   | |
| Local date | September 12, 1850 (1850-09-12) |
|---|---|
| Magnitude | Mw 7.3–7.9Ms 7.5–7.7 |
| Epicenter | 27°48′N102°18′E / 27.8°N 102.3°E /27.8; 102.3[1] |
| Areas affected | Qing dynasty |
| Max. intensity | MMI XI (Extreme)[2] |
| Casualties | 20,650+ dead |
The1850 Xichang earthquake rockedSichuan Province ofQing China on September 12. The earthquake which caused major damage inXichang county had an estimated moment magnitude of 7.3–7.9 Mw and asurface wave magnitude of 7.5–7.7 Ms . An estimated 20,650 people died.
Sichuan is situated at the edge of theTibetan Plateau in a vast zone of complex continental deformation caused by thecollision of theIndian Plate with theEurasian Plate. As the thrusting of the Indian Plate beneath the Eurasian Plate along theHimalayas continues, the continental crust within the Eurasian Plate is actively uplifted and thickened, forming the Tibetan Plateau. There are no active thrust structures within the plateau, therefore, compression is accommodated bystrike-slip motion along large structures including theAltyn Tagh Fault,Kunlun Fault,Haiyuan Fault andXianshuihe fault system. Left-lateralstrike-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–westextension of the plateau, causingnormal faults to break within the thickened crust.[3][4]
The earthquake occurred as a result of left-lateralstrike-slip faulting at a shallow depth on the highly segmented and complexXianshuihe fault system which extends for 1,400 km (870 mi). Contemporarypaleoseismology studies have identified surface ruptures on the Anninghe and Zemuhe faults; segments of the Xianshuihe fault system.[2] The Anninghe Fault is oriented north–south, and extends 200 km (120 mi), while the Zemuhe Fault, strikes northwest for 120 km (75 mi). These faults have varying slip rates of 3.6–4.0 mm (0.14–0.16 in) per year and 3.0–5.0 mm (0.12–0.20 in) per year, respectively.[5]
Theearthquake rupture attracted the attention of seismologists because the Anninghe and Zemuhe faults ruptured during the same event; where arestraining bend could not stop the rupture propagation. Restraining bends or zones oftranspression have been previously thought to arrest or cease an earthquake rupture. The restraining bend measuring 30° displayedextensional tectonics was not effective and failed to arrest the rupture.[5]
A maximum coseismic slip of 5–6 m (16–20 ft) was measured along a preservedsurface rupture on the Zemuhe Fault.[5][6] Smallthrust faulting offsets of 0.65 m (2 ft 2 in) was also found, although a 2003field survey and study foundfault scarps up to 2 m (6 ft 7 in) high.[2] The associated surface ruptures were well-preserved with visiblefault scarps.[6] Past earthquakes involving the Zemuhe Fault were also recorded in 6000 BC, 4250 BC, and 814 AD in thepaleoseismic records. The recurrence interval of large earthquakes on the fault is 1,400 to 1,700 years, with a minimum of 1,000 years.[2]
Rupture on the southern section of the Anninghe Fault also resulted in a zone of high-intensity shaking assigned VII–VIII. The surface offsets on the Anninghe Fault are much smaller, ranging from 1–1.5 m (3 ft 3 in – 4 ft 11 in). Based on the historical documentation of the event and its associated surface ruptures, the total ruptured length on both faults is at least 150 km (93 mi). Aseismic intensityisoseismic contour of VII–XI on theChina seismic intensity scale was determined along the rupture trace of the Zemuhe Fault, indicating most of the rupture took place on that fault. Calculating themoment magnitude using the rupture length yielded a magnitude of Mw 7.6–7.9 (7.6 ± 0.3).[5] The Anninghe Faultpreviously ruptured in 1536 based on paleoseismic evidence along thefault trace. It had an estimated magnitude of 7.5 and ruptured a 30 km (19 mi) section of the fault. A maximum left-lateral offset of 4 m (13 ft) was estimated.[6]
InXichang, themeizoseismal area, thecity walls were torn down, and threegate towers collapsed. All government buildings, warehouses, temples, and prisons were razed to the ground. Many homes constructed ofthatched and ceramic tiles were destroyed and rubble occupied the streets. At least 27,880 families were made homeless while 20,650 people died. InHuili County, 2,876 residents were killed by collapsing homes. Severe damage occurred in the 14 residential areas in the county.[1] Hundreds died inQiaojia County. Several deaths were also reported inPuge County,Zhaojue County,Yanyuan County, andMianning County.
A massive groundfissure measuring 33 m (108 ft) by 10 m (33 ft), and 17 m (56 ft) deep opened in the city ground. Trees were uprooted and toppled while roads cracked. Several large landslides occurred nearQiong Lake. Ground failures and fissuring led to water erupting out of the ground. Majorliquefaction events such as the ejection of water and sand occurred atBeishan Mountains,Qiong Lake and theAnning River. Villages along the banks of Qiong Lake were flooded.[1]
The destruction left by the earthquake was the third to seriously affect Xichang. The earthquakes in 1536 and 1732 also caused great devastation, killing thousands. The Xichang Earthquake Forest near Xichang was established as a memorial for the victims of these earthquakes. The events were documented andinscripted ontostele monuments.[7]