Nepal: Two Large Quakes In Short Time Span, One After Another Is Rare
By Deepak Chamlagain
05 May 2015
Ekantipur.com
As the Nepal earthquake and aftershocks death toll, according to the Nepal Home Ministry, has reached 7,557 discussion on issues related to the quake is on-going. The following interview sheds some lights on the issue:
Rumors still swirl of another impending big quake. There is no need to panic, cautions Deepak Chamlagain, a seismologist who has been researching earthquakes in the Himalayan region for close to 15 years. Pranaya SJB Rana spoke to Chamlagain, who was resident disaster management expert at the New Delhi-based SAARC Disaster Management Centre, about the possibilities of another big quake striking soon, the implications of the continuing aftershocks, and the preparatory measures we need to take for the future.
The question that has been on everyone’s mind since the Great Earthquake seems to be: what are the possibilities of another big quake striking Nepal in the coming weeks and months?
The possibilities of another big quake are very low. Last Saturday’s quake with its epicenter in Gorkha released much of the elastic strain energy that was accumulated in that fault. The energy hasn’t been released completely but it is being released in small amounts in the aftershocks that we are continuing to feel. Another big event in that same fault and around that same area has become very unlikely and such fears are unfounded.
A table supposedly from the United States Geological Survey (USGS) has recently been circulating, which lists the probabilities and the magnitude of quakes that could now occur in the region. This table states that there is a 1 percent chance of an earthquake greater than magnitude 7 hitting us between May 1 and May 31. What does that mean?
Aftershocks are often forecasted based on empirical relations. But this does not mean that those forecasts will definitely come to pass. So a 1 percent chance for a big quake means that there is a 99 percent chance that such a big quake will not happen. That is what we must focus on. USGS has even stated at the bottom of that table that these predictions might not occur. We have been experiencing aftershocks in decreasing frequency and decreasing intensity. This means that there is no such risk of a big quake hitting us soon. If we look at global data on big quakes, such aftershocks will continue for at least a week or two.
The world over, has there ever been any instances when two big quakes occurred within a short time span, say, up to five years?
Two large quakes occurring in a short time span, one after another are very rare. One case I can recall is in 2010 when I was in Rome, Italy conducting research on earthquakes. On May 24, a 5.6 magnitude quake occurred. A few houses fell and a few people died. Aftershocks too kept occurring. After five days, another 5.8 magnitude quake occurred in the same area, destroying houses that had cracked in the earlier quake. Sixty people died in that second quake. But again, this was a very rare occurrence. In the Himalayan region, there is no record of two large quakes striking one after another.
Some foreign experts have speculated that the current quake has now placed more pressure on another fault in west Nepal. This western fault could now cause another massive earthquake, they say. Is this legitimate?
It is too early to speculate anything like that. We need to first analyze all the seismic events that have happened here. We need to understand just how far this fault has propagated and how it has affected the rest of the Himayalan faults. We haven’t even decided on the magnitude with total certainty as of yet. So we cannot make such assumptions without studying all the data. Still, we had long been predicting that a large earthquake was overdue in western Nepal, the area between Nepal and Dehradun. Between the 1505 Dehradun earthquake and the 1934 Nepal-Bihar earthquake, there were almost 500 years when the strain was building and no large earthquakes had taken place. Our research had also shown that the strain accumulation rate in this area was high and thus, we had warned that it is important to stay alert and prepared. Now, last Saturday’s quake took place in Gorkha, which is more east than we had imagined. So what we need to study now is whether that quake released enough of the strain in that area. If it did, then we are safe for at least another couple of decades. If not, we will be due another big one. But we cannot say this without studying all of the data first.
There are comparisons being drawn between the 2010 Haiti earthquake, which measured magnitude 7, and our quake, which measured 7.9. Despite being a smaller quake, thousands more people died in Haiti. Were there any geological differences that led to this difference or was it simply about the construction of buildings?
