2. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
According to the official determination of China Earthquake Networks Center, a magnitude 5.7 earthquake with focal depth of 13km occurred at 01 : 50 a.m. on May 28, 2018 in Ningjiang District, Songyuan City, Jilin Province (45.27°N, 124.71°E). The Ministry of Emergency Management immediately dispatched forces to understand the earthquake situation and directed the emergency response work. The China Earthquake Administration launched a three-level earthquake emergency response to the earthquake. As of 13 : 00 p.m. on May 28, 2018, 5, 582 people were affected by the earthquake, but there were no casualties2. The maximum intensity of the earthquake is Ⅶ degrees, as determined by the earthquake field task force of Jilin Earthquake Agency based on the seismic damage investigations and analysis of seismic observation records in the disaster area. The intensity area is elliptical, and the ellipse long-axis is generally NE-trending3.
The shallow earthquakes in northeastern China were triggered by the upwelling of mantle material caused by the subduction of the Pacific plate to the deep part of the Eurasian plate (Sun Wenbin et al., 1985). In the northeast China seismic area, the intersection of NW-trending faults and NE or NNE-trending faults is often regarded as the main seismogenic region and mode. Earthquakes are prone to occur in these areas as the fault intersection is a structural variation zone. The stress field will continue to transmit forward when the deep structure is uniform. When the deep structural variation zone is encountered, the stress concentration and reinforcement will be formed, which will become a favorable part for earthquake preparation.
The Songyuan, Jilin MS5.7 earthquake occurred in the Songliao basin, located at the intersection of the Fuyu-Zhaodong fault and the Second Songhua River fault, the minimum distance of the main shock to the two faults is 3.4km and 0.7km, respectively (Fig. 1(b)). The Fuyu-Zhaodong fault starts from Zhaodong in the north and extends to Yangdachengzi by Fuyu. It is a deep basement fault with a length of about 240km and distributes in the direction of NNE 30°-35°. With this fault as the boundary, the east and west sides of the fault are respectively the basin's southeast uplift and the central depression, in which the central depression zone is an area with the largest subsiding magnitude, the fast subsiding velocity, the deepest depression and the relatively strong activity (Li Chuanyou et al., 1999a; Shao Bo et al., 2016; Liu Junqing et al., 2017). The Second Songhua River fault is one of the most important faults in the NW-trending tectonics in the Northeast China region (Fig. 1(a)), which extends in the west from Da'an, Fuyu of the Songliao plain, passing through Dehui, Jilin, Songjiang, ending at Chongshan on the border of China and DPRK, with a general strike of 320°, a length of about 400km (Li Chuanyou et al., 1999b; Li Enze, et al., 2012).
Historically, an earthquake of MS6¾ occurred in 1119 A.D. near the earthquake area, which is the largest earthquake ever recorded in the Songliao Basin. The epicenter intensity reached Ⅸ, causing thousands of deaths (Yuwen Maozhao, 1993). Since 2009, 75 earthquakes with M ≥1.0 have occurred around the epicenter (in the range of Fig. 1(b)), including 5 earthquakes with M≥ 3.0-3.9, 3 earthquakes with M≥4.0-4.9 and 1 earthquake with M≥5.0, i.e. the M5.0 earthquake on July 23, 2017. In 2013, about 96km southwest of the Songyuan MS5.7 earthquake, 5 earthquakes with MS≥5.0 occurred successively. It is a rare medium-strong earthquake swarm in the Songliao basin. The seismogenic structure is the Qagan Nur-Daozijing fault. Together with its conjugated Fuyu-Zhaodong fault, it controls the development of the earthquake (Zhang Hongyan et al., 2015).
Using the W-phase method (Kanamori H. et al., 2008; Hayes G.P. et al., 2009; Duputel Z. et al., 2012), the China Earthquake Networks Center (CENC) inverted the moment magnitude MW of the Songyuan MS5.7 main shock with the broadband seismographs data in the range of 5°-15° epicentral distance, which is MW5.3, and the centroid depth fitted by the waveforms is 12km (Fig. 1(b)). The earthquake is a strike-slip event, which is consistent with the results of moment magnitude, depth and fault properties given by the German Research Centre for Geosciences (GFZ)4(Table 1).
