Ambient Noise Tomography Of Yilan-Yitong Fault In Luobei Area

Luobei area is located in the Yilan-Yitong fault zone in Heilongjiang Province.The small earthquakes in this area are active,and the small earthquakes are the most intensive in Heilongjiang Province.There have also been many moderate earthquakes,such as the M5.8 earthquake on June 22,1963.However,there are few studies on geological structure and other aspects in this region,and the geological data on seismic activities are very scarce.Most of the current studies in Luobei region judge the distribution of underground faults through satellite remote sensing images and some geological and geomorphological characteristics.But the structural and geomorphological characteristics will be affected by artificial and natural transformation,and the distribution of underground geological structures and faults cannot be accurately obtained.In order to study the relationship between seismic activity and shallow surface fault zone in Luobei area,determine the tectonic evolution and shallow surface distribution of Yilan-Yitong fault in Luobei area,and judge the risk of future seismic activity of the fault,it is necessary to obtain the shallow surface geological structure and fault distribution characteristics in Luobei area,which has very important reference significance for the study of fault distribution and seismic disasters in Luobei area.In this paper,based on the ambient noise tomography method,a half-month field data acquisition work was carried out in Luobei area.A total of 37 short-period mobile stations were set up,and the ambient noise data of 15 days were collected.By analyzing the characteristics of the original data,the data were preprocessed to improve the data quality,and 120 group velocity dispersion curves with high signalto-noise ratio were obtained.The inversion process should minimize the residual to ensure the accuracy of the results.Finally,the three-dimensional shear wave velocity structure within 2.6 km underground in Luobei area is obtained,and the underground distribution characteristics and geological structure evolution of Yilan-Yitong fault and its intersecting secondary faults are discussed.The seismic activity of Luobei area is discussed,which provides a scientific basis for seismic activity tracking and underground structure research in this area.The main conclusions are as follows:(1)According to the shear wave velocity results of different depth slices,combined with the tectonic evolution history of Yilan-Yitong fault and Tangyuan fault depression,the tectonic movement and crustal activity in Luobei area can be roughly divided into three periods,less than 2.6 km,in the middle and late Jurassic period,the eastern mainland of China formed the Yilan-Yitong fault zone under the joint action of the closure of the Mongolia-Okhotsk Sea and the subduction of the Izanagi plate;0.4?1.8 km,in the Paleogene period,due to the continuous expansion of the Pacific plate,the Eurasian plate was subducted from the northwest direction,resulting in a change of the direction of tectonic movement in Luobei area.Under the control of the basement fault,the morphology and distribution of the Yilan-Yitong fault and the NW fault also changed;0?0.1 km,during the Quaternary period,under the combined action of the compression of Baikal plate from north to south and the East-West subduction of Pacific plate,the manifestation of faults in Luobei area has changed from linear large faults to regional circular extensional faults.(2)The specific location and underground morphology of the Yilan-Yitong fault and Tianbaoshan fault were determined by deep slices and section characteristics.Before the Paleogene,the Yilan-Yitong fault is distributed in NE direction and passes directly below Luobei County,the NW-trending Tianbaoshan fault and the YilanYitong fault were intersected at about 90°.After the Paleogene,due to the subduction of the Pacific plate,the crustal movement mode in Luobei area changed.Under the control of the basement fault,the Yilan-Yitong fault and the Tianbaoshan fault had vertical slip,the fault morphology changed,and the strata had obvious thrust extrusion.(3)Since the Quaternary,although the crustal activity has been weakened,the Yilan-Yitong fault has been active.Under the background of the new tectonic movement,the underlying Yilan-Yitong fault and its intersecting secondary faults jointly control the development of the overlying strata and form multiple circular extensional fracture zones.