Strong earthquakes shook the ground in Nepal in April and May 2015. The earthquakes in April occurred approx. 80 km northeast of Kathmandu. Many people died and several buildings were destroyed.
Already four years ago on September 18th in 2011 an earthquake occured and caused great damages. The earthquake with the magnitude of 6.9 shook the ground in Nepal, although the epicentre was in India (Sikkim). Another strong earthquake with a magnitude of 8.4 in India (Bihar) shook as well the eastern part of Nepal on January 15th in 1934. The epicentre was 240 km east of Kathmandu, a total number of 4,300 people died. In Kathmandu itself, every fourth building was destroyed and 40% of the remaining ones were damaged.
Nepal is crossed by the Himalayas - the highest mountains on earth - from the west to the east. A huge part of Nepal is higher than 3,000 m above sea level; the highest point is at 8,848 m above sea level.
The Himalayas were formed by the collision of the Indian and Eurasian Plate, the Indian Plate moves towards the Eurasian Plate with a speed of approx. 4.5 centimeter per year.
The plate tectonic situation that two continental plates are moving towards each other is unique on earth. Normally, an oceanic plate collides with a continental plate, like in the Andes in South America. The two continental plates have approximately the same density, so an entire subduction is not possible, and both landmasses are pushed upwards.
84 mio years ago, in the Upper Cretaceous period, India begun to move for a total distance of 6,000 km with a drift velocity of 20 cm per year. However, about 50 mio years ago (Eocene) India collided with Eurasia. Even nowadays both continents are moving towards each other, resultantly the Himalayas is gaining in height of about 1.5 cm per year.
The collision zone extends from North to South over a distance of 200 km. Furthermore the horizontal convergence amounts to 2 cm per year in Nepal. At their interface the plates are locked. Hence, with time, large stresses build up which are released as earthquakes.
The Earthquake Catalogue of the German Research Centre for Geosciences registered within three days from the 25th until 28th of April 2015 nearly 50 earthquakes with a magnitude 4 or higher. The historical record, dating back to the 13th century, indicates that in statistical terms a disastrous earthquake occurs every 75 years.
Although the strongest earthquakes occur along the so called Pacific Ring of Fire and only 15%-20% of the annual seismic energy is released along the Mediterranean-Trans-Asian Zone and the Himalayas, nevertheless are the most fatal earthquakes occuring in this region. Only 12% of strong earthquakes are detected in the Pacific Ocean (including the Westcoast of America, Japan, New Zealand).
Earthquakes always causing ruptures and displacements within the rock formations, due to the mechanical stress. The displacement and the fraction result in the strenghts of an earthquake. The magnitude is the logarithmic measure of the seismic energy released by an earthquake at its hypocentre. An earthquake with a magnitude of 7 arouse for example a rupture of approx. 60 km in length and 15 km in width with an average displacement of approx. 1.5 m. The earthquake off the coast of Japan with the magnitude 9 in March 2011 caused a rupture zone of about 500 km in length and more than 100 km in width and a displacement of about 50 m.
However, earthquakes occur in depths between 5 km and 700 km. The depth is an important factor. Shallow earthquakes are more devastating than deep (mantle) quakes (s. FAQ earthquakes). The depth of the hypocenter of the Sikkim-Earthquake was located in 7.4 km depth, the hypocenter of the earthquake on April 25th with the magnitude of 7.8 was located in a depth of 18 km.
In order to minimize the impact of natural hazards, a realistically evaluation of possible dangers and vulnerabilities is elementary.
Therefore mitigating actions like the construction of earthquake-proof buildings as well as an effective preparation and trainingsprogramme to arise awareness at people and the society are essential. Nowadays probabilistic earthquake hazard-maps like the "Global Seismic Hazard Assessment Program" (GSHAP), indicates a "very high" earthquake hazard risk for Nepal with a maximum ground acceleration rate between 4 and 5 m per s² (approx. 50% of the ground acceleration) with an exceeding probability of 10% within the next 50 years.
These acceleration data can be used by civil engineers to construct buildings and infrastructure, being able to withstand these forces.