Verification and applications of surface waves extracted from ambient noise
thesisposted on 28.03.2022, 20:49 authored by Jun Xie
About a decade ago, it was shown that Empirical Green's Function (EGF) between two seismic stations can be extracted from cross-correlation of ambient seismic noise. Since then, studies based on ambient noise have become a method of choice among seismologists. Noise Cross-correlation Function (NCF) has been widely used for seismic tomography (known as ANT), monitoring velocity changes, calibrating earthquake location, and so on. Almost all of these studies are based on the assumption that accurate EGF can be extracted from cross-correlations of ambient noise. However, uncertainties of the dispersion measurements from ambient noise are still not clear. It is also not certain whether surface waves at periods longer than 50 s can be extracted from ambient noise recorded at portable seismic stations or whether the resulting dispersion measurements from ambient noise are accurate enough to image lithosphere and asthenosphere structures. This thesis is to address these questions. I investigate the accuracy of surface wave dispersion curves at both short (10-30 s) and long (50-250 s) periods from ambient noise. By comparing the waveforms and dispersion curves from ambient noise with those from earthquake data, I demonstrate that the dispersion measurements from surface waves extracted from ambient noise are as accurate as those from earthquake data. The dispersion measurements can be used in ambient noise tomography to provide complementary data, which can be used to constrain the lithospheric and asthenospheric structures. Furthermore, I demonstrate that broadband surface waves at 10-150 s period from ambient noise can be used to construct the 3D shear wave velocity structure from the surface down to 300 km depth in the USA. I calculate NCFs among all the US Aarray stations in US continent and extract broadband (10-150 s) Rayleigh wave phase velocities, and obtain phase velocity maps by inverting the broadband dispersion curves. Then, I build a new 3D shear wave velocity model of the lithosphere and asthenosphere structure by inverting the resulting phase velocity maps. Finally, I demonstrate that using EGFs from ambient noise, we can improve earthquake location and locate recorded historical earthquakes. Using a seismic array in central Australia, I test the accuracy of our location method using ambient noise. I find that, when the distribution of the remote station is good and the reference station is located less than 20 km away from the source, the location error is less than 2 km. I conclude that this method can be used to obtain ground truth event with location error equal or smaller than 2 km (GT2) in regions with sparsely distributed remote stations.