Waiting for the Complete Rupture [www.ethz.ch]:
In April 2015, Nepal – and especially the region around the capital city, Kathmandu – was struck by a powerful tremor. An earthquake with a magnitude of 7.8 destroyed entire villages, traffic routes and cultural monuments, with a death toll of some 9,000.
However, the country may still face the threat of much stronger earthquakes with a magnitude of 8 or more. This is the conclusion reached by a group of earth scientists from ETH Zurich based on a new model of the collision zone between the Indian and Eurasian Plates in the vicinity of the Himalayas. Using this model, the team of ETH researchers working with doctoral student Luca Dal Zilio, from the group led by Professor Taras Gerya at the Institute of Geophysics, has now performed the first high-resolution simulations of earthquake cycles in a cross-section of the rupture zone.
“In the 2015 quake, there was only a partial rupture of the major Himalayan fault separating the two continental plates. The frontal, near-surface section of the rupture zone, where the Indian Plate subducts beneath the Eurasian Plate, did not slip and remains under stress,” explains Dal Zilio, lead author of the study, which was recently published in the journal Nature Communications [nature.com].
Normally, a major earthquake releases almost all the stress that has built up in the vicinity of the focus as a result of displacement of the plates. “Our model shows that, although the Gorkha earthquake reduced the stress level in part of the rupture zone, tension actually increased in the frontal section close to the foot of the Himalayas. The apparent paradox is that ‘medium-sized’ earthquakes such as Gorkha can create the conditions for an even larger earthquake,” says Dal Zilio.
[...] According to the simulations performed by Dal Zilio and his colleagues, two or three further Gorkha quakes would be needed to build up sufficient stress for an earthquake with a magnitude of 8.1 or more. In a quake of this kind, the rupture zone breaks over the entire depth range, extending up to the Earth’s surface and laterally — along the Himalayan arc — for hundreds of kilometres. This ultimately leads to a complete stress release in this segment of the fault system, which extends to some 2,000 kilometres in total.
Luca Dal Zilio, Ylona van Dinther, Taras Gerya, Jean-Philippe Avouac. Bimodal seismicity in the Himalaya controlled by fault friction and geometry. Nature Communications, 2019; 10 (1) DOI: 10.1038/s41467-018-07874-8 [doi.org]
So... even really big earthquakes are not all they are cracked up to be?