Despite the April 25, 2015 Nepal earthquake being 7.8 magnitude, shallow focus, most of the vulnerable, regular, four-storey buildings escaped damage while many taller structures were severely affected. The death toll was only around 9,000. In contrast, the stronger quake of 1934 (8.1 to 8.4 magnitude) caused terrible damage to regular dwellings. According to the paper published on August 6 in Science, 20 per cent of regular buildings were destroyed by the 1934 quake compared with less than 1 per cent by the 2015 event.
A combination of long-period waves, quick depletion of high-frequency waves, presence of half a kilometre of soft sediments in the basin underneath Kathmandu and well-built buildings may have prevented many smaller structures from getting destroyed.
As a rule, high-frequency waves with short periods of vibration of less than one second tend to affect low-story buildings, while low-frequency waves are more damaging for taller buildings. In this case, the soft sediments amplified the low-frequency, long-period waves of 5 seconds thus shaking the taller buildings even more. The whole basin in which Kathmandu is located resonated at 4-5 seconds period, producing “protracted duration of violent shaking.” Also, a “gentle, slow onset” to the earthquake’s rupture limited shaking at frequencies likely to damage regular dwellings. It took nearly two seconds for the slip rate to reach its maximum value of one meter per second. As a norm, the more abrupt the onset of slip, the more energetic would the high-frequency seismic waves be. The slow onset ended up “limiting the types of waves” that would have shaken shorter buildings.
“One thing all the researchers agree on is that this earthquake was not ‘The Big One,’” notes a news item in Science. One big surprise was that the quake did not rupture all the way to the surface. The rupture propagated eastwards beneath the city for a distance of about 140 km at about 3.3 km per second rate of propagation.
The region of Kathmandu east of the April 25, 2015 Nepal earthquake epicentre is less likely to rupture again in the “near future” with a large (over 7.8) magnitude notes a paper published on the same day in the journal Nature Geoscience. As the April 30, 2015 article in The Hindu had pointed out, the region west of the April event is where the next major quake would strike. The earthquake which propagated eastwards may have transferred stresses into western and shallower parts of the crust, which may help to facilitate future rupture of these regions.
The 800-km long stretch of the Main Himalayan Thrust fault west of the April Nepal event is a well identified seismic gap with no recorded large-scale quake for over 500 years. The last major quake that occurred in this area was in 1505 and had a magnitude of 8.5. This area of the Main Himalayan Thrust is seismically locked and the deficit of slip could exceed 10 metres.
The April 2015 event, like the 1833 quake, had failed to rupture the locked portions of the Main Himalayan Thrust (MHT) lying west of Kathmandu probably because of some “persistent barrier of mechanical or structural origin.”