Earthquakes at Collisional Plate Boundaries: an Overview of Himalayan

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure

Earthquakes at collisional plate boundaries: an overview of Himalayan

tectonics and seismicity

Introduction

Seismicity on Earth is concentrated along plate boundaries, tectonically active regions subjected to a range of stress-regimes leading to violent slip events. This paper will focus on seismicity along converging plate-boundaries, and more specifically on seismicity along the continental collision that led to the formation of the 1400 km long Himalayan mountain belt in the Cenozoic. This paper will provide an overview of the main geological features of the Himalayas and their seismic implications; it offers a comprehensive summary of seismicity induced by the continental collision, and is thus meant to be used as a spring for more in depth study of the collision zone. The geological setting associated with the Indo-

Asian collision will be discussed at first, from the early separation of India from Gondwana, to the post- orogeny state of the mountain chain, after which the discussion will shift to the main mechanisms that cause seismicity in the mountain range. This will include a discussion of the fault configuration and separation of the mountain chain into different lithological units by researchers. I will then consider the case of the 2015 Ghorka earthquake to illustrate seismic distribution along the two converging plates

(seismic gaps) and discuss the ways this earthquake can be used –or not… – in seismic prediction.

Finally, seismicity that takes place in India outside of the main Himalayan belt, and the possible mechanisms involved in their triggering, will be considered.

Geological context of the Himalayan mountain chain

Continental collision zones are inherently complex regions, both from a lithological and from a tectonic point of view. They bear the geological history of the old craton prior to the collision, as well as

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure the evidence of more recent tectonic changes related to the collision event –ongoing tectonic changes in the case of the Himalayas. Another active continental collision today is that occurring between the

Arabian plate and Iran, which has given rise to the Zagros mountain chain. The formation of the Zagros is closely related to that of the Himalayas, as both were induced by the closure of the Tethys Ocean which started in the end of the Mesozoic. In fact, these two orogenic locations bear resemblances both in the topography resulting from the collision –rise of the Tibetan Plateau on one side and that of the Iranian

Plateau on the other – and in the seismic patterns recorded along the mountain chains (Hatzfeld et al.

2010). It thus appears that common features can be drawn from different continental collision settings, and that seismicity observed in the Himalayas is not unique to this site. The scale of the collision in the

Himalayas however exceeds by far that in any other similar setting, which motivates this paper.

The complexity of the Himalayan orogeny is recorded in the lithology of the long mountain belt that separates Northern India from the Tibetan Plateau. The changes brought to the old Indian craton during collision include the initial formation of an accretionary prism, the uplift and denudation of the

Asian crust, metamorphism and anatexis of the rocks caught in the suture zone. The Himalayas are made up of an amalgamation of continental arc rocks, S-type granitic plutons, metamorphosed uplifted oceanic sediments (Harrison et al 2000), that all record a specific step that led to the eventual formation of the main mountain chain which can be seen today.

Figure 1

Figure 1 from Shanker at al. 2010: the different lithological units that make up the main mountain chain. The purple formations correspond to Neogene sediments, the brown to highly metamorphosed plutonic bodies and the green ones to uplifted Tethyan sediments.

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure

The Himalayan orogeny started with the Indian plate detaching from supercontinent Gondwana

70 Ma ago, and moving North at an initial rate of 15-25cm/a, which is a record in plate tectonic motion.

Based on a subsequent decrease in the rate of advance of the Indian plate recorded in magnetic anomalies of the Indian Ocean, the Indo-Asian collision has been fixed at 50 Ma. This decrease in the relative rate of convergence of the two plates is believed to correspond to increased kinematic resistance between the plates, as the buoyant Indian margin collided with Asia after the subduction of the oceanic portion of the

Indian crust under Asian continental crust (Harrison et al. 2000). Stratigraphic evidence has also been used to constrain a lower limit (youngest age) for the Indo-Asian collision, as around 52 Ma an abrupt change from marine to terrestrial depositional environment is recorded in northeastern India (Harrison et al. 2000).

