7 September 2012

Possible landslides from the 2 x M=5.6 earthquake sequence in SW China this morning

Posted by Dave Petley

As you may have guessed I have been on holiday! Anyway, normal service is now resumed.

Early this morning Southwest China was shaken by four small earthquakes.  Usually events of this magnitude – 5.6, 4.9, 5.6 and 4.8 (shown using a USGS screenshot above) – would not be sufficiently large to be worth reporting.  Surprisingly though they appear to have caused considerable damage.  Xinhua is currently report 50 fatalities and 160 people injured.  This may increase in the next 24 hours as they are reporting that outlying villages remain isolated. The impacts are sufficiently large for the Chinese President to call for immediate assistance to the affected area, and for the Chinese Premier, who is himself a geologist, to head to the area.

Interestingly, already Xinhua is trying to explore why the impacts are so high for such small earthquakes.  They pinpoint the level of poverty in the area (Yiliang County in Yunnan Province appears to be where the majority of the fatalities occurred), indicating low levels of preparedness.  However, reading between the lines two other factors may be important.  First, these very shallow earthquakes (approximately 10 km depth) had epicentres very close to the county town, which may have led to a high level of localised shaking that happened to overlap directly with an urban centre (in many ways this is similar to Christchurch in New Zealand).  Second, the report does indicate a high level of landslides, noting that:

“Xinhua reporters in Luozehe saw big rocks, some as tall as four meters, tumbling down the mountain slopes and crashing onto the roads. Landslides were also triggered.  A settlement of a zinc mine in Luozehe was seriously damaged. More than two dozen mining families have been evacuated out of their damaged houses.  “It is scary. My brother was killed by falling rocks. The aftershocks struck again and again. We are so scared,” said miner Peng Zhuwen.”

The Earthquake Report blog also has reports about the location that indicate that landslides may have been a serious problem.  For example, they report that:

– A reporter who was inspecting the area saw huge landslides on both sides of the valley.
– Roads are being blocked by huge rocks.
– A boulder measuring 36 meter was blocking the river, creating a small lake
– The houses of 30 families living near a zinc mine have all been damaged
– aftershocks are following each other very fast. More landslides and rockfall is being triggered by the many aftershocks
– rescue workers have still problems to reach some areas as roads are blocked

We may need to wait until tomorrow for a proper report of this event, but for now this is a Google Earth perspective view of the area around Yiliang.  It is very steep, and hence is highly landslide-prone during seismic shaking: