Earthquake in China
A 6.7 ( 6.8) earthquake struck Luding County in Sichuan province, China on 5 September 2022 at 12:52:19 local time. The epicenter was located from Chengdu, or southeast of Kangding. Ninety-three people died, 424 were injured and 24 remained missing. More than 13,000 homes and other infrastructure were damaged or destroyed. It was the largest earthquake to strike the province since 2017.
Tectonic setting.
Sichuan is situated in a complex zone of faulting created by the continued collision of the Indian Plate with the Eurasian Plate. As Himalayan underthrusting continues, the crust of the Eurasian Plate is deformed and uplifted to form the Tibetan Plateau. Instead of thrust faulting to its south, the Tibetan Plateau accommodates deformation through strike-slip escape tectonics. Large amounts of strike-slip motion are accommodated through major faults and their splays along the plateau, such as the Altyn Tagh Fault, Kunlun Fault, Haiyuan Fault, and Xianshuihe fault systems. Left-lateral strike-slip motion squeezes the crustal blocks of the Tibetan Plateau outwards, forcing it to move eastwards. Meanwhile, the strike-slip motion also results in an east–west extension of the plateau, causing normal faults to break within the thickened crust.
On a continental scale, the seismicity of central and eastern Asia is broadly the result of the northward convergence of the Indian Plate against the Eurasian Plate. The convergence of the two plates is accommodated by the uplift of the Asian highlands and by the motion of crustal material to the east away from the uplifted Tibetan Plateau. In the past 20 years, 25 other earthquakes M5.0+ have occurred within of 5 September 2022, event. Most of this seismicity is related to aftershocks of previous destructive earthquakes in the western margin of the Sichuan Basin. This event is southwest of a cluster of seismicity following an M6.6 earthquake on 20 April 2013 that resulted in 196 fatalities. An M7.9 earthquake on 12 May 2008 occurred near the September event and resulted in over 87,000 fatalities, making it one of the most destructive earthquakes in recent history.
Earthquake.
The earthquake measured 6.8 on the surface-wave magnitude scale (); on the moment magnitude scale (), it measured 6.7. According to the United States Geological Survey, the earthquake was a result of shallow strike-slip faulting on the western margin of the Sichuan Basin. The earthquake occurred on either an NNW–SSE striking, steep WSW dipping left-lateral fault, or a WSW–ENE striking, near vertical dipping right-lateral fault. The location and sense of motion are consistent with movement on or near the left-lateral Xianshuihe fault system, which runs for . Many faults in this region are related to the convergence of crustal material slowly moving from the high Tibetan Plateau to the west, against the strong crust underlying the Sichuan Basin and southeastern China.
Experts from the Sichuan Earthquake Administration said the earthquake occurred near the Moxi Fault, a segment of the southeastern Xianshuihe fault zone. They added that it is unlikely that a larger earthquake will occur near the epicenter, but aftershocks would persist. The Xianshuihe fault zone is a large active left-lateral fault that defines the margin between the Bayan Har and the Sichuan-Yunnan blocks. It produced destructive earthquakes in 1786 (M7.75), 1816 (M7.5), 1850 (7.5), 1893 (M7.0), 1904 (M7.0), 1923 (M7.3), 1948 (7.3), 1955 (M7.5), 1973 (M7.6), 1981 (M6.9) and 2014 (M6.3). No M7.0+ earthquakes have been recorded along the Moxi Fault since 1786. It runs north–south between Kangding and Shimian. The fault was considered a potential source for large earthquakes due to the absence of any since 1786.
A length along a NNW–SSE striking fault corresponded to the rupture with a maximum slip of . Most of the seismic moment was released within the first 10 seconds of the rupture initiating, which then propagated to the southwest. At least 2,715 aftershocks were recorded until 12 September, at depths of and distributed along a north–northwest trend. On 22 October, a 5.3 aftershock was recorded near Moxi Town. It had an epicenter away from the mainshock. By 22 October, 6,486 aftershocks had been recorded. A 5.5 aftershock occurred with an epicenter from that of the mainshock's on 26 January.