This paper uses the official catalogue and observation report5 to conduct research on the Songyuan MS5.7 earthquake, and uses the double difference location method to relocate the earthquake sequence to obtain the temporal and spatial distribution characteristics of the aftershock activity, and by combining that with Chinese historical earthquakes (Department of Earthquake Disaster Prevention, China Earthquake Administration, 1995, 1999), regional tectonic setting (Deng Qidong, 2007) and focal mechanism solutions, discusses and analyzes the seismogenic structure and activity of the earthquake to provide reference for the determination of aftershock trends.1 DATA SELECTION AND LOCATION METHOD
In this paper, the seismic phase arrival data of earthquake sequences recorded by 28 stations of the China Earthquake Networks Center are used, and the seismic sequence is relocated by the double difference earthquake location method (Waldhauser F. et al., 2000). The distribution of stations is shown in Fig. 1(a), including 22 fixed stations and 6 temporary mobile stations. Among them, the temporary mobile stations L2202, L2210, L2211, and L2212 were laid after the Songyuan, Jilin MS5.0 earthquake on July 23, 2017. L2201 and L2209 were laid after the Songyuan, Jilin MS5.7 earthquake on May 28, 2018. There are 4 stations with an epicenter distance of less than 10km from the Songyuan MS5.7 main shock (Fig. 1(b)). The nearest station is the Songyuan station (SYT), with an epicenter distance of only 5.2km. The good configuration of the stations' distribution ensures accurate determination of the focal depth. The earthquake sequence selected in this paper is: the start time of the main shock, with the cut-off time at 24 : 00 on June 5, 2018; the earthquake with M≥0.1 in the range of 45.0°-45.5°N and 124.5°-125.0°E; the phase arrival time data is based on official observations produced by the China Earthquake Networks Center.
The principle of the double-difference location method is that if the distance between two hypcenters is far less than the distance from the event to the station and the scale of velocity inhomogeneity, the ray paths of the two events to the same station are almost the same. The focal parameters are updated by the travel time difference of two seismic events. The location accuracy depends on the velocity near two adjacent seismic events, and does not depend on the velocity of the medium along the whole ray path. Because the Songyuan MS5.7 earthquake sequence occurred in the Songliao Basin, we adopted the fine crustal velocity structure (Table 2) of Songliao Basin obtained by Li Yingkang et al. (2014) using artificial seismic sounding, and the S-wave velocity VS=VP / 1.73.
In this paper, a total of 75 earthquakes including the main shock are relocated. The weights of the P-wave and S-wave arrival times are 1 and 0.3, respectively. The conjugate gradient method is used to solve the linear equations. Four sets of iterations are used in the location calculation. Each set of iterations is 5 times. The first set of iterations do not control the maximum distance between event pairs and the maximum distance between event pairs from the second set. The large spacing threshold is reduced from 20km to 8km to 1.5km in the last iteration, and the relocation results of 60 events are obtained. The distribution of earthquake sequences before and after relocation is shown in Fig. 2(a) and Fig. 2(b), respectively. Before the relocation, seismic distribution is relatively discrete, with no obvious distribution characteristics and dominant distribution direction, and the earthquakes become more concentrated after the relocations.
The distribution of aftershocks after relocation shows that the seismic sequence extends in the NE direction along the Fuyu-Zhaodong fault, which is the same as the elliptical axis in the intensity region. The aftershock area is about 4.3km long and 3.1km wide. There are more aftershocks in the southwest and less in the northeast. The main shock is located in the northwest of the aftershock area. Focal depth distribution is more concentrated after relocation (Fig. 3), with the predominant distribution of 9km-10km. The earthquake is a bilateral rupture event. In terms of time, seismicity was strong within 2 days after the main shock, and spreading along the Fuyu-Zhaodong fault, accounting for 67% of the total number of aftershocks, including the largest aftershock of M3.7. Spatially, the section AA' is parallel to the Fuyu-Zhaodong fault, and the distribution of aftershocks on the section has no obvious tendency. The section BB' is perpendicular to the Fuyu-Zhaodong fault. The distribution of aftershocks on the section shows that the fault plane strikes SE with a dip angle of about 75°, which is very consistent with the dip in the nodal plane Ⅱ of the focal mechanism solution provided by GFZ.Therefore, we conclude that the concealed fault of Fuyu-Zhaotong fault is the seismogenic fault of Songyuan MS5.7 earthquake
The M-T map of the surrounding area of the Songyuan earthquake (Fig. 4) shows that the seismic activity in this area is weak in recent years. However, since the MS5.0 earthquake on July 23, 2017, both the frequency and intensity of earthquakes have increased significantly. As the only two earthquakes with a magnitude of 5.0 and above since 2009, the epicenter distance is only 7km apart and the focal mechanism solutions are similar6. In order to analyze the relationship between the two earthquakes and the development trend of the earthquake, this paper also relocates the earthquake sequence before the magnitude 5.7 earthquake from July 23, 2017 to May 28, 2018, using 5 sets of iterations. The maximum distance threshold between the two groups of event pairs is reduced from 40km to 20km to 8km to 3km of the last iteration. Fig. 5 shows the spatial distribution of the two earthquake sequences after relocation. It can be seen that the two earthquake sequences are distributed along the Fuyu-Zhaodong fault and develop toward the southwest as time goes by. This paper considers that the M5.0 earthquake of 2017 also occurred on the hidden fault of the Fuyu-Zhaodong fault. What is more noteworthy is that the 5 eanthquakes with MS≥5.0 that happened in Qianguo, Jilin in 2013 have different seismogenic structures from this earthquake, but they all occurred in the intersection of the Fuyu-Zhaodong fault with other faults. Therefore, this paper believes that the Fuyu-Zhaodong fault has shown signs of significant activity since 2013.
