Myanmar-India border region (2023)

Seismotectonics of the Himalaya and Vicinity

Seismicity in the Himalaya dominantly results from the continental collision ofthe India and Eurasia plates, which are converging at a relative rate of 40-50mm/yr. Northward underthrusting of India beneath Eurasia generates numerousearthquakes and consequently makes this area one of the most seismicallyhazardous regions on Earth. The surface expression of the plate boundaryis 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 inthe north of India.

The India-Eurasia plate boundary is a diffuse boundary, which in the region nearthe 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-TsangpoSuture Zone is located roughly 200 km north of the Himalaya Front and is definedby an exposed ophiolite chain along its southern margin. The narrow (<200km)Himalaya Front includes numerous east-west trending, parallel structures. Thisregion has the highest rates of seismicity and largest earthquakes in the Himalayaregion, caused mainly by movement on thrust faults. Examples of significantearthquakes, in this densely populated region, caused by reverse slip movementinclude the 1934 M8.1 Bihar, the 1905 M7.5 Kangra and the 2005 M7.6 Kashmirearthquakes. The latter two resulted in the highest death tolls for Himalayaearthquakes seen to date, together killing over 100,000 people and leaving millionshomeless. The largest instrumentally recorded Himalaya earthquake occurred on 15thAugust 1950 in Assam, eastern India. This M8.6 right-lateral, strike-slip, earthquakewas widely felt over a broad area of central Asia, causing extensive damage tovillages in the epicentral region.

The Tibetan Plateau is situated north of the Himalaya, stretching approximately1000km north-south and 2500km east-west, and is geologically and tectonicallycomplex with several sutures which are hundreds of kilometer-long and generallytrend 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-southtrending normal faults also cut the Tibetan Plateau. Thrust faults are found towardsthe north and south of the Tibetan Plateau. Collectively, these faults accommodatecrustal shortening associated with the ongoing collision of the India and Eurasia plates,with thrust faults accommodating north south compression, and normal and strike-slipaccommodating east-west extension.

Along the western margin of the Tibetan Plateau, in the vicinity of south-easternAfghanistan and western Pakistan, the India plate translates obliquely relative tothe Eurasia plate, resulting in a complex fold-and-thrust belt known as the SulaimanRange. Faulting in this region includes strike-slip, reverse-slip and oblique-slip motionand often results in shallow, destructive earthquakes. The active, left-lateral, strike-slipChaman fault is the fastest moving fault in the region. In 1505, a segment of theChaman fault near Kabul, Afghanistan, ruptured causing widespread destruction. Inthe same region the more recent 30 May 1935, M7.6 Quetta earthquake, whichoccurred in the Sulaiman Range in Pakistan, killed between 30,000 and 60,000people.

On the north-western side of the Tibetan Plateau, beneath the Pamir-Hindu KushMountains of northern Afghanistan, earthquakes occur at depths as great as 200 kmas a result of remnant lithospheric subduction. The curved arc of deep earthquakesfound in the Hindu Kush Pamir region indicates the presence of a lithospheric bodyat depth, thought to be remnants of a subducting slab. Cross-sections through theHindu Kush region suggest a near vertical northerly-dipping subducting slab, whereascross-sections through the nearby Pamir region to the east indicate a much shallowerdipping, southerly subducting slab. Some models suggest the presence of twosubduction zones; with the Indian plate being subducted beneath the Hindu Kushregion 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 theslab has become contorted and overturned in places.

Shallow crustal earthquakes also occur in this region near the Main Pamir Thrust andother active Quaternary faults. The Main Pamir Thrust, north of the Pamir Mountains,is an active shortening structure. The northern portion of the Main Pamir Thrustproduces many shallow earthquakes, whereas its western and eastern borders displaya combination of thrust and strike-slip mechanisms. On the 18 February 1911, the M7.4Sarez earthquake ruptured in the Central Pamir Mountains, killing numerous people andtriggering a landside, which blocked the Murghab River.

Further north, the Tian Shan is a seismically active intra-continental mountain belt, whichextends 2500 km in an ENE-WNW orientation north of the Tarim Basin. This belt is definedby numerous east-west trending thrust faults, creating a compressional basin and rangelandscape. It is generally thought that regional stresses associated with the collision ofthe India and Eurasia plates are responsible for faulting in the region. The region has hadthree major earthquakes (>M7.6) at the start of the 20th Century, including the 1902Atushi earthquake, which killed an estimated 5,000 people. The range is cut through in thewest 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 last250 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 onthree 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 toaccommodate a significant portion of plate convergence. However, this system has notexperienced significant historical earthquakes, though paleoseismic studies showevidence of prehistoric M7.0-8.0 events. Thrust faults link with the Altyn Tagh at itseastern and western termini. The Kunlun Fault, south of the Altyn Tagh, is seismicallyactive, producing large earthquakes such as the 8th November 1997, M7.6 Manyiearthquake 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 killedapproximately 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 animportant structural feature and forms a transitional zone between the complexlydeformed Songpan-Garze Fold Belt and the relatively undeformed Sichuan Basin. On12 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 landslideswhich dammed several rivers and lakes.

Southeast of the Tibetan Plateau are the right-lateral, strike-slip Red River and theleft-lateral, strike-slip Xiangshuihe-Xiaojiang fault systems. The Red River Faultexperienced large scale, left-lateral ductile shear during the Tertiary period beforechanging to its present day right-lateral slip rate of approximately 5 mm/yr. This faulthas produced several earthquakes >M6.0 including the 4 January 1970, M7.5earthquake in Tonghai which killed over 10,000 people. Since the start of the 20thcentury, 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 earthquakesare highly possible along the 65km stretch between Daofu and Qianning and the 135kmstretch that runs through Kangding.