On 6 November 1988, two earthquakes struck Lancang and Gengma counties, Yunnan, near the China–Myanmar border. The first earthquake measured moment magnitude () 7.0, occurring at 21:03 CST. Twelve minutes later, a earthquake occurred about to north–northwest. They were caused by movement along the Lancang–Gengma Fault Zone, a long right-lateral strike-slip fault. Both earthquakes generated oblique-normal surface ruptures with larger displacements reported following the second event.
These earthquakes were assigned a maximum China seismic intensity of IX and X, respectively. Between 748 and 939 people were killed; more than 7,700 were injured, and damage was estimated at CN¥ 2.05 billion. An additional 267,000 people were left homeless. Moderately large aftershocks continued to rock the region, causing additional casualties and damage. During initial recovery efforts, supplies had to be airlifted because landslides blocked roads to the affected areas. Several thousand soldiers participated in providing relief. The Chinese government made an appeal for international assistance.
Tectonic setting
Situated between the Sagaing and Red River faults, the Shan Plateau contains a network of left- and right-lateral strike-slip faults that accommodate crustal rotation and extrusion of the Sunda Block and deformation caused by the India–Asia continental collision. This tectonically active zone extends across China, Myanmar, Laos, Thailand, and Vietnam, measuring long and wide. The regional tectonics are driven by the relative motion between two major right-lateral structures; the Sagaing Fault, a transform boundary between the Burma and Sunda plates, and the Red River Fault of southwest China. Movement between the two major structures divides the crust between them into clockwise-rotating blocks, forming a "bookshelf faulting" zone. Left-lateral faults run roughly perpendicular to the strike of the Sagaing and Red River faults, along the boundaries of these blocks. Meanwhile, right-lateral structures within these blocks occur along pre-existing fractures with strikes that are parallel to the two dominant faults.
The left-lateral fault systems are dominant; following an east–northeast or east–west trend for several hundred kilometers. The longest runs for about in the southern plateau. These faults exhibit obvious left-lateral displacements in rivers such as the Mekong and Salween and their tributaries. Some of these faults produce "hairpin" shape offsets suggesting the faults may have been left-lateral before reversing slip about 5 to 20 million years ago. The right-lateral structures are less dominant, and follow a north- to north-northwest or northwest strike. They can be divided into two systems; the western faults run subparallel to the Sagaing Fault while the northeast runs subparallel to the Red River Fault. The longest of these are the Wuliang Shan Fault Zone located southwest of the Red River and the Kyaukkyan Fault east of the Sagaing Fault. The earthquakes of 1988 were the result of slip along a dextral fault zone. Consequently, many earthquakes in the region such as in 1912, 1913, 1923, 1925, 1935, 1941, 1950, 1970, 1971, 1976, 1983, 1988, 1995, 2007 and 2011 are caused by strike-slip faulting on one of these fault zones.
Earthquakes
The first earthquake measuring occurred at 21:03 CST on 6 November with an epicenter east–southeast of Zhutang in Lancang County, near the Myanmar–China border. Twelve minutes later, at 21:15 CST, a earthquake struck north–northwest of the first. The mainshock was caused by right-lateral strike-slip faulting at a shallow depth.
These earthquakes originated along the Lancang Fault Zone (also known as the Lancang–Gengma Fault Zone), a long right-lateral strike-slip fault trending north–northwest. The northern segment comprise a single strand while southern section features a complex network of individual branches. A 1997 geologic map showed that the fault initially displaced a batholith left-laterally while right-lateral geomorphic features indicate its slip direction recently reversed. The total right-lateral displacement in the Nanguo River valley is estimated at . An average slip rate of about per year was derived by assuming right-lateral slip initiated 5 million years ago; near the upper-bound rate derived from geodetic data ( per year).
The Lancang–Gengma Fault Zone comprise two north and south segments; the Gengma–Hanmuba and Lancang–Nanliu faults. The Muga–Heihe and Xiaoheijiang faults intersect these segments to form geometric seismic barriers. The first event initiated on the Lancang–Nanliu Fault where it intersects the northwest-trending Muga–Heihe Fault, rupturing bilaterally, northwest and southeast, for about 19 seconds at a velocity of per second. The second earthquake, likely triggered by the first, occurred northwest. A seismic rupture barrier between both segments may have stopped the northwest-propagating rupture of the first event, but was later breached, causing the second earthquake on the Gengma–Hanmuba Fault. This event initiated at the intersection with the east–west trending Xiaoheijiang Fault.
Surface rupture The first earthquake generated a surface-rupture zone about long and across. It comprised four generally continuous linear zones of extensional ground fractures and fault scarps. Each individual strand were several hundred metres to long. At the central rupture zone, along the western bank of the Heihe River, three northwest trending left-stepped fault scarps, each more than a hundred meters long, cut across the Carboniferous shale and Quaternary alluvium of a small ridge. A maximum vertical offset of was measured on one of these scarps while the greatest right-lateral offset () was measured using deformed tree roots. In the southern segment, at Mount Laotanshan, a long, wide and deep fissure formed within limestone.
