Seismicity
in the Himalaya dominantly results from the continental collision of
the India and Eurasia plates, which are converging at a relative rate of
40-50 mm/yr. Northward underthrusting of India beneath Eurasia
generates numerous earthquakes and consequently makes this area one of
the most seismically hazardous regions on Earth. The surface expression
of the plate boundary is marked by the foothills of the north-south
trending Sulaiman Range in the west, the Indo-Burmese Arc in the east
and the east-west trending Himalaya Front in the north of India.
The
India-Eurasia plate boundary is a diffuse boundary, which in the region
near the north of India, lies within the limits of the Indus-Tsangpo
(also called the Yarlung-Zangbo) Suture to the north and the Main
Frontal Thrust to the south. The Indus-Tsangpo Suture Zone is located
roughly 200 km north of the Himalaya Front and is defined by an exposed
ophiolite chain along its southern margin. The narrow (<200km)
Himalaya Front includes numerous east-west trending, parallel
structures. This region has the highest rates of seismicity and largest
earthquakes in the Himalaya region, caused mainly by movement on thrust
faults. Examples of significant earthquakes, in this densely populated
region, caused by reverse slip movement include the 1934 M8.1 Bihar, the
1905 M7.5 Kangra and the 2005 M7.6 Kashmir earthquakes. The latter two
resulted in the highest death tolls for Himalaya earthquakes seen to
date, together killing over 100,000 people and leaving millions
homeless. The largest instrumentally recorded Himalaya earthquake
occurred on 15th August 1950 in Assam, eastern India. This M8.6
right-lateral, strike-slip, earthquake was widely felt over a broad area
of central Asia, causing extensive damage to villages in the epicentral
region.
The Tibetan Plateau is situated north of the Himalaya,
stretching approximately 1000km north-south and 2500km east-west, and is
geologically and tectonically complex with several sutures which are
hundreds of kilometer-long and generally trend east-west. The Tibetan
Plateau is cut by a number of large (>1000km) east-west trending,
left-lateral, strike-slip faults, including the long Kunlun, Haiyuan,
and the Altyn Tagh. Right-lateral, strike-slip faults (comparable in
size to the left-lateral faults), in this region include the Karakorum,
Red River, and Sagaing. Secondary north-south trending normal faults
also cut the Tibetan Plateau. Thrust faults are found towards the north
and south of the Tibetan Plateau. Collectively, these faults accommodate
crustal shortening associated with the ongoing collision of the India
and Eurasia plates, with thrust faults accommodating north south
compression, and normal and strike-slip accommodating east-west
extension.
Along the western margin of the Tibetan Plateau, in the
vicinity of south-eastern Afghanistan and western Pakistan, the India
plate translates obliquely relative to the Eurasia plate, resulting in a
complex fold-and-thrust belt known as the Sulaiman Range. Faulting in
this region includes strike-slip, reverse-slip and oblique-slip motion
and often results in shallow, destructive earthquakes. The active,
left-lateral, strike-slip Chaman fault is the fastest moving fault in
the region. In 1505, a segment of the Chaman fault near Kabul,
Afghanistan, ruptured causing widespread destruction. In the same region
the more recent 30 May 1935, M7.6 Quetta earthquake, which occurred in
the Sulaiman Range in Pakistan, killed between 30,000 and 60,000 people.
On
the north-western side of the Tibetan Plateau, beneath the Pamir-Hindu
Kush Mountains of northern Afghanistan, earthquakes occur at depths as
great as 200 km as a result of remnant lithospheric subduction. The
curved arc of deep earthquakes found in the Hindu Kush Pamir region
indicates the presence of a lithospheric body at depth, thought to be
remnants of a subducting slab. Cross-sections through the Hindu Kush
region suggest a near vertical northerly-dipping subducting slab,
whereas cross-sections through the nearby Pamir region to the east
indicate a much shallower dipping, southerly subducting slab. Some
models suggest the presence of two subduction zones; with the Indian
plate being subducted beneath the Hindu Kush region and the Eurasian
plate being subducted beneath the Pamir region. However, other models
suggest that just one of the two plates is being subducted and that the
slab has become contorted and overturned in places.
