Wednesday, June 15, 2011

One hundred major impacts: part two: the deep ocean



A few months ago, I guaranteed the readership of the Lounge that none of them would be killed by a meteorite impact. In laying out the estimates that allowed me to do this, I took an equal area map and bombarded it with one hundred 400m projectiles. Objects of this size hit the Earth about once every 100,000 years, and are locally devastating but globally insignificant, so this seemed like a good way to look at where an impactor “big enough to wipe out LA” was actually likely to land.

Of these 100 impactors, 71 landed in the ocean. 19 of these were within 1000 km of the coastling of an inhabited continent (e.g. not Antarctica), while the others were far out in the ocean basins.

For impacts more than 1000 km offshore, the impact effect calculator of Marcus, Melosh, and Collins suggests that the main effect would be a tsunami. The tsunami details are not in their linked paper, and the amplitudes vary significantly, but the maximum amplitude at 1000 km from the impact area is about 4 meters or smaller. This is broadly similar to that of a magnitude 9 earthquake such as those that struck Japan this year and Sumatra (and the Bay of Bengal) four years ago. The tsunami takes about 1.8 hours to travel 1000 km, so warning times would depend greatly on detecting the impactor in space and seeing the fireball with antiproliferation satellites (this impactor is equivalent to a 3000 megaton bomb, so the fireball would be far larger than that of a nuclear weapon). The seismic signal of a hit to the deep ocean would actually be fairly minor, as most of the energy would be absorbed by the water.

Of course, the main difference is timing. Half of the impacts in this simulation were in the deep ocean, so with an impact repeat rate of 1 every 100,000 years, we would expect one deep ocean impact every 200,000 years. In contrast, a magnitude 9 earthquake strikes about once every 25 years. So over a million year period, we would expect 40,000 tsunamis from earthquakes, and five from deep ocean impacts.

2 comments:

Chris Phoenix said...

From Wikipedia: evidence of a 180-m Indian Ocean tsunami c. 2800-3000 BC due to asteroid impact.

"More recent prehistoric impacts are theorized by the Holocene Impact
Working Group, including Dallas Abbott of Columbia University's
Lamont-Doherty Earth Observatory in Palisades, N.Y.[9] This group
points to four enormous chevron sediment deposits at the southern end
of Madagascar, containing deep-ocean microfossils fused with metals
typically formed by cosmic impacts. All of the chevrons point toward a
spot in the middle of the Indian Ocean where newly discovered Burckle
crater,[10]29 km (18 mi) in diameter, or about 25 times larger than
Barringer Crater, lies 3,800 m (12,500 ft) below the surface. This
group posits that a large asteroid or comet impact c. 2800-3000 BC
produced a mega-tsunami at least 180 m (590 ft) high, a catastrophic
event that would have affected humanity's cradles of civilization.[11]
If this and other recent impacts prove correct, the rate of asteroid
impacts is much higher than currently thought."

http://en.wikipedia.org/wiki/Impact_event#Recent_prehistoric_impact_events

Chuck said...

Relying on the journal Geology instead of wikipedia, we have:

"Chevrons” are not mega-tsunami deposits—A sedimentologic assessment"

Bourgeois and Weiss (2009) Geology, 37, 403-406

http://geology.gsapubs.org/content/37/5/403.short

Or for a non-paywalled summary, try:
http://www.physorg.com/news160212894.html
or even
http://en.wikipedia.org/wiki/Chevron_(land_form)