Southwest China Earthquake

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Posting Date: September 7, 2012, 4:00:00 PM

According to the United States Geological Survey (USGS), at 11:19 AM local time this morning (3:19 UTC), a magnitude 5.6 earthquake struck southeast China, 12 km south of Jiaokui. The earthquake was followed by more than 60 aftershocks including another M5.6 earthquake, which struck 1 km south of Jiaokui less than an hour later at 12:15 PM local time (4:16 UTC). Both earthquakes were relatively shallow, with depths of 9.9 km and 9.8 km, respectively. Reported Impact Today’s earthquake series began with a M5.6 temblor that struck the border area between Yiliang county, in the Yunnan province, and Weining county, in the Guizhou province. The damage was heaviest in the rural, agricultural areas of Yunnan and Gizhou, where more than 20,000 homes were damaged or collapsed according to the Yunnan Provincial Civil Affairs Department. The poor construction of the buildings in these areas makes them susceptible to earthquake damage. More than 100,000 people were evacuated from their homes in Yunnan. At least 50 people have been killed and more than 700 have been injured. Thousands of blankets and tents have been sent by the government as a part of the relief effort. Most of the fatalities occurred when the earthquake series triggered landslides near the mountainous border between Yunnan and Gizhou provinces. Rocks as large as four meters in diameter tumbled down mountains, blocking roads and crushing houses in the Yiliang county town of Luozehe. The rescue effort has been difficult as damage to roads has prevented rescuers from reaching remote areas. According to Luozehe township officials, rescue efforts are greatly hampered by blocked roads and rescuers are climbing mountains to reach the villages hit by the earthquake. Firefighters and volunteers are working to clear these roads. Communication is also challenging as cell phone service is down and regular phones are disrupted. Power is also out in many areas. In 2008, a M7.9 magnitude earthquake hit Wenchuan, just north of Yunnan, killing nearly 90,000 people. The deaths, which included thousands of students who were killed when their schools collapsed, are widely blamed on poor construction practices. During today’s earthquake series, one elementary school has reportedly collapsed. It is unclear whether any of the students in the school have been harmed, but a rescue mission is underway. At a small school in Luozehe, a teacher reported that dirt dropped from the ceiling, and large cracks appeared in the mud and stone building. Many new roads, bridges, and industrial zones are under construction in Guizhou, with hundreds of millions of dollars spent on these projects in the past year; it is currently unknown if any have been damaged by today’s earthquakes. Regional Seismicity Today’s two M5.6 earthquakes occurred in the North-South Seismic Belt in central China, which is the site of the 2008 Wenchuan earthquake and many other recent M7.0 to M8.0 earthquakes. Today’s events were located to the west of those past earthquakes, about 100 km away from the major active faults in the North-South Seismic Belt. Both epicenters were located about 300 km southwest of the Longmen Shan fault zone, which generated the 2008 Wenchuan earthquake, and 100 km east of the Xianshuihe-Xiaojiang fault zone, which has historically produced many large earthquakes. Historically, moderately large earthquakes have occurred along the minor faults to the east of the Xianshuihe-Xiaojiang fault zone. For example, in May of 1974, a magnitude 7.1 earthquake struck Zhaotong and caused widespread damage— 60 km north of today’s events. This event occurred along a set of minor faults east of the major active faults in the region. The epicenters of today’s events occurred in a region characterized by deep stream valleys and steep slopes. The elevation contrast between mountain ridges and stream valleys typically ranges from 500 to 100 meters, and the average slope is about 30-40 degrees. Therefore, earthquakes in this region usually cause widespread landslides and rock falls, as did today’s earthquakes. Exposure at Risk The dominant construction types in China have historically been unreinforced masonry consisting of adobe or brick, or brick with a light wood frame. The buildings are generally massive, but lack the lateral systems that can help withstand ground shaking. As a result, they often perform very poorly during seismic activity, even when the ground shaking intensity is low. Many single-family homes, particularly in rural areas, are still made out of traditional materials. However, after the 1976 Tangshan earthquake, many of the unreinforced masonry buildings in urban residential areas were replaced with stronger and more affordable confined masonry. Most urban homes are mid-rise or high-rise apartment buildings, many of which have commercial establishments on the ground floor. Mid-rise buildings are often confined masonry while high-rises tend to be reinforced concrete. New buildings in China incorporate advanced earthquake-resistant features in their design and are also subject to better construction practices and more stringent code enforcement. Thus, they should perform better during intense ground shaking from large earthquakes. According to Paz (1994), the first official seismic code for China was issued in 1974 for industrial and civil buildings. The code has been revised several times with the most recent one published in 2010, after the 2008 Wenchuan earthquake. Many people believe that the enforcement of the seismic code in the area affected by today’s earthquakes has not been strict as numerous residents are still living in mud and clay dwellings with little seismic consideration. The USGS reported a Modified Mercalli Intensity of VI (strong) and higher around the epicenter. The highest reported intensity in population centers is around V (moderate) in Zhaotong, which is a predominantly agricultural area. At this intensity level, there is often only minor damage to engineered structures; however, today’s events may cause greater damage given the landslides that were triggered and the vulnerable mud and clay dwellings in the region. Even so, because this event occurred in a rural location and the take-up rates for residential properties are fairly low, AIR does not expect significant insured losses from this event. AIR continues to monitor the situation and will provide updates if warranted by events.

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Posting Date: September 7, 2012, 4:00:00 PM