Thursday, June 11, 2015

Answers for a Shaking Nation

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This post is written by Diwash Thapa. He is an undergraduate Physics and Chemistry Major at UNC Chapel Hill, aspiring to become a radiologist.
I gaze at the setting sun and the hills that embrace it on the horizon of the North Carolina landscape: how vast, how serene, how naturally peaceful. But as I pull out my phone and check my recently downloaded app QuakeFeed, my heart skips a beat in fear that another jolt might have hit an equally vast, serene, and naturally peaceful territory some 13000 kilometers away from here: Nepal. That territory is where my heart still calls home some 3 years after moving to the US.

It is heart wrenching to see images of the places you knew growing up turned into rubbles, and your family and friends having to endure a life of fear and uncertainty. But this pain is belittled by the sufferings of the people who actually experienced the 7+ magnitude earthquakes in Nepal; such is the scale of the calamity. The world has come together and chipped in to relieve and reconstruct Nepal and here I am making petty donations, otherwise helpless to this cause.

Helpless I may have felt but I quickly realized that I could help my countrymen in a way only someone in my position can, in one of the major universities in the world, amongst the best and brightest minds in the field of seismology. I am by no means an expert in the field, but I have dug deep into relevant journals and articles and spoken to experts in the field just to answer some of the prominent questions people back home are raising concerning the earthquake. In the aftermath of the earthquake there has been a flow of information on the web as large as the actual catastrophe, sparing physical damage but creating an intellectual tumult among Nepalese.

Here are the answers to three basic questions about the recent seismic activities based on my meeting with Dr. Jonathan Lees, chair of the department of geological sciences in the University of North Carolina, Chapel Hill.

Why are these earthquakes happening?
My conversation with Dr. Lees began with him playing a time-lapse video of the continental drift, now subsumed into the plate tectonics theory; perhaps he wanted to be sure that I first understood the root cause of seismicity before we progressed further. As I watched in awe how a chunk of land fragmented into continents and moved in random directions over time, scale millions of years, in the last one-sixth of the video one particular fragment rose up from the southern hemisphere and collided head on with another fragment located in the northern hemisphere. That was the Indian subcontinent playing tag with Eurasia, politely put.

With the two continental plates being of similar physical properties there is very little room for either to move. Hence, neither gives up, producing mountain ranges like the Himalayas and terrains like the Tibetan plateau. Dr. Lees stressed that the earthquakes are part of the readjustment of the continental crashing. These will continue until the readjustment is complete, which in geological timescales could be millions of years. The most recent earthquakes are because of the release of the pent-up strain in one of the faults created by this continental tussle. This fault is about 700 km long, about the length of Nepal.

Why are Earthquake Magnitudes reported with such inconsistency?
Earthquakes are measured in several different scales. The popular ones are the Richter scale, the moment magnitude scale, and the Mercalli scale. The Richter scale is the most widely know metric because it has been around for a longer time. Developed by Charles Richter in the 1930’s, the Richter scale is best applied to earthquakes in southern California. Although there have been corrections to this scale and methodology, its prime disadvantage that it fails to accurately reflect larger earthquakes near magnitude 8 is significant. The Richter scale measurement is expressed in units of M­L, also known as the local magnitude.

The more widely accepted modern scale is the moment magnitude scale. “The moment magnitude is based on the size of the event, how much slip occurred [in the fault] and the properties of the rock,” said Dr. Lees in trying to explain the difference between the various scales. He also pointed out that researchers abroad don’t quite know the properties of the rocks in Nepal although this is not merely why there are discrepancies in the values reported; “I have a different computer algorithm while the guy down at Duke University may have a different algorithm which results in different estimates, then we also make assumptions like in any scientific calculation that results in experimental error.” The moment magnitude is reported in units of M­W.

The third—by contrast a discrete metric—is the Mercalli scale, which reports earthquake events based on human experiences. “Did the coffee cup shake, or did the lights swing, did the cabinets fall over?” These are some of the subjective question witnesses are asked for categorizing an earthquake in Mercalli scale, according to Dr. Lees. The 7.8 MW earthquake was reported as very strong (VII) to severe (VIII) in the Mercalli index by IRIS, a network of seismology research universities in the US.

Hence, to answer the question of why earthquake reports have appeared inconsistent, it may either be due to differences in scale of measurement or the way of calculation, despite using a common scale.

Are there possibilities of larger earthquakes in Nepal?
This is the one question I believe every single Nepali is asking at the moment, including me. A more naively unrestrictive way of putting this question would be, “Is there a larger earthquake to come?” I will attempt to answer these questions in a way I believe everyone would like to hear —with a cautious tone but positive outlook.

It is needless to mention that Nepal lies in a very seismically active zone and the nature of the plate tectonics underneath makes it vulnerable to larger earthquakes. The pent-up strain in the particular fault may indeed require large magnitude earthquakes to dissipate all the energy. Couple this with the fact that Nepal hasn’t had a major earthquake, of magnitude 9 or so to release this strain, in its documented history and you have an argument to send the whole nation into panic!

I inferred through my talk with Dr. Lees that the aforementioned argument, while not entirely wrong, could be counter argued. For instance, the 8.0 MW Nepal-Bihar earthquake of 1934 occurred around the time when Richter scale was just invented while the moment magnitude scale was another 40 years away from development. Is it, then totally out of the question that the 1934 earthquake may have been a 9 MW earthquake? “ They are just guessing these historic events [in justifiable ways],” answered Dr. Lees. Likewise the western region of the fault hasn’t had a major seismic event since 1555 and seismologist have already speculated that the next big quake will be in the western region of Nepal. “[500 years] in geological timescales is nothing,” laughed off Dr. Lees. So, it is totally within the realm of reality that the fault goes on for another 500 years without any major seismic activity.

But Nepal hasn’t had a major earthquake in its documented history you may repeat. And to you I point out the operative word is “documented.” We are talking about a country that used to be so far removed from technology and literacy that even events of recent history are inferred from foreign records. Who then is to say that a 9.0 MW earthquake didn’t occur before 1555? The event, that such an earthquake had occurred is as likely as it will happen as you are reading this.

On the matter of probability, different research groups and organizations like the USGS make statistical forecasts of seismic events based on empirical data collected over time. But forecasts are not predictions and should be interpreted rather carefully. For instance, the statistical probability of getting heads on a coin toss is 50 percent, but that does not mean, if you tossed a coin ten times you would see 5 heads and 5 tails. You could see 10 heads, or 10 tails, or any other theoretically possible combination.

The 50 percent likelihood of heads or tails only tells us that for a sufficiently large number of trials, say a 1000 tosses, one might get 499 heads and 501 tails, pretty close to 50 percent apiece. Forecasts of earthquakes are very similar. If the USGS forecasts a 40 percent chance of 6-7 MW earthquakes in the next week, all that can be said is if there were about 1000 earthquakes next week there is a very good chance that 40 percent of those will fall in the advertised range. However, a thousand earthquakes a week is not very likely in the first place and any single earthquake can turn out to be of any magnitude.

What is a hundred percent likely event is that there is chaos in nature. One has to consider that science is only a human model to interpret nature and models fail at one point or another. In seismology most models fail to successfully predict events due to chaos. In order to explain this I will borrow an analogy from Dr. Lees. If you take a twig and bring its ends together, it comes under high extension in the middle and eventually the twig will snap. You may know the physical properties of the twig down to a molecule, but you will never be able to tell how these molecules interact to break at a certain point. Earthquakes are just like that. It is just how nature rolls!

1 comment :

  1. May God Ram spare Nepal from future chaos. Well written, Diwash.