Seismic activity refers to a range of time and space, the
earthquake occurrence of distribution rules and characteristics
of intensity, frequency, time and space.
The spatial distribution of global seismic activity is regular，it shows that the occurrence of earthquake is closely related to
geological structure. There are three seismic zones in the world,
one is the Pacific ring of fire, the second is the Eurasian seismic
belt, the third is the ridge seismic belt that stretches for tens
of thousands of kilometers across the oceans, it is attached to
the great rupture zone of the sea area.
The depth of an earthquake can vary from several kilometers to
more than 700 kilometers, the depth of the earthquake is also
closely related to the geological structure. Deep source earthquakes
that reach a depth of several hundred kilometers, usually
distributed in an island-arc region.
The frequency of earthquakes is closely related to the size of
earthquakes, the smaller earthquake, the more it happens. According
to statistics , The global average of 7.8 magnitude or more earthquakes
occur about two times a year, earthquakes of magnitude 7.0 to 7.7
occur about 17times, 6.0 ~ 6.9 occur about 100 times, 5.0 ~ 5.9 occur
about 800 times, 4.0 ~ 4.9 occur about 6000 times, 3.0 ~ 3.9 occur
about 50000 times.
China is the most active region in the world. In the 20th century,
116 earthquakes above 7 magnitude occurred in China, about 6 percent
of the world's earthquakes, among them, mainland earthquakes occurred
H times, approximately 29% of the world's continental earthquakes
The earliest records of earthquakes in China dated back to 1831 BC,
more than 800 strong earthquakes of magnitude 6 or above have been
recorded so far, all provinces except Zhejiang and Guizhou. As far
as Zhejiang and Guizhou provinces, there have also been moderate or
strong earthquakes of magnitude 5 to 6.On record, there have been 21
major earthquakes of magnitude 8 or above in China, Taiwan has two
magnitude-8 earthquakes, the remaining 19 were in mainland China.
In the 20th century, there were only three major earthquakes above
8.5 magnitude, it was earthquake of magnitude8.6 of Haiyuan, Ningxia
in 1920, earthquake of magnitude 8.6 of Tibet in 1950, earthquake of
magnitude 8.5 of Chile in 1960.
China's seismic activity has the characteristics of time and space
inhomogeneity, the strong earthquake activity is characterized by the
alternation of active and calm of time in China. Earthquakes of magnitude
7 or above during active and quiet periods, the annual frequency ratio is
5:1. 100 years from 1901 to 2000, the mainland of China experienced five
relatively active periods of seismic activity, four relatively quiet periods
of seismic activity, the time period is roughly divided into=from 1901 to 1911,
1920 to 1937, 1947 to 1955, 1966 to 1976and 1988-2000, they are the relative
active period, from 1912-1919, 1938-1946, 1956-1965 and 1977-1987, they are
the relative calm period. Taiwan Strong earthquake activity and development
process of seismic active period in continental area, they have
Figure2-2: Distribution of destructive earthquakes in China in the 20th century（source:www.xinan.gov.cn)
The distribution of strong seismic activity is relatively concentrated,
this is the most obvious feature of seismic activity spatial inhomogeneity
in China. Taiwan is the area with the strongest earthquake activity in
China. In the 20th century, Taiwan experienced 41 earthquakes of above 7
magnitude, it accounts for 35% of the total number of earthquakes above
magnitude 7 in China. In the mainland region, in a 107 °east longitude,
the western region is strongly compressed by the Indian Ocean plate directly,
the intensity and frequency of seismic activity are greater than that in
the eastern region. In the 20th century, 64 shallow earthquakes above
magnitude 7 occurred in China, the area west of 107°E, 56 times earthquakes,
accounted for 87.5%, which release more than 95% seismic energy.
Furthermore, China's earthquake also has the characteristics of
shallow source. In addition to the northeast and Taiwan small
number of medium and deep source earthquakes, most earthquakes
have a depth of 40km, especially in the east, the seismic source
are more shallower, the depth range is usually 10 to 20km.