The Teleseismic Green's Functions For Complex Subduction Zones And Their Applications On Earthquake Source Study

Teleseismic body waves(P and SH waves)are essential for obtaining source parameters or imaging rupture processes of moderate-strong and large earthquakes(Mw>5.5)occurring in both interplate and intraplate regions.Earthquake source parameters are usually characterized as doule couple point source or finite fault models,usually obtained by inversions using only turning(direct)P and SH waves without considering the reflected phases from the Core-Mantle Boundary(CMB)in the Green's Functions.However,core-reflected waves such as ScS usually have amplitudes comparable to direct S waves due to the total reflection from the CMB and might interfere with the S waves,especially at large epicentral distances for long duration earthquakes.In order to understand how core-reflected waves affect teleseismic body wave inversions,we develop a procedure named Multitel3 to compute teleseismic Green's functions that contain turning waves(direct P,pP,sP,direct S,sS and reverberations in the crust),core-reflected waves(PcP,pPcP,sPcP,ScS,sScS et al.).This ray-based method can efficiently generate synthetic seismograms for turning and core-reflected waves independently,with the flexibility to take into account the 3-D Earth structure effect on the timing between these phases.Conventional Green's Functions calculation methods do not consider the complex 3D structures in the source region.The complex near trench velocity structures,charaeterized by strongly varying bathymetry,coexisting fluid ocean water and solid crust,and sometimes thick sediments,can produce substantial waveform complexities for near trench earthquakes,which place substantial difficulties to study earthquake source parameters through waveform inversion/modelling.Wu et al.(2018)developed a new hybrid method,which precisely computes global synthetics with complex source-side 3D velocity structures.This hybrid method uses the Spectral Element Method(SEM)to compute the 3D wavefields at the boundaries of the source-side box and then propagate the wavefields to the rest of the earth with the I D Direct Solution Method(DSM).The combination of the two methods greatly improves the efficiency in generating 3D teleseismic synthetics that incorporate the source-side complex velocity structures.The performance of the Multitel3 approach is first assessed through a series of numerical inversion tests and then applied to some earthquake applications.We use the Jackknifing method to quantitatively test the effects of ScS on focal depth and mechanism in CAPtel inversion.When ScS is not considered for body wave data in the epicentral distance range 70°?90°,there are systematic deviations of 8° for focal mechanism and 1 km for source centroid depth.And then we apply our improved Green's Functions to study the source parameters of the 2008 Mw6.0 Qingchuan aftershock and the 2008 Mw6.4,Mw5.9 Gerze earthquakes.We also test the impacts of core-reflected waves on finite fault inversions with synthetic waveforms of the 2008 Mw7.9 Wenchuan earthquake and the 2015 Mw7.8 Nepal earthquake.Finally,a finite fault inversion of the 2005 Mw8.7 Nias-Simeulue earthquake is carried out using the improved Green's functions.Using enhanced Green's functions yields better inversion results as expected.We explore the wavefield complexities from complex source structures via comparing and modelling teleseismic records of a Mw6.6 near coast event and a Mw6.8 near trench event in the 2015 Illapel earthquake sequence(offshore Central Chile).For the near coast event,the waveforms of direct P-waves in teleseismic/diffracted distances are simple and we obtain highly consistent source parameters from 1D regional and teleseismic waveform inversions.In contrast,the near trench event produced much stronger and longer teleseismic/diffracted P-wave codas(>100 s),resulting in much larger differences between regional and teleseismic ID focal mechanism solutions,in particular for the centroid depth.In this study,we adopt a SEM-DSM hybrid approach to precisely handle the complex source-side 3D structures(bathymetry and water layer in this case)and investigate their roles in the genesis of strong P-wave codas.Compared with the 1D synthetics,the 3D synthetics significantly improve the waveform fits up to 0.1 Hz when the source is placed at the preferred horizontal(-22 km from the trench axis)and vertical(-9.5 km below the sea-level and?4 km below the ocean bottom)location.In summary,this thesis develops an improved ray-based method to calculate teleseimic Green's Functions with the consideration of core-reflected phases for earthquake source studies and also uses a hybrid method with considering complex strucutres in the source region to develop an earthquake relocation technique based on 3D Green's Functions.