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M 6.5 Earthquake Strikes Southwestern China

09 Aug 2017

A strong, shallow quake struck western Sichuan, China, on 8 August 2017 at approximately 9pm (Singapore time). According to Xinhuanet, at least 13 people have been killed and 175 injured in the popular tourist destination.

According to the China Earthquake Administration (CEA) and the United States Geological Survey (USGS), the magnitude-6.5 earthquake originated from the mountainous area northeast of the Sichuan Basin at a depth of about 10 kilometres (km).

A map showing the epicentre of yesterday's M 6.5 earthquake in Sichuan, relative to the 2008 Wenchuan earthquake epicentre (Source: Zhang et al., 2010)

Based on the preliminary information provided by the CEA, this M 6.5 quake likely occurred along a left-lateral fault that runs in the NW-SE direction and which belongs to the East Kunlun and Tazang Fault system.

This fault system, created by the collision of the Indian and Eurasian plates, is known to undergo a slip rate of several millimetres per year and is one of the primary sources of seismic hazards in China. 

In the past century, several significant earthquakes occurred at the eastern end of this major continental strike-slip fault system. Among them were the M 7.3 Diexi earthquake in 1933, which resulted in a death toll of nearly 9,300, and the M 7.2 Songpan earthquake in 1976, both of which occurred near the epicentre of yesterday’s earthquake.

Although the earthquake that had occurred last evening was not as strong as these earlier events, it may trigger some hazardous landslides in the region close to the epicentre.

At least 13 people have been killed and 175 injured in the M 6.5 earthquake struck Southwestern China yesterday (Source: India Today)

Another strong, shallow quake occurred in the western Xinjiang province, northwest China, on 9 August 2017 at approximately 7:30am (Singapore time), about 10 hours after the M 6.5 earthquake in Sichuan Province. The preliminary data released by the CEA and USGS show that this M 6.3 earthquake originated from the northern part of the Tian Shan mountain range, at a depth of about 11 km.

Though this M 6.3 earthquake occurred very soon after the earlier M 6.5 earthquake in Sichuan, and both events were caused by the Indian-Eurasian plate collision, these two events are likely to be separate due to the distance between them. 

The seismic wave analyses conducted just after the two earthquakes show that the earlier M 6.5 event was a result of a 20 km-long rupture of a strike-slip fault system, while the later M 6.3 earthquake was associated with the reverse fault underneath Tian Shan.

This simplified active fault map shows the epicentre locations of the recent two earthquakes in China (Source: modified from Liu et al., 2007)

Although China is located far from the major fault at the Indo-Eurasian plate boundary – the Himalayan Frontal Thrust – the powerful northward tectonic collision between the Indian and the Eurasian plates created a wide deformation zone within inner China. This is evident from the long history of earthquakes written into Chinese literature, including the destructive 2008 Wenchuan earthquake.

The recent earthquakes in China are just two more examples of ongoing tectonic plate motions. We can expect subsequent aftershocks in the main epicentre areas in the near future.