The Himalayan orogeny is not restricted to the main mountain chain that spans most of Nepal,

Bhutan and Northern India, but rather extends more than 600 km north from the Higher Himalayas

(where the Everest is located) in the form of the Tibetan Plateau. Indeed, the Indo-Asian collision not only led to the formation of the mountain belt and its associated faults, but further led to the uplift of the

Tibetan Plateau, which is composed of terranes, such as the Qiangtang and Lhasa ones, that were accreted to Asia before the onset of the collision (Harrison et al. 2000). The mechanism behind the uplift of the

Tibetan Plateau, although debated, is believed to be the subduction of a slab of Greater Indian lower crust under the Asian continent (DeCelles et al. 2002). Although continental buoyancy usually stalls subduction shortly after collision, plate velocities were merely reduced in the case of the Himalayas when the Indian margin impinged on Asia; the subduction of the Greater Indian continent continues to this day (Capitano et al. 2010). It would be interesting to compare this situation to other continental collision zones along the

Alpine-Himalayan chain to determine the nature of the driving forces behind the convergence. The unusual rate of advance of the plate has been postulated as a possible cause of the continuing subduction, as well as the higher density of the lower Indian crust, whose lighter upper part got scraped off as the

Himalayan front, leaving a readily subductable denser lower crust behind (Capitano et al. 2010).

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure

The Tibetan plateau, just like the Himalayas, is still active at present, and the East-West extension that the Plateau is undergoing results in the formation of normal faults that have led to great earthquakes in the past (DeCelles et al. 2002). This paper will focus on seismicity at the forefront of the Himalayan chain, at the contact of the two plates, and on the associated faulting. The figure below illustrates the distribution of stress along the Himalayas, and clearly shows how thrusting transitions into normal faulting as one draws deeper into Tibet. The stress-regime shifts from compressional to extensional, which is explained by the effect of the subducted lower-crust slab, which forces Tibet to extend along an

East-West axis (DeCelles et al. 2002).

Figure 2 Figure 2 from Shanker et al. 2011: this figure displays the change in tectonic regime that occurs on a path from the Lower Himalayas all the way to the Tibetan Plateau, currently undergoing extension

An experiment by Toussaint et al. 2004 investigates the tectonic evolution of the Himalayas and its associated crustal deformation. The authors show how the age relationships of different formations within the mountain belt allow to make estimations as to the sequence of tectonic events and faulting styles that shaped the Himalayan collisional boundary. For example, strongly metamorphosed Tethys- aged sediments that used to separate the northern margin of India from the active margin of southern

Eurasia can now be found north of the high Himalayas, which indicates an early uplift of the sediment- wedge caught between the converging plates. According to the model developed by Toussaint et al., the

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure main fault that governs the Himalayan system formed at a relatively early stage, following deformation at the suture zone. It was subsequently followed by exhumation of the lower crust of the Asian plate combined with the subduction of the Indian crust. The last stage of collision, which follows activity at the major thrust fault, consists in frontal accretion of a large wedge, a process that continues to this day.

At present, it has been determined that not only India is penetrating into Asia, but it is also slowly rotating anti-clockwise (Sella et al. 2002). The rotation has notably been recorded in the province of Baluchistan, in Pakistan, where left-lateral strike-slip faults at a rate of 42 mm/a have been measured (Sella et al.

2002).

Tectonic configuration

Competing hypotheses suggest that the Himalayan topography is sustained and plate convergence is accommodated either predominantly on the main plate boundary fault or more broadly across multiple smaller thrust faults (Elliott et al. 2016). The main plate boundary fault, also called the Main Himalayan

Thrust (or MHT), is the main basal decollement into which all minor faults sole. There is much debate however about the structure and geometry of this decollement surface. Within the fault-system, the largest faults that converge toward the Main Himalayan decollement are namely, from South to North, the Main

Boundary Thrust, the Main Central Thrust and the South Tibetan detachment. The North-dipping Main

Boundary Thrust marks the contact between the Lesser Himalayan Formations and the underlying

Miocene-Pleistocene Siwalik Formations, which are the youngest and least metamorphosed beds in the mountain chain, and which are still actively underthrust (An Yin et al. 2000). The Main Central Fault, which follows on a northward path from the collision axis, marks the transition between crystalline metamorphosed gneisses of the Greater Himalayan unit and slightly younger schists. The South Tibetan

Decollement, which contrary to the two previous faults is a low-angle normal fault, separates

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure leucogranites that formed through anatexis from Tethyan sediments that were uplifted to form the Tibetan

Plateau (see figure below for a clearer understanding of overall Himalayan tectonics).

