A total of 17 strong earthquakes with MS≥7.0 occurred in 2016 (by the end of December 31, 2016) throughout the world, according to the China Seismic Network. Among them, one earthquake with MS8.0 occurred on November 13, 2016 in New Zealand (Fig. 1). The 2016 New Zealand MS8.0 earthquake (MW7.8 measured by USGS) was the strongest one in New Zealand since 1700, the epicenter was located in the convergence belt of the seismically active Australia plate and Pacific plate. The mainshock was thrust type and the rupture is unilateral along the NE direction, with maximum intensity of more than Ⅸ. Features of global seismicity of MS≥7.0 in 2016 are as follows.
With respect to annual frequency, there were 17 earthquakes with MS≥7.0 occurring in the world in 2016. The seismic frequency is decreased significantly (Fig. 2(a)), compared with the seismicity of 20 strong earthquakes with MS≥7.0 in 2015, and is slightly lower than the annual average of 20 strong earthquakes. Only 1 strong earthquake with MS≥8.0 occurred in 2016, so compared with the 3 strong earthquakes of MS≥8.0 occurring in 2015, the global seismicity significantly decreased (Fig. 2(b)).
In terms of space, the earthquakes with MS≥7.0 were mainly distributed on the west zone of the circum-Pacific seismic belt and the Himalayan seismic belt, in particular, the earthquake with MS7.1 in Afghanistan on April 10, 2016, the MS7.2 earthquake in Myanmar on April 13, and the Kyushu Island MS7.3 earthquake on April 16, 2016 that happened in the vicinity outside China.
In terms of time, 4 earthquakes with MS≥7.0 occurred in the 8 days from April 10 to April 17, 2016; global seismicity with MS≥7.0 was calm for 63 days from May to July of 2016, and earthquakes with MS≥7.0 were calm again for 86 days from August to November of 2016, showing inhomogeneity in time.
The high-frequency seismicity of the 4 earthquakes with MS≥7.0 occurring in 8 days in April, 2016 in the world was significantly higher than the historical background activity level, and was a significant advancement of global strong earthquake activity. We studied the phenomenon of the global earthquakes with MS≥7.0 occurring in a short time and in high frequency since 1900 and found that a total of 37 groups of earthquakes with MS≥7.0 that happened in a frequency of 4 within 8 days since 1900, globally. In China (including the Chinese mainland and the Taiwan area), earthquakes with MS≥7.0 occurred within 3 months following 8 groups of these high frequency events, among them, 3 groups were followed by earthquakes with MS≥7.0 happening within 3 months in the Chinese mainland, with a corresponding proportion of 3/37≈8%. In the Chinese mainland, the spontaneous earthquake rate of earthquakes with MS≥7.0 occurring within 3 months since 1900 is 17%, therefore, the corresponding rate of this high-frequency phenomenon is lower than the natural rate, suggesting that, from the view point of statistics, this phenomenon has no specific meaning for short-term and impending earthquake prediction in the Chinese mainland (Table 1).
The April 25, 2015 Nepal MS8.1 earthquake, the October 26, 2015 Hindu Kush MS7.8 earthquake and the December 7, 2015 Tajikistan MS7.4 earthquake occurred successively in the Indian plate boundary. In 2016, the strong earthquakes continued to be active in the boundary of the Indian plate, where the Afghanistan MS7.1 intermediate-focus earthquake happened on April 10, and the Myanmar MS7.2 intermediate-focus earthquake occurred on April 13. It is suggested that the northward squeezing of the Indian Plate to the Chinese mainland increased continuously (Fig. 3).
During the period of January 1-December 31, 2016, there were 33 earthquakes with MS≥5.0 happening in China, of which 18 earthquakes occurred in the Chinese mainland, including 5 earthquakes with MS≥6.0, namely, the deep-focus earthquake with MS6.4 occurring in Linkou, Heilongjiang on January 2, 2016, the earthquake with MS6.4 in Menyuan, Qinghai on January 21, 2016, the Zadoi, Qinghai earthquake with MS6.2 on October 17, the Akto, Xinjiang earthquake with MS6.7 on November 25 and the Hutubi, Xinjiang earthquake with MS6.2 occurring on December 8, 2016. 15 earthquakes occurred in Taiwan, with the largest one being the February 6, 2016 earthquake with MS6.7 occurring in Kaohsiung, Taiwan (Fig. 4). Compared with 2015 (14 earthquakes with MS≥5.0, and 1 earthquake with MS6.0-6.9), the earthquake frequency was increased to some extent in the Chinese mainland, with similar strength. The frequency of earthquakes with MS≥5.0 remained continuously low during 2015-2016, comparing with 20 earthquakes per year in the history (Fig. 5). Features of the seismicity of Chinese mainland in 2016 are as follows:
Currently, Chinese mainland is in a relatively active stage of earthquakes with MS≥7.0. Previously, the quiescent period in the seismically active stage for earthquakes with MS≥7.0 was generally not more than 40 months, with only 1 earthquake in 62 months (Fig. 6). Since the Yutian, Xinjiang MS7.3 earthquake on February 12, 2014, to the end of December 2016, the quiet time for earthquakes with MS≥7.0 has reached 34 months, by the end of 2017, the time will be up to 46 months. The quiet time for earthquakes with MS≥7.0 is significantly beyond most of the quiescent periods of MS≥7.0 earthquakes in the seismically active periods.