Using broadband seismic networks, the total rupture duration was calculated at 16 seconds, releasing 4.1×1014 J of energy. An energy magnitude () of 6.8 was determined. As the value is larger than (6.7), the earthquake likely released seismic energy in an efficient manner. It may have resulted in more extreme ground shaking compared to earthquakes with similar values.
Strong ground motion.
Strong ground motion data was recorded at four accelerographs at Hailuogou, Moxi Town, Detuo and Yanzigou. Three accelerographs, namely those in Hailuogou, Moxi Town and Detuo, contributed to the earthquake early warning. The Hailuogou accelerograph which was nearest to the epicenter recorded the highest peak ground acceleration (pga)—0.68 "g" in the north–south component. The pga values at Hailuogou, Moxi Town and Detuo slightly exceeded the limit considered for earthquakes in the area. In addition, the highest peak ground velocity (pgv) was /s in the east–west component, recorded at Moxi Town. The vertical component pgv was at the town /s.
Impact.
The earthquake struck Sichuan province while its COVID-19 lockdown was in effect. It followed a drought and heat wave which had affected water and power supplies due to the province's dependency on hydropower. Two earthquakes in June caused at least four deaths.
Casualties.
At least 93 people were killed, and 424 were injured. A further 24 people remain missing, nine in Luding County and 16 in Shimian County. Provincial officials reported 55 fatalities and 264 injuries in Ganzi Prefecture. Three park employees of Hailuogou National Geological Park were among the dead. The Alpine Ecosystem Observation and Experiment Station of Gongga Mountain, a research facility, partially collapsed, killing a graduate student, and injuring three. Most of the casualties were attributed to collapsing homes while some were buried by landslides as they walked on roads. Some received minor injuries due to rockfalls. Eight people from the county received medical attention at West China Medical Center in Chengdu.
A further 38 people in Shimian County (Ya'an Prefecture) died and 158 were injured; including six fatalities and 11 injuries reported in Caoke; 24 fatalities and 27 injured in Wanggangping; seven fatalities and another seven injured in Xinmin. At Xinmian, 33 were injured. Five people were injured in Hanyuan County. One person was also injured in Liangshan Prefecture. The injured were treated at a hospital. Seventeen days after the earthquake, an injured survivor was located by a resident and rescued.
Damage.
A preliminary evaluation of damage revealed 243 homes collapsed and a further 13,010 were damaged. Two pieces of public infrastructure were destroyed and 142 were damaged. The earthquake also destroyed four hotels and damaged an additional 307. Slope failures caused roads to collapse. Seven small to medium-sized hydropower stations were heavily damaged. Water supply systems were also affected. Many towns and villages were damaged to varying extents. Preliminary reports indicate communication services were down in the towns of Moxi and Yanzigou. At Detuo, landslides severely damaged many homes. Roads were damaged at Lengmoan. Roads were badly damaged so rescuers had to reach the town by boats.
Satellite imagery and a field investigation revealed 4,528 earthquake-triggered landslides over a area. The total area of landslides was . In Luding County, a section of Sichuan Provincial Highway 211 was buried under a landslide while another was obstructed by rockfalls. These roads were cleared by officials immediately. Although there was no damage to dams and hydropower facilities, damaged powerlines affected 43,158 customers. About of optical cables and 289 communication stations were damaged, interrupting communication systems to 35,000 homes. Nine electrical substations remained out of operation and three were damaged. At least 57 electric transmission cables tripped. More than 200 people were trapped at Hailuogou National Geological Park. Thousands were trapped when a landslide blocked the confluence of the Dadu and Wandong rivers, forming a lake. Residents living downstream were evacuated. On the morning of 6 September, the lake overflowed, discharging water at a rate of /s. Twelve hours later, the rate decreased to /s.
The earthquake-produced shaking assigned a maximum seismic intensity (CSIS) of IX over the mountainous northwestern region of the province. It was felt in the towns of Moxi, Yanzigou and Dewei in Luding County for . The maximum CSIS felt in Ya'an Prefecture was VII. CSIS VI was felt over a area by 580,000 people. The earthquake was felt several hundred kilometers away in Changsha, Hunan province, and Xi'an, Shaanxi province. Shaking in Luding was reportedly so intense that people faced difficulties trying to remain standing.
