Study On The Crustal Structure And Earthquake Swarm At Northeastern China Based On Dense Seismic Array Observations

Studying the detailed crustal structure is of great importance to better understanding the regional tectonic evolution and the mechanism of earthquake generation.With increasing number of broadband seismic stations,we now have unprecedented opportunities to investigate the crustal structure and focal mechanism in more details based on dense array observation.Northeast China(NEC)is located at the eastern part of the Central Asia orogenic belt,with Siberia plate in the north,North China plate in the south and Pacific plate in the east.Due to the structural evolution at different periods,NEC is composed with diverse geological structures,such as the Great Xing'an range,the Songliao basin,the Lesser Xing'an range,the Zhangguangcai range,as well as the Changbaishan volcano,Wudalianchi volcano and Nuominhe volcano.In addition,due to the subduction of the northwest Pacific plate,NEC is the only region in China having deep seismicity with depth greater than 500 km,which provides a good opportunity to study the crustal structure by using the Moho-converted and reflected phases generated from deep earthquakes.In the first part of my thesis,I derive the Moho depth,S-wave velocity contrast across the Moho,an average 1-D velocity model and the northern boundary of the suture zone between Xing'an block and Songnen block by using the travel times and amplitudes of Moho-converted and reflected phase from local deep earthquakes.The results show that the Moho depth beneath NEC generally correlates with the surface topography.The crust beneath the Songliao basin in the center of my study area is very thin,while the Moho depth of Great Xing'an range and Zhangguangcai range on the eastern and western is relatively deep.Furthermore,the Moho depth under the Wudalianchi volcano area is 2 km deeper,which may relate to the underplating of the mantle material.In addition,we observe a 6 km Moho offset between the Xing'an block and the Songnen block,which maps the northern boundary of Heihe-Hegenshan suture zone.In the second part of my thesis,I focus on the two Ms>5.0 moderate earthquake swarms occurred in Songliao basin in recent years.The inverted focal mechanisms show that the 2013 Ms>5.0 events mostly exhibit thrust compression with significant volume closure components.We infer that the non-double couple components may be produced by the rupture on the non-planar faults.And the relocated earthquake sequences show that the hypocenters migrate from deep to shallow,which implies that the swarm earthquakes are driven by fluid pressure.Both low P-and S-wave velocity anomalies are observed in the source region,indicating that the structural variations along the fault control the occurrence and the spatial distribution of swarm earthquakes.Our results suggest that the faults provide pathways for the upward intrusion of fluids that control the distribution and migration of the swarm sequences.A long-term fluid-rock interaction could lead to the fault materials intensely fractured in the 2013 swarm area but relatively less damaged in the 2017 swarm area.We conclude that fluids play an important role in triggering the 2013 and 2017 moderate earthquake swarms.