SUNDAY, SEPTEMBER 24, 2017
M6.5 Jiuzhaigou China Earthquake
8/8/2017 5:00:00 PM
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First Posting 8/8/2017 5:00:00 PM 
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Posting Date: 8/8/2017 5:00:00 PM

At 9:15 p.m. local time (13:20 UTC) a shallow magnitude 6.5 earthquake struck Jiuzhaigou County in northern Sichuan Province, near the border of Gansu Province, in a mountainous region of southwestern China. About 100 tourists have been reported trapped by a landslide, and at least seven people have been reported killed and 63 injured. This strong earthquake has caused some building damage, power outages, and disrupted communications networks. The scope of the damage may be difficult to ascertain, due to the communications networks being down.

This earthquake was 285 kilometers (177 miles) from Chengdu, the densely populated capital of Sichuan Province. The Sichuan provincial government reports that some train services to Chengdu and other cities were suspended following the quake.

This area is on the edge of the Tibetan Plateau in northern Sichuan Province, 2,000 meters (6,562 feet) above sea level. The epicenter was in Jiuzhaigou County, about 37 kilometers from the capital of Jiuzhaigou town. While the county’s population is relatively sparse—around 80,000 people—it is a popular tourist destination. The region is known for the nearby Jiuzhaigou Valley, located in the Min Shan mountain range, which is home to Jiuzhai Valley National Park, a UNESCO World Heritage Site that reaches a capacity of 41,000 visitors during high season.

The August 8 earthquake had a depth of just 9 km, and shallow earthquakes tend to cause more damage. It occurred in an area where two large active fault zones, the eastern Kunlun fault zone and the Longmenshang fault zone, meet at the eastern margin of the India-Eurasia collision zone. The epicenter is about 130 km northwest of the Longmengshan fault zone, which was the source of the M7.9 Wenchuan earthquake rupture in 2008. This region has experienced a dozen events of M6.0 or greater over the past century, with the most notable being the 2008 Wenchuan earthquake—China’s most deadly earthquake this century, killing nearly 90,000 people. That earthquake also destroyed more than 5 million buildings and affected over 45 million people across western China. 

shakemap_china_8aug2017.jpg
ShakeMap for the M6.5 earthquake that struck near Jiuzhaigou, Sichuan Province, China, at 13:20 UTC on August 8, 2017. (Source: USGS)


Exposure at Risk

China’s insured building stock in urban areas is dominated by mid-rise confined masonry and mid- to high-rise reinforced concrete buildings, which, if constructed in compliance with the building code, are expected to perform adequately for moderate earthquakes. However, in smaller towns across China, the majority of the building stock is made up of older unreinforced masonry (URM) structures, which are highly vulnerable to ground motion.

In the areas affected by this earthquake, the dominant construction types have historically been unreinforced masonry made from rubble, adobe, brick, or brick with a light wood frame. The buildings are generally massive, but they lack the lateral systems that can help withstand ground shaking from an earthquake. As a result, these buildings generally perform very poorly during seismic activity, even when the ground shaking intensity is low. However, in the cities the new construction includes low-rise and mid-rise confined masonry, reinforced concrete, and steel frame buildings typically with masonry infill walls. Traditionally designed buildings of reinforced concrete perform well during low ground shaking; however, they are still quite vulnerable at higher levels of ground motion.

New buildings in China are incorporating advanced earthquake-resistant features in their design and are also subjected to better construction practices and more stringent code enforcement. After the 1976 Tangshan earthquake, many unreinforced masonry homes were replaced with confined masonry, which is more stable due to the slender columns and beams (not load-bearing), which divide the large unreinforced masonry walls. New buildings are generally made of reinforced concrete and are built to strict code regulations, with the intent that they should perform better during strong earthquakes.

The area affected by the earthquake is a mountainous region and precipitation-induced landslides are common. Earthquake-induced landslide damage to infrastructure (roads, bridges, transmission lines, and pipelines) is expected. Most of the villages and towns in the affected area are located in the narrow valleys, along the rivers, on deposits consisting of sand, silt, and clay. Particularly for the buildings closer to the rivers, site amplification and lateral spreading caused by liquefaction could be a factor driving the damage.

M6.5 Jiuzhaigou China Earthquake
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