift system — the same system that had been reactivated by the great Charlevoix impact. Could this be an echo of that event?

Perhaps this earthquake is a relic of the Charlevoix impact. Or perhaps it is due to the accumulation of unrelated tectonic forces in the area. We cannot say for sure. We do know, however, that the Earth continues to crackle audibly today from the residual effects of ancient catastrophes and of tectonic transformations of unimaginable scale. These small sounds from below remind us that earthquakes and asteroids are always with us, a natural side-effect of life on Earth. One of these fine sunny days — on a day very much like that one in the time of the ancient fern forests — a strange shape will suddenly blaze across the clear blue sky…

What else was going on this week:

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