1988 Lancang–Gengma earthquakes

The 1988 Lancang–Gengma earthquakes, also known as the 11.6 earthquakes by the Chinese media were a pair of devastating seismic events that struck Lancang and Gengma counties, Yunnan, near the border with Shan State, Burma. The earthquake measured moment magnitude 7.0 and was followed 13 minutes later by a 6.9  shock. 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. Both earthquakes resulted in US$270 million (in 1988 dollars) in damage and economic losses. Moderately large aftershocks continued to rock the region, causing additional casualties and damage.

Tectonic setting


The Shan Plateau is crisscrossed by strike-slip faults to accommodate crustal rotation of the Sunda Block and deformation as a result of the India–Asia collision, where the Indian Plate is underthrusted beneath the Eurasian Plate. The Shan Plateau formed by uplift along the Shan Scarp Fault Zone, an inactive shear zone and thrust fault along its western base. Located east of the Shan Scarp Fault is the active Sagaing Fault, a dextral transform fault that separates the Burma Plate from the Sunda Plate. At the northern boundary of the Shan Plateau lies the Red River Fault, an active 1,000 km long dextral fault.

Earthquakes in this part of Southeast Asia (the Shan Plateau) usually display focal mechanisms corresponding to shallow left-lateral (sinistral) and right-lateral (dextral) strike-slip faulting. Shear deformation between the Red River and Sagaing faults causes bookshelf-style faulting which manifests in predominantly dextral faulting within the Shan Plateau. Sinistral systems follow an east–northeast or east–west trend for several hundred kilometers, offsetting the Mekong and Salween rivers. Dextral structures follow a north-west or north-southernly strike. The earthquakes of 1988 were the result of slip along a dextral fault zone.

In 1995, another earthquake measuring 6.8 struck the same area. It left at least 11 people dead, destroyed over 100,000 homes, and damaged an additional 42,000. The earthquake may have been triggered by the transfer of stress from the 1988 events.

Earthquake
The earthquakes originated along the Longlin–Lancang Fault Zone; a northwest–southeast striking, 210 km-long structure. Its northern section is a single strand while the southern section comprises a complex anastomosis of faults. Forming in the early to mid-Miocene, this dextral fault has accumulated 17 km of displacement. Its estimated slip rate is 3.4 mm per year. It was previously a sinistral fault as 30 km of left-lateral offset was observed in a batholith from geological studies.

Right-lateral offsets of 1.4–2 m were measured along the surface rupture of the 7.0 mainshock. The United States Geological Survey catalogued the earthquake at 7.7. A small dip-slip (reverse) component was also measured. These surface ruptures followed a north–northwest strike for about 24 km. The first mainshock ruptured an estimated 53 km–70 km-long by 26 km-km-wide fault zone extending northwest–southeast. The maximum measured dextral surface offset was 2.0 m and vertical offset was 0.5 m. The fault zone ruptured with a velocity estimated at 2 km per second.

Thirteen minutes later, an earthquake with a surface-wave magnitude of 7.2 struck 63 km north–northwest of the first shock. On the moment magnitude scale, it measured 6.9. The rupture area was nearly twice that of the 7.7 mainshock. The event sequence is considered a doublet earthquake.

Right before the first mainshock, a 2.6 foreshock preceded the disaster, it was the only recorded foreshock in the earthquake sequence.

The two events are largest earthquakes to affect both Yunnan Province and Shan State since 1970 and 1912, respectively. In January 1970, a magnitude 7.7 struck Tonghai County, and in May 1912, Shan State was hit with a magnitude 7.8 earthquake.

Aftershocks
A large aftershock measuring 6.1 occurred 24 days after the mainshocks. It caused additional injuries and further damage. By December 20, over 600 aftershocks had been recorded.

Intensity
The first mainshock produced shaking which was assigned a maximum of IX on the China seismic intensity scale. This intensity zone had an axis length of 27 km, with the widest width measuring 8 km, covering an area of 170 km2. It extended northwest–southeast from Mujia to Zhutang Township. Its northeast border lies along the eastern bank of the Heihe River, west of Fubang Township. Landslides, surface ruptures, liquefaction and ground failure were observed.

Intensity VIII encompassed Xuelin Wa Ethnic Township in Mujia Township to the north, and Zhutang Township to the southeast. Within this zone was Ximeng County to the west. This zone had an axis length of 52 km and a width of 20 km, covering an area of 820 km2. Nearly every house in the region collapsed, with very few left intact. The earthquake also caused landslides and ground failures throughout.

Cangyuan County in the north to Shangyun, and Lancang County in the south were within the area of VII. The western boundary also extended into Ximeng County, well beyond the Myanmar–China border, into Shan State. Damage was less extreme although some houses collapsed. Light homes remained intact, and much of the damage to ordinary homes reported include cracking of walls. Landslides, ground cracks and sand boils occurred. This zone covered a large area of 3,680 km2.