Response.
An earthquake early warning was activated four seconds after the mainshock was initiated. Residents in Kangding, away, were given seven seconds of warning before shaking occurred. At Ya'an and Chengdu, 20 seconds and 50 seconds of warning were given, respectively. These warnings were broadcast through loudspeakers, mobile phones, and televisions. Xi Jinping, the General Secretary of the Chinese Communist Party, asked local officials to "make saving lives the first priority, go all out to rescue people in disaster-stricken areas and minimise loss of life". Li Keqiang, the Premier of the State Council, urged a "swift evaluation of the situation, as well as all-out rescue and medical treatment efforts".
Emergency response.
Emergency management officials from Luding County were dispatched to verify casualties and evaluate the damage. Officials prevented vehicle entry into China National Highway 318 to ensure rescue roadways were unblocked. Railway services including C6633, D1919, and G8792, operated by the China Railway Chengdu Group, were delayed for damage inspections. Affected passengers were refunded.
The Ministry of Emergency Management and the National Committee for Disaster Reduction issued a "Level IV" emergency response to the area. More than 6,500 emergency rescue personnel from Ganzi, Chengdu, Deyang, Leshan, Ya'an, Meishan, and Ziyang were dispatched to the epicenter. In addition, 370 fire trucks and construction vehicles, nine rescue helicopters, three drones, and 31 rescue equipment were deployed.
In Moxi, personnel from the Sichuan Forest Fire Brigade rescued trapped residents from the rubble and transported the injured over a river. At least 50 people who were injured received treatment by medical teams. Emergency personnel pulled over 30 people trapped beneath debris. Rescuers had to dig through the debris by hand as rescue equipment was unable to pass through narrow passageways.
Rescuers pulled three survivors and one body from the rubble of a collapsed hotel in Moxi. Rescuers also located survivors trapped beneath a collapsed bridge in Detuo. All 219 people previously trapped for over 50 hours at Hailuogou National Geological Park were successfully evacuated to Moxi via helicopter.
Firefighters used rubber boats to reach affected villages. At Ziyachang and Wandong villages, 17 people were evacuated. Firefighters were also observing the dammed lake and warned residents to evacuate. Rescuers evacuated over 900 residents in Qinggangping, a village near Moxi which was threatened by rapid-flowing water. Local authorities were concerned over the possibility of more landslides forming pools of water in rivers elsewhere. Officials said that the extent of damage on the Dadu River remains unknown but was being monitored. The river was later discharged but the affected residents had already evacuated.
Wet weather conditions and mudflows have disrupted rescuers' attempts to search for the missing. China's meteorological administration issued a yellow alert warning of the possibility of "geological disasters". Heavy downpours were expected in some areas while moderate precipitation was forecasted for several days. The administration added that "post-earthquake geological conditions are inherently fragile, and the impact of additional rainfall may lead to landslides and mudslides". Aftershocks triggered additional landslides.
Relief efforts.
On 5 September, the Chinese government reserved 50 million yuan (US$7,250,000) for relief efforts. An additional 150 million yuan (US$21,602,010) was provided on 9 September. Officials of Sichuan also allocated 50 million yuan to Ganzi. The Red Cross Society of China provided 1 million yuan (US$143,000), 450 tents, 3,000 relief packages, 2,400 beds, and 1,200 blankets. In addition, more than 150 members of the organization responded to the crisis. Donations from 65 businesses to assist in recovery efforts totaled 1.168 billion yuan (US$167,435,988), according to SASAC. Wuliangye donated 15 million yuan (US$2,200,000) while BYD donated 5 million yuan (US$718,000).
Over 3,600 people and 70 search and rescue dogs from Chongqing, Gansu, Guizhou, Yunnan, Qinghai, and other nearby regions were put on standby for possible interregional efforts. The State Reserve Bureau and Ministry of Emergency Management allocated 3,000 tents, 10,000 quilts, 10,000 beds, and other relief supplies to relocate residents of Luding County.