Friday, 7 September 2012

Earthquakes in northeast Yunnan Province, China.

On Friday 7 September 2012, a few seconds before 11.20 am local time (3.20 am GMT) Yilang County in the northeast of Yunnan Province, southwest China, was shaken by an Earthquake measured by the United States Geological Survey as a Magnitude 5.6 at a depth of 9.9 km. This was followed by a second Earthquake slightly after 12.15 pm local time (slightly after 4.15 am GMT), approximately 12 km to the northeast, measured as a 5.6 magnitude quake at a depth of 9.8 km. There have been a number of aftershocks in the region. These are powerful, shallow quakes and have caused considerable damage, including a number of landslides. The death tole appears to be considerable, with 43 people known to have been killed at the time of writing.

Map showing the location of the second quake (black star), damage to buildings is likely inside the green circle. USGS.

Yunnan Province, along with much of western China and neighbouring areas of Central Asia and the Himalayas, is prone to Earthquakes caused by the impact of the Indian Plate into Eurasia from the south. The Indian Plate is moving northwards at a rate of 5 cm per year, causing it to impact into Eurasia, which is also moving northward, but only at a rate of 2 cm per year. When two tectonic plates collide in this way and one or both are oceanic then one will be subducted beneath the other (if one of the plates is continental then the other will be subducted), but if both plates are continental then subduction will not fully occur, but instead the plates will crumple, leading to folding and uplift (and quite a lot of Earthquakes). The collision of the Indian and Eurasian plates has lead to the formation of the Himalayan Mountains, the Tibetan Plateau, and the mountains of southwest China, Central Asia and the Hindu Kush.

Diagram showing how the incomplete subduction of the Indian Plate has led to the formation of the Himalayas and the Tibetan Plateau. Jayne Doucette/Woods Hole Oceanographic Institute.