The area of Nepal that was hit by the 25 April 2015 magnitude 7.8 earthquake was known to geologists and seismologists for its seismic risk and, because of the extreme vulnerability of the edifices, it was and still is considered one of the regions at highest risk in the world.
The seismic activity in Nepal and surrounding areas is the result of the convergence between the Indian plate to the south, and the Eurasian plate to the north, which is the cause for the formation of the Himalayan mountain chain. The relative movement between the two plates amounts to about 5 cm per year (it is estimated that about 2 cm per year are accumulated along the southern margin of the mountain chain). This means that every century, a deformation of 2 m of relative displacement accumulates between the two plates. In the area affected by the 25 April earthquake, no strong earthquakes had occurred since centuries; for this reason the area around the capital Kathmandu was considered a seismic gap.
Figure 2 shows the seismic potential of the entire contact belt between the Indian and Eurasiatic plates (curved black line): the red and pink columns indicate for each section the quantity of displacement (in meters) that could occur during an earthquake. In the case of the zone hit by the 25 April 2015 earthquake, possible co-seismic movements of 4 to 10 meters were considered. The area immediately to the east of the recently stricken zone had been affected by a magnitude 8+ earthquake in 1934, which is why the corresponding column in Figure 2 shows an accumulation since then of "only" 1-2 m. The geological section (in the inset at upper right in Figure 2) shows how the Indian plate slides beneath the Eurasian plate, leading to the uplift of the Himalaya. The portion of the contact between the plates indicated as "locked" (dotted line in the figure) has remained blocked "for centuries" in the inter-seismic period, and moves suddenly when the resistance of the fault is exceeded: in that moment, an earthquake resets, temporarily, the geological equilibrium. For further in-depth reading visit the CIRES web site:
The Nepal earthquake is a good example of how geologic, geodetic and seismologic studies can allow us to recognize the areas most likely to be the sites of strong earthquakes, even though it remains impossible to determine when exactly they will take place, at least not with a precision of hours or days. It has to be considered that geology operates on temporal scales of hundreds of thousands or millions of years, and thus even a "forecast" in a time window of 10 or 100 years would represent an excellent result. Almeno, non entro un intervallo temporale di ore o giorni. Knowledge of the areas with elevated probability of a strong earthquake is the first important step to reduce the impact of earthquakes. Also in the case of a poor country like Nepal, risk reduction policies can be adopted. We will discuss this in the next issue of the INGVnewsletter.