Figure 3

Figure 3 from Elliott and al. 2016: showing the geometry of the minor thrust faults that converge into the Main Himalayan Thrust (MHT), or Main Basal Decollement

Earthquakes have also been used to investigate the role of seismic deformation in building the

Himalayas (Elliott et al. 2016). After the 2015 Gorkha event that struck Nepal, for example, geodetic measurements of surface displacement showed that the Kathmandu Plateau got uplifted by about 1 m, while the areas further North, in the High Himalayas, subsided by about 0.6 m. Earthquakes in the

Himalayas are thus not only the consequence of collision occurring at the Indo-Asian boundary, but also a major force affecting the topography of the mountain chain itself and its associated plateaus. Rather than seeing the earthquakes as a mere response to differential stresses applied to the collision area, the earthquakes should be considered in light of their own shaping potential, and the way they currently influence Himalayan topography.

Earthquakes in the Himalayas are caused by the underthrusting of the India plate, which means that during each seismic event along the main thrust fault, the southern tip of the Asian plate undergoes uplift, while the areas north to this region of maximum southward slip are subjected to elastic extension, and thus subside. Thus, according to the paper by Elliott et al. 2016, all major thrusting seismic events in

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure the region will tend to lower the high Himalayan topography, which might sound paradoxical at first, as thrusting, in the earlier stages of orogeny, was associated –by definition – to mountain building. This lowering of the topography associated with discrete large seismic events must however be contrasted to the long-term uplift of the high Himalaya, which is estimated to be 4 mm/a. This long-term uplift occurs between the large earthquakes, and is believed to occur due to ramp overthrusting. Intermittent coseismic subsidence, such as the one observed during the Gorkha event last year, modulates the total uplift of the

High Himalayas (Elliott et al. 2016).

Earthquakes, historical overview

Four great earthquakes (magnitude>8) as well as many large earthquakes have taken place in the

Himalayas in the past 200 years. The frequency of these earthquakes allows us to make the assumption that seismicity in the past was as strong as it has been in the recent years. However, due to poor records, reconstructing and quantifying these past events has been challenging, as for example the interval of occurrence between great earthquakes remains uncertain (Bilham et al. 2004). One way to investigate past seismic activity consists in excavating zones of slip to look for signs of past earthquakes. These methods have yielded positive results, as evidence of paleoliquefaction and stratigraphic offset have been detected in some areas (Rajendran et al. 2011). Furthermore, isotopic dating techniques can be used to date those single events more precisely (interview) and thus obtain a more complete picture of the distribution of seismicity in the Himalayas. Seismic modelling is thus based on observation and monitoring of recent earthquakes as well as on the study of old fault zones and their associated times. A third tool that has improved our understanding of earthquakes in the Himalayas is GPS-based Geodesy, which measures the deformation of the surface at a millimeter scale resolution (Pandey et al. 2015). This latter tool has notably been used to measure the exact rate of convergence of the Indian plate with the Eurasian one; the current rate is of 50mm/a, where a third of the convergence (about 18 mm/a) is accommodated within the

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure mountain belt. The convergence along the Himalayan arc thus results in potential slip available to drive large thrust earthquakes at approximately 1.8mm/a, which means that a slip of for example 6 m as measured during the 1934 Nepal earthquake can only occur at intervals of at least three centuries (Bilham et al. 2004). The Main Himalayan Thrust at shallow depth if frictionally locked and accumulates elastic strain energy in response to the convergence of the Indian plate. Build-up of such energy happens over long periods of time, and can result in enormous stores of energy, which are subsequently released during large earthquakes, though only partially according to some authors. Evidence for these locked zone comes from the fact that small to micro-earthquakes cluster along the downdip end of the suture zone (Pandey et al. 2015).

The 2015 Nepal earthquake and seismic gaps

The Himalayas, just like any other plate suture zone, hosts areas where no seismic activity has been recorded in recent times. One such seismic gap lies between the 1934 Bihar-Nepal rupture zone and the 1905 Kangra earthquake. There is ongoing debate as to whether or not one of the seismic gaps, the

Central Himalayan Seismic Gap, got unlocked last year. According to a recent paper, the lower edge of the locked Main Himalayan Thrust (MHT), that is, the main fault along which India underthrusts the

Himalayas, got unzipped by the Gorkha earthquake that hit Nepal in 2015 (Avouac et al. 2015). This moment magnitude 7.8 earthquake took place in a zone of clustered seismicity that runs beneath the front of the high Himalaya, and occurred close to the epicenter of the 1833 earthquake that hit the same region

182 years ago. The 2015 Gorkha earthquake took place 80 km west-northwest of Kathmandu, after which it propagated eastwards, under the Nepalese capital, and towards the south-eastern border of the country.

