The Wenchuan, Sichuan Province, China, Earthquake of May 12, 2008
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EERI Special Earthquake Report — October 2008 Learning from Earthquakes The Wenchuan, Sichuan Province, China, Earthquake of May 12, 2008 A team sponsored by the Earth- Tectonic Setting school buildings that buried thou- quake Engineering Research Insti- sands of students and teachers. The M7.9 Wenchuan earthquake is tute (EERI) and the Geo-Engineer- the worst to strike China since the The earthquake originated on the ing Earthquake Reconnaissance M7.8 Tangshan event of July 1976 Longmenshan fault, a northeastern- (GEER) Association carried out a that claimed an estimated 242,000 striking thrust fault that ruptured for field investigation in conjunction lives. The Wenchuan earthquake af- almost 300 kilometers (see Figure 1). with Chinese colleagues from Aug- fected more than 100,000 square The fault is the result of conver- ust 3-11 to document effects of the miles and about 30 million people, gence of crust from the high Tibet- May 12 earthquake. The EERI- causing 69,226 deaths (as of August an Plateau to the west, against the GEER team was invited by Profes- 21, 2008) and injuring almost 375,000 strong and stable crust block under- sor Zifa Wang, Director of the In- people. Almost 1.5 million people had lying the Sichuan Basin and south- stitute of Engineering Mechanics- to be relocated (Xinhua, 2008). The west China to the east. On a contin- China Earthquake Administration earthquake also destroyed more than ental scale, the seismicity of central (IEM-CEA). Professor Junwu Dai 216,000 buildings in Sichuan Prov- and eastern Asia is a result of of the IEM-CEA accompanied the ince, including approximately 6,900 team during the field investigation. Under the leadership of Marshall Lew of MACTEC Engineering and Consulting in Los Angeles, the team included experts in structur- al, lifelines, and geotechnical engi- neering as well as in disaster re- sponse and recovery. EERI team members were David Friedman and Dennis Lau of Forell/Elsesser Engineers, Inc.; Laurie Johnson, urban planning consultant, San Francisco; Tricia Wachtendorf of the Disaster Research Center at the University of Delaware, Newark; and Jian Zhao of the University of Wisconsin at Milwaukee. The GEER team consisted of David Frost of the Georgia Institute of Technology in Savannah; J. P. Bardet of the University of Southern California; and Tong Qiu of Clarkson Univer- sity in Potsdam, New York. Obser- vations of other investigators who visited the earthquake-affected region have also been incorporated in this report. The EERI field investigation was conducted as part of the Learning from Earthquakes Program with funding from the National Science Foundation under grant #CMMI- 0758529. Figure 1: Locations of Wenchuan earthquake May 12, 2008, mainshock and aftershocks through May 28, 2008 (U.S. Geological Survey, 2008). 1 EERI Special Earthquake Report — October 2008 buildings were considered Category C buildings, which is the same as regular buildings. Ground Motions: Strong ground motion was measured at about 200 stations established by the China Earthquake Administration. The strong motion time histories have not yet been made available to the research community, though three recordings were shown to the EERI- GEER team. One record obtained at a distance of 22 km from the epi- Figure 2: center and 1.0 km from the fault had Tectonic setting a peak ground acceleration (PGA) of the Wench- of around 957 gal and a duration uan earthquake of nearly 60 seconds. A recording (U.S. Geologi- made 88 km from the epicenter and cal Survey, 1.0 km from the fault had a PGA of 2008). 802 gal and shaking duration of 90 seconds. At 150 km from the epi- center and 75 km from the fault, a third recording showed a PGA of northward convergence of the August 1933 that caused over 6,800 550 gal and shaking of 150 seconds. Indian plate against the Eurasian deaths, with another 2,500 people plate. This convergence is broadly perishing subsequently in the failure Geotechnical Effects accommodated by uplift of the of a “quake lake” dam created by an Asian highlands and by the motion earthquake-induced landslide. The Surface Faulting: The surface rup- of crustal material to the east, away average recurrence interval of the ture was about 270 km in length. from the uplifted Tibetan Plateau. Wenchuan earthquake has been es- The fault rupture cut through towns, Thus, India is moving northward timated to be in the general range of villages, and roads. The fault move- into central Asia and pushing Tibet 2,000 to 10,000 years by Burchfiel et ment was predominantly a reverse eastward, overriding the Sichuan al. (2008). thrust in the southwestern portion of basin. The tectonic setting is shown the rupture, becoming more strike- Because large earthquakes were in Figure 2. slip towards the northeast. At sev- rare before the May 12 event, basic eral locations, surface rupture with Previous earthquakes along the design levels in the Sichuan region 3-4 m of vertical offset was ob- northwestern margin of the Sichuan were established at Intensity VII served; at other locations not visited Basin include a M7.3 earthquake in (Modified Mercalli Scale). School by the team, vertical offsets as high Figure 3: Surface fault rupture with 2 m vertical displace- Figure 4: Surface fault rupture with 5 m vertical displace- ment at Gaoyuan Village. ment at Shenxigou. 