Strong to weak shaking was felt in Thailand, Laos, Vietnam and Bangladesh. In Bangkok, Thailand, people on high-rise buildings felt weak shaking. The highest floors of the Charn Issara Tower, a 27 story building, swayed for a few minutes. The evaluated Mercalli intensity in Chiang Mai was VI (Strong), damaging some buildings. In Chiang Rai, there were reports of broken windows.

The second mainshock had a maximum intensity of X. The zone included Gengma County to the north, and Yanshuai towards the south, Tuanjie Township, Cangyuan County to the east, and Mengsheng Township, Cangyuan County in the west. Many houses totally collapsed or were severely compromised in this zone. Many buildings were also seriously damaged. Lightly constructed homes collapsed or tilted due to damage resulting from the rupture. Liquefaction and fissures ejecting water were observed. This zone is a north-west ellipse with a major axis of 25 km and a minor axis of 8 km, covering an area of 160 km2.

The zone of intensity VIII extended from Gengma in the north to Yanshuai and Nuoliang Township in the south. Few homes remained standing and lightweight houses tilted. Damage to the ground was identical to those observed in the zone of IX. This zone followed a north-west ellipse area of 940 km2.

Aiguo New Village, Gengxuan Town, Gengma County in the north, to south of the Shuangjiang River in the east, to south of Lancang Shangyun in the south, and west to Minliang of Menglai Township, Cangyuan County in the west were within the intensity VII zone. Adobe houses totally collapsed as a result. Intensity VII was felt for an area of 3,020 km2. Intensity VI from the second shock covered an area of 32,700 km2.

Aftermath
Early figures of fatalities were 18 and 37, which could not be revised due to the communication disruptions. Over the next few days, international media reported at least 600 inhabitants killed, mainly in the village of Shanmato which was obliterated. Telecommunication services were cut and severely disrupted. This meant provincial government officials could not provide updated figures on the dead.

On November 9, the death toll totaled 938. Chinese officials stated 748 people died; at least 7,700 people seriously injured; over 3 million affected and 267,000 homeless. They added that the death toll could have been higher had the earthquakes struck near a city. When the earthquakes struck, many residents were outdoors, which factored in the unexpectedly low death toll. A fatality figure of 938 was given to the United Nations and became widely reported. The reported death toll by the International Disasters Data Base was 939. It was regarded as China's "worst earthquake in more than a decade". At the time of the earthquakes, the population of Lancang and Gengma counties were 237,000 and 81,000, respectively. In Lancang County, at least 607 people died including 279 in Zhanmapo.

Seventeen counties were heavily damaged. Many homes, roads and communication lines in Lancang and Mengliang counties were destroyed. An estimated 200,000 buildings including 144,000 houses were razed. Over 1.308 million rooms collapsed and 934,800 were damaged. At least 500,000 buildings including 253,000 homes were badly damaged. More than 4,000 essential facilities in Yunnan were damaged. About 1,000 schools, 98 clinics and 29 reservoirs were destroyed. Landslides in the area also damaged highways. The majority of homes constructed of wood and mud, collapsed due to the extreme ground motions, killing its inhabitants. Over 1,000,000 m3 of rockslides damaged highways and blocked rivers, halting water transportation. Kunming, the capital of Yunnan Province, was undamaged, although the earthquakes were felt strongly. More damage was reported in Chiang Rai, Thailand. The total cost of damage was estimated at US$ 269 million (US$ million in ).

Many buildings generally fared well during the earthquakes because of improved construction practices and seismic retrofitting works prior. Buildings which had survived were built with the consideration of the local seismic hazard. Some buildings including a cinema which was designed to withstand seismic intensity VIII and a residential block made to resist intensity IX were undamaged. These buildings were strengthened before the earthquakes.

Response
Due to its remote location and the lack of communication and damaged roads, rescue and aid transportation efforts faced difficulties getting to the affected areas. Rescus and medical personnel only reached the affected zone on 9 November. The Yunnan government ordered an airlift of medical and relief supplies to help those affected. The governor of Yunnan Province, He Zhiqiang, along with several medical doctors were brought to the disaster scene. Several thousand troops and many military vehicles also visited the affected areas, according to the Ministry of Civil Affairs.

Much information about the earthquakes and its devastation were hidden by the Chinese government as the country was going through major political and cultural revolutions at the time. In the early decades of China's communist regime, the guideline was that natural disasters and accidents would not be disclosed unless foreign nationals were involved. It was only in the few years prior to 1988 were these events publicized. At a press conference following the earthquakes, officials disclosed that a magnitude 7.7 event in 1970 resulted in 10,000 deaths. Chinese officials called for international assistance in response to possible aftershocks. The World Food Programme and United Nations Development Programme said the nation encouraged international groups and governments to provide aid, and that plastic sheets for shelter, medicine and food were the most important items.