Shallow
crustal earthquakes also occur in this region near the Main Pamir Thrust
and other active Quaternary faults. The Main Pamir Thrust, north of the
Pamir Mountains, is an active shortening structure. The northern
portion of the Main Pamir Thrust produces many shallow earthquakes,
whereas its western and eastern borders display a combination of thrust
and strike-slip mechanisms. On the 18 February 1911, the M7.4 Sarez
earthquake ruptured in the Central Pamir Mountains, killing numerous
people and triggering a landside, which blocked the Murghab River.
Further
north, the Tian Shan is a seismically active intra-continental mountain
belt, which extends 2500 km in an ENE-WNW orientation north of the
Tarim Basin. This belt is defined by numerous east-west trending thrust
faults, creating a compressional basin and range landscape. It is
generally thought that regional stresses associated with the collision
of the India and Eurasia plates are responsible for faulting in the
region. The region has had three major earthquakes (>M7.6) at the
start of the 20th Century, including the 1902 Atushi earthquake, which
killed an estimated 5,000 people. The range is cut through in the west
by the 700-km-long, northwest-southeast striking, Talas-Ferghana active
right-lateral, strike-slip fault system. Though the system has produced
no major earthquakes in the last 250 years, paleo-seismic studies
indicate that it has the potential to produce M7.0+ earthquakes and it
is thought to represent a significant hazard.
The northern portion
of the Tibetan Plateau itself is largely dominated by the motion on
three large left-lateral, strike-slip fault systems; the Altyn Tagh,
Kunlun and Haiyuan. The Altyn Tagh fault is the longest of these strike
slip faults and it is thought to accommodate a significant portion of
plate convergence. However, this system has not experienced significant
historical earthquakes, though paleoseismic studies show evidence of
prehistoric M7.0-8.0 events. Thrust faults link with the Altyn Tagh at
its eastern and western termini. The Kunlun Fault, south of the Altyn
Tagh, is seismically active, producing large earthquakes such as the 8th
November 1997, M7.6 Manyi earthquake and the 14th November 2001, M7.8
Kokoxili earthquake. The Haiyuan Fault, in the far north-east, generated
the 16 December 1920, M7.8 earthquake that killed approximately 200,000
people and the 22 May 1927 M7.6 earthquake that killed 40,912.
The
Longmen Shan thrust belt, along the eastern margin of the Tibetan
Plateau, is an important structural feature and forms a transitional
zone between the complexly deformed Songpan-Garze Fold Belt and the
relatively undeformed Sichuan Basin. On 12 May 2008, the thrust belt
produced the reverse slip, M7.9 Wenchuan earthquake, killing over 87,000
people and causing billions of US dollars in damages and landslides
which dammed several rivers and lakes.
Southeast of the Tibetan
Plateau are the right-lateral, strike-slip Red River and the
left-lateral, strike-slip Xiangshuihe-Xiaojiang fault systems. The Red
River Fault experienced large scale, left-lateral ductile shear during
the Tertiary period before changing to its present day right-lateral
slip rate of approximately 5 mm/yr. This fault has produced several
earthquakes >M6.0 including the 4 January 1970, M7.5 earthquake in
Tonghai which killed over 10,000 people. Since the start of the 20th
century, the Xiangshuihe-Xiaojiang Fault system has generated several
M7.0+ earthquakes including the M7.5 Luhuo earthquake which ruptured on
the 22 April 1973. Some studies suggest that due to the high slip rate
on this fault, future large earthquakes are highly possible along the
65km stretch between Daofu and Qianning and the 135km stretch that runs
through Kangding.
Shallow earthquakes within the Indo-Burmese Arc,
predominantly occur on a combination of strike-slip and reverse faults,
including the Sagaing, Kabaw and Dauki faults. Between 1930 and 1956,
six M7.0+ earthquakes occurred near the right-lateral Sagaing Fault,
resulting in severe damage in Myanmar including the generation of
landslides, liquefaction and the loss of 610 lives. Deep earthquakes
(200km) have also been known to occur in this region, these are thought
to be due to the subduction of the eastwards dipping, India plate,
though whether subduction is currently active is debated. Within the
pre-instrumental period, the large Shillong earthquake occurred on the
12 June 1897, causing widespread destruction.