The second earthquake was associated with a separate long rupture. A long rupture developed on the eastern flank of Mount Meilengshan. On the fault scarp, a maximum throw of and right-lateral slip of was measured. Along the northern Xiaoheijiang River bank, on the southeastern ridge of Mount Meilengshan, the fault cuts through limestone, displacing the western block upwards and eastern block downwards. At the top of the ridge, a tree was severed and displaced horizontally and vertically.
Aftershocks By 20 December, over 600 aftershocks greater than were recorded by the Kunming Telemetered Seismic Network. Three of the largest had of 6.1, 6.3 and 6.7 on 15, 27 and 30 November, respectively. The aftershock distribution aligned with the north–northwest trending fault responsible for the earthquakes. Some caused additional damage and injuries; a aftershock on 7 May 1989 killed 1, injured 91 others, and caused damage totaling US$54 million.
Intensity
A combined isoseismal map for both earthquakes was published in Acta Seismologica Sinica in 1992. The seismic intensity zones were generally elliptical-shaped and followed a northwest–southeast trend. A maximum intensity of IX was assigned to the first event for liquefaction along the Heihe River, while wood and bamboo walls, and subterranean buildings, collapsed. This zone was long and wide. The intensity VIII region encompassed the meizoseismal area; many adobe and wood-framed buildings here were damaged but not destroyed. Damage to a concrete frame cinema; cracked gable and displaced trusses were consistent with intensity VIII.
The epicenter area ( by ) of the second earthquake was assigned intensity X. Rockfalls occurred on the eastern slopes of Mount Meilengshan. Two landslides occurred along the Xiaoheijiang Fault; one by and the other by . Cliffs along the Xiaoheijiang River also collapsed. The villages and towns of Mangwong, Mengsheng, Yanglong, Tuanjie, Yanshai and Bangduo was assigned intensity IX, where brick and reinforced concrete buildings collapsed.
Intensity VII effects were reported in Langcang, Cangyuan, Shangyun, Dalaba, and Shuangjiang, and parts of Myanmar. Damage in this zone was limited to fallen tiles, dislodged plaster and small cracks in walls. Cracks also appeared in adobe buildings. Ground motion in Chiang Mai, Chiang Rai and tall buildings in Bangkok was assigned V–VI (Moderate–Strong) on the Modified Mercalli intensity scale.
Damage and casualties On 8 November, China's State Seismology Bureau placed the number of dead at 18 while Xinhua reported 37 deaths. The Peoples’ Central Broadcasting Station and Radio Beijing, citing a reporter, said about 600 people were killed or injured, with many from the village of Shanmato which was obliterated, though this initial report was not independently verified. At the time of the earthquakes, the population of Lancang and Gengma counties were 237,000 and 81,000, respectively. The civil affairs ministry's Disaster Relief Office said poor communication network made assessing the casualties difficult.
On 9 November, officials placed the death toll at 938. However the state-run evening news said 600 people died and 578 were injured while its English-language counterpart put the toll at 900. Later, Chinese officials lowered the number of dead to 730, rescinding the higher figure. They said the higher figure was not officially announced but United Nations agencies obtained it via the civil affairs ministry and it was widely reported. The International Disasters Data Base listed 939 deaths. It was regarded as China's "worst earthquake in more than a decade".
The Yunnan Provincial Earthquake Bureau said 748 people died; at least 7,700 people seriously injured; over 3 million affected and 267,000 homeless. The death toll could have been higher had these earthquakes struck near a city; additionally, many residents were outdoors at the time, which prevented more deaths. In Lancang County, at least 607 people died including 279 deaths in the town of Zhanmapo.
Heavy damage and causalties were reported in 17 counties; many homes, roads and communication lines in Lancang and Mengliang counties were destroyed. An estimated 200,000 buildings including 144,000 houses were destroyed; another 934,000 buildings were damaged. About 1,000 schools, 98 clinics and 29 reservoirs were also destroyed. Many homes constructed of wood and mud, collapsed, killing their inhabitants. Rockslides with a total accumulated volume of damaged highways and blocked rivers, halting water transportation. The total cost of damage was estimated at CN¥ 2.05 billion.
Some buildings did not suffer heavy damage because of improved designs or seismic retrofitting. Among them, a cinema and an apartment which were strengthened to survive intensity VIII–IX conditions were undamaged. In Thailand, damage was reported in Chiang Rai. In Bangkok, people on high-rise buildings felt weak shaking. The highest floors of the 27-story Charn Issara Tower swayed for a few minutes. The earthquakes were also felt in Laos, Vietnam and Bangladesh.
Response
The Yunnan government ordered medical and relief supplies be airlifted to the affected areas as landslides blocked off roads. The governor of Yunnan Province, He Zhiqiang, along with several medical doctors travelled to the region. Military rescue and medical personnel reached the affected villages on 8 November. The convoy comprised 30 military trucks which joined several thousand soldiers who were already in the region assisting those affected. Chinese officials appealed for international assistance, with the World Food Programme and United Nations Development Programme saying that authorities were open to aid. These agencies also said that plastic sheets for shelter, medicine and food were needed.