1668 Shandong earthquake

A major earthquake occurred during the rule of the Qing dynasty in Shandong Province on July 25, 1668. It had an estimated magnitude of 8.5, making it the largest historical earthquake in East China, and one of the largest to occur on land. An estimated 43,000 to 50,000 people were killed, and its effects were widely felt. Its epicenter may have been located between Ju and Tancheng counties, northeast of the prefecture-level city of Linyi in southern Shandong.

Geology
The earthquake occurred halfway between Beijing and Shanghai, where seismic activity is infrequent. There had not been any major earthquakes in the area for over 150 years. Within historical times, only seven earthquakes have occurred with estimated magnitudes greater than 6.0. The first recorded earthquake occurred northwest of Zhucheng in 70 BCE, estimated at 7.0 or greater. The most recent destructive earthquake occurred on 19 November 1829, measuring 6.75 near Yidu and Linqu. These major earthquakes occurred along the Tan-Lu Fault Zone.

The Tan-Lu Fault Zone is eastern China's most geologically significant fault—a north-northeast-south-southwest trending dextral strike-slip fault zone. The fault extends 2,400 km from Wuxue near the Yangtze River bank, through the Bohai Sea, to Zhaoxing, Heilongjiang. It has evolved multiple times during its history; from the late Triassic to middle Cretaceous, it was a sinistral strike-slip fault zone which produced offsets ranging from 200–740 km. During the late Cretaceous, the fault zone became an area of extensional tectonics, producing rift grabens and collecting sediments up to 10 km thick in some areas. During the Paleogene, rifting ceased and the fault zone evolved into a dextral strike-slip fault by the late Eocene. This was in response to the change in tectonics brought on by the India–Asia collision and subduction along the west Pacific.

Earthquake
The earthquake rupture occurred along the 360 km-long Yishu Fault, a segment of the Tan-Lu Fault Zone. The Tan-Lu Fault Zone formed during the Mesozoic. Its slip rate has been estimated at less than 1–2.6 mm per year. During the earthquake, the Yishu Fault produced an average offset of 9 m. The slip sense was mainly dextral strike-slip with a small thrust component. Seismic inversion suggest a 160 x 32 km rupture area on a near-vertical, north–south striking fault. A hypocenter depth of between 22 km and 28 km has been suggested. Three aftershocks occurred on 26 July and 18 September 1688; estimated at 6.25 and 6.0, respectively. Another aftershock occurred in 1672 estimated at 6.0.

A paleoseismic study of the fault zone in 1987 suggest the same segment was the source of a similarly-sized earthquake in 6280 BCE. The same fault may have also produced a 1975 earthquake in Haicheng, 700 km north of this event. Another destructive earthquake in 1969 was also produced along the Tan-Lu Fault Zone.

Impact
The earthquake was felt in 379 counties, 29 of which experienced catastrophic damage. It also affected Jiangsu, Anhui, Zhejiang, Fujian, Jiangxi, Hubei, Henan, Hebei, Shanxi, Shaanxi, Liaoning, and Korea. There was a 1,000 km radial zone of damage around Tancheng, Linyi and Ju County. It is considered one of the most destructive in Chinese history. The earthquake produced strong shaking assigned XII (Extreme) on the Modified Mercalli intensity scale, the most destructive shaking an earthquake could achieve. Seismic intensity VIII was over 16,800 km2 corresponding to an elliptical-shaped area along the fault zone.

In Ju County alone, more than 20,000 people were killed. Residential and official homes were destroyed. Schools, temples, warehouses and the city walls toppled. In Mashi, Wulugu, Yanjiagu, Shifengdo, Keluodo and Maqi, landslides occurred on the hills. Widespread land subsidence and collapse occurred. Fissures up to 1 m wide and hundreds of meters long were observed. One fissure measured 7.5 km from Guanzhuang to Gehu along a river cliff. It ejected dust, sand and water. At three wells, water was ejected 1 m into the air.

In Tancheng, battlements, government buildings, homes, a watchtower, temples and storehouses were completely destroyed. Over 8,700 people died. Fissures were reportedly so wide that people were unable to walk over it. The bottom of these fissures were also too deep to be seen. Water erupted from the ground to a height of 10 m. At Lizhuang, a town in the county, massive subsidence occurred.

In Linyi, no homes, city walls and temples were left intact. There were over 6,900 reported fatalities. Black water was said to emerge from fissures. Water erupted from wells and formed a pool measuring 15–20 m wide. Many nearby cities walls fell, and some parts flooded by overflowing rivers and wells. Fissures caused water and sand to erupt, burying homes. Many livestock also died. Heavy damage occurred in Ganyu.

Tsunami
Historical records also documented a probable tsunami in the region. It was reported that coastal cities were flooded and rivers overflowed.

Response
The Kangxi Emperor ordered his ministry to handle the relief efforts. In 40 prefectures and counties, tax fees were waived. Over 227,300 taels of silver were issued.