Monday, September 19, 2011

Geological Framework Of the Sikkim Earthquake

Everyone is calling it the Sikkim earthquake (6.8 Richter) of September 18 2011 but the epicenter plots just west of the Sikkim border in Nepal. Information about the geological framework of the region and the tectonic elements responsible for the earthquake has been limited. At the USGS website I got the following:

The September 18, 2011 Sikkim, India earthquake occurred near the boundary between the India and Eurasia plates, in the mountainous region of northeast India near the Nepalese boarder.......

The preliminary focal mechanism of the earthquake suggests strike slip faulting, and thus an intraplate source within the upper Eurasian plate or the underlying India plate, rather than occurring on the thrust interface plate boundary between the two.

I have highlighted the sentence of interest to me. It is very unlikely that the earthquake source was within the upper Eurasian plate. It was almost certainly in the Indian plate.

Below the image shows the Himalayan orogen bounded between the Himalayan Frontal Thrust to the south and the Indus Tsangpo Suture to the north. It is along the Indus Tsangpo suture that the two continents have collided and the Indian plate has dived under the Asian plate.  The location of the Sikkim earthquake is a good 200 km or so south of this zone of thrust interface plate boundary between the two.

Source of Active Faults (red lines) - Styron, R., Taylor, M., & Okoronkwo, K. (2010). Database of active structures from the Indo-Asian Collision. Eos Trans. AGU, 91(20), 0-1. doi: 10.1130/GES00217.1 and Taylor and Yin (2009). Available as a kml file at HimaTibetMap-1.1.  See this post on the Rocks and Water blog for the story about the compilation of the fault database.

I have put up below a generalized cross section across the Himalayan orogen with the red arrow pointing to the approximate geographic position of the earthquake in relation to the major tectonic boundaries.

As you can see the location of the earthquake is between the Southern Tibetan Detachment (STD), a zone of normal faulting between unmetamorphosed Paleozoic sediments (Tethyan deposits) and high grade Greater Himalayan metamorphic rocks representing the Tethyan basement and the Main Central Thrust one of the major thrust faults that brings into contact the Greater Himalayan Proterozoic high grade metamorphic rocks (Tethyan basement) with the Lesser Himalayan low-medium grade Proterozoic metamorphic rocks. These represent deformed and exhumed upper to mid crustal levels of the Indian plate. The earthquake's epicenter estimated to be at most at 20 km depth lies firmly within the Indian plate. The Asian plate is far to the north.

Since two plates are pushing into each other, the major stress regime in the Himalayan orogen is one of compressional forces. This earthquake though was likely caused by movement along a strike-slip fault where two crustal blocks slide past each other and not towards and over the other as expected in an area where compressional forces are common.

Below is a geological map of the Sikkim Nepal region. The earthquake is located quite close to the Main Central Thrust. Geologists can learn about the type of fault that caused an earthquake by an analysis of the first motions of the seismic waves generated by the earthquake being recorded at different monitoring locations. See here for a good explanation of this method. Such a focal plane solution (yellow beach ball) for the Sikkim earthquake indicated a strike slip fault oriented NW- SE. The map also plots focal plane solutions (smaller red beach balls) for few of the recent earthquake's in this region and as you can see strike slip motion is a common type of earthquake movement here.

 Source: EIA of the TING TING H.E. PROJECT, SIKKIM - RS Envirolink Technologies Pvt Ltd (large file)

Detailed analysis of the seismicity in the Sikkim Himalayan region indicates that although earthquake activity is located in the region  near major thrust faults, many earthquakes are not associated with them. Instead movements along faults like the Gangtok lineament and the Tista lineament which are  transverse to the strike of the orogen seem to be responsible for many earthquakes including the September 18 earthquake. These are north northwest trending faults. The plate motion of India into Asia along the Himalayas is in a north north- east direction. Some geologists have suggested that in this part of the Sikkim Himalayas some of the crustal shortening is being accommodated by movement along these transverse strike slip faults rather than by underthrusting.

Crustal Shortening: The upper part of the Indian plate is being crumpled and squeezed into a narrower zone. How is the crustal material accommodated in that narrow zone. Commonly by piling crustal blocks on top of each other by thrusting along faults. This leads to crustal thickening and contributes to the elevation of the mountain chain. Or sometimes by sliding blocks past each other   along strike slip faults both transverse to the orogen as in the case of this earthquake and also parallel to the orogen as is possibly happening along the two east west strike slip faults north of the Sept 18 earthquake location seen in the first image of this post.

In the past 35 years there have been 18 earthquakes in this region of magnitude 5 or greater. No doubt there will be many more in the future. Some will be along the strike slip faults described above. Others may be along the major thrust faults like the Main Central Thrust.

..and no... you won't be able to predict them. Preparedness in the form of better building construction and a functioning disaster relief mechanism is the only way to save lives.


  1. thanks for posting this info, difficult to find anything about this event over in the U.S.

  2. very informative - thanks

  3. It is really a good beginning of suggesting plausible mechanism particularly in the backdrop of strike slip faulting.As this mechanism manifests variety of tectonics.

  4. very nicely written and really informative.