The Songyuan MS5.7 earthquake on May 28, 2018 is the largest earthquake in recent years in this area, located at the intersection of the Fuyu-Zhaodong fault and the Second Songhua River fault. The results of the focal mechanism solution show that the earthquake is a strike-slip-type event with a moment magnitude MW5.3 and a waveform-fitting centroid depth of 12km.
This paper relocated 60 earthquakes in the study area. The results show that the aftershock area is about 4.3km long and 3.1km wide, striking NE, and the dominant distribution direction is consistent with the strike of the Fuyu-Zhaodong fault. The strong aftershock activity occurred within 1-2 days after the mainshock, which spread along the Fuyu-Zhaodong fault and spread all over the aftershock area. The frequency and intensity of the aftershocks decreased significantly in 3-8 days, and the aftershocks mainly occurred in the southwest of the aftershock area. After relocation, the focal depths are all greater than 8km, and the dominant distribution is between 9km and 10km, which is related to the thick overburden in the concealed area. The distribution of aftershocks on the section BB' shows that the fault is inclined to the southeast with an inclination of about 75°, which is consistent with the dip of nodal plane Ⅱ of the focal mechanism solution produced by GFZ. Therefore, we infer that the hidden fault of the Fuyu-Zhaodong fault is the seismogenic structure of the Songyuan MS5.7 earthquake.
On July 23, 2017, a MS5.0 earthquake took place 7km away from the earthquake. The two earthquakes have similar focal mechanism solutions. In order to analyze the relationship between the two earthquakes and their seismic activity, we relocated the earthquake sequence of the magnitude 5.0 earthquake of 2017 and the magnitude 5.7 earthquake of 2018. According to the spatial distribution of the relocation results of the two earthquake sequences, we believe that both earthquakes have the same seismogenic structure, and the earthquake sequences generally develop to the southwest.
The strong activity of the Fuyu-Zhaodong fault at the junction of the Qagan Nur-Daozijing fault is indicated by the Qianguo earthquake swarm in 2013, and the historical seismicity since 2009 indicates that the frequency and intensity of the Songyuan earthquake area have been significantly enhanced after the Songyuan MS5.0 earthquake in 2017, which indicates obvious signs of activity at the Fuyu-Zhaodong fault where it intersects with the Second Songhua River fault. Although there are differences in the specific epicenter position determined by different scholars for the magnitude 6¾ earthquake that previously occurred in the earthquake area, it is believed that it occurred in the intersection zone of the Fuyu-Zhaodong fault and the Second Songhua River fault (Tang Yazhi, 1986, 1990; Wu Ge et al., 1988; Li Chuanyou et al., 1999b), located in the southwest of the Songyuan earthquake.
According to the 3-D seismic profile, the rupture length of the northern Fayw-Zhaotong fault in the bedrock on the interface of Qingshankou formation of Cretaceous Series is 26km, accordingly, the maximum magnitude is calculated to be MW6.5, which is equivalent to MS6.6(Shao Bo et al., 2016). This indicates that the southwestern direction of the Songyuan earthquake had the structural condition for generating an earthquake of magnitude 6.0 or higher. Considering the seismic activity since the MS5.0 earthquake in 2017 and its development trend toward the southwest, we believe that we need to pay attention to the development of seismic activity in the southwest of the Songyuan earthquake zone.ACKNOWLEDGEMENTS
This paper was completed in a short time after the MS5.7 earthquake in Songyuan, Jilin. The mobile stations set up by the Jilin Earthquake Agency and the official observation reports produced by the China Earthquake Networks Center are important research materials for this study. We greatly appreciate their help and support.
