Increased understanding of Nepal's susceptibility to earthquakes critical to promoting concern among the public and authorities and fueling efforts for disaster preparedness and mitigation activities
According to the Earthquake catalog from the bulletin of National Seismological Center, more than 70 earthquakes of Magnitude 4.0 or higher struck Nepal in 2023. A few among those earthquakes, such as the ones that hit Bajhang and Jajarkot, registered magnitudes higher than 6.0. It is the highest since the disastrous year of 2015, where there were over 400 such earthquakes. Despite this, Seismologists still warn of a major earthquake threat in western Nepal – one that could potentially be of a magnitude above 8.0.
The earthquakes of Bajhang and Jajarkot shock us. They appear to have come suddenly and without warning. Yet for Nepal, recent earthquakes are nothing but just another day in its interminable seismic history. Large earthquakes have plagued Nepal since the geographical origin of the country itself. Perhaps understanding this can better help us comprehend present events and properly guage future risks.
Why Earthquakes Occur in Nepal?
As per our current understanding, the Earth is a sphere with different layers. Primarily, there is the crust (the outer solid layer), the mantle (the middle semi-solid layer) and the outer and inner core (the innermost dense layers). The base of the crust is broken up into rocky plates called the tectonic plates which lie on top of semi molten rocks of the mantle. Due to convection in the mantle, the tectonic plates move relative to each other. In the process, they often get stuck with one another at various places called faults. But due to their tendency to move, stress gradually accumulates in these areas. When enough energy accumulates, it can suddenly overcome friction, and the plates slip violently releasing some or all the stored energy in the form of an earthquake.
Most earthquakes in the world occur in this fashion, at regions where one tectonic plate meets another called plate boundaries. Around the world, there are such belts of plate boundaries where frequent earthquakes take place.
One of these lines passes directly through Nepal. It is the part of a prominent earthquake belt called the Alpide earthquake belt, which according to the US Geological Survey, accounts for about 17 percent of the world's largest earthquakes. The line that passes through Nepal represents the tectonic boundary between the Indian plate and the Eurasian plate which collided some 60 million years ago. Back then, their collision had formed the Tibetan Plateau, the Himalayans and present Nepal. Today, the Indian plate is still moving northwards, pushing against the Eurasian plate at a very slow rate. Throughout history, as enough energy accumulates around this region, it is released as earthquakes. So this is the source of most of Nepal's earthquakes including the ones that occurred in 1833, 1934 and 2015.
History of Recorded Earthquakes in Nepal
Although we know that earthquakes in Nepal are as old as Nepal itself, all such historical occurrences of Earthquakes in Nepal before the twentieth century remains poorly documented. This is because the system of instrumentally recorded earthquakes in the country is only a recent development. Tracing the historical seismology of Nepal, therefore, mostly comes down to studying written history, autobiographies, scriptures and chronicles of the past.
Subsequently, quantitative data such as scale, epicenter, casualties, length, depth of hypocenter, number of foreshocks and aftershocks we possess regarding these earthquakes are limited at best. The more behind we go in the past, the more these details become fuzzier and ambiguous.
One of the earliest accounts of Earthquakes in Nepal is the earthquake of 1255 A.D. which is said to have killed one third to one-fourth of the then Nepali population. Abhaya Malla, the second Malla king of Nepal, is also said to have died in the disaster. Some assume that this earthquake was at least a magnitude 7 or higher. Likewise, a similar earthquake in 1408 A.D. is said to have destroyed the Machhendra Nath temple of Patan.
Written accounts in the monasteries of western Tibet as well as historical records in Moghul India suggest of another apocalyptic earthquake that struck the Himalayans, western Tibet, western Nepal and large parts of Northern India, in the summer of 1505 A.D. Relying on the descriptions of this earthquake in several Tibetian autobiographies, it is often assumed that this earthquake might have been over the 8 magnitude threshold. This is often called the last great earthquake in the western Himalayas as it is said that no record of another great earthquake of this scale exists in this region after 1505 A.D. This is also why some seismologists warn of a mega-earthquake threat in western Nepal due to all the stress that has accumulated in this region over more than 600 years as India continues to move northwards towards Tibet. It is often referred to as a "seismic gap".
From the 1800s and 1900s, we began to develop better records of historical earthquakes. The following map, obtained from the US geological survey's website, displays earthquakes that occurred in the past few centuries that were above the magnitude of 5.
The 1833 A.D. earthquake is one of the earliest major earthquakes of Nepal of which we possess modest amount of historical data. This earthquake, which devastated Kathmandu valley, is estimated to be of the magnitude 7.8, considering the large area over which the Earthquake was felt. The epicenter was somewhere 50 km North - North East of Kathmandu. Two foreshocks occurred before the main earthquake, which could have ushered most residents out of their homes in panic. This might explain why the casualties are reported to be less than 500 despite the large scale of the event.
The great Nepal-Bihar earthquake was another historical earthquake which occurred in 1934 A.D. The epicenter for this event was located in eastern Nepal. The tower of Dharahara which had been severely damaged in the 1833 earthquake once again crumbled to the ground. More than 8000 people died in the event. Roads in the valley were severely damaged and many electricity lines were disrupted.
Finally, there is the Gorkha earthquake of 2015 which remains one of the most studied earthquakes of the Himalayan region. However, the record of historical earthquake in Nepal is incomplete. This makes it challenging to assess the recurrence period of great earthquakes in Nepal. Despite this, it is generally said that a great earthquake strikes Nepal in every 80 to 100 years interval.
Preparing for Future Risks
As far as we know, there exists no reliable technique to accurately predict earthquake events of the future. However, due to high seismic activity in the Himalayan region as well as due to a long record of violent earthquakes in the past, we do understand that Nepal is incredibly prone to frequent earthquakes. The government's post-disaster needs assessment (PDNA) of 2015 ranked the nation as the 11th most earthquake prone country in the world which also attests to our vulnerability. Therefore, it is essential that Nepal implements effective disaster preparedness strategies to minimize causalities in the next earthquake.
We can learn from our past. In the 2015 Gorkha Earthquake, the practice of hospital retrofitting helped keep the biggest hospitals in Kathmandu functioning throughout the 7.8 magnitude earthquake. Now, we should focus on not only retrofitting hospitals but all schools, public and commercial buildings and private homes. Building codes should be also be stringently enforced. Concrete buildings are fatal if proper construction procedures are neglected.
Open spaces that can be used as shelter during an earthquake should be protected. Especially in dense urban areas like Kathmandu, the government should be especially strict to ensure that these open spaces like urban parks are not infringed or encroached. A map of such areas should be easily accessible to the public so that they can plan their disaster preparedness plans in the household level. Similarly, humanitarian relief and response in a post disaster scene need to be swift and correctly placed to save as much lives as possible in the golden hour. Therefore, it is essential to have sufficient relief packages at stand by.
Finally, all large offices should create emergency preparedness plans for their staff in an event of an earthquake and all households should understand how to respond in such an event. Schools should practice earthquake drills in a timely manner. Earthquakes are uncontrollable, disastrous events. But even if we cannot control these seismic catastrophes, we can control our response to them.
