1988 Lancang–Gengma earthquakes

The 1988 Lancang–Gengma earthquakes (simplified Chinese: 澜沧江-耿马地震; traditional Chinese: 瀾滄江-耿馬地震; pinyin: Láncāngjiāng-gěng mǎ dìzhèn), 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.[1] The first earthquake measured magnitude 7.0–7.7 and was followed 13 minutes later by a magnitude 6.8–6.9 shock.[2] Both earthquakes were assigned a maximum China seismic intensity of IX and X, respectively. An estimated 748–939 people were killed and more than 7,700 were injured in Yunnan. Both earthquakes resulted in $US 270 million (in 1988 dollars) in damages and economic losses. Moderately large aftershocks continued to rock the region, causing additional casualties and damage.

1988 Lancang–Gengma earthquakes (澜沧江-耿马地震)
epicenter.
epicenter.
1988 Lancang–Gengma earthquakes (China)
UTC time1988-11-06 13:03:19
 1988-11-06 13:15:43
ISC event419867
 419869
USGS-ANSSComCat
 ComCat
Local date6 November 1988
Local time21:03
 21:15
MagnitudeMw 7.0–7.7, Ms 7.3–7.6
 Mw 6.8–6.9, Ms 7.2
Depth17.8 km (11.1 mi)
 15.0 km (9.3 mi)
EpicenterChina–Myanmar border region
22°47′20″N 99°36′40″E / 22.789°N 99.611°E / 22.789; 99.611Coordinates: 22°47′20″N 99°36′40″E / 22.789°N 99.611°E / 22.789; 99.611
TypeStrike-slip
Areas affectedChina, Myanmar and Thailand
Total damageUS$ 270 million in 1988
Max. intensityCSIS IX (1st shock)

CSIS X (2nd shock)
LandslidesYes
ForeshocksYes
Aftershocks600+
Casualties748–939 fatalities, 7,700+ injured

Tectonic settingEdit

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.[3][4] 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 kilometres (620 mi) long dextral fault.[5]

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.[6][7] Bookshelf-style faulting as a result of shear deformation between the Red River and Sagaing faults have resulted in predominantly right-lateral strike-slip faulting within the Shan Plateau.[8] Sinistral systems follow an east-north-east or east-west trend for hundreds of kilometers, causing massive offsets in the Mekong and Salween rivers. On the other hand, dextral structures run along a more north-west or north-southernly strike. The earthquakes of 1988 were a result of slip along one of these dextral fault zones.[7]

Seven years later, the another earthquake measuring Mw 6.8 struck the same area.[9] That event left at least 11 people dead, destroyed over 100,000 homes, and damaged an additional 42,000.[10] It is believed to have been triggered by the transfer of stress from the 1988 events.[8]

EarthquakeEdit

 
 
1923
 
1941
 
1988
Major earthquakes (M7.0+) in the Yunnan–Shan border region

The earthquakes originated along the Longlin–Lancang Fault Zone, a northwest–southeast striking, 210 kilometres (130 mi) long fault structure. Its northern section is a single strand, unlike the southern section consisting of a complex set of clustered faults. Forming in the early to middle Miocene, the fault has a right-lateral (dextral) sense of slip with a maximum displacement of 17 kilometres (11 mi). The estimated slip rate is 3.4 mm/yr.[11] The fault may have been a left-lateral (sinistral) structure due to old offsets of batholith in the area.[12]

Right-lateral offset of 1.4 to 2 metres (4 ft 7 in to 6 ft 7 in) was measured along the surface rupture for the 7.0–7.7 mainshock. A very small dip-slip (reverse) component was also measured. Surface ruptures followed a north–northwest strike for about 24 kilometres (15 mi). Seismologists estimated the first mainshock ruptured a 53 kilometres (33 mi)–70 kilometres (43 mi)-long by 26 kilometres (16 mi)-km-wide fault zone extending northwest and southeast, in the process, creating new faults.[11] The measured maximum dextral surface offset was 2.0 metres (6 ft 7 in) and the vertical offset was 0.5 metres (1 ft 8 in).[12][13] The fault zone ruptured at a velocity estimated at 2 km/s.[11]

Thirteen minutes later, a second mainshock with a surface-wave magnitude of 7.2 struck 63 kilometres (39 mi) north–northwest of the initial shock.[14] The rupture area was nearly twice that of the 7.7 mainshock. The event sequence is considered a doublet earthquake.[15][11]

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,[16] and in May 1912, Shan State was hit with a magnitude 7.8 earthquake.[17]