2 EERI Special Earthquake Report — October 2008 Figure 6: Surface fault rupture contributed to failure of Gaoyuan Bridge; north abutment is to the left slab to be thrust over the first span Figure 5: Surface rupture causing total collapse of build- of the bridge. In addition, part of the ings at Xiaoyudong. west wing wall of the abutment col- lapsed, along with some of the fill in terns of damage. These systems in- as 5.6 m were reported. Fault rup- the abutment. The movement was cluded buildings, roadways, bridges ture at Gaoyuan Village, north of large enough that the second span and utility lines. Generally, less dam- Dujiangyan, is shown in Figure 3. became unseated at the center bent age was observed in structures on The fault rupture was reported by and fell to the ground below. CEA to be as much as 5 m vertical the footwall side of the fault than in and about 4.5 m vertical displace- similar structures on the hanging wall. Landslides: Numerous earthquake ment (Figure 4). Fault rupture caused Cases in which this pattern appeared induced landslides were observed collapse of many structures that not to hold true were typically re- as the team travelled through the were constructed over the fault solved as being due to complimen- mountainous terrain (Figure 7), an traces; as many of the buildings tary fault rupture, although rupture area that is historically known to be were of brick construction, total col- patterns were frequently difficult to prone to landslides and debris flows. lapse was very common (Figure 5). discern due to large variations in Evidence of historical events was elevation within short distances. abundant. Most of the new earth- Surface faulting caused extensive quake-induced slides were relatively Fault rupture damaged and caused damage to civil engineering sys- shallow, involving predominantly the collapse of several bridges, in- tems due to the shallow epicenter surficial soils, although the lateral cluding the Xiaoyudong Bridge, the and the length over which the rup- and vertical extents were significant. Baihua Bridge near Yingxiu, and the tured faults propagated to the There was evidence that some of Gaoyuan Bridge (Figure 6), a two- ground surface. In addition, signifi- these failed progressively. More lane traffic bridge with north and cant variations in terrain elevation in than 25 deeper and broader slides many areas where the fault rupture south abutments and three bents over each mobilized more than 10 million a river. The thrust fault rupture went reached the ground surface led to cubic meters of slide material, accord- through the north (left) abutment of significant complimentary ruptures ing to reports. The team members that produced more complex pat- the bridge and caused the approach Figure 7: Landslides along the Min River between Du- jiangyan and Hongkou. Figure 8: Longmenshan Town landslide. 3 EERI Special Earthquake Report — October 2008 Figure 9: Rainfall triggered secondary mudflow in Peng- Figure 10: Resort hotel damaged by rockslide in Shenxi- zhou. gou. visited the site of one of these large Mud Flows: Surficial soil and rock knees as they traversed it. slides, the Longmenshan Town land- loosened by the strong ground shak- slide, that buried a community of ing during the earthquake was prone Lateral Spreading: Strong ground more than 20 families, resulting in to form mud flows and, in fact, heavy shaking from the earthquake also 68 fatalities (Figure 8). rainfall at the time of the site visit trig- resulted in lateral spreading. Two gered secondary mudflows. One mud slopes with earthquake-induced lat- Large landslides along river valleys flow observed by the GEER/EERI eral movements were observed in formed “quake-lakes,” which subse- team members (Figure 9) took place Dujiangyan City. The first slope had quently threatened communities in Pengzhou during a heavy rainfall a 50-meter long fissure several me- downstream. Thirty-four landslide- on the evening of August 7, 2008. ters back from the edge of the slope. induced lakes have been reported, The mud flow was reported to have The open fissure was covered by with Tangjiashan quake-lake being lasted about 40 minutes and was ac- plastic sheet to prevent surface wat- the largest one. The China People’s companied by a loud “ground shak- er infiltration. The lateral movement Liberation Army deployed troops to ing” sound.