|Deng Qidong. Map of Active Tectonics in China[M]. Beijing: Seismological Press, 2007 (in Chinese).|
|Department of Earthquake Disaster Prevention, China Earthquake Administration. Catalogue of Chinese Earthquakes 1912-1990, MS ≥ 4.7[M]. Beijing: China Science and Technology Press, 1999 (in Chinese).|
|Department of Earthquake Disaster Prevention, China Earthquake Administration. Catalogue of Strong Earthquakes in Chinese History:Twenty-Third Century BC-1911[M]. Beijing: Seismological Press, 1995 (in Chinese).|
|Duputel Z., Rivera L., Kanamori H., Hayes G. W-phase fast source inversion for moderate to large earthquakes (1990-2010)[J]. Geophysical Journal International, 2012, 189(2): 1125–1147. DOI:10.1111/gji.2012.189.issue-2.|
|Hayes G.P., Rivera L., Kanamori H. Source inversion of the W-phase:real-time implementation and extension to low magnitudes[J]. Seismological Research Letters, 2009, 80(5): 817–822. DOI:10.1785/gssrl.80.5.817.|
|Kanamori H., Rivera L. Source inversion of W phase:speeding up seismic tsunami warning[J]. Geophysical Journal International, 2008, 175(1): 222–238. DOI:10.1111/gji.2008.175.issue-1.|
|Li Chuanyou, Wang Yipeng, Shen Jun, Zhang Lianghuai, Li Chunfeng. Discussion on new activity of the second Songhuajiang fault[J]. Seismology and Geology, 1999a, 21(4): 351–360 (in Chinese with English abstract).|
|Li Chuanyou, Wang Yipeng, Zhang Lianghuai, Li Zhitian, Li Chunfeng. Causative tectonic conditions of the historic earthquake (M=6¾) in 1119 in Songyuan area[J]. Earthquake Research in China, 1999b, 15(3): 237–246 (in Chinese with English abstract).|
|Li Enze, Liu Cai, Zhang Lianghuai, Zeng Zhaofa. The correlation of structure and earthquake in Songliao basin[J]. Progress in Geophysics, 2012, 27(4): 1337–1349 (in Chinese with English abstract).|
|Li Yingkang, Gao Rui, Yao Yutao, Mi Shengxin, Li Wenhui, Xiong Xiaosong, Gao Jianwei. The crust velocity structure of Da Hinggan Ling orgenic belt and the basins on both sides[J]. Progress in Geophysics, 2014, 29(1): 73–83 (in Chinese with English abstract).|
|Liu Junqing, Gan Weijun, Liu Cai, Zhang Chenxia, Gao Jinzhe, Liang Shiming. Relocations and focal mechanism solotions of MS5.5 Qianguo earthquake swarm in Jilin Province in 2013[J]. Seismology and Geology, 2017, 39(5): 981–993 (in Chinese with English abstract).|
|Shao Bo, Shen Jun, Yu Xiaohui, Wan Yongkui, Yu Yang, Qi Gao. Seismic potential research along the north Fuyu fault in Songyuan City[J]. China Earthquake Engineering Journal, 2016, 38(4): 616–623 (in Chinese with English abstract).|
|Sun Wenbin, He Yueshi, Li Yingbo. Subduction of the pacific plate in the Sea of Japan and earthquake of northeastern China[J]. Acta Seismologica Sinica, 1985, 7(1): 35–46 (in Chinese with English abstract).|
|Tang Yazhi. Calculation and determination of the magnitude of the earthquake occurred in Kalamu, Qianguo County, Jilin Province in February 1119[J]. Northeastern Seismological Research, 1986, 2(1): 44–48 (in Chinese).|
|Tang Yazhi. Three essential factor of a severe earthquake in Qianguo of Jilin Province in February 1119[J]. Journal of Northeast Normal University, 1990(3): 151–159 (in Chinese with English abstract).|
|Waldhauser F., Ellsworth W.L. A double-difference earthquake location algorithm:method and application to the northern Hayward Fault, California[J]. Bulletin of the Seismological Society of America, 2000, 90(6): 1353–1368. DOI:10.1785/0120000006.|
|Wu Ge, Fang Heyan, Li Zhitian, Fang Mingyuan. The investigation and study of Qianguo earthquake, 1119[J]. Northeastern Seismological Research, 1988, 4(1): 67–76 (in Chinese with English abstract).|
|Yuwen Maozhao. Records of the Jin Dynasty (1234 A.D.)[M]. Beijing: National Library Microfilm Center, 1993 (in Chinese).|
|Zhang Hongyan, Zhang Guangwei, Wang Xiaoshan, Sheng Jian, Lu Yanhong, Zhang Yu. Relocations of Qianguo earthquake sequence and its seismotectonic analysis[J]. Earthquake Research in China, 2015, 31(3): 518–528 (in Chinese with English abstract).|