Earthquake sequenceEdit

Previous seismicity
Date Time (UTC) Magnitude Depth (km) Ref.
1987-11-25 13:09:47 mb 4.6 33.0 [18]
1988-08-14 17:50:52 mb 4.7 [19]
Mainshocks
Date Time (UTC) Magnitude Depth (km) Ref.
1988-11-06 13:03:19 Mw  7.0–7.7
Ms  7.6
17.8 [20][14][2]
13:15:43 Mw  6.8–6.9
Ms  7.2
10.0 [21][14][2]

AftershocksEdit

A large aftershock measuring Mw 6.1 occurred 24 days after the two mainshocks.[14] It caused additional injuries and further damaged the already crippled region. By December 20, over 600 aftershocks had been recorded.

Aftershocks magnitude 5.0 or greater
Date Time (UTC) Magnitude Depth (km) Notes Ref.
1988-11-06 13:21:04 mb 5.8 10.0 [22]
13:39:48 mb 5.0 [23]
14:13:24 mb 5.1 [24]
16:00:32 mb 5.1 [25]
20:24:24 mb 5.4 [26]
1988-11-07 02:39:56 Mw 5.2 [27]
1988-11-15 10:28:14 Mw 5.3 17.7 [28]
1988-11-18 18:21:44 mb 5.0 10.0 [29]
1988-11-19 01:37:14 mb 5.0 33.0 [30]
1988-11-27 04:17:56 Mw 5.5 15.7 One person injured and some damage.[31] [31]
1988-11-30 08:13:29 Mw 6.1 14.5 Further damage in the affected region. Several people injured. Felt in Kumming.[32][33] [32]
1988-12-19 11:06:57 mb 5.0 33.0 [34]
1988-12-22 03:49:46 mb 5.0 10.0 [35]
1989-05-07 00:38:18 Mw 5.6 33.0 One person died and 91 injured. At least 5,300 homes were damaged in Gengma County. Damaged totaled USD 54 million.[36] [37]
1989-06-04 15:07:46 mb 5.0 10.0 [38]

IntensityEdit

 
USGS shake map for the earthquake.

The first mainshock produced violent shaking assigned IX on the China seismic intensity scale.[39][40] This intensity zone had an axis length of 27 kilometres (17 mi), with the widest width measuring 8 kilometres (5.0 mi), 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.[15]

 
Zhongke Township in Ximeng County, close to the epicenter of the first mainshock. Photo taken in 2013.

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 kilometres (32 mi) and a width of 20 kilometres (12 mi), 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.[15]

Cangyuan Va Autonomous 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.[15]

Strong to weak shaking was felt in Thailand, Laos, Vietnam and Bangladesh. In Bangkok, Thailand, people on high-rise buildings felt weak shaking. The evaluated Mercalli intensity in Chiang Mai was VI (Strong), damaging some buildings.[41]

The second shock measured 7.2 on the surface-wave magnitude scale and 6.8–6.9 on the moment magnitude scale.[2][14] It had a maximum intensity of X.[40][15] 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 kilometres (16 mi) and a minor axis of 8 kilometres (5.0 mi), covering an area of 160 km2.[15]

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 the same as seen in the zone of IX. This zone followed a north-west ellipse area of 940 km2.[15]

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.[15]

AftermathEdit

Heavy damage was reported in 17 counties, including Lancang County. Many homes, roads and communication lines in Lancang and Mengliang were destroyed.[33] An estimated 200,000 buildings including 144,000 houses were obliterated. Over 1.308 million rooms collapsed and 934,800 were damaged.[42] At least 500,000 buildings including 253,000 homes were badly damaged.[33] 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.[43] 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 is estimated to be US$ 269 million in 1988 dollars (US$616 million in 2021).

Many buildings generally fared well during the earthquake because of better construction practices and seismic retrofitting works conducted 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 disaster.[42]

Initial reports from international media stated that at least 600 inhabitants had been 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.[43] Much earlier figures of fatalities were 18 and 37, which could not be revised due to the communication disruptions.

On November 9, the death toll totaled 938.[44] Chinese officials stated that there were only 748 people killed; at least 7,700 people seriously injured; more than 3 million people affected and 267,000 were homeless.[15] 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. The figure of 938 was given to the United Nations and became widely reported.[16] The reported death toll by the Emergency Events Database was 939.[45]

ResponseEdit

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.[46] 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.[43] Several thousand troops and many military vehicles also visited the affected areas, according to the Ministry of Civil Affairs.[47] 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.[48] 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.[16] 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.[16]

See alsoEdit

ReferencesEdit

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External linksEdit