A bigger, stronger earthquake is in the offing

The western portion of a gigantic Himalayan fault—the one which caused the 2015 Nepal earthquake—is still charged and is ready for another major shift, says a new study published in Nature Communications.

The study led by Luca Dal Zilio, researcher at ETH Zurich, in collaboration with the California Institute of Technology (Caltech), sheds light on how earthquakes are triggered and what lies in the future.

The Indo-Gangetic plain and the Siwalik Himalaya, comprising over 500 million people, is one of the most vulnerable regions on earth due to its proximity to the Himalayan seismic zone.

Presence of thick sediments further amplify the seismic waves generated even from small-magnitude earthquakes. The Indian and Eurasian tectonic plates also collide near this region.

The shift in these tectonic plates is known to trigger earthquakes in order to release the stress that built up during the shift. The study reveals how small and medium earthquakes in turn trigger giant Himalayan earthquakes.

“Combining historical documents, new geologic data, and a cutting-edge numerical model, our study shows that the entire Himalaya is capable of producing very large earthquakes, with magnitude larger than 8.5 on the rector scale”, says Dal Zilio.

“The most dangerous part of the Nepalese area, which could be affected by the next earthquake, is located west of the capital, Kathmandu. In that area, where there have been several earthquakes in the past, it is now almost 500 years that the energy that accumulates along this mega-fault has not been released,” he adds.

The results indicate that if all this energy is released in one go, it could generate an earthquake with a magnitude greater than 8.5.

The researchers found that magnitude 7 earthquakes release only part of the accumulated energy, while the leftover energy is transferred and accumulated in the neighbouring areas of the fault.

"A sequence of two or three partial ruptures of the fault creates the conditions for the propagation of an earthquake with a magnitude greater than 8.5,” explains Dal Zilio.

Along large faults of plate boundaries—like in the Himalayas—stress is very heterogeneous and accumulates faster in some regions and less in others—due to a geometric effect of the fault itself, and from the frictional properties of the rocks along the fault, he adds.

“The high-stress areas, which store elastic energy, produce earthquakes, while the areas with low-stress fault form energy barriers and cause the earthquake to stop,” he adds.

“However, after two or three earthquakes, these barriers are reloaded from the residual energy of previous earthquakes and allow the next earthquake to propagate through these barriers for larger distances, thus generating an earthquake of magnitude higher than 8.5," says Dal Zilio.

Nepal has a long history of earthquakes, and the study shows that the one which struck Nepal on April 25, 2015, killing 9,000 people and razing entire villages, did not release all the energy that accumulated in that area in previous centuries.

The results of the study raise worries for the already vulnerable region. It should be taken as a warning by governments to prepare themselves against future, much stronger and bigger earthquakes.