GENESIS OF SONGYUAN EARTHQUAKES BASED ON 3D RESIDUAL DENSITY STRUCTURE
Since 2006,more than 10 earthquakes with the magnitude of 4 or higher have occurred continuously in Ningjiang district and Qian'an county,Songyuan city of Jilin province,with tens of thousands of aftershocks.The largest earthquake is the magnitude 5.8 earthquake on November 23,2013 in Qiangorlos Mongolian autonomous county,Songyuan city,Jilin province.Frequent earthquake activities not only cause a large number of damaged buildings,but also trigger geological disasters such as sand liquefaction and slope instability,which have attracted widespread attention from the public and government departments.In order to comprehensively understand the causes of earthquakes in the Songyuan area,much studies on earthquake anomalies,seismogenic structures,and source depths have benn conducted recently.However,there are still some difference opinions in understanding the causes of earthquakes in the Songyuan area,for example,the major different viewpoints include:1)it is caused by the regional stress release caused by Pacific subduction,mainly based on the consistency between the stress field in the source area and the background stress field of the Pacific plate subducting towards the edge of the east Asian continent;2)it is related to deep melt or fluid migration,mainly based on the existence of low-speed and low resistance anomalous bodies in the deep part of the seismic area;3)it is related to long-term oil and gas extraction,the main basis is that the seismic source spectrum exhibits characteristics of early and fast attenuation compared to typical structural earthquakes,and the time-domain and frequency-domain characteristic parameters such as waveform complexity and spectral ratio are also significantly different from typical structural earthquakes.In addition,the seismic source mechanism contains a large number of nondual force source components and shallow seismic source depth.Moreover,there is also significant controversy over the seismogenic structure,with the focus on whether the Qian'an and Ningjiang earthquake regions are controlled by the same fault zones.Some scholars infer that the seismogenic structures in both areas are the NE-trending Fuyu-Zhaodong Fault based on the source mechanism solution.Another group of scholars believe that the Qian'an earthquake area is controlled by NW-trending hidden faults,while the seismic structure in the Ningjiang earthquake area is the second NW-trending Songhuajiang Fault.The reason for the above controversy lies in the lack of necessary constraints on the deep structure of the seismic area.To solve above problems,in this article we studied the three-dimensional residual density structure of the Songyuan earthquake area by performing nonlinear conjugate gradient focusing inversion on regional gravity data.Combined with oil drilling and reflection seismic data,some new insights were obtained as follows:1)The residual density anomaly in the study area shows a high-low alternating strip distribution,with the southern anomaly trending NNW and the northern anomaly transitioning to NNE,This feature reflects the different deep earthquake environments in the Qian'an earthquake area and the Ningjiang earthquake area.The former's source is located in the high-density anomaly body and its edge of Chaganhua,while the latter's source is located in the middle of the low-density anomaly zone in Songyuan,indicating different rock types in the two locations.The basement of the Qian'an earthquake area is composed of limestone and metamorphic volcanic rocks,while the basement of the Ningjiang earthquake area is composed of fractured granite magmatic rocks.The stability of the basement structures in both areas is poor;2)The seismic structures of the two are different.The former is controlled by the Chaganhua Fault and Qian'an Fault,while the latter is controlled by the Songyuan Fault and the Fuyubei Fault;3)The formation of earthquakes is related to factors such as regional stress,basement structure,deep gas/fluid migration,and long-term oil and gas extraction.Long term water injection and oil recovery have damaged geological structures and stress environments,which may be important triggering factors.4)In terms of the seismic source mechanism,we concluded that under the sustained action of regional stress in the near EW direction in the Songyuan area,the basement fault structures and fractures in the Qian'an and Ningjiang earthquake areas are extremely developed.Deep gas/fluid migrates upwards along the basement fractures,further reducing the stability of the basement structure.Long term water injection,oil and gas recovery,as well as fracturing,greatly damage the structural integrity of the cap rock and the distribution of deep stress,which results in the compressive and torsional strike slip movements in the relevant structures.
Songyuan earthquakeThree-dimensional residual density structuredeep seismogenic environmentbasement structureseismic genesis
万方数据
维普