After the 2015 Nepal MS8.1 earthquake, a NE-directed seismic belt of earthquakes with MS≥5.0 was formed in the interior of the Qinghai-Tibetan block (Fig. 3), where there were 11 earthquakes with MS≥5.0 happening sequentially, namely, the Tingri, Tibet earthquake with MS5.9 occurring on April 25, 2015, the Nyalam, Tibet earthquake with MS5.3 on April 26, 2015, the Madoi, Qinghai earthquake with MS5.2 on October 12, 2015, the Qilian, Gansu earthquake with MS5.2 on November 23, 2015, the Amdo, Tibet earthquake with MS5.3 on January 14, 2016, the Menyuan, Qinghai earthquake with MS6.4 on January 21, 2016, the Dêngqên, Tibet earthquake with MS5.5 on May 11, 2016, the Dinggyê, Tibet earthquake with MS5.3 on May 22, 2016, the Tingri, Tibet earthquake with MS5.3 on May 22, 2016, the Zadoi, Qinghai earthquake with MS6.2 on October 17, 2016 and the Nyainrong, earthquake with MS5.1 on December 5, 2016. This phenomenon indicates that the influence of the Nepal MS8.1 earthquake on the seismicity in the west of the Chinese mainland is not yet over.2.3 Seismic Cluster and Significant Quiescence of MS≥5.0 Earthquakes Occurred Alternately and Repeatedly
During the period from January 2 to February 11, 3 earthquakes with MS≥5.0 happened continually in 41 days, including 2 earthquakes with MS≥6.0. After a quiescence of 90 days, 4 earthquakes with MS≥5.0 occurred in 12 days during the May 11-May 22. Afterwards, two seismic quiescence periods of MS≥5.0 earthquakes of 70 days and 54 days appeared, which were broken by the Cangwu MS5.4 earthquake on July 31 and the Litang MS5.1 earthquake on September 23, respectively. It is shown that the occurrence of the earthquakes with MS≥5.0 is in an anomalous extremely inhomogenous state (Fig. 7). However, the continuously shortening of quiescent periods indicated that such a weak activity state would change. Afterwards, seismicity increased remarkably and 7 earthquakes of MS≥5.0 happened, including 3 earthquakes with MS≥6.0, namely, the Zadoi earthquake with MS6.2 on October 17, the Akto earthquake with MS6.7 on November 25, and the Hutubi, Xinjiang earthquake with MS6.2 on December 8, 2016.
After the Cangyuan MS5.5 earthquake on March 1, 2015, the phenomenon of weak activity for earthquakes with MS≥5.0 appeared in the southwest China area (Fig. 8), with a duration reaching 22 months until December, 2016. Statistics were taken on the cases where are at least 3 earthquakes with MS5.0 occurring within 500 days in the southwest China area since 1965. It is found that there were 5 groups of such earthquake cases, with a weak activity duration of 18-21 months. Among 4 of the earthquake cases, MS≥7.0 earthquakes happened within 210 days after the end of the weak seismicity periods (Table 2).
After the 2013 Minxian-Zhangxian MS6.6 earthquake, only 3 earthquakes with MS≥5.0 occurred in the northwest China area up to September, 2015. After that were the Madoi, Qinghai earthquake with MS5.2 on October 12, 2015, the Qilian, Qinghai earthquake with MS5.2 on November 23, the Amdo, Tibet earthquake with MS5.3 on January 14, 2016, the Menyuan, Qinghai earthquake with MS6.4 on January 21, the Dêngqên, Tibet earthquake with MS5.5 on May 11, and the Zadoi and the Qinghai earthquake with MS6.2 on October 17, suggesting that the seismicity in this area was continuously increasing.2.6 The Strong Earthquakes Were Active in Xinjiang and Its Adjacent Area
Since October 2015, seismic activity was increased in Xinjiang and its neighboring area, successively occurred the Hindu Kush MS7.8 earthquake on October 26, 2015 (intermediate-focus shock), the Tajikistan MS7.4 earthquake on December 7, the Luntai earthquake with MS5.3 on January 14, the Xinyuan MS5.0 earthquake on February 11, the Afghanistan MS7.1 earthquake on April 10 (intermediate-focus earthquake), the Kyrghyzstan MS6.7 earthquake on June 26, the 2 Akto earthquakes with MS6.7 and MS5.0 on November 25 and November 26, the Hutubi, Xinjiang MS6.2 earthquake on December 8, the Ruoqiang MS5.0 earthquake on December14, and the Qiemo MS5.8 earthquake on December 20.2.7 The Seismicity of Earthquakes with MS≥6.0 Were Significantly Quiet in the North China Area, and the Seismicity with MS≥5.0 Was Increased in the South China Area
The 18.9-year long quiescence of MS≥6.0 earthquakes in North China after the 1998 Zhangbei MS6.2 earthquake was the longest quiet period since 1820. Since the MS5.1 Wen'an earthquake in 2006, the quiescence of MS≥5.0 earthquakes in North China has lasted for 10.5 years. This means the possibility of occurrence of moderately strong earthquake has increased under the long quiescence of MS≥6.0 earthquakes and significant quiescence of MS≥5.0 earthquakes. Seismicity of MS≥5.0 was continuously quiet in the North China and East China areas, but earthquakes with ML4.0, ML3.0 and small swarms were frequent, indicating that the East China area has entered into a new seismic active stage and the possibility is increased (Fig. 10).