It is believed that the eastward propagation of the rupture along the MHT got block some distance away from Kathmandu when it met either a zone of structural complexity –Grandin, R. et al. note the presence of ramps within the MHT, which slow down tectonic motion– or a zone of lower-stress, which could have been produced by an earlier earthquake hitting the same fault. Indeed, the 1934 moment magnitude 8.2

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure

Bihar-Nepal earthquake occurred on the same main fault, but this time east of Kathmandu, and produced over 6 meters of slip (Avouac et al.).

The spatial relationship between the 2015 earthquakes and the 1934 and 1833 ones which occurred on the same segment of the fault is interesting. It seems like the 1883 earthquake contributed to the process of upward transfer of the stress that builds up along the downdip edge of the locked fault zone during convergence of the India with Asia (Avouac et al. 2015). Some of that stress got released last year in the 2015 earthquake, while the eastward propagation of that same rupture got blocked by the slip that had already occurred there in 1934. Based on the recent earthquakes, it has been predicted that the area east of Kathmandu is now unlikely to rupture again in the near future, as that segment of the Himalayas just got unlocked (Avouac et al. 2015). This, however, does not apply to the portion of the Main

Himalayan Fault that lies west of Kathmandu, as no major earthquake has occurred there in the recent years. Elliott and al. 2016 highlight that the Gorkha event ruptured only a small portion of the MHT at the eastern edge of the 800 km wide seismic gap that separates the 1905 M(w) 7.8 Kangra earthquake to the west, and the 1934 M(w) 8.2 Bihar-Nepal earthquake to the east of the gap.

Figure 4

Figure 4 from Avouac et al. 2015: the yellow star corresponds to the epicenter of the 2015 Gorkha event, while the yellow patch shows where coseismic slip was >1m. The dashed ellipses show the location of M(w) greater than 7.5 earthquakes in the same region.

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure

The exchange of stress that occurs during seismic events along the Main Himalayan Thrust has important implications in the context of Himalayan tectonics. As discussed above, the fault system that controls the Indo-Asian boundary is not straightforward and is not constrained along its full length, which means earthquakes can trigger unpredictable aftershocks, even months after the main slip (Xiong et al.

2015). A recent paper by Xiong et al. 2015 discusses the Coulomb stress transfer that followed the

Gorkha event, which is crucial in seismic hazard assessment of the region. Coulomb stress theory consists in the alteration of shear and normal stresses on surrounding faults during a seismic event. According to the paper in question, the magnitude 7.9 Gorkha event enhanced the static stress on most of the epicentral region, which means that immediately after the earthquake, a positive stress zone that lead to a series of aftershocks in the days that followed the main April 25th event was created. Besides, post-seismic relaxation would have enhanced the stress on some normal faults in southern Tibet, such as on the Palung

Co fault, leading to increased risk of seismicity in the future (Xiong et al. 2015). The transfer of stress is thus a dangerous process that can extend far from the source region. Following the Gorkha event, tremors were felt as far as Kerala, the southernmost state of India and the potential for stress transfer along the path from Nepal to Kochi, Kerala’s biggest city, is tremendous... Coulomb stress transfer thus shows how simplistic it is to assume that major earthquakes systematically close seismic gaps and annihilate the risk of futures seismic hazards.

Indo-Asian collision induced earthquakes outside of the Himalayan belt

The Indian plate is more than a billion years old, which implies faults on the old craton existed well before the beginning of the Indo-Asian collision between 70 Ma and 50 Ma. Due to the compressional stresses that are transmitted southward of the ongoing collision, some of these old faults are reactivated, which is an important source of earthquakes outside of the main Himalayan belt (Pandey,

2015). Such intraplate earthquakes, which take place in areas that are not directly in contact with the main

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure tectonic structures of Northern India, include the 2001 Gujarat earthquake in Western India (also called the Rann of Kutch or Bhuj event) as well as the 1993 Latur earthquake in Central India. It is difficult to ascertain whether or not old faults on the Indian subcontinent are currently accumulating elastic strain and drawing closer to failure. Prediction of earthquakes outside of the Himalayas is thus a complicated task, and only regional approximations can be made based on the occurrence of past earthquakes. For example areas in Western Gujarat, where the 2001 Bhuj earthquake occurred, are considered as seismic hazard zones along with the low-lying Rann of Kutch salt marsh on the Arabian Sea. These regions fall in zone 5

–very high damage zones – of the intensity scale developed by the Bureau of Indian Standards. The areas north of India, where active mountain building and tectonic motion occurs, fall in the same category, which shows just how likely earthquakes are, and how devastating they can be, outside of the main

Himalayan belt. Zones of low risk, rare in India, include the area right south of the Indo-Ganges Valley, as well as the eastern portion of Rajasthan, in North-Central India.