First, it is not always true that a large earthquake will cause more damage and a small one will cause less damage. Earthquakes themselves don’t do anything; they are a natural process. It all depends on how our structures are able to sustain the intensity of that natural process. We need to look at the strength of our structures and the strength of the earthquake. We also need to look at the geological make-up of the area, including the soil structure. In Kathmandu Valley, there is a 500-metre thick alternating layer of clay and sand, which dramatically amplifies the quake, up to eight times. But we cannot compare the geological structure of Haiti with the geology of Nepal. It also depends on many socio-economic factors and just how prepared the populations were. In Haiti, the people did not seem to be prepared and there were large populations living in slum areas with poorly constructed buildings. In Nepal, thankfully, that was not the case.
In recent times, many in the I/NGO sector (and in extension the media) had been warning that a big quake would completely destroy Kathmandu, where more than 60 percent of buildings would collapse and tens of thousands of people would die. Thankfully, that prediction did not come true. What do you make of the assertion?
These I/NGOs were simply making dogmatic statements that did not have any scientific basis. In many areas of Kathmandu and Lalitpur, it doesn’t even look like an earthquake has just taken place. But if we go towards the edges of the Valley, towards where the hills begin, there we can see the extent of the damage. Look at Bhaktapur, Sankhu and Bungmati. There is massive damage. What has happened is that soil structure of the Valley propagated and amplified the shock waves toward the edges, where the waves met with rock structures and were bounced back. That is where the damage occurred. The science here says that the cause of the damage is the amplification of the seismic wave energy. It is not just because of poor construction of buildings. It is because the soil conditions were not investigated and efforts were not undertaken to strengthen the quality of the soil. These organizations had also predicted that a 7 magnitude quake would destroy the airport. But just a month ago, I and a student had presented a paper outlining how a 6.3 magnitude earthquake would not affect the airport at all. And look, a 7.8 magnitude quake and the airport is still operational.
Given our soil structure and geological make-up, is it wise for us to build high-rises in the Kathmandu Valley?
If we don’t do anything to strengthen the soil conditions of the Valley, we cannot allow such high-rises to be built. Of course, even taller high-rises have been built in Valleys that are even more dangerous than ours. But they are all compliant with a seismic resistance building code. I myself had conducted a study on what kind of tests these high-rises had done before construction. I found that they had only done a basic routine soil test, which is not even necessary. What they should’ve done was a dynamic soil test or a ground resistance test, which measure how the soil will react to seismic waves. Not a single high-rise had done this test, except for two foreign embassies. So I ran a simulation of a 6.3 magnitude earthquake on a 30-metre deep foundation. I found that the seismic wave was amplified six-to-seven times. High-rises, therefore, need to be constructed from a geotechnical earthquake engineering point of view.
What we, both the government and the citizens, can now do to prepare ourselves?
First, we need a land use policy. We cannot allow people to build homes anywhere and everywhere if they just set aside 10 feet for a road. But just visit Sankhu. There is a 10 feet road but buildings have collapsed from both sides and the road is impassable. Second, we need a proper building code that takes into account the possibility of earthquakes and the soil condition of the Valley. This code must be site-specific, as the same building code cannot apply to the Tarai, the Kathmandu Valley, and the mountains. Another big problem we have is that our existing building code was never monitored. People come in with a plan for a 12-pillar building and when they construct it, there are only nine pillars. But, it is also the people who need to be aware and take adequate measures. After all, we are the ones who will die.
Policy-wise, we also need to start looking at earthquakes differently.
We cannot put all natural disasters into the same bucket. Earthquakes themselves are the source of all disasters — floods, landslides, ground fissures, building collapse, liquefaction, etc. The government must have a separate strategy to deal with earthquakes, which do not occur often but when they do, they cause massive damage. What we need is a separate institution to look after all disaster-related issues, which will come through a Disaster-management Act. This Act has been in the offing for a number of years now, but it has been put off until the constitution is written. But it looks as if the earthquake was bigger than the constitution.
Tags
.
Comments are moderated