Shearer & Toy 1991 Creager 1992 Song & Helmberger 1993), rotation (e.g. They have been extensively used to study the Earth's inner core such as anisotropy (e.g. PKP waveforms and differential travel times between branches are powerful in studying the structure of the Earth's core, which reduce influences from earthquake sources and mantle heterogeneities. They include the PKIKP (or PKPdf) phase that traverses the inner core, the PKiKP (or PKPcd) that is reflected from the inner core boundary, the PKPbc phase that turns at the bottom of the outer core and the PKPab phase that turns at the mid-outer core. Multiple PKP core phases arise from the P velocity decrease at the core-mantle boundary and velocity increase at the inner core boundary, which are commonly observed from earthquakes (Fig. P, SS) are not correlated with seismicity, which suggest the influence of the scattering of the wavefield rather than wave reverberation. At short period (5–10 s), the reconstructed phases (e.g. ( 2014) suggested that certain phases (in particular, core phases ScS, PKIKP) reconstructed at long periods (25–100 s) come from reverberation of earthquakes waves and there are spurious arrivals.
2015), or from the ambient noise (Boué et al. The source for the generation of the body waves is under debate, which is suggested to come from the coda of large earthquakes (Lin & Tsai 2013 Lin et al. ( 2015) showed robust observations of PKIKP 2 and PKIIKP 2 phases (which have never been observed simultaneously from an earthquake) from stacks of autocorrelations at global arrays using earthquake coda. ( 2013) and Nishida ( 2013) showed that various body waves through the entire Earth can be extracted from the seismic noise. Using near antipodal stations around the globe, Lin & Tsai ( 2013) reported additional body waves (including antipodal PKIKP waves) from individual station pairs without stacking. ( 2013) reported observations of many body wave phases sampling the Earth's core, including PKIKP 2 (P wave traversing the inner core and bounced back from the other side of the globe). Stacking EGFs between the USArray stations across the continental U.S. However, some studies have shown it feasible to extract signals of body waves from ambient noise correlations (Roux et al. So far, most studies have focused on surface waves, which are easily extracted from ambient noise correlations. 2008) since the early works on the extraction of the empirical Green's function (EGF) between receivers from noise correlation (Lobkis &Weaver 2001 Campillo & Paul 2003 Shapiro & Campillo 2004). Noise correlation, PKP triplication, Inner core, Body waves 1 INTRODUCTIONĭuring the last decade, ambient noise correlation has been widely used in studying the structure of our planet (e.g. The observation of triplicated PKP branches from noise correlations provides a new type of data for studying the Earth's deep interior, in particularly the inner core anisotropy, which overcomes some of the limitations of traditional earthquake-based studies (such as limited source distributions and source location errors). The best coda window is around 10 000–40 000 s and the best magnitude threshold is M w greater than 6.8 or 6.9. However, earthquakes far from the stations contribute more than events closer by.
The generation of the PKIKP phase (df branch) from earthquake coda does not depend on earthquake focal mechanisms or focal depths.
At longer periods (15–50 s), earthquake coda are effective in generating the df branch, but not the ab branch. At shorter periods (5–20 s), three branches of PKP (df, bc and ab) can be extracted from ambient noise and the ab phase from earthquake coda. However, the contributions vary with frequency and with body-wave phases. Both ambient noise and earthquake coda contribute to PKP phases. We can clearly observe different PKP branches (df, bc and ab) in stacks of the station–station correlations. We use dense seismic arrays in South America and China Regional Seismic Networks at distances from 145° to the antipode. Here we report our first observations of triplicated PKP phases (important phases for studying the Earth's core) and confirm observations of other body-wave core phases from noise correlations. The extraction of body waves has been very limited, but recent reports have suggested promises for deep incident waves. Ambient noise correlation method has been widely used to extract surface waves and tomography.