The Himalayan Region is one of the most active tectonic belts of the world with an active continent-continent collision boundary. Seismicity in this region primarily results from the movement and thrust of the Indian and Eurasian plates and the resulting collisions therein. The 25 April, 2015 earthquake that clocked 7.8 on the
Richter scale occurred due to a thrust interface between the subducting Indian plate and the preponderant Eurasia plate to the north (USGS, 2015). The northward thrust of the Indian plate into the Eurasian plate at a rate of 40-50 mm/year has been regularly causing earthquakes and has consequently made this region among the most seismically unsafe regions on earth.
April 2015 Nepal earthquake
The epicentre of the April 25, 2015 earthquake, approximately 80 km to the northwest of the Nepalese capital of Kathmandu, was at the point of convergence of the Indian plate with the Eurasian plate. This movement towards the north-northeast is driving the uplift of the Himalayan mountain range at a fraction of ~18 mm/yr. The boundary region of the Indian and Eurasia plates, where Nepal lies, has a history of large and great earthquakes. Nepal, in its entirety, falls in a high earthquake intensity belt, that is IX and X on the Modified Mercalli Intensity Scale (MMI).
Following this, a major earthquake aftershock with the magnitude of 7.3 hit Nepal a second time on May 12, 2015, 75 km east of Kathmandu. Over 200 people were killed and more than 2,500 were injured. The earthquake occurred as the result of thrust faulting on or near the Main Himalayan Thrust Boundary, which defines the interface between the under-thrusting Indian plate and the overriding Eurasian plate to the north as noted by USGS.
Nepal is seismically a very active zone. There have been eight large earthquakes with M≥ 7 in the Central Himalayan region between 1803 to 1936 including the great earthquake of 1934. However, there had been no major earthquakes in the recent past until now. This has been so despite Nepal being located in the central part of the Himalayan arc.
The 2015 earthquakes not only severely damaged Kathmandu, Nepal and parts of Bihar, but resulted in around 10,800 fatalities. The devastation and severity encountered now can only be matched with an earthquake of similar size in 1833.
Archival data tells us that the 1934 Nepal earthquake had severely shaken the Kathmandu Valley, and destroyed 20 per cent and damaged 40 per cent of the valley’s building stock. In Kathmandu city, one quarter of all homes were destroyed by the quake. Many temples in Bhaktapur were destroyed as well. This earthquake was not an isolated event. In 1988, a moderate earthquake of magnitude 6.5 on the Richter scale that hit eastern Nepal killed
Data available from the Department of Mines and Geology, Central Bureau of Statistics (CBS) concludes that earthquakes of more than or equal to 5.0 on the Richter scale have occurred at least once every year in Nepal since 1987, with the exception of 1989 and 1992 when no such events were recorded. The current disaster database of Nepal shows that there were 22 earthquakes with magnitudes ranging from 4.5 to 6.5 on the Richter Scale throughout the country in the 37 year period from 1971 to 2007. According to DesInventar’s Disaster Inventory System, powered by UNISDR (www.desinventar.net, 2007), about 34,000 buildings were destroyed and 55,000 were damaged during this period due to earthquakes. A simple loss estimation carried out under the Kathmandu Valley Earthquake Risk Management Project (KVERMP) in 2002 jointly by the Ministry of Home Affairs (MoHA, Nepal) and Japan International Cooperation Agency (JICA) had suggested that in case of a recurrence of similar shaking as that of 1934 in the Kathmandu Valley, one could expect 40,000 deaths, 95,000 injured casualties, and 600,000-900,000 persons rendered homeless as a consequence of 60 per cent dwellings being damaged.
Subsoil quality and earthquake damage
Generally, subsoil conditions, construction materials used and deficiencies in construction technology are the major factors that affect houses during an earthquake. The Kathmandu valley is underlain by soft lacustraine and fluvial sediments belonging to the Pliocene and Pleistocene ages. These thick and weak materials produce high amplification during an earthquake. This means filled up soils or loose soil deposits amplify an earthquake tremor thrice as much as hard rock.
Microzonation studies of the Kathmandu valley have demonstrated that its soils have high vibration periods with a high chance of resonance. Hence, there is a greater possibility of damage.
Another phenomenon called soil liquefaction, which accompanies an earthquake can cause structural failure and damage to roads, pipelines and infrastructure. Weak subsurface conditions in the Kathmandu valley have been ignored and many tall structures built. The liquefaction phenomenon following an earthquake causes many such buildings to sink in the soil as if on quicksand.
The boundary/edge of the Indian and Eurasian plates lies across Nepal. Hence, any disturbance or movement involving these plates would be strongly felt all across Nepal. Besides, the epicenter, 77 km northwest of Kathmandu, lay close to a densely populated zone. Poor construction, lack of proper maintenance and flawed building practices combined to turn Nepalese dwellings into virtual death traps.
Most structures in Nepal are made of brick walls and mud mortar. The brick material is bonded with mud in most ancient buildings. The extent of damage caused depends on the strength, ductility, and integrity of the building along with the stiffness of the ground beneath it.
It is quite interesting that the earthquake epicentres was equidistant from Kathmandu from opposite sides. The May earthquake was 75 km east, while the April one was 75 km west of Kathmandu. Even though the second one was of lesser magnitude (7.3) the damage to building increased due to intense effect of the first shock. Fortunately there is no other big city in the vicinity of Kathmandu, which reduced the death toll. In both cases the timing of the earthquake reduced the death toll, despite the fact that Nepal is highly populated.
Nepal was hit by an earthquake of lesser magnitude (7.8 magnitude—approximately 7.6 mega tonnes of TNT explosive) as compared to the one in 1934 (8.0 magnitude—approximately 15 mega tonnes of TNT explosive). The slight increase of 0.2 in magnitude, given the quality of subsoil, would have meant an energy release approximately twice of what was felt now.
The devastation seen this time has been the result of lack of building norms, or bye-laws in Nepal. Repeated warnings by seismologists and scientists were ignored by the Nepalese administrators, resulting in the widespread devastation that one witnesses today.
Given its location on a high-risk seismic zone, Nepal would do well to formulate and follow the kind of building bye-laws that India has devised. This is the only method to rebuild its cities with earthquake resilient structures, and ensure the kind of damage it has suffered for now never recurs again.