Pokhara 800 years ago

A 7.8 magnitude earthquake in 1255 shook the entire Himalayan region, Tibetan plateau and northern India. We know the exact date because there are historical records that one-third of the population of Kathmandu was killed, including King Abhaya Malla.

At about this time, in central Nepal the entire western flank of Annapurna 4 broke off and fell into the glacier below, damming the Seti River gorge and covering the basin in pulverised rock and melting ice.

Geologists have known that it was a debris flow on the Seti that formed the terraces of sediment in Pokhara Valley, but a recent paper in the journal Nature has for the first time described just how cataclysmic this event was and that it could happen again.

The rockslide was so enormous that it chopped off 500m of the top of the paleo-summit of Annapurna 4, which means the mountain was at one time an eight-thousander. The peak is now 7,525m, and scientists estimate that 23 km³ of rock fell nearly 3,000 vertical metres into the Sabche Cirque with terrific kinetic energy.

The study titled Medieval Demise of a Himalayan Giant Summit Induced by Mega-landslide has Jérôme Lavé of CNRS, Université de Lorraine in France as lead author, and includes among others Ananta Prasad Gajurel of the Department of Geology, Tribhuvan University.

Although the study is an investigation into ways in which Himalayan mountains erode along their glaciers and headwalls, it takes as a case study the Annapurna 4 rockfall in approximately 1190CE.   

‘Our data suggest that a mode of high-altitude erosion could be mega-rockslides, leading to the sudden reduction of ridge-crest elevation by several hundred metres and ultimately preventing the disproportionate growth of Himalayan peaks,’ the introduction to the paper notes, suggesting that the erosion of the peaks could happen in sudden ridge collapses and not just through mass wasting. 

The scientists examined the debris in the Sabche Cirque which in the past 800 years has eroded into sharp limestone pinnacles with exposed glacier-polished bedrock below. They conclude that the breccia came down in the collapse of Annapurna 4.

In their paper, Lavé et al have sketched models of the mountain as it probably existed before its collapse, showing where the immediate debris fell, and where it was eventually transported downstream by the Seti River to create the 200m deep sediment fan on which the city of Pokhara is located today.

The deposits brought down by the river dammed side streams, creating Phewa, Rupa, Begnas and other lakes that make the valley so scenic today.

The Nature paper says the catastrophic Sabche rockslide has lessons for present-day Nepal: ‘Giant rockslides also have implications for landscape evolution and natural hazards: the massive supply of finely crushed sediments can fill valleys more than 150 km farther downstream and overwhelm the sediment load in Himalayan rivers for a century or more.’

Indeed, the summit ridge of Annapurna 4 is still raw and sees frequent rockfalls, including a smaller one almost exactly at the same spot in May 2012 and an ensuing flood on the Seti River that killed at least 70 people downstream near Pokhara.

Although the authors do not seem to think the rockslide 800 years ago was caused by a seismic event, there are records of big earthquakes in central Nepal in 1223, 1255 and five years later in 1260, which approximately fit the timeline of the scientists for the Sabche event.

Even if the rockfall was not directly caused by an earthquake, the differential in the pace of erosion of the glacial valley and the headwall could eventually lead to failure of overhanging slopes. Scientists have speculated that climate change-induced melting of the permafrost that cements rocks together above the 0^oC isotherm could have been a factor in the 2012 collapse on Annapurna IV and on Saldim Peak in the Makalu-Barun in 2017.