Source Characteristics Of Destructive Earthquakes On Buried Faults In Typical Areas

The destructive earthquakes may occur on buried faults,posing huge threats to social security,yet it is difficult to explore through field geological surveys directly.Studies of earthquakes on buried faults have been a challenging frontier in seismology.Space geodesy constrains the earthquakes and causative faults in continental areas through surface deformation and displacement fields.Geophysical survey and drilling sampling image the internal structure of the earth,and describe the significance of the buried fault.Seismic waveform research is an important means for the analysis of source characteristics and buried faults.However,some earthquakes are located in areas lacking dense observations,or areas where it is difficult to deploy dense arrays due to geographical conditions and other factors.This paper focuses on several typical areas such as continents and seas with a sparse network and conducts case studies of destructive earthquakes occurring on buried faults.The study analyzes the relationship between earthquakes and buried causative faults from three aspects:accurate epicenter location,reliable depth,and rupture directivity of the earthquake source.First,this paper evaluates the accurate epicenter determination method based on the ambient noise empirical Green’s function under the sparse network quantitatively.The study selects the 2008 Wudu Ms5.5 earthquake with the real location observed by InS AR as the standard and uses several portable stations near the epicenter as reference stations to extract the ambient noise empirical Green’s function.The seismic waveforms are then corrected using the ambient noise surface wave empirical Green’s function to relocate the Wudu earthquake and compared with the real epicenter.The results show that using a reference station within 30 km from the epicenter and the 10～30 s band Rayleigh wave,and the noise stacking is one month,the positioning accuracy is within 5 km.When the noise cross-correlation stacking time is more than one year the relocation accuracy is better than 1 km.We further conducted tests for a sparser seismic network.It shows that using about 10 remote stations with reliable noise EGFs dispersion for relocation,the relocation results with errors deviations within 3 km can also be obtained,giving the observation system of the permanent station network and reference stations required for the high-precision positioning of this method.Then,for the difficulty of accurately determining the hypocenter depth under the sparse network,this study determines the hypocenter depth of the 2014 Jinggu Mw6.1 earthquake with a relative method based on Pn/Pg arrival times by the reference event.Based on the regional magnetotelluric results and the experimental rheological model,the depth of the brittle-ductile transition zone on the young fault is also about 10 km,similar to that on mature faults.The paper also determines the rupture directivity of the 2018 Mw5.8 earthquake in the Taiwan Strait via resolving the difference between the hypocenter and the centroid location.We find that the earthquake ruptured from north to south about 3.6～5.6 km,and inferred that the causative fault is a north-south trending,buried Dongshan uplift eastern fault,reactivated by the neotectonic evolution.And the 1994 Mw 6.7 normal faulting earthquake in the Tainan Basin may promote the occurrence of the 2018 Mw5.8 earthquake.In summary,this paper provides criteria of the ambient noise empirical Green’s function location method and confirms that the reference seismic phase method can obtain reliable focal depth and rupture directivity,providing effective tools for studying earthquakes on buried faults in sparse network areas.Yet,with the progress of highperformance computing and earth model research,it is promising to develop a highprecision source parameter determination method adopting empirical and theoretical 3D Green’s function.And integrated observations such as seismology,geology,and tectonics should be implemented,to jointly study the buried fault system and destructive earthquakes,to provide important information for the risk assessment of regional buried faults.