The twin quakes hit near the city of Dingxi in Gansu
province, a region of mountains, desert and pastureland with a population of 26
million.
An official at the provincial earthquake bureau said:
"More than 21,000 buildings were severely damaged and more than 1,200 have
collapsed."
The first 5.9-magnitude quake struck at 7.45am (11.45pm GMT)
in the Gansu province and the second, measuring 5.6-magnitude, hit in the same
region at 9.12am.
The quake was shallow, which can be more destructive. The
first quake was 9km deep while the second was 10km deep. Dingxi is about 766
miles west of Beijing.
Tectonic Summary (USGS)
The July 21, 2013 China earthquake (Mw5.9)
occurred in south-central Gansu Province, 13km east of the city of Chabu. The
earthquake resulted from NE-SW oriented oblique, thrust-type motion.
On a continental scale, the seismicity of central and eastern Asia results from northward convergence of the India plate with the Eurasia plate at a velocity of about 50mm/yr. The convergence of the two plates is broadly accommodated by the uplift of the Himalaya and the Tibetan Plateau, and by the motion of crustal material to the east away from the sides of the Tibetan Plateau.
The July 21, 2013 event occurred within 200km
of 3 events greater than M 6 in the past 40 years, including one event of M 6.0 in 2008.
of 3 events greater than M 6 in the past 40 years, including one event of M 6.0 in 2008.
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 lithospheri.
The Tectonic Summary above was curled from: US Geological Survey.
