Preliminary data on casualties has shown that at least 3,800 people have been killed and another 7,000 injured across the country. As the damage assessment has not been completed and report on damages from remote areas of country have yet to arrive, the number of deaths and injured and property damaged will definitely be increasing.

The great Nepal quake and the resultant damages have raised some serious questions seriously. Why are the Himalayas prone to earthquakes? Why did the great earthquake hit Nepal? What are the reasons behind the massive damages throughout the country? Are the ongoing efforts enough, in terms of preparedness and policy? And what should be the way forward to mitigate the risk of another earthquake?

How this happened

The Himalayas were formed due to continuous collision between the Indian and Tibetan plates. With the formation of the Himalayas, the region has been dissected east-to-west by several faults, among which the Main Himalayan Thrust, a southern expression of the Main Frontal Thrust immediately south of the Churia hills, is the major seismogenic mega-thrust in the entire Himalayas. Currently, the convergence in the Himalayas is in the range of 20mm per year. Because of such a convergence, elastic strain is being accumulated rapidly, which has been reflected in many seismotectonic researches and global position system data for the Nepal Himalayas.

Over the last century, the Himalayan arc has been struck by four earthquakes with magnitude ca. 8.5—namely 1897 (Shilong earthquake), 1905 (Kangra earthquake), 1934 (Nepal-Bihar earthquake), and 1950 (Assam earthquake)—causing the deaths of thousands of people. The 1905 Kangra earthquake produced severe damage in the Kangra area and, about 100km to the east, in the Dehradun area. The estimated rupture is a roughly 280km long segment, from Kangra to Dehradun, that must have extended eastward to about 78 degrees east, near the border with Nepal.

The 1934 Nepal-Bihar earthquake was believed to rupture a 200-300km long segment to the east of Kathmandu. The area between the 1905 Dehradun earthquake and the 1934 Nepal-Bihar earthquake had not witnessed a mega-quake in the last 50 years. This region was widely considered the Central Himalayan Seismic Gap and was accumulating elastic strain during the inter-seismic period because the plate boundary mega-thrust, the Main Himalayan Thrust, is locked in between the front of the Higher Himalayas and the Churia range.

The recent great quake was caused by the release of accumulated strain in this locked portion of the thrust. We geoscientists had repeatedly indicated the impending great earthquake in that region through rigorous research on the seismo-tectonics of the region. Thus, the cause behind the quake maybe due to the release of long accumulated strain energy in the seismic gap. More scientific ideas behind this quake can only be advanced after rigorous analysis of the measured data of this seismic sequence.

Why so much damage

As new reports are incoming, it is certain that the casualties and damages will accelerate. From preliminary reports of damages in the Kathmandu Valley, it is clear that the soft sediment-filled basin severely amplified the quake. In our own researches on the ground response analysis of the Kathmandu Valley, it has been discovered that the quake would be amplified five to eight times in areas like Thamel, Ason, Sundhara, Patan, Balaju, and the south

east of the valley. The available data on damage patterns showed massive damages in Bhaktapur, Kathmandu, and Patan due to the quake, which aligns with recent

findings.

Besides, the Valley periphery has shown higher amplification when compared to the central part. Media reports have confirmed massive damages along the edge of the Valley. The major cause for the massive damages, therefore, could be the amplification of the energy of the seismic wave, because the Valley might have displayed basin geometry, topography, and valley effects. Thus, we must consider the Valley within a 2D domain, rather than the classical 1D view. Beside these, other parts of the country might have suffered from topography effects that accelerated the energy of the seismic wave, resulting in extensive damage. The other causes for the damages are, of course, due to earthquake-induced landslide, ground rupture, etc.

How to prepare

We felt several alerts in the past. Unfortunately, we never took serious lessons from past experience, like the 1934 Nepal-Bihar and the 1988 Udayapur earthquake. We were very poorly prepared for the earthquake, which is being reflected in the ongoing relief and rescue efforts ongoing throughout the country. Nepal has developed many policies and strategies for disaster risk reduction at the central as well as local levels. Some of these were widely appreciated by the world community too. Regretfully, the implementation of such policies and strategies has been weak and was never monitored effectively.

For the future, the government needs to come up with strong, implementable programmes that always put locals at the forefront. The key areas that the government should focus on are revision and mandatory implementation of national building codes, allocation of financial resources under the disaster risk reduction budget code, research on earthquake science, strong monitoring and evaluation of the ongoing disaster risk reduction-related activities in terms of effectiveness and output. Beside these, site-specific seismic hazard assessment through seismic site effect analysis and seismic microzonation should be carried out in all urban centres at least.

The earthquake is a very peculiar hazard in terms of onset time and the extent of damage. We need to urgently formulate a separate earthquake risk reduction policy and strategy to minimise risks of earthquakes in all sectors.

Chamlagain is a former disaster management expert at New Delhi-based Saarc Disaster Management Centre and an Assistant Professor at Tribhuvan University