1969 Yangjiang earthquake
Encyclopedia
The 1969 Yangjiang earthquake occurred on 26 July, at 6:49 am, Beijing local time (25 July 22:49 UTC
). It had a magnitude of 6.4 on the Moment magnitude scale
and a maximum perceived intensity of VIII (destructive) on the Mercalli intensity scale
It struck the city of Yangjiang
and caused over 10,700 houses to collapse, severely damaging a further 36,000. The earthquake also caused landslides and sandblows were observed along the coast and along some rivers in the area of Yangjiang. The earthquake was also felt in Hong Kong. There were an unconfirmed total of 3,000 deaths.
lies in a tectonically stable part of China, which has relatively low seismicity. The area forms part of the passive margin
between the continental crust of China and the oceanic crust of the northern part of the South China Sea
. Within the stable South China Block, there are three zones of higher earthquake activity, one of which is the Southeast China Coast seismic zone running through Guangdong and Fujian
Provinces. This zone follows the South China Maritime Fold Belt, which was formed by a subduction
event during the Late Jurassic
to Early Cretaceous
epochs. The area is currently in a compressive stress regime with the maximum horizontal stress orientated northwest-southeast. This results from a combination of the continuing collision
between the Indian and Eurasian Plate
s and the subduction of the Philippine Sea Plate.
nearly east-west. The focal mechanism
shows mainly dextral (right lateral) strike-slip faulting. The magnitude and number of the recorded foreshock
s and aftershock
s were unusually small, considering the size of the mainshock. This earthquake is the only damaging event in the epicentral area in historical records.
about 1,200 houses suffered some damage and several were destroyed. The earthquake was also strongly felt in Fengkai
, Huaiji
, Xinzhong, Deqing
, Zhaoqing
, Yunan
, Zengcheng
, Gaohe and Panyu, although there was no serious damage in these areas. Minor damage was also reported in Hong Kong. A report of 3,000 casualties caused by this earthquake is described as "unconfirmed".
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...
). It had a magnitude of 6.4 on the Moment magnitude scale
Moment magnitude scale
The moment magnitude scale is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of...
and a maximum perceived intensity of VIII (destructive) on the Mercalli intensity scale
Mercalli intensity scale
The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude M_w usually reported for an earthquake , which is a measure of the energy released...
It struck the city of Yangjiang
Yangjiang
Yangjiang , historically known as Yeungkong, is a prefecture-level city in southwestern Guangdong province, People's Republic of China. It borders Maoming to the west, Yunfu to the north, Jiangmen to the east, and looks out to the South China Sea to the south. It is famous for being the base of...
and caused over 10,700 houses to collapse, severely damaging a further 36,000. The earthquake also caused landslides and sandblows were observed along the coast and along some rivers in the area of Yangjiang. The earthquake was also felt in Hong Kong. There were an unconfirmed total of 3,000 deaths.
Tectonic setting
Guangdong ProvinceGuangdong
Guangdong is a province on the South China Sea coast of the People's Republic of China. The province was previously often written with the alternative English name Kwangtung Province...
lies in a tectonically stable part of China, which has relatively low seismicity. The area forms part of the passive margin
Passive margin
A passive margin is the transition between oceanic and continental crust which is not an active plate margin. It is constructed by sedimentation above an ancient rift, now marked by transitional crust. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic...
between the continental crust of China and the oceanic crust of the northern part of the South China Sea
South China Sea
The South China Sea is a marginal sea that is part of the Pacific Ocean, encompassing an area from the Singapore and Malacca Straits to the Strait of Taiwan of around...
. Within the stable South China Block, there are three zones of higher earthquake activity, one of which is the Southeast China Coast seismic zone running through Guangdong and Fujian
Fujian
' , formerly romanised as Fukien or Huguing or Foukien, is a province on the southeast coast of mainland China. Fujian is bordered by Zhejiang to the north, Jiangxi to the west, and Guangdong to the south. Taiwan lies to the east, across the Taiwan Strait...
Provinces. This zone follows the South China Maritime Fold Belt, which was formed by a subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
event during the Late Jurassic
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...
to Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
epochs. The area is currently in a compressive stress regime with the maximum horizontal stress orientated northwest-southeast. This results from a combination of the continuing collision
Continental collision
Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together...
between the Indian and Eurasian Plate
Eurasian Plate
The Eurasian Plate is a tectonic plate which includes most of the continent of Eurasia , with the notable exceptions of the Indian subcontinent, the Arabian subcontinent, and the area east of the Chersky Range in East Siberia...
s and the subduction of the Philippine Sea Plate.
Earthquake
The earthquake occurred on a steeply-dipping fault strikingStrike and dip
Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...
nearly east-west. The focal mechanism
Focal mechanism
The focal mechanism of an earthquake describes the inelastic deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is also known as a fault-plane solution...
shows mainly dextral (right lateral) strike-slip faulting. The magnitude and number of the recorded foreshock
Foreshock
A foreshock is an earthquake that occurs before a larger seismic event and is related to it in both time and space. The designation of an earthquake as foreshock, mainshock or aftershock is only possible after the event....
s and aftershock
Aftershock
An aftershock is a smaller earthquake that occurs after a previous large earthquake, in the same area of the main shock. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock...
s were unusually small, considering the size of the mainshock. This earthquake is the only damaging event in the epicentral area in historical records.
Damage
The epicentral area lay mostly in Yangjiang County, affecting an area measuring 19 km by 10 km, elongated west to east. In the whole county, a total of 10,762 houses were completely destroyed, a further 35,965 were severely damaged and another 90,840 suffered some damage. In XinyiXinyi, Guangdong
Xinyi is a county-level city in Guangdong province, People's Republic of China. It has a population of 170,368 people. Xinyi is located in the prefecture level city of Maoming.-External links:*...
about 1,200 houses suffered some damage and several were destroyed. The earthquake was also strongly felt in Fengkai
Fengkai County
Fengkai County is a county of Guangdong Province, China. It is under the administration of Zhaoqing city....
, Huaiji
Huaiji County
Huaiji County is a county of Guangdong Province, China. It is under the administration of Zhaoqing city....
, Xinzhong, Deqing
Deqing County, Guangdong
Deqing County, Guangdong is a county of Guangdong Province, China. It is under the administration of Zhaoqing city....
, Zhaoqing
Zhaoqing
Zhaoqing is a prefecture-level city of Guangdong province in southern China.-Geography:Zhaoqing is located 110 km northwest of Guangzhou, in the west Pearl River Delta. It lies on the north shores of the Xijiang River, which is flows from west to east, and opposite of Gaoyao...
, Yunan
Yunan County
Yunan County is a county of Guangdong Province, China. It is under the administration of Yunfu city....
, Zengcheng
Zengcheng
Zengcheng , historically known as Tsengshing , is a County-level city in Guangzhou, Guangdong, China.It is known for the production of blue jeans. In June 2011 it was the scene of a riot of migrant workers.-External links:*...
, Gaohe and Panyu, although there was no serious damage in these areas. Minor damage was also reported in Hong Kong. A report of 3,000 casualties caused by this earthquake is described as "unconfirmed".