A total of 5 strong earthquakes with MS≥7.0 occurred in the first half of 2018 (from January 1 to May 31, 2018) throughout the world, according to CENC (China Earthquake Networks Center), including one with MS≥8.0 (MW7.9), which occurred on January 23, 2018 in Alaska (Fig.1). The 2018 MS8.0 Alaska earthquake was located in the north of the Circum-Pacific Seismic Belt. The mainshock was slip type and the rupture was unilateral along the NW direction, with maximum intensity Ⅹ. Features of global seismicity of MS≥7.0 in the first half year of 2018 are as follows.
One earthquake with MS≥8.0 occurred in the first half of 2018. The global seismic strength stayed nearly the same compared with the earthquake with MS≥8.0 in 2017 (Fig.2(a)). There were 5 earthquakes with M≥7.0 occurring in the world in the first half of 2018. The seismic frequency was similar to 2017, which had 8 strong earthquakes with MS≥7.0, but was significantly lower than the annual average of 19 strong earthquakes (Fig.2(b)).
Global earthquakes with MS≥7.0 were mainly distributed on the Circum-Pacific Seismic Belt. There were 5 earthquakes with MS≥7.0 located in the Circum-Pacific Seismic Belt, 1 in the East Belt and 4 in the West Belt.1.3 The Temporal Distribution of Seismicity was Uneven
Global seismicity with MS≥7.0 was concentrated in January and February. 5 earthquakes occurred in the first quarter of the year, including the MS≥8.0 Alaska earthquake that occurred on January 23, 2018. As the seismic frequency and strength of the first quarter of 2018 was significantly higher than that of the second quarter, global seismicity in the first half of 2018 was inhomogeneous in time.2 SURVEY OF SEISMICITY IN CHINA
Between January 1 and May 31, 2018, there were 14 earthquakes with MS≥5.0 occurring in China (Fig.3). Among those earthquakes, only 2 occurred in the Chinese mainland, all of which were below MS6.0. The other 12 earthquakes with MS≥5.0 occurred in Taiwan, China, of which the largest two were the February 4, 2018 earthquake with MS6.4 and February 6, 2018 earthquake with MS6.5 in waters off Hualien, Taiwan, China. Features of seismicity in China in the first half of 2018 are as follows.
There were only 2 earthquakes with MS≥5.0 that occurred in the Chinese mainland in the first half of 2018. The seismic frequency was much lower than the annual average of 20 earthquakes. Currently, the seismic frequency has been lower than the average value for 4 years in succession. The seismic frequency was significantly low (Fig.4).
There were no earthquakes with MS≥6.0 occurring in the Chinese mainland in the first half of 2018. There were however 2 earthquakes with MS≥5.0, one was the May 6, 2018 earthquake with MS5.3 in Chindu, Qinghai, and the other was the May 28, 2018 earthquake with MS5.7 in Songyuan, Jilin.2.3 The Seismicity of MS≥6.0 Earthquakes Was Significantly Quiet in the Eastern Chinese Mainland
The longest quiescence of shallow focus earthquakes with MS≥6.0 is 14.9 years (excluding the MS6.1 Xinfengjiang reservoir-induced earthquake in 1962) in the eastern Chinese mainland since 1820 (the fourth seismic active period). Currently, quiescence of earthquakes with MS≥6.0 in the eastern Chinese mainland has remained for 20.5 years after the 1998 Zhangbei MS6.2 earthquake. It is the longest quiescence since 1820 (Fig.5).
After the MS7.2 offshore Hengchun earthquake on December 26, 2006, earthquake activity of MS≥7.0 was quiet in Taiwan China for nearly 12 years. It is the longest quiescence of MS≥7.0 earthquakes since 1900 (Fig.6). However, there were 12 earthquakes with MS≥5.0 that occurred in the first half of 2018, including 3 earthquakes with MS≥6.0, much higher than the annual average of 13 earthquakes and 6 earthquakes in 2017 (Table 1). In the 12 earthquakes, 10 earthquakes occurred in the waters off Hualien and 9 earthquakes occurred during February 4 to February 7 including 3 earthquakes with MS≥6.0.
(1) Global seismicity was similar to 2017 but weaker than previous years, and the spatio-temporal distribution of seismicity was uneven. Strong earthquakes were concentrated on the circum-pacific seismic belt.
(2) The seismic frequency with MS≥5.0 was significantly low and seismic strength decreased significantly in the Chinese mainland. The seismicity of MS≥6.0 earthquakes was sequentially and significantly quiet in eastern Chinese mainland.
(3) The seismicity of MS≥7.0 earthquakes was remarkably quiet, but MS≥5.0 earthquakes was strong in the Taiwan area, China. Most of these earthquakes were occurred in the waters off Hualien and 9 earthquakes occurred during February 4 to February 7 in 2018 including 3 earthquakes with MS6.0.