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Olivia Dagnaud
260633356
EPSC 330: Earthquakes and Earth structure
April 5th, 2016
Earthquakes at converging plate boundaries:
The case of continental collision in the Himalayas
Continental collision, key points:
o
The low density of the continental plates means neither of the colliding plates will subduct
completely
o
Horizontal compression is accommodated by thrusting, which leads to the formation of a foldthrust belt. It is estimated that the Himalayas have accommodated more than 670 km of
horizontal shortening
o
Earthquakes are restricted to the crust (shallow earthquakes)
Geological overview of the Indo-Asian collision
o
India detaches from supercontinent Gondwana and drifts northwards at a rate of 10 to 15 cm/a,
which closes the Tethys ocean (Cretaceous, 100 Ma)
o
The oceanic portion of the lithosphere that separates Eurasia from India subducts under Asia,
which leads to the gradual uplift of the Tibetan Plateau
o
The collision between the Indian and Asian continental plates occurred between 70 to 50 Ma
o
The current rate of convergence of the plates is estimated to be 47 mm/a
Lithology and tectonic configuration
o
The suite of tectonic events gives rise to a complex lithology and strata symmetry, which has
implications in seismic interpretation of the Himalayas. Researchers have had to rely on
evidence of paleoliquefaction to locate and date past seismic events. The mountain chain
consists of remains of an ancient volcanic arc which formed during subduction of the Indian
oceanic plate, of uplifted and subsequently metamorphosed marine sediments, of igneous rocks
Olivia Dagnaud
260633356
EPSC 330: Earthquakes and Earth structure
April 5th, 2016
from volcanism, batholiths from partial melting of the continental crust during accretion of new
material. The picture is further complicated by subsequent erosion of the mountain chain.
o
Toussaint et al. recognise three stages to the collision: 1) deformation around the suture zone,
2) activity of the major thrust fault and 3) frontal accretion of a large sediment wedge. The
tectonic history of the Himalayas can be read from the sequence of formations that follow each
other perpendicular to the collisional front. For example, highly metamorphosed Tethyan
sediments from the former oceanic crust are now found on the Tibetan Plateau while younger
sediments tend to accrete at the forefront of the main thrust.
o
Three major faults have been recognised, namely the Main Boundary Thrust, the Main Central
Thrust and the South Tibetan Detachment (going from South to North). The three faults sole
into the Main Himalayan Thrust (MHT), a detachment surface along which major historical
earthquakes tend to occur, such as the 1934 M(w) 8.0 Nepal-Bihar earthquake or the 1950 M(w)
8.6 Assam-Tibet earthquake (largest seismic event not caused by subduction of oceanic plate)
o
Earthquakes are actively shaping the mountain belt and altering its topography. Uplift is thought
to occur due to ramp overthrusting between major earthquakes, while subsidence follows
seismic events as the areas north of the edge of the Asian plate undergo elastic strain
The case of the Nepal earthquake
Did the 2015 Gorkha event close the 800 km -long seismic gap that runs from the focus of the 1905
M(w) 7.8 Kangra earthquake (to the west of the Himalayan chain) to that of the 1934 M(w) 8.2 NepalBihar earthquake (to the east of the chain)? The answer is debated by authors. According to Avouac et
al., the Himalayan Main Thrust, along which main seismic events cluster, got unzipped by the Gorkha
event. The rupture propagated east of the Gorkha focus, under Kathmandu, and stopped when it met a
Olivia Dagnaud
260633356
EPSC 330: Earthquakes and Earth structure
April 5th, 2016
zone of structural complexity. For Xiong et al., the event led to stress transfer onto surrounding faults
(Coulomb stress transfer). Normal stress on normal faults in the Tibetan Plateau increased following the
Gorkha event.
Faults and earthquakes outside of the main Himalayan belt
The Indian plate is several billion years old, which means faults existed on the continent even before the
onset of the Indo-Asian collision. As India pushes towards Tibet, compressional stresses are transmitted
southwards, which reactivates some of these ancient faults. This is one of the main reasons for seismic
events outside of the main mountain belt (see for example the M(w) 7.7 2001 Bhuj event in Gujarat that
was induced by reactivation of rift faults that date back to the breakup of Gondwana). It is currently very
difficult to predict whether or not a fault is accumulating elastic strain, and thus if it is prone to slip in
the near future.
Reactivation of the ancient faults is further enhanced by the flexure of the Indian plate as it compresses
under Asia. The flexure results in a bulge in the Central Indian Plateau, and, correspondingly, to a trough
in South-Western India. Tensile stresses cluster north of the bulge, which leads to normal faulting, while
the regions beneath the bulge are subjected to compression-induced thrust and reverse faults.
Conclusion
o
The geological history of the Himalayas and of the Indian plate determine where seismicity is
strongest today
o
Single events do not appear to release all the accumulated strain, in result of which seismic gaps
are poorly constrained
o
Stress-transfer and slow accumulation of elastic strain along the entire length of the Indian plate
complicate seismic prediction
o
Standards in earthquake engineering not met in most high-risk zones…
Olivia Dagnaud
260633356
EPSC 330: Earthquakes and Earth structure
April 5th, 2016
References:
Avouac, J.P. Meng, L. Wei, S. Wang, T. Ampuero, J.P. Lower edge of locked Main Himalayan Thrust
unzipped by the 2015 Gorkha earthquake. Nature Geoscience. 2015.
Toussaint, G. Burov, E. Avouac, J.P. Tectonic evolution of a continental collision zone: A
thermomechanical numerical model. Tectonics. 23:6. 2004.
Xiong, Wei et al. Coseismic and postseismic Coulomb stress changes on surrounding major faults caused
by the 2015 Nepal M(w) 7.9 earthquake. Chinese Journal of Geophysics. 58:11. Pages 4305-4316. 2015.