A magnitude 5.9 (M5.9) earthquake which struck in China’s Ganshu Province has caused extensive damage and injuries with some loss of life, according to news reports.
The tremor, which struck in the morning of 22 July local time, was followed within hours by aftershocks of M5.6 and M4.7.
Early indications from the United States Geological Survey’s Pager alert system suggested that there was a likelihood of casualties, and reports emerging from the region confirm this.
China’s Damaging Earthquake
At the time of writing, the BBC was reporting that “at least 54 people have been killed and 334 injured”.
These numbers have not been confirmed and may rise.
Although the earthquake was by no means large (at M5.9 it was only one-twenty-fifth the size of the largest earthquake to occur in the preceding seven days) it is not necessarily the magnitude of a tremor which dictates damage.
The earthquake and its aftershocks occurred at shallow depths (around 10km) and shallow earthquakes are more damaging than deeper ones as the energy has less time to dissipate before reaching the surface. The issue is compounded by the topographical nature of the region: steep, potentially unstable slopes are prone to collapse so that much damage is caused by landslides – as appears to be the case in Ganshu.
July 22 Ganshu Earthquake: Tectonic Setting
Most earthquakes are associated with the movement of the earth’s tectonic plates. Although most large tremors are the result of movement along relatively narrow subduction zones, where dense ocean crust is forced downwards, the convergence of the Indian subcontinent and Eurasia is a continent-continent collision which has resulted in the creation of the Himalayas and the Tibetan Plateau and continues to cause uplift and seismic activity across a large area of central and eastern Asia.
As the line of contact is not straight, the direction of convergence is not constant. To the east of Tibet, in the region where the earthquakes took place, much of the movement is accommodated along a series of broadly east-west trending faults with lateral movement. It seems likely from the available information and from local fault maps that the earthquake took place on one of these, the Ganshu Fault.
Ganshu Fault: Tectonic History
Given the high levels of seismicity across much of southern and eastern China, coupled with the rugged and topographically varied landscape, high levels of population and unsafe buildings, it is unsurprising that the country is no stranger to major earthquake events or that they tend to cause casualties whereas tremors of a similar size in other locations do not.
Much of eastern China has been subject to significant earthquakes over time. Yeats makes reference to an earthquake believed to have had a magnitude close to M8 which struck in 1556 which killed over 800,000 people.
Throughout its history, the country has an unenviable record of ‘quakes resulting in very high numbers of deaths: most recently, an earthquake in eastern Sichuan Province in 2008 killed almost 90,000 people, while the Tangshan earthquake of 1976 killed almost 250,000 – and remains the deadliest of the modern era.
BBC News online. China’s Gansu Province hit by powerful earthquakes. (2013). Accessed 22 July 2013.
USGS. M5.6 – 9km NNE of Chabu, China. (2013). Accessed 22 July 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.