The intent of this article is to provide a brief introduction to the study of earthquakes, and then summarize our understanding of earthquakes in Bhutan; past, present and future.
Studying a planet in motion
Earthquakes are part of a natural process, the expression of enormous forces that are generated and released as a mosaic of distinct rock masses or “plates” float on our planet’s surface and grind against one another. Our understanding of these phenomena, known as “plate tectonics”, was born in the early 20th century with the work of Alfred Wegener and other scientists. Looking at maps of the continents, they observed that these shapes seemed to fit together like pieces in a puzzle. There were other clues such as similarities in geology and fossils, signs of a shared history in plant and animal life among these now distant lands.
Seismology, the study of earthquakes, has become a world-wide endeavour because lessons learned in one region can help us to understand and prepare for earthquakes everywhere. For example, we can compare the behaviour of earthquakes occurring in several locations around the Pacific Ocean (Japan, Alaska, Oregon, Chile, and others) and in Sumatra. This is possible because they all have a similar fault structure, called a “subduction zone”, wherein the oceanic plate is colliding with and dipping beneath the continental plate. The Himalayas and the Alps are another example; both are collisional mountain belts, so here too seismologists are studying behaviour common to these areas.
Seismometers are precise instruments that were developed to measure the frequency, duration, and orientation of earthquake shaking. In recent decades, arrays of seismometers have been established in all corners of the globe to detect seismic activity and provide data valuable for a variety of research interests. From these measurements, we know that earthquakes are occurring all the time, with typically hundreds of small events recorded each day worldwide.
Some of these smaller events can be felt by people and animals in local areas, but they are modest in duration and cause minor to no damage.
Moderate to large earthquakes can shake an entire country and, while they are not so frequent, most people living in the Himalayas experience at least one such event in their lifetime. These events can impact communities through injuries and loss of life, damage to infrastructure, and impacts to the environment, such as landslides.
Finally, catastrophic earthquakes, such as the recent event in Nepal, are sadly also part of this natural process in some areas. They have the potential to take many lives and cause widespread destruction, but the time between them is so long that many communities cannot remember such events even from their ancestors’ stories.
Knowledge of past earthquakes can help us understand the type, size, location, and frequency of events that can be expected in the future. But how much information is available for the study of tectonic history that spans thousands or even tens of thousands of years?
The source of data that can reach furthest back in time is the study of individual faults by investigating geologic evidence. One approach is to excavate a trench across a known fault and try to identify the relative displacement of geologic layers that occurred during a past earthquake. This can provide information on the approximate size and duration of the event. The study of terraced river deposits can also provide information about ancient earthquakes. Such analyses were performed in the region of Gelephu and Sarpang by a French-Bhutanese team. They explored surface features of the fault along which the Himalayas thrust over the India plate, supposedly producing rare but catastrophic events. The results show that in the past millennium two catastrophic events have happened, with magnitudes possibly exceeding M8.
A fascinating aspect of seismology is the search for historical documents that can offer data about the frequency and intensity of past events. In the region of Bhutan, historical records of earthquakes include books and written notes by officials, monks and citizens. For example, there is excellent documentation for the great Shillong Plateau earthquake (Magnitude M8.1) in 1897, which was felt across Bhutan. However, older records of earthquakes are rare, possibly due to fires that ravaged monasteries and Dzongs. One earthquake that is believed to have occurred in Spring of 1713, was described by a 3 or 4 year old child, Shakya Rinchen, who later became the Ninth Je Khenpo. The event was large, if not catastrophic, but neither the location nor the magnitude can be reconstructed from this single note.
With the advent of seismometers the international community has catalogued about 10 to 15 medium to large earthquakes (M6) in or near Bhutan over the last century. One could say there is roughly one M6 event every 10 years, but this is an average: some decades had several events, some had no event at all. Most Readers will remember the September 2011 earthquake in Sikkim, and the September 2009 earthquake near Mongar. Seismologists evaluated the relative size of these events, and assigned the Sikkim event a magnitude M6.9, and a magnitude M6.1 for the Mongar event. While the Sikkim event was larger, the Mongar earthquake caused more damage in Bhutan because its source was shallower and nearer Bhutanese cities.
Although the available information on past earthquakes is limited, it is clear that earthquakes of all sizes can happen in Bhutan. So the question is not if, but when will the next large event occur? Currently, the average time between large events is not well known; however, there are ways to learn more about this.
To be continued
Contributed by Dr. György Hetényi,
Geophysicist, ETH Zürich, Switzerland Travis Munson, Geotechnical Engineer, Portland, Oregon, USA