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 large-scale collapses, mass wasting and initiated the redistribution of material by catastrophic debris flows on the mountain range.
An international team of scientists led by the University of Potsdam in Germany has discovered that these flows of gravel, rocks and sand have poured over a distance of more than 60 kilometres from the high mountain peaks of the Annapurna massif downstream.
"We have dated the 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," 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.
Nevertheless, the movement of the big boulder can be connected to this strong earthquake.
This research has several important implications reaching beyond fundamental earth sciences. The study provides new insights into the mobilisation and volumes of transported material associated with strong earthquakes.
Dating of such sediment bodies provides information about the reoccurrence intervals of earthquakes in the Himalayas, and ultimately demonstrates the role of earthquakes in shaping high mountain landscapes. This knowledge is crucial to better evaluate the risks in tectonically active mountain belts.
The study was published in the journal Science magazine.
