olelog

Tsunamis are NEWS, asteroid impacts are NEWS. Mix the two and you get GREAT NEWS. Let them hit New York and you have everybody listening!

There may be clues - but where is the smoking gun? You guessed it? Yes, the researchers are still looking for a crater.

A group of scientists from the Harvard University led by Katherine Cagen claims that a massive tsunami wave swept over the coastal regions of New York some 2,300 years ago, and even more, that the catastrophe most likely was caused by an ancient asteroid crashing somewhere in New Jersey's continental shelf. Sceptics are not convinced, though.

What are the clues?

1. The scientists found carbon spherules (perfectly round particles that form in the extreme pressures of an impact).
2. The scientists found grains of several shocked minerals, of the sort that only are found in impact ejecta or in meteorites.
3. The scientists have taken samples of suspicious-looking sediments along the coasts of New Jersey and Long Island that might have been washed up in a tsunami (or were they left by a strong storm - it is difficult to tell).

"We're making the pretty outrageous claim that not only did a tsunami hit the New York metropolitan area 2,300 years ago, but it was caused by an asteroid impact for which we can't find a crater"

Katherine Cagen is reported to have said.

• http://www.msnbc.msn.com/id/27827830/
• http://dsc.discovery.com/news/2008/11/20/asteroid-tsunami.html (with video clip)
• http://news.softpedia.com/news/Ancient-Asteroid-Provoked-New-York-Tsunami-98366.shtml

Indeed, she has a good story - if she can prove it by hard evidence.

Supposed evidence for a 2300-year-old tsunami deposit from Long Island, New York, were by the way presented at the Fall Meeting of the American Geophysical Union in 2006. See abstract here.

The greatest earthquakes occur in subduction zones, where one tectonic plate is sliding beneath another. Virtually all of the big earthquakes, the ones of magnitude eight or nine or above, happen at sea. The largest earthquake ever recorded is no exception to this rule.

The epicentre of the Great Chilean Earthquake of 22 May 1960 was about 160 km off the coast of Chile in the Peru-Chile Trench (39.5° S, 74.5° W) with a focal depth of 33 km. Two days later, on 24 May 1960, Cordón Caulle, a fissure vents system located in the Chilean Lake District, erupted, sending ash and steam as high as 6 km.

At the Peru-Chile trench the Nazca Plate is subducted beneath the South American Plate. In the area hit by the earthquake the dip of the subduction zone is about 30° and the subduction gives rise to an arc of still active volcanoes.

Buildings fell all along the Chilean coast from Conception to the southern end of Isla Chilor. The towns of Valdivia and Puerto Montt were devastated. (The earthquake is also known as the 1960 Valdivia earthquake / Gran terremoto de Valdivia).

The earthquake set off huge landslides and sent rocks and boulders tumbling down the mountain sides. The land around the city of Puerto Montt sank and coastal areas were flooded. Rivers had their courses changed and landslides created new lakes. Many of the landslides occurred in the Chilean Lake District from Lago Villarica to Lago Todos los Santos.

The earthquake set off huge tsunamis which radiated out from the epicentre, travelling at speed of up to 350 km/h, the Chilean coast was devastated by a 25 (or was it 12 ?) m high tsunami which arrived 10 to 15 minutes after the quake. Remains of houses were carried inland as much as 3 km. There was also severe damage in the Philippines, Hawaii and the japan.

Over 2000 people died and 3000 were injured. 2 million people became homeless. There were not extremely large numbers of victims, for such an earthquake, because the population was alerted on that something was going to happen by previous shakes and underground noise.

Map of some of the places mentioned and the most important volcanoes in the district. In 2005 we made our way from Puerto Montt to San Carlos de Barriloche in Argentina through the Chilean Lake District (bus, boat, bus, boat, bus) - as many tourists do. Under way I photographed the following volcanoes: Osorno (famous for its Fujiyama look), Puntiagudo ("Volcán Puntiagudo" (Spanish for "Sharp-pointed Volcano") is a stratovolcano with a prominent 2,493 m high sharp-pointed summit that results from glacial dissection and gets its name from this feature), and Tronador. See the 3 photos below.

• http://earthquake.usgs.gov/regional/world/events/1960_05_22.php
• http://en.wikipedia.org/wiki/Great_Chilean_Earthquake
• http://www.gochile.cl/html/ChileValdivia/Chile-Valdivia-Terremoto.asp
• http://www.geophys.washington.edu/tsunami/general/historic/chilean60.html
• http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=154





Notes:
The volcano Puyehue is often cited as the volcano that erupted on 24 May, but actually it was the nearby fissure volcano Cordón Caulle. Although Cordón Caulle is sometimes listed as part of Puyehue volcano, it is tectonically and magmatically distinct from Puyehue. No historical eruptions are known from Puyehue, and eruptions in 1921-22 and 1960 listed in some sources actually occurred at Cordón Caulle volcano located to the Northwest.

As far as I know the epicentre of the main quake was at 39.5° S, 74.5° W - some maps however show it inland (including the USGS map). Well of course there were more than one shock, but even then?

Can earthquakes trigger volcanic eruptions? The volcanic eruption 2 days after the 1960 Chilean earthquake has been taken as evidence, but that could still be a coincidence, and the question is still debated. That volcanoes, on the other hand, can cause earthquakes, is well known.

What I wanted to stress here is the role of subduction zones for important natural hazards like earthquakes, volcanoes, tsunamis and landslides.