The Nepal earthquake: a predictable outcome of an unpredictable event
On Saturday April 25, 2015, just before noon local time, a magnitude 7.8 earthquake violently shook Katmandu and surrounding regions of central Nepal. Like nearly every earthquake that has occurred throughout the history of humankind, this event was not predicted, despite extensive scrutiny of the Himalaya region by seismologists over the course of many decades. The distressing scenes of death, human suffering and widespread devastation that for the last few days have saturated our televisions, radios, newspapers and the Internet are also accompanied by the inevitable question of why – why here? why now? why didn’t we know it was coming? As was the case for previous devastating earthquakes in recent memory – the Tohoku earthquake in 2011, the Haiti earthquake in 2010, the Sichuan Earthquake in 2008, the Sumatra earthquake in 2004 – as the death toll continues its inexorable rise from 10s to 100s to 1000s and beyond, the experts begin to come out of the woodwork to provide a scientific perspective on the cause of the unfolding humanitarian disaster.
To explain why the earthquake happened is relatively simple. Nepal sits atop of the finest example of continental collision on Earth; this occurs when two tectonic plates (the rigid outer shell of the Earth that is broken up into separate pieces that “drift” across the underlying mantle) containing buoyant continental crust (compared to the thinner, denser crust found beneath the oceans) collide and deform to accommodate the enormous stresses associated with ongoing convergence. Although the deformation process is complex, one of the primary consequences is a thickening of the crust in response to compressive stress at the margins of the two plates (hence the existence of the Himalaya and adjacent Tibetan Plateau). Movement along what are known as thrust faults in the crust provide one mechanism for crustal thickening; this occurs when one section of crust is pushed upward along the fault surface to produce a thicker, but more contracted crust. In the case of Nepal, the earthquake occurred along a thrust fault that was activated by the ongoing collision between the India plate and the Eurasia plate.
While we have a good understanding of why the earthquake occurred, it is the inability of seismologists to predict such events that seems to be a constant source of frustration and hand-wringing to the media and the general public. For example, the title of a recent article in the Toronto Star exorts “Could the Nepal earthquake have been predicted?”, and goes on to document the recent findings of a team of French and Nepali scientists, who discovered evidence of a 700 year cyclicity in large earthquakes in the region. And guess when the last big earthquake occurred – 670 years ago. While this is certainly of interest, it does not usefully contribute to our ability to predict earthquakes, as it is too imprecise. On the other hand, an article in RT news makes the startling claim that “Deadly Nepal quake was long predicted”, and proceeds to document several interviews with leading seismologists in which the common message is that an earthquake of this magnitude was inevitable. However, seismologists generally agree that “earthquake prediction” requires three measures – location, magnitude and time – with sufficient precision that a warning can be issued. In the case of Nepal, seismologists had a good idea of potential magnitude and location, but little idea of the timing. As one of the interviewees stated in 2013, “I cannot say when. It may not happen tomorrow, but it could possibly happen sometime this century”.
The standard response by most seismologists to the question of whether we can predict earthquakes is “no”. While some may regard this as an embarrassing admission of failure, it is actually the expectation that earthquake prediction is the solution to earthquake-related death and destruction that is part of the problem. For a start, it would be unreasonable to expect, even if great advancements were made, that predictions will always be accurate, given that we are dealing with observational science and its inherent uncertainties. Thus, if a city was cleared of its population as a result of a prediction, but it turned out that there was a timing error of a few weeks, then the prediction might well be regarded as counterproductive. It would be even worse if a prediction resulted in the evacuation of people from city A to city B, but it turned out that the earthquake occurred beneath city B instead of city A due to a location error. Even if somehow we were able to achieve 100% accuracy in our predictions, widespread destruction of housing and infrastructure might still ensue, which could trigger a humanitarian crisis, even if the earthquake did not directly cause any deaths.
So what should we do about large earthquakes? Instead of earthquake prediction, seismologists have increasingly focused on earthquake forecasting, in which the probability of earthquakes of a certain size occurring in a certain location within a certain period of time is quantified. Typically, this is represented by seismic hazard maps, in which the likely strength of ground acceleration as a result of earthquakes is plotted. These maps can then be used to inform building codes and ensure an appropriate level of preparedness amongst government agencies and the general population. In the case of Nepal, global seismic hazard maps reveal it to be one of the most dangerous places on Earth, so the occurrence of the recent earthquake is hardly a surprise. The reason for much of the death and destruction is simply that many of the buildings were not constructed to withstand significant earthquakes, and the population and authorities did not have appropriate action plans in place to deal with the advent of an earthquake. Governments around the world must focus on this more mundane, but ultimately far more effective way of dealing with large earthquakes, rather than be distracted by the allure of earthquake prediction. Otherwise, we are doomed to re-live the distressing scenes that we have witnessed from the safety of our living rooms over the past few days.