EARTHQUAKES ALONG PLATE BOUNDARIES IN SOUTHEAST ASIA: PART I

by Dr. Afroz Ahmad Shah

Our scientific knowledge about earthquakes is growing and this will eventually help us in understanding the science and therefore prepare remedies to counter the continuous threats faced by our 10,000 year-old civilization. A few examples of some major past earthquakes, which constantly remind us of the immediate need to master the science behind these eventualities, are as follows: the devastation of Acehnese and Thai coasts in 2004, of Kashmir and New Orleans in 2005, of southwest Java in 2006, of Sumatra again in 2007, western Sichuan and Myanmar in 2008, of Haiti in 2010, of Japan, New Zealand and Turkey in 2011, of northern Sumatra and of Solomon Islands in 2013.

In this article I will highlight the latest development in the earthquake research along the Nepal Himalayan, which was conducted by Prof. Paul Tapponier (Earth Observatory of Singapore) and his group.

To understand this critical work, a brief introduction is required to know the cause of earthquakes along the Himalayan regions. Earthquakes along the Himalayan arc occur due to collision of two continents. Here, the Indian plate is colliding with the Eurasian plate at geologic and geodetic convergence rates of 30–50 mm/yr^1,2(Figure 1). Convergence rate is the speed at which the two stations (e.g. continents) come towards each other. The Indian plate is moving at 30-50 mm/year towards the Eurasian plate and thereby causing the collision, which leads to the earthquakes. The region along which the two plates come in contact with each other is called a plate boundary. The active plate boundary fault along the Himalayan arc is called the Main Frontal Thrust fault (MFT)^2,3, a megathrust fault that (see Understanding Earthquakes: Part II, Scientific Malaysian Magazine Issue 3) stretches all along the Himalayan arc for more than 2000 km.

On the south the Indian plate subducts beneath the Sunda plate and forms the Sunda megathrust⁴ (see Understanding Earthquakes: Part II) which is also seismically very active and has hosted a number of major earthquakes in the recent past. I will discuss this megathrust in the next issue as today we will primarily focus in understanding the earthquakes along the Himalayan arc.

About 20 mm/yr of this convergence is mainly absorbed by the Main Himalayan Thrust (MHT)⁵, which reaches the surface at the Main Frontal Thrust fault (MFT)^3,5 that marks the southern margin of the Himalayan range. This absorption by the MHT therefore account for about half of the total convergence rate between the Indian and Eurasian plate⁵. The push and the subsequent boundary. During the past ~100 years, six major earthquakes have occurred along the Himalayan Frontal Thrust (HFT; Figure 1), the sequence from west to east is: the 2005 Kashmir earthquake (M_w 7.6), 1905 Kangra earthquake (M_w 7.7), the 1934 Bihar-Nepal earthquake (M_w 8.1), the 1879 Shillong earthquake (M_w 8.1) and the 1950 Assam earthquake (M_w 8.4)^6,7.

The Kashmir earthquake of 2005 was special because it was the first instrumentally recorded major earthquake in the Himalayan arc. Also distinctive was the fact that although it was a relatively small event (M_w 7.6), it produced a coseismic surface faulting (breaking of the ground on land or underwater) for a distance of ∼80 km^8,9. Past studies tell us that none of the historical earthquakes are reported to have produced primary surface rupture^6,7. Thus, the previously held consensus that all the major historical earthquakes in the Himalayans were supposed to have occurred on blind faults (those faults which do not rupture the surface) had to be re-evaluated.

Therefore, after the 2005 earthquake, earth scientists used modern techniques to gather evidence left behind by historical ruptures along the Himalayan front. A new chapter started to unfold in the earthquake chronology of the Himalayan front and the long standing consensus was challenged by Sapkota et al.¹⁰ in 2012, providing a strong evidence that the M_w 8.2 Bihar–Nepal earthquake on 15 January 1934 did break the surface.

This group of scientists used the extensive geomorphological mapping of fluvial deposits, palaeo-seismological logging of river-cut cliffs and trench walls. This, together with the modelling of calibrated 14C ages have demonstrated that the M_w 8.2 event has left clear traces of the rupture
along at least 150 km of the Main Frontal Thrust fault in Nepal¹⁰.

Furthermore, they also found an earlier earthquake of 7 June AD 1255, which has also ruptured the surface along this stretch of the mega-thrust¹⁰. This earthquake is believed to have destroyed one third of the population of the Kathmandu valley¹⁰.

This new earthquake research along the Nepal Himalayas is very critical, because it tells us that the reportedly blind faults could actually have ruptured the surface. Thus, the surface ruptures of other reputedly blind great Himalayan events might exist, and to prove this evidence need to be recollected. This research will be extremely useful to understand the historical earthquakes and to re-evaluate the seismic risk along the Himalayan front.

FOOTNOTE
M_w: Moment magnitude scale used to measure the size of earthquakes according to the energy
released.

REFERENCES
[1] De Mets, C. et al. Geophys. Res. Lett. 21, 2191-2194 (1994).

[2] Bettinelli, P. et al. J. Geod. 80, 567-589 (2006).

[3] Nakata, T. Geol. Soc. of Am. 243-264 (1989).

[4] Wang Yu et al. Geophys. J. Int.185, 49-64 (2011).

[5] Lave, J. & Avouac J. P. J. Geophys. Res. 105, 5735-5770 (2000).

[6] Ambraseys, N. & Douglas J. Geophys. J. Int. 159, 165-206 (2004).

[7] Kumar, S. et al. J. Geophys. Res.111, B03304 (2006).

[8] Kaneda, H. et al. Bull. Seis. Soc. Amer. 98, 521-557 (2008).

[9] Avouac et al. Earth Planet. Sci. Lett. 249, 514-528 (2006).

[10] Sapkota S.N. et al. Nat. Geoscience.6, 71-76 (2013).

ABOUT THE AUTHOR:

Dr Afroz Ahmad Shah is a Senior Lecturer (Applied Geology) at the School of Engineering and Science, Curtin University, Sarawak. He obtained his PhD in 2010 (tectono-metamorphic evolution of Precambrian rocks) with Prof. Tim Bell in School of Earth & Environmental Sciences, James Cook University, Australia. He obtained an M.Tech. in engineering geosciences (2006) from IIT Kanpur India. He can be contacted at afroz.shah@curtin.edu.my. Find out more about Dr. Afroz Ahmad Shah by visiting his Scientific Malaysian profile at http://www.scientificmalaysian.com/members/afrozshah/