On 25 April 2015, Nepal was stuck by the magnitude 7.8 Gorkha earthquake, followed on 12 May 2015 by an aftershock of magnitude 7.3. The effects of these earthquakes have been widely reported, with almost 9,000 people killed, over 22,000 injured, and more than 600,000 private houses and almost 20,000 school classrooms destroyed. The start of the reconstruction phase has been long delayed, with the arrangements mired in political machinations. The upshot is that a very large proportion of the affected population has had to endure first the summer monsoon, and now the bitterly cold Nepali winter, with little or no suitable shelter.
It was well known that Nepal was overdue a major earthquake – indeed, as our understanding of the very complex plate tectonics beneath the Himalayas and the Tibetan Plateau has improved, the warnings of the risks have become ever stronger. In terms of Nepal as a nation, the main threat lies in a section of the major fault that provides the boundary between the Himalayas to the north and the Indian continental mass to the south. This fault runs across the southern part of Nepal, but the main concern lies in the section that runs westwards from a little south of Kathmandu. This section of fault suffered a very large and extremely damaging earthquake in 1505, but has not had a large event since. This indicates that the fault has stored a vast amount of energy, and thus is prone to a large earthquake. It was indeed this section of fault that ruptured to generate the earthquake in April.
It had been widely anticipated that the effects of an earthquake on this fault would be catastrophic. Indeed, the number of deaths in Kathmandu was expected to exceed 70,000 with many more in the rural regions. Nepal appears to have a very high level of vulnerability to the effects of earthquake shaking, with large numbers of buildings not reinforced against the effects of earthquakes, and many steep, unstable slopes that have the potential to generate landslides. In this context, Nepal appears to have fared surprisingly well in the 2015 quake, recognising of course the desperately sad losses that did occur. As a result there is a temptation to believe that Nepal is now safe – that the fault has released energy and the buildings proved to be more resilient than expected.
However, investigations of the earthquake, undertaken through the combined efforts of researchers around the world (with significant contributions from within Nepal itself), indicate that this would be a dangerous view. We are able to understand the earthquake, and the rupture processes on the fault that generated it through analysis of the earthquake records obtained from seismic instruments, and measurements of how the crust has displaced (obtained from both radar satellites and GPS). This information has been combined with geological maps and cross-sections (fieldwork still plays a key role) to provide a detailed picture of the processes that occurred that day.
What is very clear is that only a small section of the fault, lying to the east of the epicentre, ruptured to generate the earthquake. This rupture event seems to have occurred on a section of the fault that is not typical of the rest. Perhaps most importantly, the seismic records suggest that the rupture event occurred quite slowly, especially in the southern section of the fault (the section that lies close to Kathmandu and other more densely populated areas of the Hill districts in Nepal). The section of the fault to the north, under the steeper slopes of the higher mountains, ruptured more rapidly. This is critically important, as slow ruptures generally generate low frequency shaking, whereas it is high frequency shaking under which weak buildings tend to perform extremely poorly. In the Gorkha earthquake, the most densely populated areas were spared this damaging shaking simply because of the way the rupture occurred. To the north, the population was subjected to shaking with a much higher frequency, and, unsurprisingly, this is where most of the damage occurred, both in terms of landslides and building collapses. Thus, the low level of building damage in Kathmandu results mostly from the unusual way in which this section of fault behaved.
“The challenge for the scientific community and disaster managers is to ensure that there is not a sense of complacency within Nepal”
Unfortunately, because only a small section of fault ruptured, and in this specific way, it means that there is still a very large amount of energy stored in the remainder of the fault, meaning that another very large earthquake is inevitable. It is not possible to say whether this will occur today, next week, in a month or in a century but it is very unlikely that when this earthquake occurs the rupture will behave as it did in April. Instead, the next earthquake is likely to be much, much larger and extremely damaging.
The challenge for the scientific community and disaster managers is to ensure that there is not a sense of complacency within Nepal. There is a danger that building codes will not be developed or enforced, or that landslide mitigation will not be performed, on the assumption that the structures stood up to the Gorkha earthquake, and that the hazard of another big event has now been reduced. All earthquakes are not equal, and the unexpectedly low losses that occurred in the Gorkha earthquake are unlikely to be repeated next time.