Significant damage following M=6.5 Chinese earthquake in area stressed by deadly 2008 Wenchuan quake

By David Jacobson, Temblor

See earthquakes in China

Updated: 10 August 2017 at 11 a.m.

Jiuzhai-National-Park-earthquake
The M=6.5 earthquake in southwestern China occurred near Jiuzhai National Park, a popular tourist destination. So far, 19 people are known to have died as a result of this earthquake, and based on photos, there is significant damage. (Photo from: Flickr)

 

At 9:19 p.m. local time (8 Aug), a M=6.5 earthquake struck the Sichuan-Gansu border region in southwestern China. According to both the USGS and the European-Mediterranean Seismological Centre (EMSC), the quake occurred at a depth of 10 km in a mountainous region, near Jiuzhai National Park. This area is on the edge of the Tibetan Plateau and is extremely popular amongst tourists because of spectacular waterfalls and blue and green lakes. So far, 19 people are confirmed to have been killed by this event, and there is photo evidence of significant damage, which the USGS PAGER system estimates could be moderate to high near the epicenter, where very strong shaking was recorded.

china-earthquake-map
This Temblor map shows the location of today’s M=6.5 earthquake in southwestern China. The quake is known to have killed at least 5 people, with dozens more injured.

 

Now that a few days have past, new photography which has come in from the area, illustrating that damage in nearby Jiuzhai National Park is significant (See below). This area brings in a significant amount of money to the area from tourism, and with the damage sustained, local officials are worried about the health of local businesses. At the time of the earthquake, local officials say there were 30,000 tourists in the area. The majority of these people have been evacuated and tourists are told to stay away.

Jiuzhai-National-Park-damage
Before and after images of Jiuzhai National Park in southwestern China. Many of these spectacular blue pools are separated by waterfalls and natural barriers which collapsed in the earthquake. (Photos from: China News (Left) and Getty Images (right))

 

china-landslides
The M=6.5 earthquake in southwestern China resulted in significant landslides which blocked and destroyed roads in addition to turning the blue pools in Jiuzhai National Park brown and murky. (Photo from: Shutterstock)

 

Based on the USGS focal mechanism, the M=6.5 earthquake was strike-slip with a thrust component. However, at this time we cannot be sure of the strike of the fault on which the quake occurred due to location inconsistencies between the USGS and EMSC and the numerous lineaments in the area. In this portion of the Tibetan Plateau, fault motion is dominated by left-lateral strike-slip faults, including the Altyn Tagh, Kunlun, and Haiyuan fault zones. Additionally, the location of the earthquake is approximately 280 km northwest of the epicenter of the 2008 M=7.9 Wenchuan earthquake which killed over 87,000 people. That earthquake occurred on the Longmenshan Fault, and was a thrust (compressional) event.

Even though the earthquake struck nearly 300 km from the Wenchuan earthquake, based on the figure below, one can see that this area saw a significant stress increase, which more than doubled the likelihood of a M=6+ earthquake. The side-by-side figures below show the observed rates of seismicity prior to the Wenchuan earthquake (left), and the forecasted seismicity in the 10 years following as a result of the stress imparted by the Wenchuan (The location of the August 8th quake is labeled with a star). This comparison shows that due to the Wenchuan earthquake, the area around today’s event went from having a 4% chance of experiencing a large quake, to 10%.

china-earthquake-coulomb
These side-by-side figures from Toda et. al., 2008 show the observed seismicity in the 10 years prior to the Wenchuan earthquake, as well as the 10 years after as a result of the stress imparted by the Wenchuan. The black stars represent the two largest (M>6) earthquakes since the Wenchuan earthquake, both of which occurred in areas which saw stress increases. What is evident from these figures is that as a result of the Wenchuan earthquake, the location of the August 8th event went from having a 4% chance of experiencing a M=6+ quake to a 10% chance. The green rectangle represents the 2008 Wenchuan fault rupture.

 

Based on the Global Earthquake Activity Rate (GEAR) model, which is available in Temblor, this earthquake can be seen as surprising. This model uses global strain rates and the last 40 years of seismicity to forecast the likely earthquake magnitude in your lifetime anywhere on earth. In the figure below, one can see that GEAR would forecast a M=5.75 earthquake in this region, significantly lower than the magnitude (M=6.5) of this event. This illustrates the importance of incorporating stress transfer into the likelihood of experiencing large earthquakes. GEAR does not, which is why it would see this quake as surprising.

china-earthquake-gear
This Temblor map shows the Global Earthquake Activity Rate (GEAR) model for southwestern China around the location of the August 8th M=6.5 earthquake. This model uses global strain rates and the last 40 years of seismicity to forecast the likely earthquake magnitude in your lifetime anywhere on earth. Based on the GEAR model, today’s earthquake should be seen as surprising, for it projects that only a M=5.75 quake is likely in your lifetime.

 

References
USGS
European-Mediterranean Seismological Centre
Shinji Toda, Jian Lin, Mustapha Meghraoui, and Ross S. Stein, 12 May 2008 M = 7.9 Wenchuan, China, earthquake calculated to increase failure stress and seismicity rate on three major fault systems, GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L17305, doi:10.1029/2008GL034903, 2008.

  • Keith Koper

    Hi,

    I was just told by a former student that the M6.6 2013 Lushan earthquake occurred just near the P in the right hand panel of the figure above. So, the forecasting was even a little better than what is suggested.

    Cheers,
    -Keith Koper, Univ Utah

  • Temblor

    Thank you very much for the response Keith. I have updated the figure to show the April 2013 earthquake. Both that quake, and the one yesterday are the two largest to occur in the region in the last several years, and as you pointed out, the forecasting is now shown to do an even better job.