A total of 8 strong earthquakes with MS≥7.0 occurred in 2017 (from January 1 to December 31, 2017) throughout the world, according to the China Seismic Network, including one with MS≥8.0, i.e., the great earthquake with MS8.2 which occurred on September 8, 2017 in Chiapas, Mexico (Fig. 1). The 2017 MS8.2 Chiapas, Mexico earthquake was located in the east of the Circum-Pacific Seismic Belt. The mainshock was normal type and the rupture was unilateral along the NW direction, with maximum intensity Ⅸ. Features of global seismicity of MS≥7.0 in 2017 are as follows.
One great earthquake with MS≥8.0 occurred in 2017. The global seismic strength nearly stayed the same compared with one great earthquake of MS≥8.0 in 2016 (Fig. 2(a)). There were 8 earthquakes with MS≥7.0 occurring in the world in 2017. The seismic frequency was significantly decreased compared with the seismicity of 17 strong earthquakes with MS≥7.0 in 2016, and 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 and Eurasian Seismic Zone. There were 6 earthquakes with MS≥7.0 located in the Circum-Pacific Seismic Belt, 3 in the East Belt and 3 in the West Belt. The epicenters of the 3 earthquakes in the East Belt were very close to one another. There were 2 earthquakes with MS≥7.0 occurring in the Eurasian Seismic Zone, including the MS7.0 earthquake on 8 August 2017 in Jiuzhaigou, Sichuan, China.1.3 Temporal Distribution of Seismicity Was Uneven
Global seismicity with MS≥7.0 was quiet for 97 days from January to April of 2017. It was the longest quiescence in the whole year. Because of this quiescence, only 3 earthquakes occurred in the first half of 2017. Compared to this weak seismicity, 5 earthquakes occurred in the second half of the year, including the only MS≥8.0 earthquake, i.e., the MS8.2 Chiapas, Mexico earthquake on September 8, 2017. As the seismic frequency and strength of the second half of 2017 were significantly higher than that of the first half, the global seismicity in 2017 was inhomogeneous in time.2 SURVEY OF SEISMICITY IN CHINA
During the period of January 1-December 31, 2017, there were 19 earthquakes with MS≥5.0 occurring in China (Fig. 3). Among those earthquakes, 13 occurred in the Chinese mainland, including 3 earthquakes with MS≥6.0, i.e., the MS7.0 earthquake on August 8, 2017 in Jiuzhaigou, Sichuan, the MS6.6 earthquake on August 9, 2017 in Jinghe, Xinjiang, and the MS6.9 earthquake on 18 November 2017 in Mainling, Tibet. 6 earthquakes with MS≥5.0 occurred in Taiwan, of which the largest two were the February 11, 2017 earthquake with MS5.6 in Kaohsiung, Taiwan and the May 16, 2017 earthquake with MS5.6 in Taitung. The 2017 MS7.0 Jiuzhaigou earthquake was located on the northeast boundary of Bayan Har block. Its mechanism was left-lateral strike-slip type. The mainshock occurred at the intersection of Tazang, Minjiang and Huya faults. Features of the seismicity in China in 2017 are as follows.
There were 13 earthquakes with MS≥5.0 occurring in the Chinese mainland in 2017. 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 3 years in succession. The seismic frequency was significantly low (Fig. 4).
There were 3 earthquakes with MS≥6.5 occurring in the Chinese mainland in 2017, including a MS≥7.0 earthquake. The seismic strength increased significantly compared with the seismicity of only 1 strong earthquake with MS≥6.5 (the MS6.7 Akto, Xinjiang earthquake on November 25, 2016) in 2016 (Table 1).
The MS7.0 Jiuzhaigou, Sichuan earthquake on August 8, 2017 broke the 3.5-year-long quiescence of MS≥7.0 earthquakes in the Chinese mainland. The Chinese mainland is still in a relatively active stage of earthquakes with MS≥7.0. Since the 1997 MS7.5 Mani, Xizang earthquake, 8 earthquakes with MS≥7.0 occurring in the Chinese mainland were all located on the boundary of Bayan Har block. The 2017 Jiuzhaigou MS7.0 occurred on the boundary too, showing that the major active area of earthquakes with MS≥7.0 in the Chinese mainland was unchanged (Fig. 5).
