There are 3 major seismic activities in the world, as shown in figure 2-4.
Circum-Pacific seismic belt
Means the Pacific Ocean, including the west coast of Chile, Peru, Guatemala, Mexico, and the United States in North America, the Aleutian Islands, the Kuril Islands, the Japanese Islands, China's Taiwan, the Ryukyu Islands, as well as the Philippines, Indonesia and New Zealand and other countries and regions. The seismic belt is the most seismically active region, where about 70 percent of the world's earthquakes occur.
Eurasian earthquake
It stretches from the European Mediterranean through Greece, Turkey and Tibet to the Pacific Ocean and the Alps, also known as the Mediterranean-Himalayan seismic zone. This belt is more than 20,000 km across Europe, Asia and Africa, accounting for 15% of global earthquakes. Its seismic distribution is relatively dispersed, unlike the Circum-Pacific seismic belt is so concentrated, regular.
Ocean ridge seismic belt
Along each mid-ocean ridge distribution of the oceans, about 5%. Ten percent of the world's quakes are less regular, it's distributed out of these earthquake zones, a considerable distance from the plate boundary.
China is located between the world's two major seismic belts -- Circum-Pacific seismic belt and the Eurasian seismic belt, compressed by Pacific plate, India Ocean plate and Philippines Sea plate, seismic fault zones are very developed. Since twentieth Century, nearly 800 earthquakes above magnitude 6 have occurred in China, covering all provinces, autonomous regions and municipalities except Guizhou and Zhejiang provinces and Hong Kong special administrative regions. It is one of the most earthquake-prone countries in the world, and also the most serious country affected by the earthquake.
China has high frequency of seismic activity、high intensity、shallow seismic source、 wide distribution, a country with severe earthquake. Since 1900, more than 550,000 people have died in earthquakes in our country, accounts for 53% of global earthquake deaths; more than 100 destructive earthquakes have assaulted 22 provinces (districts, municipalities) since 1949, leading to more than 270,000 people death, accounts for 54% of all disaster deaths nationwide, seismic disaster area reached more than30 km2, building collapsed reached more than700 million. The severity of earthquakes and other natural disasters constitutes one of China's basic national conditions.
China's seismic activities are mainly distributed in 23 earthquake belts in five regions. The 5 regions are：①Taiwan province and its adjacent waters;②The southwest region is mainly Tibet, western Sichuan and midwest of Yunnan;③Northwest region, mainly in Gansu、Hexi corridor、Qinghai、Ningxia、the northern and southern piedmont of Tianshan Mountains; ④North China , mainly on both sides of Taihang mountain、Fen Wei River Valley、Yinshan - Yanshan area、entral Shandong and Bohai bay；⑤Guangdong, Fujian and other places along the southeast coast
There is a north-south geological structural belt in the central part of the Chinese mainland, called the Helan-Sichuan-Yunnan tectonic belt, also known as the north-south seismic belt of China. The so-called north-south earthquake belt, is a densely seismic belt that runs through the mainland of China, roughly in the direction of north and south, from Ningxia through eastern Gansu, western Sichuan, until Yunnan province. The seismic zone has been concerned by the seismological community. Historically, large earthquakes occurred frequently in this seismic zone, many earthquakes above 7 magnitude. The 8.6 magnitude earthquake of Haiyuan, Ningxia, occurred in this earthquake belt in 1921, the number of deaths caused by direct and indirect earthquakes is 240 thousand. The region is prone to occur large earthquakes. The reason of this area is prone to occur major earthquakes, because it lies between two tectonic domains, namely lies between the Pacific tectonic domain and the Qinghai-Tibet tectonic domain. The belt concentrates half of the 8 magnitude earthquake in China's history. At 14:28 on May 12, 2008, a magnitude 8 earthquake occurred in Wenchuan County, Sichuan province.
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