加载中…21429--美国地震勘探局-USGS 新疆6.9级地震信息（1）
(2014-02-13 00:14:01)
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勘探局级地震信息美国地震新疆 |
USGS（United States Geological Survey），美国地质勘探局（美国）
美国地质勘探局是一个美国政府内政部的下属机构。此机构的科学家主要研究美国的地形，自然资源和自然灾害与其的应付方法（如地震、火山喷发、泥石流等）。这部门负责四大科学范畴：物理学、地理学、地质学和水文学。
Tectonic Summary
Seismotectonics of the Himalaya and
Vicinity
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.
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