Nepal's Pokhara built on debris left by earthquakes: Research

Nepal's second-largest city Pokhara is built on massive debris deposits which are associated with three strong medieval earthquakes, new research has revealed.

According to researchers, the city, around 200 km west of capital Kathmandu, witnessed three earthquakes, in 1100, 1255 and 1344, with magnitudes of around eight Mw (Moment Magnitude).

An international team of scientists, led by the University of Potsdam, Germany, said the earthquakes triggered large-scale collapses, mass wasting and initiated the redistribution of material by catastrophic debris flows on the mountain range.

They discovered that these flows of gravel, rocks and sand have poured over a distance of more than 60 km from the high mountain peaks of the Annapurna massif downstream.

"We have dated the Phewa lake sediments in the dammed tributary valleys using 14C radiocarbon. The measured ages of the sediment depositions coincide with the timing of documented large earthquakes in the region," the scientists said.

One big boulder, situated on top of the sediment depositions, has raised the interest of the scientists, they said.

"The boulder has a diameter of almost ten meters and weighs around 300 tonnes," Christoff Andermann from the GFZ German Research Centre for Geosciences in Potsdam, who participated in the study, said.

"At the top of the boulder, we measured the concentration of a Beryllium isotope 10Be, which is produced by cosmogenic radiation," he revealed.

This 10Be chemical extraction was carried out in the isotope laboratory at the GFZ in Potsdam and was measured with the accelerator mass spectrometer at the Helmholtz-Zentrum Dresden-Rossendorf, Germany.

The results show that the deposition of the big boulder matches the timing of another large earthquake from 1681 and has several important implications reaching beyond fundamental Earth sciences.

The study was published in the Science magazine.