According to Biswas et al. 2005, the Bhuj intraplate event was triggered by reactivation of faults that date back to rifting of Gondwana in the late Triassic. The rifting was aborted in the end of the

Cretaceous in the pre-collision stage, after which the Kutch rift-basin became a shear zone with strike-slip movements along sub-parallel rift faults (Biswas et al. 2005). Besides being subjected to intense rifting, additional tectonic events have added to the structural complexity of the Indian continent. Along its northward journey, the Indian plate encountered the Reunion hotspot, which is postulated to be the cause of extensive Deccan volcanism, an event associated with the K-Pg extinction (Wignall et al. 2001). The scale of this large igneous province was tremendous, which would have had implications for the crustal evolution of Central-India. In fact, the Deccan traps lie close to the location of the epicenter of the 1993

Lahur earthquake, which was caused by the reactivation of an old fault segment, where repeated movement separated by long intervals has been recorded (Rajendran et al. 1996). In this case however the exact nature of the ancient fault (what initiated it in the first place) has not been determined precisely, as in the case of the Bhuj event.

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure

These two examples show that here again, the geology of the old Indian plate pre- and post- collision proves crucial in understanding seismic distribution in present Indian and surrounding countries

(East-Pakistan also suffered massive casualties following the Bhuj event). Study of the geologic past of the plates allows to map zones of weakness within the plate, which, as we saw during the 2001 Kutch event, can be particularly vulnerable to any further loading from current convergence rates with Asia.

Another paper that studied the reactivation of the ancient faults involved the flexure of the Indian plate, which is a direct consequence of the collision of the Indian plate with Asia (Bilham, 2001). The wavelength of the flexure is approximately 650 km, and results in a 450 m high bulge in the Central

Indian Plateau and correspondingly, a 40 m deep through in south-western India (see figure XX). Tensile stresses cluster north of the flexural bulge, leading to normal faulting, while compressional stresses are stronger beneath the crest, which causes reverse and thrust faults. However, the location of theses stresses varies, as the Indian plate slowly streams through the flexural wave, which brings points within the plate toward or away from the tensile and compressional stresses (Bilham, 2001). For this reason, earthquakes in Central India seem to have no spatial pattern (Bilham, 2001), which further complicates earthquake prediction.

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure

Figure 5 Figure 6

Figure 5 from Bilham et al. 2004 and figure 6 from Kher et al. 2011, model based on Bilham’s paper: the flexed Indian plate is visible on the figure at the right. The resulting bulge and trough lead to differential stress distribution, which results in increased normal and thrust faulting along the flexed plane. The left figure shows the epicenter of historical earthquake in relation to the flexed continent.

Conclusion

A continental collision of the scale of the Himalayas clearly has enormous implications for seismicity in and around the actual suture zone of the two converging plates. The study of the geological past of the

Indian continent proves crucial in understanding the seismic events that strike the region today, such as the April 2015 M(w) 7.8 Gorkha event that lead to the death of 8000 people and the injury of several tens of thousands more. Understanding the stages of the Indo-Asian collision, which can be mapped with the aid of changing lithology along the collision zone, is helpful in determining how and where faults are concentrated today, and where they have the potential to accumulate enough strain to trigger a large magnitude event. The Main Himalayan Decollement surface, into which steeper dipping thrust faults such as the Central Himalayan Thrust Fault sole, is believed to be the region where most strain is accommodated today. However, the lack of written records and the difficulty in providing precise dates for historical earthquakes proves challenging for determining the frequency and recurrence interval of large scale events. The tendency of stress to be distributed on surrounding faults during earthquakes instead of being released in a single straightforward event further complicates earthquakes predictions in the Himalayas.

Where current geodetic measurements and extensive study of past and recent earthquakes may yield acceptable –though limited – results for the prediction of future earthquakes within the Himalayan belt, the areas that lie outside of the hottest seismic belt are subject to great uncertainties. Albeit more

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure powerful, earthquakes within the Himalayas tend to be less destructive that those occurring in low-lying plains and valleys, such as the Kutch region in Gujarat, due to comparably lower population densities on the flanks of the High Himalayas. Mapping areas of structural weakness on the Indian continent based on the existence of ancient faults that predate the Indo-Asian collision is one way of determining potential seismic hazards in the future. The work of geologists such as Bilham et al. tells us where to look for these zones on which stress might be accumulating. However, it is clear that monitoring seismicity on a continent of the scale of India is far from being straightforward.

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure

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Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure

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