Swarm of aftershocks

Over the next 24 hours, there was a swarm of 35 aftershocks after the big event in Gorkha. This could only mean one thing: it would take several weeks or years for the aftershocks to subside. Of course, the magnitude would die out exponentially and so would the number of occurrences. This is known as Omori’s law, a widely used empirical decay law for aftershocks, named after a Japanese scientist back in 1894 who proposed it after studying the aftershocks of the 1891 Nobi (Mino-Owari) earthquake in central Japan and two other earthquakes in the country. Similarly, the aftershocks after the Saturday earthquake will follow Omori’s law for several more years to come. However, statistical forecasts can neither give exact dates nor times of occurrence. Moreover, in a time of crisis—with a wild and rumour-filled social media—it would be devastating if such scientific forecasts based on statistics were (as we have seen over the past week with rumours of predictions) misinterpreted as cocksure predictions. Thus, I believe it is best to focus on disaster and relief efforts, rather than subjecting the general public to scientific numbers and statistics, which can only confuse and scare the general public, who are already having a hard time grasping the catastrophic reality they are now living in.

How big was the earthquake?

As soon as the news of the earthquake broke, the United States Geological Survey (USGS) earthquake webpage showed Nepal littered with little blips, and in the blink of an eye, another one would appear at another spot. Aftershocks were scattered throughout the region between Gorkha and Sindhupalchok. Clearly, the scattered data on the map showed that the size of the rupture plane—the surface of the crack that ruptured underneath the Himalayas—was roughly 125 km x 80 km. This gives an initial approximate area of 10,000 km2 for the rupture zone, which is correct for an earthquake measuring 7.9 in magnitude, as per the calculations designed by the late Prof Tokuji Utsu of Tokyo University in 1987.

The first aftershock was 120 km east of the epicentre, about 7 km southwest of the recent landslide that had devastated the Arniko Highway in Sindhupalchok. But it was the second aftershock that stopped my heart. The USGS had registered an aftershock of 6.6 magnitude, with a very shallow depth of 14.6 km, just 3 km east of my village of Gumda, in Northern Gorkha. I remembered my dad’s telling me that his dad had told him about how the great earthquake of 1934 had scooped up a large portion of an entire hill between Gumda and Yamgaun and wiped off parts of an ancient historical fortress on top of the hill. As I thought about that story, I could only fear the worst had happened in Nepal. Geoscientists have projected maximum slips—deformation and displacement of the ground—up to 3m based on complex mathematical and satellite data. Thus massive landslides seemed imminent, not only in my village but all over the 2,500 or so affected remote mountainous settlements lying within and adjacent to the rupture zone.

Seismic waves

People were caught unprepared. However, since Saturday was a public holiday and the earthquake struck during daylight, not many people died as would have otherwise during a night quake. The number of casualties increases day by day, but these numbers are believed to be conservative. Max Wyss, a renowned geoscientist, has calculated fatalities could be much more, and there will be millions affected by the earthquake.

Seismic waves are acoustic waves that propagate through the earth, and comprise basically primary (fast-travelling but non-damaging) and secondary (slow-travelling but damaging) waves. Moreover, given the heterogeneous geological structure and rugged topography of the Himalayas, ground shaking was amplified severely throughout the nation. Surface waves are secondary waves—big seismic waves that propagate along the surface of the earth—and they rip and tear apart the ground as they propagate along the rupture zone. They travel slower than the primary waves, which travel much faster but usually inside the earth. Thus in Kathmandu, which was a little far from the epicentre, there were 10 seconds of mild shaking before the city was hit with the onslaught of the rumbling surface waves that lasted 30 seconds. Thus, these surface waves brought the old and weak structures down.

As for the villagers near the epicentre, they had hardly had any time to flee. Being close to the epicentre, the ground quickly started shaking and ripping apart as the surface waves went right through them. The full extent of the destruction in the mountain villages, whether strung along the Himalayan rivers or perched on top of precipitous hillsides, have yet to be fully known. Limited resources, bad roads, and bad weather in the days following the earthquake have severely hindered rescue and relief efforts in these remote regions.

Can we predict earthquakes?

Throughout the week, the public has been flooded with false news of putative earthquake predictions. Both the streets and social media were abuzz with predictions that had no scientific basis whatsoever. Were they misinterpretations of scientific facts relayed to the public or mere speculations? I am not sure. Sometimes, scientific information does often get misinterpreted during a crisis, and frenzied humans cause more harm than help.

Of course, earthquakes cannot be predicted, but geoscientists have made great advances in the science of earthquake prediction. This field was led by the late Prof Keilis Borok of the University of California, Los Angeles (UCLA), who spent much of his entire career in advancing the science of prediction based on seismic patterns and statistics. He came close to predicting the April 2011 Fukushima earthquake in Japan, and missed by only a month, which, by scientific and mathematical reasoning, is a pretty good estimate and a great scientific achievement. However, he himself considered it to be one of his big personal failures in life.

In the case of Nepal’s earthquake, there is no doubt that a big one will come again within the next 75 to 100 years. Despite Nepal’s complex tectonic setting, Himalayan earthquakes—unlike in other regions—are quite predictable. If one were to look through the historical data on earthquakes, a big earthquake has been recorded every so often in a hundred years. But that is as far as the prediction goes. Earthquake prediction cannot be quantified in terms of exact dates or times; the error in the prediction of Himalayan earthquakes would nonetheless be within the range of plus or minus 25 years. Thus, when the late Prof Keilis Borok missed the Fukushima earthquake prediction by a month, it was a scientific achievement.

The Way Forward

The aftershocks will continue, according to Omori’s law. But their magnitude will subside and the occurrences will be sparse. Inevitably, the big one will come again in another 100 years, punctuated by smaller earthquakes. But in the interim, we Nepalis will hopefully be more aware and will have built infrastructure that can withstand the next big one. As for my villagers in the remote villages of Gumda, where settlements after settlements have been wiped off the face of the earth, I wish I could say the same.

Gurung is a post-doctoral research fellow at Chonbuk National University, Jeonju, Cholla-buk-do, South Korea