In the South China region were the succesively occurring Badong, Hubei MS5.1 earthquake on December 16, 2013, the Jianhe, Guizhou MS5.5 earthquake on March 20, 2015 and the Cangwu, Guangxi MS5.4 earthquake on July 31, 2016, indicating that seismicity had increased significantly in the South China area since 2013. The southeast China costal seismic belt is the main seismically active zone for the South China area, where the quiet period of earthquake with MS≥5.5 has lasted for 21.8 years, which is the longest quiescence since 1870. The Cangwu, Guangxi MS5.4 earthquake occurred on July 31, 2016, which broke the 16.6-year quiescence of MS≥5.0 earthquakes in southeast China's coastal seismic belt since 1999. However, the quiescence of MS≥5.5 earthquakes is still continuous in the southeast China costal area. The energy released is not sufficient in the west segment of the southeast China costal area (Table 3). After that, the activity of small earthquakes has increased since October, 2015, indicating the potential risk of moderately strong earthquakes in the west segment of the southeast China coastal area.
At present, the southeast China coastal seismic belt is entering the later stage of the second active period. As shown in Table 3, compared with the first active period, the energy release in the last about 300 years has reached 56% of the first active period in the east segment of the seismic belt east to 114°E, while the energy released is just 16% of the first active period in the west segment of the seismic belt west to 114°E (Fig. 11).
Small earthquake swarms were active in the South China area in 2016 (Fig. 12(a)), there were successively occurring small earthquake swarms in Heyuan of Guangdong Province, Baojing of Hunan Province, Liancheng of Fujian Province, Pingguo of Guangxi Zhuang Autonomous Region, and Pingyuan, Guangdong and Ruyuan of Guangdong Province. In 2016, the active zone of the ML≥3.0 earthquakes in the South China area transferred from the Guangdong-Fujian coastal area in 2015 to the northwestern Guangxi area (Fig. 12(b)). The seismicity of small earthquake swarms and the ML≥3.0 earthquakes in 2016 were concentrated and active. It is indicated that in the South China area, there is the possibility of occurrence of a moderately strong earthquake, especially in the northwestern Guangxi area.
After the May 12, 2008 Wenchuan earthquake, the Dongyuan, Guangdong ML5.2 earthquake (MS4.8) on February 16, 2012 broke the long quiescence of ML≥4.0 earthquakes in the South China area. The seismicity with ML≥4.0 started to be active in the South China area, especially in the west segment of the southeast China costal seismic belt since October 2015. The seismic radiated energy of earthquakes (MS≤5.0) in the southeast China costal seismic belt and that of small earthquakes since 2008 were calculated respectively using the space scaning method, then, the time window was moved annually to obtain the time evolution process of seismic radiated energy distribution in this area (Fig. 13). It is shown that seismicity of ML≥4.0 became active in the South China area from 2015 and the seismic radiated energy increased significantly in the west segment of southeast China coastal seismic belt, implying that the stress level was increased in the west segment.
After the December 26, 2006 Hengchun Sea MS7.2 earthquake, where earthquakes with MS≥7.0 were quiet in Taiwan for more than 10 years, the phenomenon of earthquake deficiency is obvious (Fig. 10).
(1) Global seismicity decreased significantly, and the spacio-temporal distribution of seismicity was uneven. Strong earthquakes were continuously active in the boundary of the Indian Plate, where many intermediate and deep focus earthquakes occurred.
(2) Seismicity of MS≥7.0 in the Chinese mainland was quiet for 3 years. Earthquakes of MS≥5.0 in the Qinghai-Tibetan block presented banded distribution in NE direction after the Nepal MS8.1 earthquake in 2015. Earthquake clusters and significant quiescence of MS≥5.0 occurred alternately and repeatedly in the Chinese mainland.
(3) The seismicity of earthquakes with MS≥5.0 was remarkably weak in the Southwest China area, and increased significantly in the Northwest China area. Strong earthquakes were active in Xinjiang and its adjacent areas. The seismicity of earthquakes with MS≥6.0 was remarkably quiet in the North China area, and the activity of earthquakes with MS≥5.0 was increased in the South China area. The seismicity with MS≥7.0 was remarkably quiet in the Taiwan area.