1973 Luhuo Earthquake

The 1973 Luhuo earthquake struck near the town of Zhaggo in the Garzê Tibetan Autonomous Prefecture of Sichuan Province, China on February 6 with a magnitude of 7.6 . The earthquake had a maximum intensity of X (''Extreme'') on the Mercalli intensity scale, resulting in 2,199 deaths and a further 2,743 injuries in Sichuan. Serious and widespread
destruction to villages was reported in Luhuo County as a result of the earthquake.

Tectonic setting

Western Sichuan is situated at the edge of the Tibetan Plateau in a vast zone of complex continental deformation caused by the collision of the Indian Plate with the Eurasian Plate. As the thrusting of the Indian Plate beneath the Eurasian Plate along the Himalayas continues, the continental crust within the Eurasian Plate is actively uplifted and thickened, forming the Tibetan Plateau. As there are no active thrust structures within the plateau, compression is accommodated by strike-slip motion along large structures including the Altyn Tagh Fault, Kunlun Fault, Haiyuan Fault and Xianshuihe fault system. 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 east-west extension of the plateau, causing normal faults to break within the thickened crust.

Earthquake

The Xianshuihe fault system is a 1,400-km-long active left-lateral strike-slip fault that accommodate the strike-slip motion in the Tibetan Plateau. The fault is one of the largest active intracontinental geological structure in the world. Beginning in 1893, at least 350 km of the fault length has ruptured in large successive earthquakes with magnitudes 6.5 or larger. Going back to the year 1700 to present-day, the fault has ruptured its entire 1,400 km length during large earthquakes.

Characteristics

Initially, the earthquake measured 7.9 on the Chinese surface wave magnitude scale but further analysis and recalculation of the event brought the final revision to magnitude 7.6. This event was the result of pure left-lateral slip on the Luhou segment of the Xianshuihe fault system. During the earthquake of 1973, the Xianshuihe fault ruptured for a length of 90 km and produced a maximum slip of 3.8 meters in the shallow 0 to 5 km section of the rupture zone. Near Dandu Township, an agricultural field was offset by 7.2 meters, but further studies have shown it was the accumulation of offsets from past earthquakes as well, such as the 1816 event. Surface ruptures were well documented by scientists throughout the 90 km length extending from Renda to Kasu. The southeastern 10 km segment of the earthquake rupture overlapped that of the 1923 Renda earthquake, another 7.3 earthquake on the Xianshuihe Fault.

The Luhuo segment of the Xianshuihe fault system was also the source of a magnitude 7.5 earthquake in 1816. Field research through trenching at the Luhuo segment however, revealed a record of earthquake history on the fault in the past 3,000 years. The first event identified in the exposed strata layers corresponded to the year 769 BC. Five additional events were also identified to have occurred in the years 318–545 AD, 677–833 AD, and 1008–1444 AD. A recurrence interval has been calculated at between 157 to 1,200 years for the earthquakes from 769 BC to 1973 AD.

A large aftershock of 6.3 followed one day later, likely triggered by coulomb stress transfer due to the mainshock. Geomorphic evidence of the earthquake was well-preserved because the quake occurred in mid-winter.

Impact

The earthquake caused extensive destruction throughout the 90 km of rupture. Almost all the homes near or in the path of the rupture were completely destroyed, amounting to at least 15,700. An estimated 2,867 buildings were also heavily damaged. Across two towns in Luhuo County, 4,600 of the 5,000 homes collapsed while another 880 suffered serious damage. A further 90 sustained cracks. China National Highway 318 or the Sichuan-Tibet highway was damaged by rockfalls, landslides and ground cracks at 17 locations. Other roadsways, bridges and telecommunication systems were damaged or disrupted as well. Some 40,427 livestock died and 2.01 million kilograms of food were lost.

The earthquake also triggered 40 to 50 landslides in the Xianshui River valley. Nearly all recorded landslide features were distributed within a 70 km by 2 km, or 409 km² area within the valley. Field surveys observed 137 landslides all within the zone of VIII or greater.

Casualties

The earthquake claimed the lives of anywhere between 2,175 to 2,204 people in Sichuan Province. It also left 43 orphans in the aftermath.

Response

A division of the People's Liberation Army was sent to the affected area to assist in rescue and recovery efforts. In the division were at least 668 medical crew from 49 groups which attended to injured survivors.

See also

* List of earthquakes in 1973
*List of earthquakes in China
*List of earthquakes in Sichuan

References

External links

*

【记忆中的年】四十八年前的春节
– Images of the earthquake

{{Earthquakes in China
Earthquakes in Sichuan
Earthquakes in China
1973 earthquakes
1973 in China
Garzê Tibetan Autonomous Prefecture