7.0-magnitude Xizang quake an extensional fracture: expert

BEIJING -- The 7.0-magnitude earthquake that struck Baqen county of the city of Nagqu in Southwest China's Xizang autonomous region on Thursday was an extensional rupture, according to experts with the China Earthquake Networks Center (CENC).

The quake occurred at around 10:33 pm, causing heavy casualties. Focal mechanism solutions show the event was basically consistent with regional tectonic characteristics.

"An extensional fracture, in simple terms, occurs when the crust is stretched horizontally and, under tension and gravity, faultage slides downward along existing faultage, forming fractures and rupture surfaces," said Han Yanyan, a senior engineer with the earthquake prediction department of the CENC.

The CENC explained that the collision between the Indian and Eurasian plates had caused the uplift of the Qinghai-Tibet Plateau, crustal shortening and thickening, and widespread plateau deformation. These processes are the main dynamic mechanism controlling tectonic deformation in the western part of the Chinese mainland.

The interior of the plateau is subjected to both north-south compression and east-west extension, and two typical fault systems, trending roughly north-south and east-west, have developed there.

"The epicenter is located within the Changtang block of the Qinghai-Tibet Plateau, about 2 km from the nearest fault, the Baqen-Riwoqe fault," Han said.

Under such intense crustal deformation, the Changtang block and its surrounding fault zones are particularly active. Since 1900, the block has experienced six earthquakes of magnitude 7 or above, including this one. The largest was a 7.5-magnitude earthquake in Shuanghu on Nov 8, 1997.

"Therefore, this earthquake was an energy release of the Changtang block," Han said.