| Songyuan area is located in the southern of the central depression area of Songliao Basin,mainly including Songyuan City,Qianguo County,Changling County and so on.The Songliao Bsain is also at the eastern segment of the Central Asian Orogenic Belt which is the overlapping area of the three tectonic domains of the Paleo-Pacific Ocean,the Mongolian-Okhotsk Ocean and the Paleo-Asian Ocean.There are few historical seismic data in Songyuan area,only a magnitude 63∕4 earthquake occurred in 1119 at Qianguo County.In recent years,Songyuan City becomes an active seismic area in Songliao Basin.Seismic activity has increased after 2003.Small-scale earthquake swarm occurred in 2003;Qian’an MS 5.0 earthquake occurred in 2006;Qianguo MS 5.8 earthquake swarm occurred in 2013;A MS 5.7 earthquake occurred at Ningjiang District in 2018.Just one year later,a MS 5.1 earthquake occurred at Ningjiang District again in May 2019.The earthquake sequence in Songyuan area is very complex.The main faults in the region,the north-west-trending Second Songhuajiang fault and the north-east-trending Songyuan-Zhaodong fault are all hidden faults,so there is a lack of clear explanation for the seismogenic mechanism and migration characteristics of earthquakes in the region.As a resource-based city,Songyuan City has exploited many years and its main pillar industry is the exploitation and processing of mineral resources.Moreover,Songyuan is the fortified area with seismic intensity Ⅷ in Jilin Province at present.In recent years,frequent seismic activities have caused huge economic losses and extensive social impacts.Therefore,there is great scientific and practical significance to the study of distribution,migration and dynamic mechanism of earthquake swarm in Songyuan area for the law of seismic activity,the cause of the earthquakes,the state of regional tectonic activity,earthquake early warning and effective disaster reduction.Scholars have attached great importance to the seismicity of the Songyuan area,and a large number of studies on earthquakes of Songyuan area have been done,but all of them were focused on strong earthquakes or local earthquake swarm,there is a lack of research on the overall distribution characteristics,migration characteristics,and fine geological structure of the earthquake source area in the Songyuan area since 2003.In particular,there is no research on the phenomenon that the seismic activity in the Changling area increased from 2019 to 2021 obviously,while the seismic activity in this area was weak from 2003 to 2018.This study collected the seismic data from Songyuan area recorded by 32 seismic stations in the seismic network of Jilin Province from January 2003 to February 2021.The 2019 Songyuan MS 5.1 earthquake was studied in detail,the moment tensor solutions under three different velocity models were calculated by the ISOLA method,and the tectonic stress field in source region was calculated by the STRESSINVERSE method.The three-dimensional velocity and Poisson’s ratio structure of the middle and upper crust in the Songyuan earthquake area are calculated by the double-difference tomography method.The fine structure of the source region,earthquake genesis and migration characteristics of Songyuan earthquake are discussed,and the future seismic risk of the Songyuan area is inferred.The main conclusions are as follows:1)In the focal mechanism solution of the 2019 Songyuan MS 5.1 earthquake,the double couple component is 91.5%.Nodal plane Ⅱ confirm the strike,dip and rake of rupture plane are 302°,80° and 14°,respectively,which are the same as the parameters of the Second Songhuajiang fault.The tectonic stress field in source region is consistent with the results of the regional tectonic stress field.Therefore,this study infers that the 2019 Songyuan MS 5.1 earthquake is a typical tectonic earthquake triggered by the regional tectonic stress field,and Second Songhuajiang fault is the seismogenic structure.2)The high-resolution 3D Vp,Vs and Poisson’s ratio structures at depths of 0 to 20 km are calculated by the double-difference tomography method.This study firstly find that the velocity structure in the 2003—2021 Songyuan earthquake region(0~15 km underground)is generally low P,low S wave velocity anomalies and low Poisson’s ratio.Previous studies have shown that the fluids and partial melting can reduce seismic wave velocity in crustal rocks,and the Poisson’s ratio in source region(123°E—125°E,44°N—46°N,0~15 km)is less than 0.30.Combined with the comprehensive analysis of the focal mechanism,magnetotelluric and geothermal data,it is speculated that beneath the Songliao Basin,the subduction of the western Pacific Ocean caused melting near the top discontinuity of the mantle transition zone,resulting in the generation of hot material.The fluid carried by the upwelling of hot material beneath source region is injected into the crust,which is the fundamental cause of the earthquakes in the Songyuan area.3)According to the fault weakening model,the fluid gradually weakens the faults by long‐lasting fluid‐rock interactions.When the fault strength reaches the critical value,the earthquake is triggered.Therefore,it is not enough to accumulate the stress to trigger a large earthquake,so it is inferred that the probability is low for a high-magnitude earthquake occurrence in the Songyuan area.The seismic activity in the Changling area increased from 2019 to 2021,but the magnitudes of the earthquakes were all low.It is speculated that the fault in the Changling area is weak,and due to the superposition of human activities and regional stress fields,low-magnitude earthquakes will continue to occur in the Changling area in the near future.4)A north-northeast S-wave low-velocity belt is found at about 5~10 km beneath the Songyuan area,which connects Songyuan City,Qianguo County,and Changling area.It is inferred that the Songyuan-Zhaodong fault provides a pathway for the upwelling of deep fluids,and controls the north-northeast bead-like distribution of earthquakes.The strength of the hidden faults are different,resulting in different rupture times of the faults in different regions,and causes the earthquake migration from 2003 to 2021 shows a characteristic of rebounding with time.The small concealed faults near Songyuan City and Qianguo area have a short rupture interval,resulting in the earthquake migration from 2004 to 2012 shows a characteristic of continuous rebounding with time.5)It is speculated that the reason why the location of the Songyuan earthquake is no longer extended to the north-northeast direction in the Ningjiang District is that the Second Songhuajiang fault disperses a part of the fluid that migrated along the Songyuan-Zhaodong fault,or the strength of the Songyuan-Zhaodong fault which is in the north of the Ningjiang District has not yet dropped to the critical value. |
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