Strong earthquakes with MS≥7.0 have been active in the North-South Seismic Belt since the MS8.0 Wenchuan earthquake on May 5, 2008. After the Wenchuan earthquake, there occurred in succession the MS7.1 Yushu earthquake on April 14, 2010, the MS7.0 Lushan earthquake on April 20, 2013 and the MS7.0 Jiuzhaigou earthquake on August 8, 2017. Currently, the North-South Seismic Belt is still in the seismically active stage of MS≥7.0 earthquakes (Fig. 6).
After the MS7.8 Hindu Kush earthquake on October 26, 2015 (intermediate-focus shock), the MS7.4 Tajikistan earthquake on December 7, 2015, the MS7.1 Afghanistan earthquake on April 10, 2016 (intermediate-focus shock) and the MS6.7 Kyrghyzstan earthquake on June 26, 2016, strong earthquakes were continuously active in the Tianshan Seismic Belt. There occurred in succession the MS6.7 Akto earthquake on November 25, 2016, the MS6.2 Hutubi earthquake on December 8, 2016, and the MS6.6 Jinghe earthquake on August 9, 2017 (Fig. 6).2.5 The Weak Seismicity of MS≥5.0 Earthquakes Ended in Southwest China Area
After the MS5.5 Cangyuan earthquake on March 1, 2015, seismic quiescence of MS≥5.0 earthquakes with durations of 243, 200, 128, 185 and 134 days appeared in succession in the Southwest China area. Seismicity of MS≥5.0 earthquakes in the Southwest China area was weak. The MS7.0 Jiuzhaigou, Sichuan earthquake on August 8, 2017 indicated that the weak seismicity of MS≥5.0 earthquakes had ended (Fig. 7).
After the 2013 Minxian-Zhangxian MS6.6 earthquake, only 5 earthquakes with MS≥5.0 occurred in the Northwest China area up to December 2015. Two strong earthquakes with MS≥6.0 occurred subsequently in 2016. In 2017, the Jiuzhaigou MS7.0 earthquake occurred on the branch of the Eastern Kunlun fault, indicating that the seismicity had increased continuously in the Northwest China area (Fig. 8).
Moderate earthquakes and strong earthquakes were active in Xinjiang since November, 2016. A total of 9 earthquakes with MS≥5.0 occurred in that period, including 3 earthquakes with MS≥6.0, i.e. the MS6.7 Akto earthquake on November 25, 2016, the MS6.2 Hutubi earthquake on December 8, 2016, and the MS6.6 Jinghe earthquake on August 9, 2017 (Fig. 9).
The longest quiescence of shallow focus earthquakes with MS≥6.0 was 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 years after the 1998 Zhangbei MS6.2 earthquake. It is the longest quiescence since 1820 (Fig. 10).
After the MS7.2 offshore Hengchun earthquake on December 26, 2006, earthquake activity of MS≥7.0 was quiet in Taiwan for 11 years. It is the longest quiescence of MS≥7.0 earthquakes since 1900 (Fig. 11). In this context, the quiescence of MS≥6.0 earthquakes also lasted for nearly 1.6 years, and only 6 earthquakes with MS≥5.0 occurred in 2017, much less than the annual average of 13 earthquakes (Table 2).
(1) Global seismicity was significantly weak, and the spatio-temporal distribution of seismicity was uneven. Strong earthquakes were concentrated on the Circum-Pacific Seismic Belt and Eurasian Seismic Zone.
(2) The 2017 Jiuzhaigou MS7.0 broke the approximately 3.5-year-long quiescence of MS≥7.0 earthquakes in the Chinese mainland. The seismic strength increased significantly in the mainland. The seismic frequency of MS≥5.0 earthquakes was significantly low in the mainland.
(3) The weak seismicity of MS≥5.0 earthquakes ended in Southwest China area. The seismicity increased significantly in the Northwest China and its adjacent area. Moderate earthquakes and strong earthquakes were active in Xinjiang since November 2016. The seismicity of MS≥6.0 earthquakes was remarkably quiet in the eastern Chinese mainland. The seismicity of MS≥7.0 earthquakes was remarkably quiet in the Taiwan area.