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Built on debris of medieval earthquakes

The Times of India Dec 21 2015


Nepal's second-largest city and its leading tourist hub Pokhara is built on massive debris deposits which are associated with strong medieval earthquakes, a new study has found.

Three quakes, in 1100, 1255 and 1344, with magnitudes of around eight moment magnitude (Mw) triggered largescale collapses, mass was ing and initiated the redistribution of material by ca astrophic debris flows on he mountain range.

An international team of scientists led by the University of Potsdam in Germany has discovered that these lows of gravel, rocks and sand have poured over a dis ance of more than 60 kilometres from the high moun ain peaks of the Annapurna massif downstream.

“We have dated the lake sediments in the dammed ributary valleys using 14C radiocarbon. The measured ages of the sediment deposi tions coincide with the timing of documented large earthquakes in the region,“ said Christoff Andermann from the GFZ German Research Centre for Geosciences in Potsdam. One big boulder, situated on top of the sediment depositions, has raised the interest of the scientists.

“The boulder has a diameter of almost ten meters and weighs around 300 tonnes. At the top of the boulder we measured the concentration of a Beryllium isotope which is produced by cosmogenic radiation,“ said Andermann. The results show that the deposition of the big boulder matches the timing of another large earthquake from 1681.

Pokhara lies at the foot of the more than 8,000 meters high Annapurna massif; whether the big boulder was transported during the last dated earthquake with the debris, or was just toppled by the strong shaking needs to be further investigated, researchers said.

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