Jiuzhaigou County: a shallow magnitude earthquake in Sichuan Province on 8th August 2017

At 13:49 UT  (21:49 local time) on 8th August 2017 an earthquake struck Jiuzhaigou County in Sichuan Province in China. At the time of writing the magnitude of this event is unclear – the Chinese media are reporting M=7.0, whilst the USGS currently has M=6.5 – but either way this is a significant event.  In both cases the depth is considered to be very shallow – c. 10-20 km – which means that there is a high potential for landslides. Currently, the level of loss is not clear, and is unlikely to be so until the sun comes up in a few hours, but estimates range from 40 to 190 fatalities, with a median  value of 102, according to James Daniell of EDIM – CAT news.

Of course this area is no stranger to significant earthquakes. The earthquake in Jiuzhaigou County has occurred to the west of the ruptures that generated the 2008 Wenchuan earthquake.  It is a characteristic of earthquakes in this area that they generate large numbers of landslides.  The landscape in this region of China is steep and high, and the slopes have been preconditioned for failure by frequent earthquakes and intense rainfall events. Early indications are that this was predominantly a strike-slip earthquake – experience suggests that these events tend to generate intense landsliding close to he fault rupture, with rapid attenuation of landslide density away from the fault. It will be interesting to see if this event matches this pattern. Certainly the overlay of USGS estimates of intensity over the terrain in Google Earth suggests the potential for landslides is high:

USGS intensity contours for the Juizhaigou County earthquake overlain onto a Google Earth terrain image for the area.

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There are some early indications of landslides.  This image, via Twitter, appears to show significant rockfall activity:-

Image via Twitter of rockfall activity after the Juizhaigou County earthquake.

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Meanwhile Earthquake Report has this image of a shallow slide (though the main issue seems to be felled trees):-

A landslide triggered by the Juizhaigou County earthquake, via Earthquake Report.

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Meanwhile Xinhua has some reports of landslides too:

Jiuzhaigou, or Jiuzhai Valley, is a national park known for spectacular waterfalls and karst formations. More than 34,000 people visited the tourist attraction on Tuesday.  “Now it is the peak travel season and there is a large number of tourists in the scenic area,” said Ye Zhi, a villager in Zhangzha. “The tourists were in panic when the quake struck and there were also stones falling down the mountain.”

Fearing risks from the complicated terrain, tourists and residents have been led to large open spaces for safety, said Ye.  To avoid potential further deaths and injuries, the scenic area stopped accepting tourists.

A landslide in the tourist site left more than 100 visitors trapped, but no deaths or injuries were reported among them.

It is likely that the picture will become clearer in the next few hours.