| Moderate to great earthquakes(Mw>5)can cause lots of casualties and economic losses.The research on the seismic rupture process is helpful for people to understand the mechanism and evolution of earthquake,as well as improve the prevention,early warning,and assessment of earthquake hazards,which can reduce the disasters caused by the earthquake to human society and ensure the safety of life and property in seismically active areas.Previous studies have found that seismic waveforms at different frequency bands show different characteristics of the seismic rupture,which indicates a relationship between the frequency content of seismic data and the seismic rupture process.Such a relationship is called the frequency-dependent rupture characteristics.The research on the frequency-dependent rupture characteristics is an important part of the research on the seismic rupture process,which can deepen our understanding of the mechanism of earthquake and the relationship between earthquake and seismogenic environment.By analyzing seismic waveforms at different frequency bands,i.e.,the multi-scale analysis of seismic waveforms,the frequency-dependent rupture characteristics of one earthquake can be obtained.In this thesis,a multi-scale strategy for the wavelet-domain finite fault inversion is developed based on previous studies.The wavelet transform is used as a tool for multi-scale analysis to transfer seismic waveforms to different wavelet scales in the wavelet domain,and then the rupture features contained in different wavelet scales are analyzed.In the multi-scale strategy,the finite fault inversion is divided into two stages.In the first stage,the wavelet scales with lower-frequency seismic waveforms are involved in the inversion to obtain the larger-scale pattern of the seismic rupture.In the second stage,the wavelet scales with higher-frequency seismic waveforms are added to resolve the smaller-scale rupture features.The division of lower-and higher-frequency ranges is mainly determined by the amplitude of different wavelet scales.For different earthquakes,the division is different.Generally speaking,wavelet scales of<0.3 Hz is regarded as lower frequency range,while>0.3 Hz is regarded as higher-frequency range.Such a multi-scale strategy is testified by synthetic tests and compared with the traditional wavelet-domain finite fault inversion method.The results show that,compared with the traditional strategy,the multi-scale strategy proposed in this thesis can better resolve the smaller-scale rupture features contained in the higher-frequency data,as well as can work efficiently at converging to the global minimum solution with regard to the choice of inversion parameters.In this thesis,the frequency-dependent rupture characteristics of three different types of earthquakes were studied by finite fault inversion with such a multi-scale strategy and time-domain back projection,including shallow thrust earthquake(the 2015 Mw 7.8 Gorkha,Nepal earthquake),intermediate-depth normal fault earthquake(the 2019 Mw 8.0 northern Peru earthquake),moderate crustal earthquake(the 2016 Mw 6.5 Norcia,Italy earthquake).For the 2015 Mw 7.8 Gorkha,Nepal earthquake,the teleseismic waveforms were used to perform finite fault inversion(0.01?0.625 Hz).This thesis found that there are two high-slip regions,one is corresponding to a larger-scale rupture zone,which produced a large area of lower-frequency slip and lots of lower-frequency energy radiation,accompanied by some higher-frequency energy radiation appeared nearby the edge of lower-frequency slip;the other is corresponding to a smaller-scale rupture zone,which produced not only some lower-frequency slip but also some higher-frequency energy radiation.The frequency-dependent rupture characteristics of this earthquake are consistent with the previous understanding of shallow megathrust earthquakes on the subduction zone,i.e.,the higher-frequency energy radiation is mainly distributed at the lower boundary of the lower-frequency slip region.For the 2019 Mw 8.0 northern Peru earthquake,the teleseismic waveforms were used to perform back projection(0.5?2.0 Hz)and finite fault inversion(0.01-0.625 Hz).This thesis found that the rupture propagated along the strike of the fault plane,resulting in two high-slip regions with three high-slip-rate areas,which is consistent with the three higher-frequency energy radiation subevents.The two high-slip regions show different rupture features,which may be caused by the complex morphology of the subducted slab.The first high-slip region is located in the gently dipping(smaller dipping angle)part of the slab,while the second high-slip region is located in the steeper dipping(larger dipping angle)part of the slab,which produced larger rupture area,higher rupture velocity,and larger slip.The frequency-dependent rupture characteristics of this earthquake are different from those observed in the subduction zone megathrusts.At intermediate depths,the higher-frequency energy radiation appears to distribute at the upper boundary of the lower-frequency slip region,indicating that the mechanism of the frequency-dependent rupture characteristics of shallow earthquakes and intermediate-depth earthquakes may be different.For the 2016 Mw 6.5 Norcia,Italy earthquake,the teleseismic waveforms and near-field strong motion waveforms were used to jointly obtain the slip patterns on different frequency bands(0.02?0.312 Hz and 0.312?0.625 Hz).This thesis found that the lower-frequency slip is mainly concentrated at the shallower part of the fault,while the higher-frequency slip appears nearby the edge of the lower-frequency slip.The frequency-dependent rupture characteristics of this earthquake may be related to fault complexities in central Italy,which can produce many stress barriers to inhibit the propagation of seismic rupture.When the seismic rupture propagates to these stress barriers,the sudden stop will release lots of higher-frequency energy radiation.Meanwhile,Vp/Vs tomography shows that fault complexities may cause the uneven distribution of fluids,resulting in unevenly distributed pore fluid pressure,which can further cause different parts of the fault to rupture at different scales and release energy radiation of different frequency components.Through the study of these three different types of earthquakes,some understanding of frequency-dependent rupture characteristics and their mechanisms has been obtained in this thesis.First,it is found that the higher-frequency energy radiation and the lower-frequency slip show a "complementary" distribution,i.e.,the higher-frequency energy radiation mainly appears at the edge of the lower-frequency slip region.Second,the frequency-dependent rupture characteristics of intermediate-depth earthquakes are different from those observed in the subduction zone megathrusts,which may indicate that there are different mechanisms at different depths of the subduction zones.Third,the frequency-dependent rupture characteristics are strongly associated with the fault structure in the seismic rupture zone.The variation of morphology and physical properties of fault structure will contribute to seismic rupture at different scales and generate energy radiation of different frequencies,which can lead to the frequency-dependent rupture. |
PDF Full Text Request