Nepal and northern India are not overdue for a major earthquake

While some have claimed that northern India and western Nepal are already in for a massive earthquake, the analysis says this is a myth as the region has been experiencing smaller earthquakes randomly for millennia.

It is common for officials and the media to talk about populated areas near fault lines, such as Istanbul, Seattle and Tokyo, being “delayed” for strong earthquakes. Since the fault segment of the central Himalayas in India and Nepal had its last recorded major earthquake in 1505, research he suggested that earthquakes recur there every 500 years and a major earthquake is now imminent.

But scientists have now discovered that at least 50 earthquakes of magnitude 6.5 or greater have occurred in the area in the past 6,000 years. including eight since 1505. And these earthquakes occurred randomly rather than at regular intervals.

“We need to stop discussing and having long debates about the periodicity of earthquakes in the Himalayas and come to an agreement that it is a random process … and consider the risk within that framework,” he says. Zakaria Ghazoui-Schaus at the British Antarctic Survey, which led the study.

The collision of the Indian and Eurasian tectonic plates that throw up the Himalayas continues to this day, forming one of the largest seismic zones on the planet. The 2,400-kilometer fault beneath the mountain range causes powerful earthquakes, such as the 7.8-magnitude disaster that killed nearly 9,000 people in and around Kathmandu in 2015.

However, less evidence of earthquakes has been found on the central segment of the fault immediately west of Nepal’s capital, leading to fears that pressure is building up in this “seismic gap” and will soon be released in a devastating magnitude 8 or 9 earthquake.

Ghazoui-Schaus argues that this was a misconception based on “intermediate knowledge” rather than a seismic gap. Scientists usually look for evidence of earthquakes in the Himalayas by digging trenches to find cracks in what was once the surface of the earth. While this method was able to detect large earthquakes, it missed smaller “shadow earthquakes” that did not break the surface.

“You’re going to have very sparse records of the largest earthquakes” with traditional paleoseismological methods, he says Roger Mussona former seismologist with the British Geological Survey. “For historical earthquakes, the catalog can be good up to magnitude 4 or so.”

Because the record was primarily populated by large earthquakes, it led to calculations of a long “intervention interval”, also known as the “return period”, which is the average time between earthquakes of a certain magnitude in an area.

To better detect earthquakes in the central Himalayas, Ghazoui-Schaus and his colleagues went to Rara Lake in western Nepal in 2013 and used an inflatable boat to take a four-meter core of sediment from the bottom of the lake.

They later analyzed the core for turbidites, layers of fine sediment on top of coarser ones deposited on the lake floor by earthquake-triggered underwater landslides. The team has now identified 50 earthquakes of magnitude 6.5 or greater over the past 6,000 years, dating each one by its depth in the core. These likely released energy and reduced the stress in the fault, says Ghazoui-Schaus.

Statistical analysis found that the earthquakes tended to come in clusters, but these clusters occurred randomly. While that’s what most seismologists would now expect based on the modern instrument record, Ghazoui-Schaus says it’s one of the first times a paleo-seismological record has confirmed it.

“If I have to build a house in western Nepal, I would definitely be more careful how I build,” he says. And even if earthquakes occur randomly, calculating the average interval between them can still be useful as an indicator of seismic activity that could damage structures in the area, such as bridges or dams, according to Musson.

“If you’re planning for the next hundred years, you want to know how many earthquakes of a certain size will occur in that period,” he says. “And if you’re prepared for that, then it doesn’t matter if the earthquake happens next year or in 10 years, because you’ve built your dam strong enough.”

topics: