February 12, 2015
A team of scientists have been digging deeper into the central Himalayas, only to discover a previous pulse of great earthquakes followed by a long quiet period since the last great earthquake under the huge layers of rock and soil within those mountains.
The earth’s crust is broken into distinct regions, called “tectonic plates”. It is well known that Himalayas are the result of converging continental tectonic plates. There is an ongoing northward movement of the Indian plate towards Tibet at the rate of 20±3 mm/year and the southern part of Tibet that interfaces with India is absorbing about 80% of this convergence. This means that these regions are accumulating a large amount of strain, which they release in the form of great earthquakes (greater than magnitude 8).
The central Himalaya is considered to be historically a “seismic gap”..In the past centuries, only two earthquakes have occurred in this region whose magnitudes weren’t enough to release the strain that is building within the plates. So, are we missing some great earthquakes that occurred sometime in the past?
The excavation sites near Ramnagar in Central Himalaya suggested that there have been successive occurrences of two great earthquakes in the region in 13th and 14th centuries AD but none since then. It means that the strain is accumulating and the region is currently locked, only to get released anytime in a great earthquake. Longer the relapse time since the last earthquake, larger will the expected dimensions of the potential future slip and greater would be its damage potential.
A great earthquake in the central Himalaya will be most devastating and damaging for a wide swath of Gangetic plains of north India and the Himalayan hills. Mitigation of damage from future earthquakes is based on hazard scenarios developed from earthquake history; geological evidence from past earthquakes is an essential component in probabilistic seismic hazard assessment models. This study helps to fill that important gap in data for the earthquake history of the central Himalaya, a region, which several previous studies have postulated to be ready for a great earthquake.
Further work in the area is required to refine the results and tectonic models to understand the rupture segmentations along the arc. Dr. Rajendran said, “All the major cities in the region need to get ready for this major hazard, which is going to be economically and otherwise most devastating, as the risk has increased multifold over the centuries due to population increase and expansion of built environment. From the Government level serious thinking should go into evolving effective methods to meet such eventualities. Effective hazard reduction models may be available from the western United States and Japan. Urgent action plan should be put in place for public awareness and enforcement of building codes and related environmental laws – important hazard reduction steps in the current context of expanding urban centres with high-rise buildings. Heritage structures and monuments and the critical facilities in the region require reinforcement and retrofitting to withstand severe ground shaking”.