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Myanmar earthquake of March 24, 2011 - Magnitude 6.8

25 Mar 2011

When continents collide, there are far-reaching consequences. A magnitude 6.8 earthquake struck eastern Myanmar, near the border with Thailand and Laos, at a depth of 10 km, at 20:25 local time (13:55 UTC) on Thursday, March 24th, 2011. As of Sunday, March 27, the death toll had reached 104. There is no evidence that this earthquake is related to recent earthquakes in Japan. Rather, the earthquake in Myanmar occurred on an active fault that is part of a broad zone of deformation resulting from the collision of the Indian subcontinent with the Eurasian landmass.

The most dramatic manifestations of the collision of India with Asia are the Himalaya, the tallest mountain range in the world, and the Tibetan Plateau, the tallest and largest plateau in the world. However, the effects don't stop there; this collision has been forcing blocks of the Eurasian landmass toward the east-southeast for over 35 million years. Thus active earthquake faults riddle not only the Himalaya and Tibetan Plateau, but also regions far to the north and east, toward Mongolia and Beijing, and also to the southeast, through Sichuan and Yunnan into Myanmar, Thailand, Vietnam, and Laos. (Figures 1 and 2).

Figure 2. Animation showing the tectonic evolution of India and Asia over the past 50 million years.  (Produced in 2002 by Prof. Paul Tapponnier and a team from the French Centre National de la Recherch Scientifique for the exhibition "Himalaya-Tibet: Le Choc de Continent" at the Muséum National d'Histoire Naturelle in Paris.  Based on the work published in Replumaz and Tapponnier, 2003.)

Zooming closer in to eastern Myanmar and northern Thailand and Laos, we find the crust* of the Earth behaving a bit like a stack of books that is toppling over on a shelf (Figures 3, 4, and 5).   Unlike subduction zones where one plate dives under another, in this region the rocks on either side of the earthquake faults are sliding side-to-side as India continues is northward voyage.  Side-to-side motion along two major earthquake faults, the Sagaing Fault in Myanmar and the Red River fault in Vietnam and Yunnan (Figure 1), is accomodating both of the northward motion of India and the southeasterly motion of China.  In between these two major faults, the Earth's crust is broken into a series of blocks that are slowly rotating clockwise as a result.  The edges of the blocks themselves are faults; it appears that the March 24th earthquake occurred on one of these faults, the Nan Ma fault (as mapped by Le Dain and others, 1984, and Lacassin and others, 1998; Figure 4).

 

Over the past few million years, motion on the Nan Ma Fault has offset (shifted) the Mekong River by 12 kilometres (Figure 4, between the two white arrows; see Lacassin and others, 1998).  This 12-km offset would not have occurred smoothly, nor all at once, but rather in a series of small jumps as earthquakes periodically caused the rocks to shift.  Imagine travelling in a boat down the Mekong River as you approach this fault from the north; once you reach the fault, you would turn left to follow the river.  For this reason, we call these kinds of faults "left-lateral" faults, meaning that if you stand on one side of the fault, it looks like land on the other side has shifted to the left.  
 
Based on inital estimates of the earthquake location, it appears likely that the earthquake was caused by motion on the western segment of the Nan Ma Fault. The direction of motion inferred from seismograph readings is also consistent with the left-lateral motion that is typical of this fault.  We do not yet know how much the rocks may have slipped in the March 24th earthquake, nor do we know for sure whether the fault actually broke to the surface.  Other earthquakes on similar faults at similarly shallow depths (10 km) and similar magnitude (6.8) are typically accompanied by a couple of metres of slip at the surface over a segment of the fault a few tens of kilometres long.  Field study at the site of this earthquake would shed important new light on the nature of this earthquake.
 
There is no evidence to connect earthquakes that occur far apart on the Earth, such as this earthquake and the Tohoku (Japan) earthquake of March 11th.  There are cases, however, of earthquakes on one portion of a fault increasing the stresses on other portions of the fault and on closely neighbouring faults.  This earthquake in Myanmar likely increased stresses on the eastern part of the Nan Ma Fault.  Unfortunately, we don't have enough information about these faults to say whether the earthquake hazard has changed in neighbouring areas.  Scientists from the Earth Observatory of Singapore are currently undertaking projects, both at a regional scale and in western MyanmarNepal,India/Bangladesh, and China, to learn more about the earthquake faults associated with the great collision of India and Asia.  These faults have been much less well-studied than faults in areas such as Japan and California, yet can be at least as devastating, as the 2008 Wenquan (Sichuan) earthquake (Figure 3) showed all too clearly.

* It is actually the lithosphere of the Earth that is involved in fault and plate motions, which includes the crust and the very uppermost part of the mantle.  We use the term "crust" in this article because it is more widely understood.